Microvolt 3901 User Manual

Introduction

Microvolt 3901 is a software instrument from Pittsburgh Modular. At its heart is a component level emulation of the Microvolt 3900, our single-oscillator analog synthesizer that blended east coast subtractive synthesis with west coast wave shaping. Around that core the 3901 gathers circuits from across our history: interchangeable oscillators, two modulation places that take any of six modulation modules, eight effect slots that take any of nineteen effects, a patch bay that reaches every module, a step sequencer, a playable keyboard and six-voice polyphony.

Every module on the panel can be patched to every other. Cables carry audio-rate signals at the hardware's voltages, so the panel behaves like a small modular system that happens to live inside your computer or iPad.

Formats

macOS: Formats: VST3, Audio Unit, standalone app; Requirements: macOS 12 or later, Apple silicon or Intel
Windows: Formats: VST3, standalone app; Requirements: 64-bit Windows
iPad: Formats: Standalone app with an Audio Unit (AUv3) plug-in inside; Requirements: iPadOS 15 or later

In plug-in hosts it appears as Microvolt 3901 from Pittsburgh Modular, as an instrument with a stereo output.

Hearing safety

Microvolt 3901, by itself or with amplifiers, headphones or speakers, can produce sound levels that may cause permanent hearing loss. Resonance, feedback patches, long delay repeats and self-oscillation can raise the level suddenly. Start with a low volume and raise it gradually, and lower your monitors before loading a preset or patching a feedback cable.

Installation and activation

macOS

Run the Microvolt 3901 installer. It installs the standalone app in Applications, the VST3 in /Library/Audio/Plug-Ins/VST3 and the Audio Unit in /Library/Audio/Plug-Ins/Components; choose Customize in the installer to leave any of them out. Installing a new version replaces the old one and keeps your presets.

Windows

Copy the VST3 into C:\Program Files\Common Files\VST3 and the standalone app wherever you keep your programs. After installing, rescan plug-ins in your host if Microvolt 3901 does not appear.

iPad

Install the app from the App Store. Its Audio Unit plug-in comes with it (see Microvolt 3901 on iPad).

Activation (macOS and Windows)

Until Microvolt 3901 is activated, the activation screen covers the panel and the instrument is silent. Activate online through the store page that opens in your browser, or offline by exchanging a machine file for a license file. Once activated, the screen steps aside and the sound comes back without a restart.

One activation covers every format on that computer: the standalone app, the VST3 and the Audio Unit.
SETTINGS > License… shows your license and has Deactivate, to move the license to another computer.
The license is checked online from time to time; a computer that has not been online for 7 days asks to check again.

The iPad app is bought through the App Store and needs no activation.

Getting started

The first sound

Load Microvolt 3901 on an instrument track, or open the standalone app, and play a key. The default panel plays the Microvolt oscillator's sawtooth through the Binary Filter and the Dynamic VCA, shaped by the envelope generator, with the function generator sweeping the filter. From there:

Raise the other waveform level sliders in the oscillators.
Move the binary filter's FREQUENCY and RESO sliders.
Turn on an effect with its ACTIVE button.
Open the presets window with the PRESETS button and try the factory sounds.
Patch a cable to change the routing.

Sliders, knobs and buttons

Drag a slider or knob vertically to move it. Hold Shift while dragging for fine control.
Double-click (double-tap on iPad) a control to return it to its default.
The mouse wheel moves the control under the pointer.
Click or tap a button to switch it. Buttons light while they are on.
On iPad, several controls can be moved at once with several fingers.

Readouts

While a control is touched, a small bubble beside it shows its value in real units: tuning in semitones and cents, rates in Hz (or the clock division when tempo sync is on), times in ms or seconds, balances as the share of each side, and plain levels and amounts from 0 to 100 %.

Typing a value

Right-click a tuning control (the oscillators' FINE TUNE sliders and the oscillator / LFO PITCH knob) or the TEMPO knob to type an exact value. Tuning is set in tenths of a cent: Shift-drag and the mouse wheel move a tenth of a cent at a time (the wheel moves a cent with Shift).

The ? and + buttons

The ? button in a title bar opens a short description of the module in that place; click anywhere to put it away. (The envelope generator's ? has no text yet.) The places that take more than one module also have a + button, which opens the menu of modules for that place (see Panel overview).

Lamps

Slider lamps follow the signals routed to them: the LFOs, the function generator, the envelopes, the filter cutoff and the random gates. Each glows steadily for fast signals and goes dark when its signal is at zero.

If the sound stops

A red note at the top left of the panel appears if the engine has had to restart a section ("engine restarted"), or, in the standalone app, if the audio device has stopped ("audio device is not running").

Panel overview

The panel is laid out as a hardware synthesizer. The top row is the voice: two oscillator places, the Binary Filter, the Dynamic VCA and the output. The middle row holds the scope, the MIDI / arpeggiator section, two modulation places and the envelope generator. Under them runs a row of buttons, and the bottom of the panel shows the eight effect slots, the keyboard controller or the step sequencer. The patch bay fills the right side.

Places that take more than one module

Four places on the panel can hold different modules. Press the + in a place's title bar to choose:

Oscillator 1: Microvolt Oscillator, Double Helix 6-Bit Wavetable Processor, DNA Symbiotic Waves 8-Bit Waveform Processor, Lifeforms SV-1 Oscillator
Oscillator 2: Voltage Controlled Oscillator 2, Primary Oscillator, Generator
Modulation place 1 (left): Modulation Sources, Dual Low Frequency Oscillator, Game System, Dual VILFO, Function Generator, Three Stage Envelope Generator
Modulation place 2 (right): the same six modules
Each of the eight effect slots: any of the nineteen effects

A module's patch bay jacks move with it, and the cables in them follow. The same modulation module can be in both modulation places at once: each copy has its own settings.

The button row

SETTINGS: Opens the settings window (see Settings).

CC ASSIGN: Shows the MIDI controller assigned to every slider and knob (see MIDI, clock and automation).
UNDO / REDO: Steps back and forward through the last 100 changes to the panel.
PRESETS: Opens the presets window. The display beside it shows the current preset's name.
EFFECTS / CONTROLLER / SEQUENCER: Chooses what the bottom of the panel shows: the effect slots, the keyboard controller or the step sequencer. The effects keep running whichever is shown.
UNPATCH ALL: Removes every cable.

The display shows [MODIFIED] after the preset name once anything differs from the preset as it was loaded or saved, so you know to save it to keep the changes.

Patching

Making and removing cables

Drag from any jack to a jack of the other kind (an output to an input, or an input to an output) to make a cable.
Click or right-click a jack to open its menu: remove one of its cables, or choose new cable to hang a cable from the jack and click the jack it should go to.
Drag a jack that has cables in it and all its cables come along, to be plugged into another jack of the same kind. Let go away from any jack and the cables are removed.
Shift-drag from a jack that has cables pulls a new cable from it.
While a cable is being moved, the other cables fade so the panel's labels can be read under them.
UNPATCH ALL removes every cable. Every cable change can be undone.

Up to four cables can share an input; they are added together. An output can feed any number of inputs. Inputs have their labels in a box; outputs have plain labels. Cables take one of eight colors, which follow the jacks they join.

Normals

Most inputs are normalled: with nothing patched they hear an internal source, shown by the lines printed on the panel and described in each module's page. A cable into the input replaces the normal. Patching a cable from a waveform, noise or preamp output removes that source from its mixer, as the switched jacks do on the hardware.

Voltages

Cables carry audio-rate signals, so feedback, FM and audio-rate modulation work everywhere. Signals use the hardware's levels: audio is about ±5 V, pitch is 1 V/oct, gates are 5 V, and most CV inputs add to their control's slider or knob (+5 V sweeps its full travel).

Drones

A voice's own jack patched straight into a shared input (the output section's IN jack, or an effect) is a sound path that does not pass the voice's VCA. The voice keeps running while that cable is there, so an oscillator patched into the output drones with no key held, as on the hardware.

Oscillators

The top row has two oscillator places. Choose the module in each with the + in its title bar. Each module's jacks take the top of the patch bay, under the oscillators' title bars, and the cables in them follow the module.

Microvolt Oscillator

A component level emulation of the Microvolt oscillator originally developed by Pittsburgh Modular's Analog Engineer Michael Johnsen. The Microvolt 3900 was a single oscillator, analog synth that blended east coast subtractive synthesis with west coast additive wave shaping. This oscillator mixes the classic Pittsburgh oscillator core with a Don Buchla wavefolder.

A single analog oscillator core produces four waveforms at once, each with its own level slider feeding the oscillator's mixer.

FINE TUNE SLIDER: Sets the pitch over ±24 semitones and follows the keyboard, MIDI and the PITCH jack at 1 V/oct.

PULSE WIDTH SLIDER: Sets the width of the pulse wave from 10% to 90%.

FOLD TIMBRE SLIDER: Shapes a sine wave through a wavefolder. Low settings are close to a pure sine. Higher settings fold the wave back on itself, adding bright, complex harmonics.

