MODEL A+ (flexible dual VCA)

This is a spinoff of the simpler Model A VCA, which is also based on the Alfa Par AS662D, a recreation of the classic Roland VCA in a dual package. Yet, this can do so much more, including VC mixing, VC panner and VC xfades (more on that below).

Simple Demos

Input boost on raw waveforms (tri, saw, square)
input boost on filtered sounds
audio rate amplitude modulation
VC Feedback using second VCA half for sending out back into filter input (Rides in the Storm SED Filter)

Module Description

Corny Pynchon reference, yup, but OTA symbols and post horns are not too dissimilar.

This module is 16HP as I found smaller versions crammed. In terms of functionality, you have to thank KSS on Modwiggler too, as they demanded more I/O. Also thanks to the folks who participate in the pass on shenanigans on, providing valuable feedback while toying with a proto!

This VCA is DC coupled, so you can use it for audio and CV signals. With “boost” on zero and +5V on CV, you have unity gain (output level = input level). The offset pot sweeps between -5v and +5v (just a tad more, actually, to have some leeway). The mod knob is bipolar, with zero amount at 12 o‘clock; if you pull it below 12, your input signal is inverted. Now, pull offset up and mod amount down and you have a VCA that is always open unless you send in some envelope or LFO: audio ducking here we go! A boost knob lets you double the input volume and thereby drive the VCA in a nicely sinusoidal fashion.

As for I/O, there are non-inverting and inverting inputs and outputs, which comes in handy if you want your signal polarity inverted. And, finally, there is a second CV input for adding, say velocity to your envelope.

Normalised connections: I prewired some connections between the two VCAs, so that you can set up this thing as a voltage controlled panner or as a voltage controlled crossfader. Audio -IN1is forwarded to -IN2, unless you plug a cable into the -IN2 socket, which breaks the prewired connection. Mod1 in is also forwarded to Mod2 in, so here’s a trick: send an audio signal to -in1 and an lfo to mid1 in, then pull mod1 knob up an mod2 knob down. Send -out1 to the left channel of your mixer and -out2 to the right mixer channel. You have an auto-panner!

One further prewired connection is that -out1 is mixed into -out2, unless you plug a cable into -out1. This way, you can use the models+ as a simple mixer or even voltage controlled Crossfader.

VC panner
VC Xfade

These normalisations are on the -ins and outs as this interferes least in standard independent VCA applications where you would use +in and +out and leave -I/o untouched.

Parameters and Knob Settings

Boost: This depends a bit on your taste. With R5 and R19 at 100k you double incoming volume by turning up boost to the max. This may not be enough drive for your taste, so toy around with values for these two resistors. I hate 47k at the moment which I like, but anything down to 10k should be fun!

Mod1 and Mod2 In: Mod1 is scaled so it can cancel out offset, i.e. with +5V input and max setting you have unity gain. The Mod2 socket has a stronger effect and is well suitable for vintage audio VCA applications. If you want MOD2 to match MOD1 at maximum value, simply replace R13 and R27 (both 47k) with 100k resistors.

Offset and Mod knobs: For dialling in plain and simple VCA stuff, please note that the offset knob setting interacts with incoming CV as well as with the Mod Knob. I wanted to have these knobs centre detented first but then decided against that for various reasons – you’d need to have a bunch of internal trimmers and/or you’d rely on optical settings (i.e. “this looks 12 o clock, so it must be silent” type of stuff). So, long story short, you. need to use your ears and it can happen that your zero modulation point on MOD1 knob is not exactly at 12 ‘clock.

Common Setups Illustrated

Setup as normal VCA
Set as 2:1 Mixer
Setup as auto-panner with one mono input signal
do ya thing!

For more stuff, just try and hear what happens 🙂


Jan 2024 errata: R16 and R33 need to be 47K, and R4 and R23 need to be 100K. Will update/correct schematic soon.

This is CC NC BY SA, so for non-commercial use.

Gerbers and BOM

BOM comes asap!