Voltage Processor - CGS 81 - Build notes

These are my build notes for Ken's  Cat Girl Synth 81 - voltage processor.
A very useful module with a super small footprint. It's a classic example of
a two-stage amplifier worthy of breadboarding if you like to examine how op-amps work.

It's very similar to Ken's DC mixer (CGS 04)

First the virgin PCBs.

 You can snap the PCB into 2 if you only need 1 voltage processor.

Resistors, IC headers, first.

I have the diodes (Bat 48 Schottky) on back order. "The BAT48 diodes provide a dead zone in the center of the offset pot's rotation, to allow for easier zeroing". 

I'm building this as a processor so there really is no need for trimmers (or diodes).
I've used a 100k for RS and linked the two pads of the SPAN trimmer with some resistor wire offcuts. 
However, I may at a later date add a trimmer or a larger pot (rather than a wire across the SPAN) to one of the processors so I can adjust the level of the inverting output. 

These are two SMD 100nF caps ... optional.
They are placed across the power rails.

The circuit uses just one (dual) Op-Amp - the workhorse TL 072

Check out Ken's schematic in his build notes

Both non-inverting inputs (Pin 5 & pin 3) are connected to ground.
Thus the inverting inputs (pin6 & 2) must also be ground (virtual ground)

The input signal is fed to both inverting inputs (6 & 2). The output from the first opamp (pin 7) is just
the inverted output. This is fed into the inverting input of the second op-amp (pin 2). So they cancel each other out.
"By adjusting the SPAN pot to either side, either the inverting or the non-inverting is increased with respect to the other, the difference being buffered by the second stage and fed to the output."

Such a lovely circuit ... the basic building components are just one TL072, two 47pf caps, one trimmer, two 330R  & three 100k resistors.

To Be Continued.

Links:
1. CGS 81 build notes
2. Serge voltage processor - M module
3. Musings on (Buchla) voltage processors - Buchla Tech
4. Q125 signal processor - Synthesizers.com
5. Moog CP-251 Moogerfooger
6. CGS 70 - Dev-mod Mixer/Inverter block
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