HARMONIC SINE BUTTON: Turned on, the sine output changes to a harmonically richer, half-wave rectified variation of the sine wave.

NOISE LEVEL KNOB: Adds white-ish noise to the oscillator's mixer.

INPUT GAIN KNOB: Sets the gain of the preamp for a signal patched into the INPUT jack.

INPUT LEVEL KNOB: Sets how much of the preamp's output reaches the oscillator's mixer.

SINE, SAW, PULSE and FOLD LEVEL SLIDERS: Set the level of each waveform in the mix.

Three modulation input jacks are normalled to a modulation source identified by the line leading from the associated attenuator knob.

The AM jack modulates the amplitude of the oscillator's mix. Each waveform has its own output jack, and a waveform patched from its jack is removed from the mix. The MIX jack carries the full mix.

Double Helix 6-Bit Wavetable Processor

Originally prototyped in 2014 but not released until 2026, the Double Helix Wavetable Processor is the second digital Pittsburgh Modular oscillator. The original version utilized 8-bit wavetables sent through a built-in effects processor and 6-bit DACs to create a unique, aggressive sound. This version doubles the available wavetables able to pass through the onboard effects processor and 6-bit DACs.

The Double Helix is built around two wavetable oscillators, and every waveform passes through one of 16 digital processors before it reaches the output.

FINE TUNE SLIDER: Sets the pitch over ±24 semitones and follows the keyboard, MIDI and the PITCH jack at 1 V/oct.

WAVETABLE DETUNE SLIDER: Tunes the second oscillator from unison up to one octave above the first, for anything from slow beating to wide intervals.

BANK BUTTON: Steps through 4 banks. Each bank holds 16 wavetables and each table contains 16 waves. Banks 1 and 2 are the original module's 8-bit wavetables.

WAVETABLE SELECT SLIDER: Chooses the table.

WAVETABLE SWEEP SLIDER: Moves through the 16 waves inside it.

SMOOTH BUTTON: Turned on, neighboring waves crossfade into one another. Turned off, they step from one wave to the next.

PROCESSOR SELECT SLIDER: Selects the effects processor that processes the waveforms.

PROCESSOR CONTROLLER SLIDER: Controls a value within the processor.

WAVE/MIX LEVEL SLIDER: Sets the output level of the module.

Five modulation input jacks are normalled to a modulation source identified by the line leading from the associated attenuator knob.

The BANK and SMOOTH jacks toggle their button state with a gate or trigger.

DNA Symbiotic Waves 8-Bit Waveform Processor

Originally launched in January 2014, the Symbiotic Waves is the first digital Pittsburgh Modular oscillator. It was a dual 8-bit oscillator with 16 waveforms, a unique built-in effects processor, and a distinctly digital sound. The version included here retains the original Layers preset from the effects processor and adds a new ring modulator.

The DNA is built around two 8-bit oscillators, each able to play any of 16 waveforms, blended together with the mix balance.

FINE TUNE SLIDER: Sets the pitch over ±24 semitones and follows the keyboard, MIDI and the PITCH jack at 1 V/oct.

WAVE 2 DETUNE SLIDER: Tunes the second oscillator from unison up to one octave above the first.

WAVE 1 SELECT SLIDER: Chooses one of 16 waveforms for the first oscillator.

WAVE 2 SELECT SLIDER: Chooses one of 16 waveforms for the second oscillator.

RING MODULATED BUTTON: Turned on, wave 1 is ring modulated by wave 2 for metallic, bell-like tones.

LAYERED WAVE 2 BUTTON: Turned on, wave 2 is layered with a slower copy of itself. The Layers effect is taken from the effects processor on the original module.

WAVE 1 LEVEL SLIDER: Sets the level of the first oscillator.

WAVE 2 LEVEL SLIDER: Sets the level of the second oscillator.

MIX BALANCE SLIDER: Blends the two oscillators. At the bottom only wave 1 is heard, both are at full level in the middle, and at the top only wave 2 is heard.

Five modulation input jacks are normalled to a modulation source identified by the line leading from the associated attenuator knob.

Lifeforms SV-1 Oscillator

A component level emulation of the primary oscillator included in the Lifeforms SV-1 synthesizer originally launched in January 2016, this module faithfully recreates all the waveform quirks and robust SV-1 sound that has defined the Pittsburgh sound ever since. One quirk of this oscillator is that the sub oscillator is positive-only. That means it creates even harmonics when blended with other waveforms.

A single analog oscillator core produces six waveforms at once, each with its own level slider feeding the oscillator's mixer.

FINE TUNE SLIDER: Sets the pitch over ±24 semitones and follows the keyboard, MIDI and the PITCH jack at 1 V/oct.

SUB OSC LEVEL SLIDER: Sets the level of the sub oscillator, a positive-only square wave one octave below the oscillator.

HARD SYNC BUTTON: Turned on, the oscillator is reset by the oscillator in the second oscillator slot, for classic hard sync tones.

SINE, TRIANGLE, SAW, BLADE and PULSE LEVEL SLIDERS: Set the level of each waveform in the mix. BLADE is a unique SV-1 waveform that splits into two halves as the width is modulated.

Two modulation input jacks are normalled to a modulation source identified by the line leading from the associated attenuator knob. WIDTH MOD changes both the width of the pulse and the shape of the blade.

Each waveform has its own output jack, and a waveform patched from its jack is removed from the mix. The MIX jack carries the full mix.

Voltage Controlled Oscillator 2

The original Microvolt was a single oscillator monosynth. We wanted the digital companion to reach further. This oscillator pairs a component level emulation of the original Microvolt oscillator core with a unique set of waveshapers developed by our analog team.

A single analog oscillator core produces three waveforms, each with a waveshaper that can be switched in.

FINE TUNE SLIDER: Sets the pitch over ±24 semitones and follows the keyboard, MIDI and the PITCH jack at 1 V/oct.

PRESSED SINE BUTTON: Turned on, the sine is pressed into curved slopes with a sharp notch folded into each peak.

FUZZY SAW BUTTON: Turned on, the saw is folded and saturated into a fuzzy, aggressive tone.

BROKEN SQUARE BUTTON: Turned on, the square is folded into a broken, harmonically rich shape.

SINE, SAW and SQUARE LEVEL SLIDERS: Set the level of each waveform in the mix.

The FREQ MOD jack is normalled to a modulation source identified by the line leading from its attenuator knob.

Each waveform has its own output jack, and a waveform patched from its jack is removed from the mix. The MIX jack carries the full mix.

Primary Oscillator

A component level emulation of (most of) the Voltage Lab Mini Primary Oscillator. This beautiful sounding oscillator utilizes thru-zero FM to allow high levels of frequency modulation without drifting out of tune. The Primary Oscillator also includes an asymmetry shaper and custom wavefolder developed by Pittsburgh Modular's Analog Engineer Michael Johnsen.

A single analog oscillator core with a selectable waveform passes through an asymmetry shaper and a wavefolder before it reaches the output.

FINE TUNE SLIDER: Sets the pitch over ±24 semitones and follows the keyboard, MIDI and the PITCH jack at 1 V/oct.

ASYMMETRY LEVEL SLIDER: Pushes the waveform out of balance, stretching one half of each cycle and squeezing the other.

FOLD TIMBRE SLIDER: Sets how hard the waveform is driven into the wavefolder, adding harmonics as it rises.

WAVEFORM SELECT BUTTON: Steps through sine, triangle, saw and square. The button shows the waveform selected.

OUTPUT LEVEL SLIDER: Sets the output level of the module.

Three modulation input jacks are normalled to a modulation source identified by the line leading from the associated attenuator knob. TZFM is thru-zero frequency modulation, which stays in tune even at deep settings.

The WAVEFORM jack selects the waveform by voltage, stepping from sine to triangle, saw and square as the voltage rises. The MIX jack carries the module's output.

Generator

A component level emulation of a very early Pittsburgh Modular design. The Generator intertwines a pair of triangle core oscillators in beautiful and unstable ways. Cross frequency modulation and opposing waveform shaping create a chaotic playground for drones, percussive tones, and experimental textures.

Two free running triangle core oscillators modulate each other and pass through an opposing wave shaper before they are mixed together.

GEN 1 FREQUENCY SLIDER: Sets the pitch of the first oscillator over ±5 octaves.

GEN 2 FREQUENCY SLIDER: Sets the pitch of the second oscillator over ±5 octaves.

WAVE SHAPER SLIDER: Shapes the two oscillators in opposite directions. As gen 1 moves from triangle to pulse, gen 2 moves from pulse to triangle.

RING MODULATOR BUTTON: Turned on, the two oscillators are ring modulated instead of mixed.

MIX LEVEL SLIDER: Sets the output level of the module.

Three modulation input jacks are normalled to a modulation source identified by the line leading from the associated attenuator knob. With nothing patched, each oscillator frequency modulates the other. The GEN 1 FM MOD and GEN 2 FM MOD knobs are attenuverters: turned left they invert the modulation.

The Generator does not follow the keyboard. To play it from the keyboard, patch the MIDI pitch into an FM jack and turn its knob fully right for 1 V/oct.

