I’ve built a Tetrazzi quad oscillator, a Quantussy quintuple modulator, a Roolz 5-6 dual LFO, and am about to built a Dubernator dual VCF. All of these from PC boards donated to me by Josh Rodriquez. Almost a complete system, it lacks VCAs. Here is a possible Peter Blasser designed VCA (with a small modification by me).

I looked at a number of VCA circuits published by Peter B., including ones in the Cocoquantus input circuitry, the Plumbutter AVDog, and the Rollz-5 AVDog, plus the Eurorack Denum module. The AVDog is a combination module, consisting of a VC oscillator, a low frequency pinged filter that generates an undulating envelope, and a dual VCA. Like the VCA in Denum, AVDog has two outputs, with the sound going out only one or the other at a time. I breadboarded one of the AVDog VCAs to examine how it works.

Findings

First, using one of the DIY Tetrazzi oscillator outputs as the signal to be modulated, I discovered that the maximum output was obtained at around 0.5 volts of CV, and the output became minimized at around +4 volts. Well that was unexpected! But not too surprising, because the CV driving it in the AVDog comes from the odd envelope generator and meant to work just as Peter intended. I also saw that the maximum gain is about 0.6, which seems to be the result of a compromise between gain and distortion. There is a bit of distortion, which can be noticed (in the video below) as a slight rounding of the tips of the triangle wave.

Voltage ranges

Peter’s designs all use a single 9 volt power supply. To obtain symmetry everything is biased at +4.5 volts, which Peter labels ‘Mido’ (middle). All DC-coupled outputs center on Mido, with an amplitude also of about 4.5 volts peak to peak. So the minimum voltage is about 2.25 volts and the maximum about 6.75 volts.

The external input to the AVDog VCA is AC-coupled to insure internal centering on Mido. Likewise, the output is AC-coupled. Thinking about how I would use this as a stand-alone module, I decided it would make a great two-quadrant multiplier for Mido-centered signals coming out of Tetrazzi and Quantussy. But I wanted them to be DC-coupled so that LFOs can also be processed.

So that’s what I’ve designed. The signal and CV inputs both expect a voltage range of between +2.25 and +6.75 volts. The output, also centered on Mido, will be the product of the two signals, with a maximum amplitude of about 2.7 volts peak-to-peak. In order to get zero output at +2.25 volts and maximum output at +6.75 volts of CV, I had to design a simple gain and offset buffer to put in front of the AVDog VCA CV input. The buffer is shown on the left side of the schematic diagram, below.

Schematic

I’m thinking about building a unit that contains two or four of these. It would be very simple, with just input, CV, and output banana jacks.

Video Demo

The video shows the setup I used to test it. I took triangle waves from the DIY Quantussy to use as the signal and the CV. The point is to see and hear the amplitude modulation. Don’t be confused by the pitch jumping around; that’s just how the Quantussy works. What’s interesting is that the VCA circuit performs an enveloping function at LFO rates, as well as interesting amplitude modulation at audio rate on the CV input.