Modular Synth Patching Basics (with VCV): Setting up a Classic Synth Bass

Modular synthesis patching, whlie complex, can be satisfying on a creative level with the number of possibilities for those open to sound exploration as Joe Albano demonstrates.  

Before the Minimoog introduced the world to the compact portable prewired analog synthesizer, intrepid synthesists had to negotiate the ins & outs of modular synthesizers to craft their tones and effects, connecting every component by hand with patch cables. Though modular synths never went away, prewired models dominated for years, offering easier accessibility, and eventually virtual synths began to take over. 

Fig 1 Classic Modular Synthesizers

Fast-forward to the present, and a resurgence of true analog hardware modular synthesis is in full swing, thanks to the popular Eurorack standard for assembling modules from various manufacturers—like Doepfer, the originator of the Eurorack format—into custom modular designs.

Fig 2 Eurorack Modular Synthesizer Modules

Eurorack Modular Synthesizer Modules

At the same time, virtual emulations of classic modular designs—like the Moog Modular and ARP 2600 semi-modular V.I.s from Arturia, among others—brought modular synthesis into the box as well.

Virtual Modular

While not everyone has access to a hardware modular synth, virtual modulars are available for those who want to explore the connectivity and creative possibilities of modular synthesis. Besides the Arturia classic emulations mentioned above, other companies offer V.I. versions of today’s Eurorack modulars—Softube’s Modular is a virtual version of a Eurorack with various Doepfer modules. There’s even a free emulation of a Doepfer modular synth rack from VCV. Called simply Rack, it’s available as a free standalone application for both Mac & PC, and it’s the perfect tool for learning the ins & outs of modular synthesis while conveniently staying inside the box. It’s a little bit of a CPU hog, but it should be fine on most modern computers.

For this article I’m going to use VCV Rack to go through the creation of a classic resonant synth bass patch from scratch, assembling and wiring all the modules together just as a hardware synth user would have to do.

Piece by Piece

Once the VCV Rack application is installed, opening a New file calls up a simple patch—to get to a point where you can start from scratch you just Clear Cables from the Edit menu. For this run-through I’ll remove any unnecessary modules—the SCOPE—and move the MIXER over next to the sound source, VCO-1 (Voltage- Controlled Oscillator 1), leaving space for a second VCO. For a richer sound, I’ll add that second VCO by right-clicking in the space and selecting another VCO-1 module from the collection of emulated Doepfer modules on tap.

To get sound, there’s a little software setup required before we can get back to the simulated hardware patching. In the AUDIO-8 module, you have to select your audio output or interface. To eventually play from a MIDI keyboard, you’d select it in the MIDI-CV module.

Now I can cable the Sawtooth wave audio outputs of the two VCOs into two audio inputs in the MIXER module, and the MIXER output into the left & right sides (TO DEVICE 1 & 2) of the AUDIO-8 output module (Command-drag to add a second cable to the MIXER’s MIX output); I’ll slightly detune the second VCO Frequency with the FINE tune knob for a little natural chorusing.

Fig 3

Basic Audio Path

Of course at this point all I have is a steady tone—I’ll need to set up keyboard triggering and ADSR Envelopes to turn the patch into a playable instrument, but first I’ll finish setting up the audio signal path. I’ll re-cable the MIXER module output into the audio input of the default VCF (Voltage-Controlled Filter). Now I’ll add a VCA module (Voltage-Controlled Amplifier) just after the VCF, and cable the VCF LPF output (LowPass Filter output) into the VCA audio input, and the VCA audio output into the left & right sides of the AUDIO-8 output module.

Raising the FILTER cutoff (FREQ knob) all the way up will restore the steady tone, which is now passing through the Filter and Amplifier. For the moment I’ll turn down VCF Resonance (RES) all the way down.

Fig 4

This is the complete audio path—source through audio modifiers: VCOs -> VCF -> VCA. The rest of the modules will be Modulators, which will modulate certain controls in the audio signal source and modifier modules.


Most prewired synths locate the Modulators below the audio signal path modules, so I’ll do the same. I’ll drag the default ADSR module to the lower rackspace, below the VCF which it’ll be patched into; and I’ll add a second ADSR, and position it below the VCA, which it’ll be patched into.