Each oscillator has its own output jack, GEN 1 OUT and GEN 2 OUT, and the MIX jack carries the module's output.

Filter, VCA and output

The rest of the top row shapes the voice: the oscillators' mix runs through the Binary Filter and the Dynamic VCA into the effects, then to the output section.

Binary Filter

A beautiful filter sound that has been at the heart of Pittsburgh Modular instruments for over 15 years, the Binary Filter is a component level emulation of the Pittsburgh State Variable Filter core at the center of the Microvolt 3900. A two-pole, 12 dB/octave low pass filter with two resonance modes. Stable Mode offers the traditional Pittsburgh Filter response, while Unstable Mode allows the resonance to fall into self-oscillation.

FREQUENCY SLIDER: Sets the cutoff frequency of the filter. The LED shows the cutoff including any modulation.

UNSTABLE RES BUTTON: Turned off, the filter is in Stable Mode: the resonance rings strongly but never self-oscillates. Turned on, the top of the RESONANCE slider sounds like Stable Mode at its maximum, and moving the slider down pushes the filter into self-oscillation.

RESONANCE SLIDER: Sets the resonance level of the filter.

FREQ MOD 1 and FREQ MOD 2 KNOBS: Set how far the FREQ 1 and FREQ 2 jacks move the cutoff.

Two modulation input jacks are normalled to a modulation source identified by the line leading from the associated attenuator knob.

The IN jack is normalled to the mix of the two oscillators, and OUT carries the filtered signal.

Dynamic VCA

A component level emulation of an analog hardware emulation of a low pass gate. The Dynamics circuit was developed by Pittsburgh Modular's Analog Engineer Michael Johnsen to improve the flexibility of the beautiful sounding but limited Low Pass Gate. A traditional VCA manipulates amplitude to shape a sound. A low pass gate manipulates both amplitude and harmonic content at the same time, creating a more natural sounding instrument. The Dynamics is switchable between VCA and Low Pass Gate modes allowing for a wide palette of electronic sounds.

Two modes allow the Dynamics to function as a clean VCA or as a two-pole low pass gate.

LPG (ON) VCA (OFF) BUTTON: Turned on, the module is a low pass gate, so the sound gets darker as it gets quieter. Turned off, it is a VCA that changes only the amplitude.

DYNAMICS RESPONSE KNOB: Sets how long the gate takes to close once its control voltage drops away. Opening is always fast. This adjusts the Vactrol response of the low pass gate.

PLUCK BUTTON: In LPG mode, each time the incoming DYN jack voltage rises past about 2 V the gate is struck fully open for a moment and left to ring out, like a plucked string. It does nothing in VCA mode.

INPUT BALANCE KNOB: Blends what reaches the VCA, from the binary filter only (full left) to the IN jack only (full right).

DYNAMICS PASS-THRU SLIDER: Sets a standing level that holds the VCA open without a control voltage.

DYN MOD KNOB: Sets how much the DYN jack opens the VCA. 5 V with the knob fully right opens it completely.

One modulation input jack is normalled to a modulation source identified by the line leading from the associated attenuator knob.

The RESP jack moves the DYNAMICS RESPONSE knob, with +5 V covering its full travel. The IN jack is normalled to the binary filter's output, so with nothing patched both sides of INPUT BALANCE carry the filter. The OUT jack carries the module's output.

Output

The end of the signal path. The effects chain feeds an output drive stage and the master level.

OUTPUT DRIVE KNOB: Drives the output stage from clean to heavily saturated. A make-up gain keeps the loudness about the same, so the knob changes the tone, not the level.

OUTPUT SLIDER: Sets the master output level.

The IN jack is the input of the output section. It is normalled to the end of the effects chain, and a cable patched there sends any signal straight to the output drive and level. An oscillator patched into IN drones with no key held, as on the hardware. The OUT jack carries the final output, the same signal sent to your speakers: patch it back into the panel for feedback patches.

Scope and MIDI

Scope

A built-in oscilloscope to monitor signals within the Microvolt. Any audio or CV signal can be patched into the Scope to view the waveform or spectrum. Video Music mode shows an audio reactive visualizer inspired by the vintage Atari Video Music.

The Scope shows the signal at its IN jack in one of three views. It only listens and never changes the sound.

VIDEO MUSIC BUTTON: Shows a mirrored field of diamonds where bass frequencies control the width and mid frequencies control the height. The colors jump on each beat and the pattern changes every 16 beats.

SCOPE BUTTON: An oscilloscope. A repeating wave locks steady, about three cycles wide, while slow or stepped control voltages roll across the screen.

ANALYZER BUTTON: A spectrum analyzer from 20 Hz to 20 kHz on a log scale, covering -80 to +20 dBV.

The SCOPE IN jack in the patch bay is normalled to the Microvolt output, so with nothing patched the scope shows what you hear. In polyphonic mode a voice's own jacks are shown from the last voice played.

MIDI / Arpeggiator

This section controls the clock, sets polyphony, and manages MIDI response.

TEMPO KNOB: Sets the internal clock from 30 to 300 bpm, shown under the knob. With Host Sync the host sets the tempo. With Ableton Link the knob and the session follow each other.

TAP TEMPO BUTTON: Tap the beat and the internal clock takes the tempo of the last two or three taps. It flashes on the beat, and it is dark and inactive when the clock follows the host or Ableton Link.

RUN/STOP BUTTON: The primary transport control, this can be toggled with the spacebar. Run resets and starts the arpeggiator, the step sequencer resets and starts when its own RUN is on, and the game system resets and starts. Stop pauses them all. With Ableton Link it starts and stops the session.

MONOPHONIC BUTTON: Turned on, the Microvolt plays one note at a time. Turned off, it is six-voice polyphonic with newest note priority.

ARP OPTIONS BUTTON: Opens a menu with ARP DIVISION (1/1 to 1/32), ARP DIRECTION (forward, reverse, random, as played, pendulum) and ARP RANGE (1 to 3 octaves). It also has RANDOM OCTAVE, which puts each step in a random octave of the range, and GENERATE RANDOM ARPEGGIO, which latches a new arpeggio of 3 to 16 notes, with an occasional rest, from the sequencer's key and scale.

ARPEGGIATOR BUTTON: Plays the held notes one at a time, one note per clock step at the ARP clock DIVISION. In polyphonic mode the steps cycle through the six voices, so each note maintains its tail.

SPREAD KNOB: In monophonic mode, SPREAD stacks up to six voices on the one line, spread evenly up to ±18 cents, for a wider sound at about the same level. Full left is a single voice.

HOLD BUTTON: Latches the notes, so the gate stays open after the keys are let go. With the arpeggiator on, the selected notes keep playing until a new one is played.

ARP TRANSPOSE BUTTON: Freezes the current arpeggio. Keys played then move the pattern so its first note lands on the key, instead of adding notes.

The MIDI / controller / sequencer jacks carry the notes played: PITCH at 1 V/oct, GATE, and VELOCITY from 0 to 5 V. CLOCK puts out the clock at the arp division, and it is normalled to the sample & hold's HOLD input. MOD follows the mod wheel, MIDI CC, or velocity, and CC follows an assignable controller, both set with CC ASSIGN.

Modulation

The two modulation places in the middle row each take any of six modulation modules. Choose one with the + in the place's title bar. Whatever is in either place runs, and its jacks appear in the patch bay row under the place.

The left place (the Modulation Sources' home) feeds the modulation bus: the FM and other modulation normals of the oscillators and the filter, following the lines printed on the panel.
The right place (the Function Generator's home) feeds the function normals: the oscillators' timbre and wave inputs and the filter's FREQ 2.
What each module puts out on its place's normals: the Function Generator's and Three Stage's outputs, the Modulation Sources' MIX, the Dual LFO's and Dual VILFO's balance, and the Game System's output 1. With the same module in both places, each place hears its own copy.

The envelope generator to the right of the two places, and the oscillator / LFO, sample & hold, ring modulator, VCA and inverting mixer in the patch bay, are always there.

Modulation Sources

The modulation section of the Microvolt 3900. A component level emulation of its analog LFO sits beside the random voltage generator from the Microvolt's own firmware.

An analog LFO with an adjustable shape and a clocked random voltage source are blended by the source balance into the MIX output.

RANDOM DENSITY SLIDER: Sets how often the random voltage steps in time with the Microvolt's clock. At the bottom nothing happens, and steps come more often and more reliably as the slider rises. Below the very top of the slider the steps cycle through a pattern of six voltages that slowly mutates. At the very top each step is a new random voltage, like a sample and hold.

SOURCE BALANCE KNOB: Blends the mix from the random voltage (left) to the LFO ramp (right).

LFO RANGE BUTTON: Switches the LFO between its slow range (lit, about 0.0006 Hz to 1.2 Hz) and its fast range (about 0.06 Hz to 120 Hz).

LFO SHAPES KNOB: Skews the LFO from a rising ramp through a triangle to a falling ramp without changing its rate. The pulse's width is affected as well.

