MODEL A+ (flexible dual VCA)

This is a sort of 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. In theory, you can easily adjust this for using a slightly cheaper LM13700 OTA instead but in this case I’d suggest not using the buffer stage (maybe later I’ll do a 13700 version too).

This is currently on the ways to proto board production, and as soon as the PCB is good enough, I’ll upload Gerbers. For now, here are some models and the schematic (NB this is CC NC BY SA, so for non-commercial use).

Panel render

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.

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). Per single VCA there are non-inverting and inverting inputs and outputs, a “boost” knob by means of which you can double the audio input volume, which nicely overdrives the VCA. There is an offset knob with which you can set an offset between -5V and +5V. Together with the bipolar CV input, you can do stuff like fadeins and fadeouts and, of course, adjust for any offset you have in your CV gear. A second CV in (added to the first) concludes the I/O of each channel.

Now, 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. The audio inputs of channel 2 are normalled to those of channel 1, and so is the bipolar CV. Set up VCA 1 for a simple decaying sound and then on VCA pull offset up and -/+ CV fully down – connect the +OUTS to a left and right sound card channel, and hear panning happen in your areas. As for outputs, OUT- of VCA 1 is mixed into OUT – of VCA2. VC Crossfade can be done like this: keep your offset and -/+ CV set as in VC panning, but plug wave A to VCA 1 and wave B to VCA 2. Leave OUT-1 unplugged and use OUT-2 as your output. You now blend between two inputs. The prewired connections are disengaged if you plug a jack into the respective sockets.

Schematic and board renders

https://maffez.com/wp-content/uploads/2022/11/MODEL_A_1_4.pdf

pcb front
pcb back

More soon!