Introduction:
Beside VCAs for audio purposes (control of audio volume) you need VCAs for manipulation of control voltages (ADSRs) or extremely low frequency modulation signals (LFOs) in a modular system.
The audio VCAs of the Formant are not really suitable for this purpose, as a signal being manipulated by the VCA is fed in via capacitor. This makes it difficult to process LFO signals and makes it impossible to process dc like signals (ADSR).
As there are not that quality needs for control voltages concerning linearity and noise like for audio signals I developed a tiny little circuit which can be used for VCAs processing control voltages.
Very important for me was the total lack of special chips (like the CA3080 OTA) which are in danger of extinction and the total lack of transistor arrays. Indeed the circuit works with just one field effect transistor, only input and output buffer stages in form of TL064 are provided.
Circuit:
And here you can find the circuit. Attention: Clicking the circuit means acceptance of the disclaimer on page bottom!
Two input signals can be processed simultanously. They are mixed and buffered by the TL064/1 buffer stage. This works as a not inverting input buffer as control voltages have to be processed as they are.
The buffered input signal reaches the output buffer (TL064/2) via 100k resistor and the drain pin of the fet. The output stage is just a voltage follower.
As the gate voltage of the fet remains positive the drain => source connection within the fet stays in a high impedance status and the input signal passes the drain pin of the fet without beeing influenced, as if the fet would'nt exist. If the gate voltage becomes more and more negative, the drain => source channel opens according to this change and more and more parts of the input signal voltage go 'down the drain' and the output amplitude lowers proportional. If the fet is comletely open, the output signal voltage is (nearly) zero. This means: The output signal voltage is reverse proportional to the fet gate voltage.
The module can process two control voltages. They are mixed and buffered by a this time inverting input buffer (TL064/3) to invert again the reverse proportional behaviour of the cv / gate - voltage to the output signal voltage becoming a direct positive proportional dependence between cv input and output voltage again.
To be able to process positive and negative control voltages you can add an offset voltage between +15V and -15V.
Frontend:
Each signal and cv input is controlled by a level pot. There is also one offset pot per VCA to adapt negative cvs.
The task of the module, its voltages and controls are shown by the amplification symbol (triangle) and lines on the front panel.
Board of the Simple VCAs:
Complete module:
Hints
The circuit ist not really suitable for audio signals, as even with a completely opened fet you can hear a little rest of the input voltage at the output. Additionally a changing control voltage causes a little noise in the output signal which is probably caused within the fet.
Perhaps an additional offset pot for signal voltages might be helpful, as the fet stage in the VCA can handle only positive signals. This is no problem for ADSR signals but might be a problem for LFO outputs which have positive and negative parts. I had to adapt LFO signals with the CVS module.
Sound example
Delayed vibrato