LFO RATE SLIDER: Sets the speed of the LFO. Its lamp glows with the LFO.

The MIX output is normalled to the modulation inputs identified by the line leading from the modulation slot the module sits in.

RND CV carries the random voltage (0 to 5 V) and RND GATE a short gate with each random step. LFO PULSE and LFO RAMP are the LFO's two waves. NOISE is the noise source that also feeds the oscillator's NOISE LEVEL knob and the S&H.

Function Generator

An expanded version of the Microvolt's Function Generator. This module is a complex dual stage (rise, fall) slope generator with voltage controllable rise and fall times. The cycle button retriggers the function generator at the end of the fall stage to create a voltage controlled low frequency oscillator. The Function Generator is a modular multi-tool that can be patched as a slew generator, voltage controlled LFO, two-stage (attack, decay) envelope generator, three-stage (attack, sustain, release) envelope generator, and more...

RISE SLIDER: Sets the rise time, from a few milliseconds at the bottom to minutes at the top. The slider LED lights during the rise.

FALL SLIDER: Sets the fall time over the same range as RISE. The slider LED lights during the fall.

CYCLE BUTTON: Turned on, the function generator retriggers itself at the end of each fall and cycles like an LFO.

SUSTAIN BUTTON: Turned on, the output holds at the top for as long as the input stays high (attack, sustain, release instead of attack, release).

RESPONSE KNOB: Bends the curve of both stages: exponential at the left, linear at the center and logarithmic at the right.

FALL MOD KNOB: An attenuverter for the FALL jack. Turned left it inverts the control voltage, and the center is off.

The OUT jack is normalled to the modulation inputs identified by the line leading from the modulation slot the module sits in.

The IN jack is normalled to the MIDI gate, so each note starts the function. A trigger that arrives during the rise is ignored. One that arrives during the fall starts a new rise immediately. With SUSTAIN on, the function generator becomes a slew limiter. The output glides toward the input voltage at the RISE and FALL rates.

A positive voltage at the RISE or FALL jack lengthens that stage, by about 10 times per volt (FALL goes through FALL MOD). The function cycles while the CYCLE jack is high. END OF FALL sends a short trigger each time a fall finishes.

Three Stage Envelope Generator

A four-stage envelope generator (three stages plus a switched sustain: attack, decay, switched sustain, release) that smooths the shape of incoming gates and triggers to produce a more expressive instrument. The envelope generator output can be used to control the amplitude of an oscillator, the cutoff frequency of a filter or any other function that accepts control voltage modulation.

ATTACK SLIDER: Sets the attack time, from a few milliseconds to minutes, as the function generator's RISE does.

DECAY SLIDER: Sets how quickly the output falls from the top to the sustain level.

SUSTAIN BUTTON: Turned on, the envelope holds at 70% of its peak while the gate is high. Turned off, it becomes an attack / decay envelope that falls to zero.

RELEASE SLIDER: Sets how quickly the output falls to zero once the gate ends.

The OUT jack is normalled to the modulation inputs identified by the line leading from the modulation slot the module sits in.

The IN jack is normalled to the MIDI gate, and a new gate starts the attack from wherever the output is. The DECAY and RELEASE jacks are control inputs, each set by the attenuator knob beside it in the patch bay. A positive voltage lengthens the stage, by about 10 times per volt at full.

Dual Low Frequency Oscillator

A simple dual LFO with clean triangle and square waves that can sync to the clock tempo. Mix and ring modulated outputs of the two LFOs are available.

Two independent triangle and square LFOs are blended by the source balance into one modulation mix and multiplied together using a ring modulator.

LFO 1 RATE SLIDER: Sets the speed of LFO 1 from 0.02 Hz to 50 Hz (1 Hz at the center). With tempo sync on, it chooses the length of one cycle instead, from 8 bars to 1/32. The LED follows the LFO.

SOURCE BALANCE KNOB: Blends the mix from LFO 1's triangle (left) to LFO 2's (right).

TEMPO SYNC BUTTONS: Lock the LFOs to the Microvolt's tempo: the top button locks LFO 1, the bottom button LFO 2.

LFO 2 RATE SLIDER: Sets the speed of LFO 2, as LFO 1 RATE does.

The source balance mix is normalled to the modulation inputs identified by the line leading from the modulation slot the module sits in.

The RING MOD jack carries the two triangles multiplied together.

Dual VILFO

The birth of Pittsburgh Modular and the first module developed by founder Richard Nicol. The included Voltage Influenced Low Frequency Oscillator is an early experiment full of quirky results. The original VILFO was created as a playground to test different methods of frequency modulation creating a module that responds incorrectly and almost opposite to what is expected. The result is an LFO with imperfect wave shapes that wobble and wiggle in a chaotic and musical manner.

Two VILFOs, each modeled from the VILFO schematic, push each other around through their influence inputs, and the source balance blends their triangles into one modulation mix.

VILFO 1 RATE SLIDER: Sets the speed of VILFO 1, from 0.02 Hz to 190 Hz. The LED follows the triangle.

SOURCE BALANCE KNOB: Blends the mix from VILFO 1's triangle (left) to VILFO 2's (right).

INFLUENCE MOD 1 KNOB: Sets how hard the INF 1 jack pushes VILFO 1 around, with most of its effect in the top third of its travel. A positive voltage hurries the fall and slows the rise, leaning the triangle. A strong voltage at a slow rate can hold the oscillator still until the voltage turns around.

INFLUENCE MOD 2 KNOB: Does the same for VILFO 2 and the INF 2 jack.

VILFO 2 RATE SLIDER: Sets the speed of VILFO 2, as VILFO 1 RATE does.

Two modulation input jacks are normalled to a modulation source identified by the line leading from the associated attenuator knob. With nothing patched, each VILFO's triangle influences the other.

The source balance mix is normalled to the modulation inputs identified by the line leading from the modulation slot the module sits in.

Game System

A direct port of the chaotic Game System Eurorack module. Six games and a settings page create random and sequenced gates and control voltages.

The display shows the selected game with the text labels of the four output jacks updating accordingly.

GAME BUTTON: Steps through the six games. Hold it for 2 seconds to open the settings page, and press it again to return to the game.

MODE BUTTON: Changes the current game's mode (see below).

JOYSTICK: Drag its cap or click one of its arrows. With the pointer over the module, the arrow keys work too. Clicking an LED moves the game's cursor there.

RESET BUTTON: Restarts the current game: sequences go back to their first step and the meteors are cleared.

CLOCK BUTTON: Divides the incoming clock by 1, 2, 4, 8, 16 or 32. With the internal clock selected in the Game System settings, it works as a tap tempo button.

JOYSTICK BUTTON: The joystick's own button. What it does depends on the game.

METEOR SHOWER: Steer the ship away from the falling meteors. BUTTON sets how many meteors fall and MODE puts the ship on autopilot. A hit sends a new random pitch to RAND CV and a CRASH TRIG. A meteor reaching the ground sends a GROUND TRIG.

MUSIC SEQUENCER: A 32-step pitch sequencer. The joystick moves the cursor and sets the selected step's note. BUTTON turns the step on or off and assigns the last step of the sequence by holding the button for over a second. MODE plays forward, pendulum, reverse or random. Both PITCH outputs carry the step's note, and ACTIVE TRIG fires on each step played. Click a step to type its note, or play the note from a keyboard.

DRUM SEQUENCER: Four 16-step trigger sequencers. BUTTON turns a step on or off and assigns the last step of the sequence by holding the button for over a second. MODE swaps the sequences in pairs.

TIME TRAVELLER: Each column of the matrix is a clock division, from 1/1 to 1/8. Four cursors, one for each TRIG output, can be positioned within the grid for a unique division and an offset. BUTTON chooses which output trigger cursor the joystick moves. MODE enables or disables the selected cursor's autopilot.

PROBABILITY MACHINE: The cursor's position sets the chance of a new random note on each clock at the two RAND CV outputs (with BLUE TRIG and GRAY TRIG), and the chance of extra notes between clocks. BUTTON switches those extra notes off or sets 2, 4 or 8 per clock. MODE enables or disables autopilot.

EUCLIDEAN RHYTHMS: The joystick sets the number of steps (up to 32) and the number of beats spread evenly across them. Each beat sends a new random PITCH and an ACTIVE TRIG, and each rest an INACTIVE TRIG. BUTTON swaps beats and rests, and MODE plays forward, pendulum, reverse or random.

SETTINGS: The joystick chooses a page and changes its value: clock source (the CLOCK jack or the internal clock), LED brightness, gate length (40 to 240 ms), Game System factory reset (push three times) and static voltages in octave steps at outputs 1 and 2.

The CLOCK jack is normalled to the Microvolt's clock, so the games step with the Microvolt's tempo, and the RESET jack works as the RESET button. Pitch outputs are quantized to the step sequencer's key and scale at 1 V/oct, and the trig outputs are 5 V gates. Output 1 is normalled to the modulation inputs identified by the line leading from the modulation slot the module sits in. The Game System follows the MIDI / Arpeggiator section's transport: when it is stopped, the module waits.