The ADSR Envelope Generators (EGs) will be triggered by notes played on the keyboard, and they’ll give shape to each note—the VCF Filter Envelope will shape the tone of each note over the length of the note, and the VCA Amp envelope will shape the volume of each note over the length of the note. ADSR, of course, stands for Attack time, Decay time, Sustain level, and Release time. These settings determine how quickly the note starts/builds up (Attack) and drops off (Decay); the level/tone (brightness) of a sustaining note (Sustain); and how quickly the note dies away when the key is released (Release).

Control Voltages

In a hardware analog modular synth, Modulators control parameters of the sound via Control Voltage (CV) signals—a Modulator’s CV is routed to a CV input in another module, where it affects a particular parameter. These connections are in already in place in a prewired synth, but they all have to be made by hand in a modular synth, So the first ADSR module’s CV output will be routed into the VCF’s FREQ CV input, where it’ll modulate the Cutoff Frequency (FREQ); the second ADSR module’s CV output will be routed into the VCA’s CV input—I’ll switch from a Linear (LIN) to an Exponential (EXP) response curve for a more useful CV response. Turning the VCF’s cutoff Frequency knob (FREQ) all the way down will allow the Filter ADSR to completely shape the tone, from dullest (off) to brightest—when a note is triggered the ADSR will raise the cutoff FREQ to the maximum level set by the FREQ CV knob, which serves as a Depth control for the ADSR modulation. The Amp ADSR will likewise control volume, from off—no sound—to maximum volume.

Fig 5

The Keyboard: CV/Gate

Finally, to play this patch the keyboard needs to be patched in. An analog modular synth’s keyboard is another Modulation source—it provides two control voltage signals. A control voltage usually labelled simply CV is routed to the VCOs. This CV will modulate the pitch (FREQ) of both VCOs, allowing for the playing of different notes. In analog modular synths there was always a standard of 1Volt/Octave, which calibrates the keyboard to the standard 12 chromatic notes in the western musical scale, and VCV Rack emulates that here. The keyboard also outputs a second control voltage, usually labelled Gate—the Gate signal starts and stops the notes, finally turning the modular patch into a playable instrument. Of course the keyboard connections are automatically made internally in a prewired synth, but they need to be patched by hand in a modular setup.

Since this is a virtual emulation, the virtual keyboard CVs—CV and Gate—are patched from the MIDI-CV module. The keyboard CV signal here is labelled V/OCT (Volt/Octave), and it’ll be patched into the similarly labelled V/OCT CV input on each VCO (again, Command-drag to add a second cable from the V/OCT output).

The GATE signal will be patched into the GATE input of both ADSRs, where it will trigger both ADSRs running every time a note is played/released. Now every time a key is plays a discrete note will sound—note: like the earliest modular synths, Rack is monophonic.

Fig 6

Dialing Up a Sound

The rest of the settings will turn this basic modular patched sound into a specific instrument or effect. Since I promised a classic synth bass sound, here are settings that would create that.

The VCF FREQ knob is all the way down, and to start with the FREQ CV knob is all the way up—I explained these settings a couple of paragraphs earlier. The ADSRs would be set to shape percussive bass notes: Both Attacks at minimum (fastest attack); Decays around 2 o’clock (to taste); Sustain at minimum (a plucked bass is a percussive instrument, so no sustain); and Release around 1 o’clock (again, to taste).

For a classic juicy resonant bass sound, the VCF Resonance knob (RES) would be set around 12 o’clock (straight up). If the bass notes are a little too bright, the maximum level that the Filter ADSR raises the cutoff FREQ to—the ADSR depth—can be reduced a bit by slightly lowering the VCF FREQ CV knob; set it to taste for a fatter bass sound.

The finished bass patch is shown here:

Fig 7


For a little more expression, some LFO modulation could be added and put under the further control of the keyboard’s Modwheel, to simulate a musical Vibrato effect. I’m out of room here, so that might be a subject for another day. In the meantime the screenshot below shows the patching required in VCV Rack for a standard LFO Pitch modulation Modwheel-controlled Vibrato effect.

Fig 8

Patch It Up

Patching in a modular synth can add an extra level of complexity for those used to prewired models, but once you get past basic modular synthesis patch construction, the creative possibilities can be a lot of fun for intrepid synthesists to explore.

Learn the ins and outs of eurorack modular synthesis: AskAudio | macProVideo


Joe is a musician, engineer, and producer in NYC. Over the years, as a small studio operator and freelance engineer, he's made recordings of all types from music & album production to v/o & post. He's also taught all aspects of recording and music technology at several NY audio schools, and has been writing articles for Recording magaz... Read More


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