Envelope Generator

A four stage ADSR envelope modeled from the Microvolt's switched RC circuit. With nothing patched it is played by the MIDI gate and opens the Dynamic VCA through its DYN jack.

ATTACK SLIDER: Sets the attack time, from about 11 ms to 8.4 seconds.

DECAY SLIDER: Sets the decay time constant, from about 5 ms to 4.7 seconds.

SUSTAIN SLIDER: Sets the level held while the gate stays high.

RELEASE SLIDER: Sets the release time constant after the gate ends, from about 5 ms to 4.7 seconds.

The slider lamps follow the envelope.

The RELEASE jack is a release time CV: +5 V adds the full travel of the RELEASE slider. IN is the gate input, normalled to the MIDI gate, so any gate or clock can play the envelope. OUT carries the envelope and is normalled to the Dynamic VCA's DYN jack.

Oscillator / LFO

A third oscillator core lives in the patch bay, one per voice. In oscillator range it is an audio oscillator for FM, ring modulation and sub tones; in LFO range it is a second, wide-ranging LFO. It has sine, triangle and saw outputs (±5 V).

LFO BUTTON: Lit, the oscillator runs in LFO range: the PITCH knob covers 1/16 Hz to 80 Hz and the PITCH jack scales the rate at 1 V/oct. The button pulses with the LFO. Off, it runs in oscillator range.

PITCH KNOB: In oscillator range with nothing in the PITCH jack, sets the pitch outright, ±10 octaves around middle C. With a pitch patched in (the MIDI PITCH jack, say), the oscillator follows it at 1 V/oct and the knob shifts it ±5 octaves. Set in tenths of a cent; right-click to type a value.

FREQ MOD KNOB: Attenuator for the FM jack: 1 V/oct exponential FM at full.

The PITCH jack is the 1 V/oct pitch (or rate) input, not normalled, and FM the frequency modulation input. SINE, TRIANGLE and SAW carry the three waveforms. In LFO range and in polyphonic mode the six voices' LFOs stay in step.

Utilities

Four small utilities in the patch bay make the panel more modular.

Sample & hold

HOLD: the clock input. On each rising edge the voltage at SAMPLE is taken and held. Normalled to the MIDI CLOCK output. SAMPLE: the voltage to sample, normalled to noise, so with nothing patched the output steps to a new random voltage on every clock. OUT: the held voltage. (SAMPLE is labelled SAMP on the panel.)

Ring modulator

IN X and IN Y: the two inputs of a four-quadrant multiplier; neither is normalled. OUT: X × Y / 5 V, so a 5 V carrier passes the other input at unity. Patch two oscillators for bell and metal tones, or an LFO for tremolo that swings through zero.

VCA

LEVEL: the control input, 0 to 10 V; 5 V is unity gain, and with nothing patched the VCA is closed. IN: audio or CV, not normalled. OUT: IN × LEVEL / 5 V, a plain linear VCA for tremolo, CV scaling or a second voice.

Inverting mixer / offset

IN 1 LEVEL knob: an attenuverter for the IN 1 jack (the center is off, right passes the signal, left inverts it). With nothing patched IN 1 is normalled to +5 V, so the knob is an offset from −5 V to +5 V. IN 2 LEVEL knob: the attenuverter for IN 2. MIX: the sum of both. Use it to invert an envelope, offset an LFO or combine two control voltages.

Effects

The bottom row holds eight effect slots. Each slot can hold any of the nineteen effects; the panel opens with Fragments, Crow Filter, Lo-Fi Tape, Flanger, Phaser, Local Florist, Digital Delay and Reverb.

Choose an effect with the + in the slot's title bar. The same effect can be in more than one slot.
Rearrange the slots by dragging an effect by its title bar to another slot. The modules in between shift along, each with its settings and cables.
ACTIVE switches an effect in and out. Switched off, its input passes through unchanged. (The Crow Filter has no ACTIVE button: it is always in its path.)
Each effect has a column of jacks at the bottom of the patch bay: its control inputs, then IN and OUT. Most have one control input with an attenuator knob above it; the Crow Filter, Overdrive and Bit Crush have two, with their knobs on the panel, and the Chorus and Compressor have none.

How the slots are wired

The slots are normalled by position, as the lines under them show:

Slots 3 to 8 run in series from the Dynamic VCA to the output section: Dynamic VCA → slot 3 → slot 4 → slot 5 → slot 6 → slot 7 → slot 8 → output.
Slots 1 and 2 are a pair of their own: slot 1's IN is not normalled, and slot 1's OUT feeds slot 2's IN. Patch any signal into slot 1, and slot 2's OUT wherever you want it (into the filter's IN, say).
A cable into any slot's IN replaces the signal normalled to it, so effects can be fed from any point in the synth, re-ordered with cables, or patched back into the voice.

Effects that are switched off with nothing patched cost no processing, and an effect that has had silence for a few seconds rests until sound reaches it again.

Fragments

The Fragments module is a reimagining of the announced but never released Fragments Generator. The idea behind the Fragments Generator required significantly more circuitry than we could possibly fit behind a Eurorack panel, or a large refrigerator for that matter. This digital implementation allows us to share the original vision without the significant compromises of the original prototypes. Fragments cuts every cycle of its input into pieces and plays each piece from somewhere else in the input's recent history. The pitch stays where it is while the wave shape changes.

Cycles are found from the input's zero crossings. Each one is cut into fragments that are rearranged from up to 1.2 cycles of history, and new settings crossfade in at a cycle start so the wave glides between shapes instead of jumping.

FRAGMENTS SLIDER: Sets how many fragments each cycle is cut into, from 1 to 250.

FRAGMENTS MOD KNOB: Sets how much the FRAG jack moves the FRAGMENTS slider. Fully right, +5 V covers the slider's whole travel.

WARP BUTTON: Turned on, the input passes through a circuit that changes the waveform shape without modifying the harmonic content. This adds new flavors for the Fragments to work with. The level and pitch stay the same but the shape bends.

SPREAD SLIDER: Sets how far back a fragment can be taken from, from 0.1 of a cycle up to 1.2 cycles.

MIX KNOB: Crossfades from the dry input to the fragmented signal. Fully right (the default) is fully wet.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged.

The FRAG jack is attenuated by the FRAGMENTS MOD knob. A cable patched into the IN jack replaces the signal normalled from the previous slot.

Crow Filter

A component level emulation of the Filter of Crows module. This is the classic Pittsburgh Filter paired with a strong overdrive at the input to enhance the harmonic content before filtering.

FREQUENCY SLIDER: Sets the cutoff from 20 Hz to 20 kHz.

FREQ MOD KNOB: An attenuverter for the FREQ jack. Turned left it inverts the modulation, and the center is off.

DRIVE KNOB: Raises the input gain from unity up to x75 (+37.5 dB).

RESONANCE SLIDER: Sets the resonance amount.

RESO MOD KNOB: An attenuverter for the RESO jack. Turned left it inverts the modulation, and the center is off.

MODE BUTTON: Steps through low pass, band pass and high pass. The button shows the selected response curve.

The Crow Filter has no ACTIVE button and is always in the signal path. A cable patched into the IN jack replaces the signal normalled from the previous slot.

Lo-Fi Tape

A worn cassette in a virtual effects module. A short bucket brigade line is wobbled by a smooth random LFO, and a low pass gate opens with the input's loudness, so loud notes brighten and fade into darkness.

WOBBLE SLIDER: Sets the depth of the wobble by lengthening the delay line (about 0.6 to 15 ms) that the random LFO swings. The LED follows the LFO.

RATE KNOB: Sets the speed of the random LFO from 1/16 Hz to 80 Hz. With TEMPO SYNC on, it picks the length of one LFO cycle, from 8 bars down to 1/32.

TEMPO SYNC BUTTON: Locks the LFO rate to the tempo.

FILTER SLIDER: Sets how far the input's level opens the low pass gate.

MIX KNOB: Crossfades from the dry input to the tape.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged.

The RATE jack adds to the RATE knob through the RATE MOD knob at the top of the bay column: fully right, +5 V covers the RATE knob's full travel. A cable patched into the IN jack replaces the signal normalled from the previous slot.

Flanger

A component level clone of our favorite flanger, the Flanger runs the signal through a 1024-stage MN3207 bucket brigade delay swept by a triangle LFO, with regeneration for the classic jet-plane sweep.

A triangle LFO sweeps the bucket brigade delay from about 1 ms to 13 ms of delay. The delayed signal is mixed equally with the dry signal.

RATE SLIDER: Sets the LFO speed from 0.06 Hz to 10 Hz. With TEMPO SYNC on, it picks the length of one LFO cycle, from 8 bars down to 1/32. The slider glows with the LFO.

MANUAL KNOB: Sets the center of the sweep. Turning it up shortens the delay and moves the notches up.

TEMPO SYNC BUTTON: Locks the LFO rate to the tempo (the host's, or the internal clock's in the standalone app).

DEPTH SLIDER: Sets how far the LFO sweeps the delay.

RESONANCE KNOB: Feeds the delayed signal back through the line for sharper, ringing peaks.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged.

The RATE jack adds to the RATE slider through the RATE MOD knob at the top of the bay column: fully right, +5 V covers the slider's full travel. A cable patched into the IN jack replaces the signal normalled from the previous slot.

Phaser

A component level emulation of the Pittsburgh Modular Toad phase shifter module. The Phaser has a single 12-stage audio signal path to create a very rich, full sound. Phased audio outputs are tapped from stages 2 to 12.

RATE SLIDER: Sets the LFO speed from 1/16 Hz to 80 Hz. With TEMPO SYNC on, it picks the length of one LFO cycle, from 8 bars down to 1/32. The slider glows with the LFO.

STAGES KNOB: Selects 2 to 12 all-pass stages, one per click. Each pair of stages adds a notch, and 12 is the default.

TEMPO SYNC BUTTON: Locks the LFO rate to the tempo.

DEPTH SLIDER: Sets how far the LFO sweeps the notches.

RESONANCE KNOB: Feeds the output of the stages back to the input for sharper, more vocal notches.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged.

Local Florist

A component level emulation of the original Local Florist module, the Local Florist is a bucket-brigade modulation effect: an MN3007 delay of a few milliseconds swept by a triangle LFO, with feedback, for chorus, flanging and vibrato sounds.

The signal runs through the module's filters and a 1024-stage bucket brigade line, whose clock is swept by the LFO across about 3 to 25 ms. Feedback lets it resonate strongly, and at the top of its travel the line rings on by itself.

RATE SLIDER: Sets the LFO speed from 0.11 Hz to 110 Hz. The LED follows the LFO.

TIME KNOB: Sets the delay time at rest, from about 3 ms up to 20 ms.

FEEDBACK KNOB: Feeds the delayed signal back into the input for a stronger, more resonant sound. Near the top it self-oscillates.

DEPTH SLIDER: Sets how far the LFO sweeps the delay around the TIME setting. At maximum it sweeps the full 3 to 25 ms.

MIX KNOB: Crossfades from the dry input to the delayed signal.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged.

Digital Delay

A clean delay of up to two seconds. A saturating feedback loop and a tone control on the repeats take it from a single slap-back to endless, darkening echoes.

The input is written into a delay line with soft saturation and a 60 Hz high pass in the feedback loop. Changing the time glides the delay to the new length, bending the pitch of the echoes as it moves.

TIME SLIDER: Sets the delay time from 10 ms to 2 s. With TEMPO SYNC on, it picks a note value from 1/32 to 1/2, including dotted and triplet values.

TONE KNOB: A low pass on the repeats from 600 Hz to 12 kHz. Each echo gets darker as the knob comes down.

TEMPO SYNC BUTTON: Locks the delay time to the tempo.

REPEATS SLIDER: Sets the feedback, from a single echo up to just past unity at the top, where the echoes go on endlessly, held in check by the saturation.

MIX KNOB: Crossfades from the dry input to the echoes.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged and the existing echoes are cut off.

The TIME jack adds to the TIME slider through the TIME MOD knob at the top of the bay column: fully right, +5 V covers the slider's full travel. A cable patched into the IN jack replaces the signal normalled from the previous slot.

BBD Delay

An analog bucket brigade delay modeled from our analog delay circuits. A pair of BBDs and steep filtering give dark, warm echoes that bend in pitch when the time is moved.

DELAY TIME SLIDER: Sets the delay from 50 to 500 ms, or 100 ms to 1 s with LONG RANGE on. Moving it while echoes are playing bends their pitch.

FEEDBACK SLIDER: Sets how much of each echo returns. Echoes hold steady at about 80% of the travel, build above it and self-oscillate at the top.

LONG RANGE BUTTON: Doubles the delay range and switches to a darker set of filters, with a smooth glide instead of a jump.

MIX KNOB: From the bottom to the middle the echoes come up to full with the dry signal at full. From the middle to the top the dry signal fades out.

TEMPO SYNC BUTTON: DELAY TIME picks a note value from 1/32 to 1/2 of the tempo instead of a time.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged and the existing echoes are cut off.

The TIME jack adds to the DELAY TIME slider through the TIME MOD knob at the top of the bay column: fully right, +5 V covers the slider's full travel. A cable patched into the IN jack replaces the signal normalled from the previous slot.

2399 Delay

A digital echo built around the PT2399 karaoke machine delay chip and modeled from the Voltage Lab mini schematic. Long delays bring out the chip's grainy sampling, and repeats can be pushed to near-endless.

TIME SLIDER: Sets the delay from 50 ms to 1 s. Longer times sound grainier, and moving the slider while it plays bends the pitch of the echoes.

REPEATS SLIDER: Sets the number of repeats, from a single echo at the bottom to nearly endless at the top.

FILTER KNOB: Sets the tone of the repeats, from 300 Hz at the bottom to fully open at the top.

MIX KNOB: Blends between the dry and delayed signal, held at a steady overall level.

TEMPO SYNC BUTTON: TIME picks a note value from 1/32 to 1/2 of the tempo instead of a time.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged and the existing echoes are cut off.

Reverb

Reverb algorithms based on open-source code by Emilie Gillet. The Reverb is a soft, dreamy wash of space that blurs sounds into long, cloud-like tails rather than placing them in a realistic room. It has a gentle shimmer and slow movement in the tail that keeps it feeling alive and warm, so even simple sounds bloom into something lush and ambient without ringing or turning harsh.

DECAY SLIDER: Sets the length of the tail, from about one second at the bottom to almost endless at the top.

TONE KNOB: Sets the damping inside the tail, from dark (400 Hz) to bright (18 kHz).

FEEDBACK KNOB: Sends the reverb's output back into its input, so the tail lasts longer and swells. At the top the tail holds indefinitely at any decay, settling near the input's level instead of running away.

DIFFUSION SLIDER: Sets how much the input is smeared. Low settings leave the early echoes more distinct, and high settings give a smoother wash.

MIX KNOB: Crossfades from the dry input to the reverb.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged and the existing reverb tail is cut off.

The DECAY jack adds to the DECAY slider through the DECAY MOD knob at the top of the bay column: fully right, +5 V covers the slider's full travel. A cable patched into the IN jack replaces the signal normalled from the previous slot.

Verbtronic

A component level emulation of the Pittsburgh Modular Verbtronic module. Three PT2399 echo chips feed one another so their echoes smear into a dense-ish, grainy reverb tail.

TONAL TILT SLIDER: Tilts the tone, flat at the center, brighter toward high and darker toward low, about ±12 dB at the ends.

FEEDBACK SLIDER: Lengthens the tail. Most of its effect is near the top, and the tail can ring on without running away.

MIX KNOB: Blends between the dry and wet signal.

TRONIC BUTTON: Turned off (verb), the chips use short delays for a tail of about 2 seconds with FEEDBACK down. Turned on (tronic), longer delays for a tail of about 4.5 seconds. With DENSE on, it also spreads the chips' delay times further apart.

DENSE BUTTON: Runs five copies of the three-chip circuit, every one of the fifteen PT2399 chips at its own delay time, for a much denser reverb.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged and the existing reverb tail is cut off.

The MIX jack works through the MIX MOD knob at the top of the bay column, an attenuverter: off at the center, turned left it inverts the modulation. A cable patched into the IN jack replaces the signal normalled from the previous slot.

Verb 28

An expanded emulation of our favorite hardware reverb. Verb 28 keeps the hardware's 16-bit, 31.25 kHz math, for a big, warm hall with a slightly grainy tail.

DECAY SLIDER: Sets the length of the tail, from 0.3 seconds at the bottom to 15 seconds at the top. Program 28's own 5 seconds is about three quarters of the way up.

DENSITY SLIDER: Sets the number of delay lines. At the bottom is the hardware's single loop of two delay lines. Toward the top up to eighteen loops of their own lengths join in, 36 delay lines in all, for a thicker, smoother tail.

DECAY MOD KNOB: Sets how much the DECAY jack moves the DECAY slider.

MIX KNOB: Crossfades from the dry input to the reverb.

TONAL TILT KNOB: Tilts the tone of the sound going into the reverb, flat at the center, brighter toward high and darker toward low, about ±12 dB at the ends.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged and the existing reverb tail is cut off.

With DECAY MOD fully up, +5 V at the DECAY jack covers the full travel of the DECAY slider. A cable patched into the IN jack replaces the signal normalled from the previous slot.

Texture

A granular processor based on open-source code by Emilie Gillet. It continuously records its input and scatters it into a cloud of grains taken from the recent past.

TIME SLIDER: Sets how far back in the recording the grains are taken from.

SHAPE KNOB: Sets the shape of each grain's envelope.

TEMPO SYNC BUTTON: Clocks the grains from the tempo's sixteenth notes. Below its center, DENSITY then plays a grain every 1 to 16 sixteenths. Above it, DENSITY sets the chance of a grain on each sixteenth.

DENSITY SLIDER: At the center no new grains are made. Below the center grains come at a steady rate, faster further down. Above it they come at random, more of them further up.

MIX KNOB: Crossfades from the dry input to the grains.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged.

Resonator

Resonator algorithms based on open-source code by Emilie Gillet. Input audio excites tuned modal resonances or strings.

FREQUENCY SLIDER: Sets the pitch over five octaves, C0 to C5, unquantized.

STRUCTURE SLIDER: Sets the spread and inharmonicity of the resonances.

BRIGHTNESS KNOB: Sets the brightness of the resonance.

DAMPING KNOB: Sets how quickly the resonance dies away.

MODE BUTTON: Steps through the three modes: modal resonator (MOD), sympathetic strings, vibrate (VIBE) and Karplus-Strong string (K-S).

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged.

With nothing in the FREQ jack, each onset at the input strums the resonator. The FREQ jack is a pitch input through the FREQ MOD knob at the top of the bay column (1 V/oct with the knob fully right), and with a cable in it each change of note strums it. There is no mix control: only the resonator is heard. A cable patched into the IN jack replaces the signal normalled from the previous slot.

Overdrive

A component level emulation of our favorite overdrive pedal.

OVERDRIVE SLIDER: Sets the gain of the clipping stage.

TONE SLIDER: Darkens the sound below the center and brightens it above.

DRIVE MOD KNOB: Sets how much the DRIVE jack moves the OVERDRIVE slider.

TONE MOD KNOB: Sets how much the TONE jack moves the TONE slider.

MIX KNOB: Blends the dry input with the effect. Fully left is the dry input alone, fully right the effect alone. The effect is matched to the input's level, so the mix changes the character, not the loudness.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged.

With DRIVE MOD or TONE MOD fully up, +5 V at the DRIVE or TONE jack covers the full travel of its slider. A cable patched into the IN jack replaces the signal normalled from the previous slot.

Fuzz

A component level emulation of our favorite fuzz. Two silicon transistors are DC coupled and share a feedback loop so the bias shifts with the signal, giving the sputter and gating of a real fuzz as it is turned down.

FUZZ SLIDER: Sets the amount of fuzz. Lower settings sputter and gate quiet notes.

TONE SLIDER: Moves from bass at the bottom, through a scooped middle at the center, to treble at the top.

VINTAGE BUTTON: Turned on, switches to the pedal's VINTAGE voicing with less gain and a hotter bias. Turned off, the MODERN voicing.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged.

The FUZZ jack moves the FUZZ slider through the FUZZ MOD knob at the top of the bay column: fully right, +5 V covers the slider's full travel. A cable patched into the IN jack replaces the signal normalled from the previous slot.

Chorus

A component level emulation of a classic synth chorus. Two bucket brigade lines sweep in opposite directions for a wide stereo sound. The input feeds two MN3009 bucket brigade delays, swept in opposite directions by a triangle LFO, each mixed with the dry signal to make the left and right outputs.

1 BUTTON: The slow mode, a 0.5 Hz sweep between about 1.7 and 5.4 ms.

2 BUTTON: A faster mode, the same sweep at about 0.9 Hz. With 1 and 2 both on, the chorus switches to a fast, shallow mode, about 9.75 Hz over 3.3 to 3.7 ms.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged.

Chorus 1 or 2 is always on, and changing modes crossfades smoothly. The Chorus has no control jacks. A cable patched into the IN jack replaces the signal normalled from the previous slot.

Bit Crush

A digital sample rate and bit depth reducer for everything from gentle grit to sputtering, broken blips. The signal is sampled and held at a reduced rate, then quantized to a reduced number of steps.

SAMPLE RATE SLIDER: Sets the sample rate from 100 Hz to 20 kHz, aliasing and all.

BIT DEPTH SLIDER: Sets the resolution from 0.1 to 8 bits, moving smoothly between whole bit values. Below 1 bit only the loudest peaks get through, as sputtering blips.

SAMPLE MOD KNOB: Sets how much the SAMPLE jack moves the SAMPLE RATE slider.

MIX KNOB: Blends between the dry and crushed signal.

BIT MOD KNOB: Sets how much the BIT jack moves the BIT DEPTH slider.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged.

With a MOD knob fully up, +5 V at its jack covers the full travel of its slider. A cable patched into the IN jack replaces the signal normalled from the previous slot.

Compressor

An emulation of the classic LA-2A leveling amplifier. Its heart is an electro-optical attenuator: the audio lights a panel, and a photocell beside it turns the signal down as it brightens. The light responds instantly, but the cell lets go slowly, so the leveling is smooth and musical, with a release that lengthens the harder and longer it has been working.

GAIN SLIDER: Sets the output level, from -20 dB to +20 dB, unity at the center. Use it to make up the level the compression takes away.

PEAK REDUCTION SLIDER: Sets how hard the signal drives the optical attenuator: the amount of compression. At the bottom there is none.

MIX KNOB: Blends between the dry and compressed signal, for parallel compression.

MODE BUTTON: Switches between COM (compress), about 3:1 with a gentle knee, and LIMIT, about 10:1, holding the level nearly flat above the knee.

ACTIVE BUTTON: Switches the effect in and out. Switched off, the input passes through unchanged.

The attack is about 10 ms. The release takes about 60 ms to recover half of the reduction, then 1 to 15 seconds for the rest, longer after heavy, sustained compression. In a stereo chain both sides are compressed together, so the image stays put. The Compressor has no control jacks. A cable patched into the IN jack replaces the signal normalled from the previous slot.

Keyboard controller

Press CONTROLLER in the button row to show the keyboard controller in place of the effects.

25 keys, C to C. Velocity is set by where a key is touched: the top plays at 127, the bottom at 16. Moving up and down a held key changes its velocity while it sounds.
Sliding along the keys plays each key reached. Several fingers play chords in polyphonic mode.
PITCH pad: bends from its center line over the pitch bend range, and springs back when let go.
MOD pad: a mod wheel, 0 at the bottom to 127 at the top. It stays where it is left.
VELOCITY LATCH (on by default): a released key's velocity stays at the VELOCITY jack, as on the hardware. Off, it drops to 0 when the key is let go.
PITCH GLIDE: turns the keys into a continuous controller, like a fretless instrument. The key touched starts the note, and sliding along the keys glides its pitch.
HOLD: the MIDI / arpeggiator section's HOLD.
OCTAVE UP / DOWN: move the keyboard an octave at a time. A button stays lit while the keyboard is shifted that way from its home range, C3 to C5.

The keyboard plays on the MIDI channel set in Settings, alongside your host's MIDI.

Parameter locks

Parameter locks give individual keys their own settings. Turn on PARAMETER LOCK, press a key to select it, then move any voice control: the new value is locked to that key, and the panel's own value is left alone. Whenever that note plays, from the keyboard, MIDI or the arpeggiator, it plays with its locks.

While a key is selected, the panel shows its locked values and every locked control glows blue.
Keys with locks show a small blue lamp.
Double-click a control to clear its lock on the selected key, double-click a key to clear all its locks, and double-tap PARAMETER LOCK to clear every lock.
Lockable: the oscillators and their mixers, the Binary Filter, the Dynamic VCA, the output, the modulation modules and the envelope generator. Not lockable: the scope, the MIDI / arpeggiator section, the effects, the patch bay, the Double Helix's BANK and the Primary Oscillator's WAVEFORM.
Locks are part of the sound: presets keep them.

Step sequencer

Press SEQUENCER in the button row to show the step sequencer. It has 16 steps, each with a PITCH slider, VALUE A and VALUE B knobs and a step button that turns the step on or mutes it (a muted step is a rest).

Modes

The four mode buttons at the lower left choose what VALUE A and VALUE B set on every step:

RATCHET: VALUE A: Triggers within the step, 1 to 8; VALUE B: Ratchet chance
PITCH/TRIG CHANCE: VALUE A: Pitch chance; VALUE B: Trigger chance
VELOCITY (default): VALUE A: Velocity, 1 to 127; VALUE B: Velocity chance
TRIGGER LENGTH: VALUE A: Length of each trigger, 5 to 30 ms, then 25 to 95 % of the step; VALUE B: Trigger length chance

Each chance is how often the step uses its own setting. At 100 % it always does; when the chance fails, the trigger is a rest, or the pitch, velocity, ratchet count or length is picked at random from its whole range.

Playing

The sequencer plays while both its own RUN / STOP and the master RUN / STOP in the MIDI / arpeggiator section (or the spacebar) are on. Switched on while the master transport is stopped, it waits for it. RESET starts the sequence again from the first step.
The PITCH slider picks a note of the scale between the key at the bottom and 1, 2 or 3 octaves up (the slider range). Each step's note is printed under its slider.
OCTAVE UP / DOWN move the range an octave at a time.
The playing step's button lights white.

Options

OPTIONS opens Key, Scale (chromatic, major, minor, whole tone, major and minor pentatonic, blues, dorian, lydian, harmonic major, harmonic minor), Clock Division (1/1 to 1/32), Sequence Length (1 to 16 steps), Slider Range (1 to 3 octaves), Direction (forward, reverse, random, pendulum) and Keyboard Transpose. With Keyboard Transpose on and the sequence running, a note played moves the sequence to that note's key instead of sounding.

Step locks

PARAMETER LOCK works as on the keyboard, with a step in place of a key: with it on, a step button selects its step and the voice controls set locks on that step. A step's lock wins over a lock on the same note.

Sequences

The steps, the sequencer's options and the step locks make up a sequence, kept apart from presets: loading a preset leaves the sequence as it is. Save and load sequences in the Sequences tab of the presets window.

Presets and sequences

Press PRESETS (or the display beside it) to open the presets window. The panel stays visible and playable around it, so you can hear and see each preset as you step through them.

Click a preset to load it; double-click to load it and close the window. The up and down arrow keys step through the list.
Save writes over the current preset; Save As… asks for a name.
Rename… and Delete act on the current preset.
Init loads the panel defaults with no cables.
Import… copies presets (or folders of them) into your presets folder. Export… saves a copy anywhere.
Show Folder (desktop) opens the presets folder.
Restore Factory Presets brings back any factory preset you deleted. It never overwrites one of yours.

Loading a preset fades the sound briefly to avoid clicks, and clears the undo history.

Factory presets

200 presets are built in, named by category: Lead (42), Pad (42), Poly (37), FX (16), Arp (12), Bass (11), Drone (11), Generative (11), Keys (9) and Drums (9). Many use patch cables. The Generative presets play themselves: hold one key. Hosts also list them as the instrument's programs.

Where presets live

On macOS and Windows, presets are .mv3900 files in Documents / Pittsburgh Modular / Microvolt 3901 / Presets, and sequences are in the Sequences folder beside it. A sandboxed host that cannot write to Documents (Logic runs some Audio Units this way) uses the same folders under the plug-in's Application Support folder instead; the factory presets are built in either way. On iPad, the app and the Audio Unit plug-in each keep their own presets; use Import and Export to move presets between them and to other devices.

What a preset holds

A preset holds every panel setting, every cable and the parameter locks. Loading one leaves alone:

the arpeggiator and HOLD, and the arpeggiator's options;
the settings (MIDI channel, pitch bend range, clock source, oversampling) and the master tune;
the CC assignments and the MOD and CC jacks' controllers;
the sequence.

Settings

Press SETTINGS to open the settings window over the panel.

MIDI channel: Omni (default) or 1 to 16
Pitch bend range: 2, 5 (default), 7 or 12 semitones
Clock source: Internal, Host Sync or Ableton Link
Engine oversampling (macOS and Windows): Eco (2x), Standard (4x, default) or High (8x)

On macOS and Windows the window also has Check for Updates…, and in the standalone app Audio/MIDI Settings… for your audio interface and MIDI inputs.

Higher oversampling reduces aliasing from the wave folders, drives and resonance at the cost of more processing. On iPad the engine always runs at Eco (2x).

These settings are part of the setup, not the sound: UNDO and presets leave them alone. Master tune (±100 cents) is available as a host parameter.

MIDI, clock and automation

MIDI messages received

Note on / note off: Play the instrument. Velocity goes to the VELOCITY jack. Note 60 is middle C.
Pitch bend: Bends the pitch over the pitch bend range set in Settings.
Control change: Moves the controls assigned with CC ASSIGN, and drives the MOD and CC jacks.
All notes off: Releases every note (notes latched by HOLD keep playing).
All sound off: Silences everything, including notes latched by HOLD.

CC assign

Press CC ASSIGN and a small box appears above every slider and knob, showing the MIDI controller that moves it. Type a number (0 to 127; empty for none), or click a box and move a controller to learn it. Several controls can share a controller. The assignments are kept with your project and as the default for new instances; presets and undo never change them.

The MIDI section's MOD and CC jacks get boxes too. The MOD jack follows CC 1 by default; type a controller number, or vel to make it follow velocity. The CC jack follows CC 74 by default. Both put out 0 to 5 V.

Clock

Internal (default in the standalone app): The TEMPO knob (30 to 300 bpm) or TAP TEMPO sets the tempo. RUN / STOP (or the spacebar) starts and stops.
Host Sync (default in plug-ins): The arpeggiator, sequencer, clock output and tempo-synced effects follow the host's tempo and transport.
Ableton Link: Joins a Link session on your local network. Tempo goes both ways, and RUN / STOP starts and stops the session. The tempo readout shows the number of peers.

On iPad, the first time Ableton Link is chosen, iPadOS asks permission to use the local network.

Automation

Every slider, knob and module choice on the panel is a host parameter, so it can be automated or mapped with your host's MIDI learn. (A few switches that set up the panel rather than the sound are not: VELOCITY LATCH, PITCH GLIDE, PARAMETER LOCK, the EFFECTS / CONTROLLER / SEQUENCER buttons and the sequencer's mode buttons.) The whole panel, including every cable, is saved with your project.

What MIDI does not do

Microvolt 3901 does not respond to the sustain pedal (CC 64) or to MIDI clock, and it has no audio input. The spacebar works as RUN / STOP once you have clicked on the panel.

Voices and polyphony

Monophonic and polyphonic

With MONOPHONIC lit, Microvolt 3901 plays one note at a time, as the hardware does. Turned off, it plays six complete voices that share the panel settings and cables. Each held note gets its own oscillators, filter, VCA, envelopes and LFOs; a seventh note takes the oldest voice. With the arpeggiator on, its steps are dealt round the six voices in turn, so each note can ring out under the next.

Spread

In monophonic mode, SPREAD stacks up to six voices on the one line, spread evenly up to ±18 cents, for a wider sound at about the same level. Full left is a single voice. Each added voice drifts gently on its own to keep the sound alive. Spread multiplies the processing: six voices cost about six times one.

Processing

Silent voices stop being processed, so the load follows the number of notes sounding. Effects that are off with nothing patched cost nothing.

Microvolt 3901 on iPad

The standalone app runs on its own with the panel filling the screen, in either orientation, in Split View or in a resized window. The panel is drawn at the largest size that fits.
The Audio Unit (AUv3) plug-in is installed with the app. Open the app once, then find Microvolt 3901 under Pittsburgh Modular in any host that supports Audio Unit instruments, such as AUM, GarageBand, Cubasis or Logic Pro for iPad.
Touch: drag controls with a finger, double-tap to reset, and use several fingers to move several controls or play chords on the keyboard controller.
The engine runs at Eco (2x) oversampling to save power.
The app and the plug-in keep separate presets: use Import and Export in the presets window to move them.
Ableton Link asks permission to use the local network the first time you choose it.

Specifications

Formats: VST3, Audio Unit (macOS), AUv3 (iPadOS), standalone (macOS, Windows, iPadOS)
Systems: macOS 12 or later (Apple silicon and Intel); 64-bit Windows; iPadOS 15 or later
Voices: 1 (monophonic, with up to 6 unison voices) or 6 (polyphonic)
Oscillator places: 4 modules for oscillator 1, 3 for oscillator 2, plus the oscillator / LFO in the patch bay
Modulation: 2 places, each taking any of 6 modules; envelope generator; sample & hold; ring modulator; VCA; inverting mixer
Effects: 8 slots, each taking any of 19 effects
Patch bay: Every module's inputs and outputs; up to four cables per input
Sequencer: 16 steps with ratchets, chances, trigger lengths and per-step parameter locks
Presets: 200 factory presets
Undo: 100 steps
Oversampling: 2x, 4x (default) or 8x on the desktop; 2x on iPad

Measured

Tuning: within 0.01 cent from A1 to A6 through the whole signal path.
Binary filter: cutoff 20 Hz to above 20 kHz; resonance Q 0.66 to about 58 in stable mode.
Envelope generator: attack 11 ms to 8.4 s; decay and release time constants 5 ms to 4.7 s.
Modulation sources LFO: 0.0006 to 1.2 Hz (slow range), 0.06 to 120 Hz (fast range).

Support and notices

Updates

On macOS and Windows, Check for Updates… in the settings window tells you when a new version is available. The iPad app updates through the App Store. Installing a new version keeps your presets.

Contact

Please contact us for support or other information about Microvolt 3901 or any other Pittsburgh Modular product: pittsburghmodular.com/contact.

Third-party notices

The Reverb, Texture and Resonator effects are based on open-source code by Emilie Gillet (Mutable Instruments Clouds, Beads and Rings), used under the MIT License. Copyright (c) Emilie Gillet.
Ableton Link. Copyright Ableton AG.
Built with the JUCE framework.
Interface text set in Inter Tight (SIL Open Font License).

Trademarks

VST is a registered trademark of Steinberg Media Technologies GmbH. Audio Units, macOS, iPadOS and iPad are trademarks of Apple Inc. Windows is a trademark of Microsoft Corporation. Ableton and Link are trademarks of Ableton AG. Other names are used only to describe the circuits that were modeled.

Disclaimer

Microvolt 3901 is a component level model of Pittsburgh Modular hardware and the circuits named in this manual. Measurements describe the software's behavior and are not specifications of any hardware product. Specifications and features are subject to change without notice.