[sdiy] Yet more questions: best VC drive approach

Tom Wiltshire tom at electricdruid.net
Fri Nov 9 01:45:20 CET 2018

I’ve pencilled in designs using OTAs for this. After all, it’s specifically a voltage-controlled (ok, current…) gain element, so it’s pretty much ideal, limitations notwithstanding. Since it can be used like an op-amp, you could take a “tube screamer”-style design and adjust it for an OTA in place of the op-amp.

Alternatively, the V2164 can provide up to 20dB of gain, or more with gain in front of it (e.g. use attenuation to get to unity) which would give you another option potentially with lower noise.

There’s plenty of options for CV-controlled overdrive/distortion/fuzz.

(My background in this: Big Muff with CV -




       Electric Druid
Synth & Stompbox DIY

> On 8 Nov 2018, at 20:31, Rutger Vlek <rutgervlek at gmail.com> wrote:
> Hi guys,
> I've been wondering about many things lately, hence the flood of emails to the list :). I also have to admit feeling a bit stupid about having to ask this.. but here goes:
> What's the best approach to designing a voltage controlled overdrive? The obvious solution I can think of is having a saturation element preceded by a VCA. While I haven't fully done my homework on it yet, my guts tell me that this isn't the best circuit in terms of noise behaviour, as it would require the saturation element to be at high gain constantly while the VCA various input level. Meaning that any VCA noise would be amplified by the full gain of the saturation element. In guitar stomp boxes, some design place a pot in the feedback loop of an opamp to change gain. Could a similar approach work well for a VC-drive unit and would it perform better/worse than the first solution I proposed?
> Finally, I've been thinking about making drive level voltage controlled via power supply to the saturation element (transistor in this case). Feeding the control voltage into a buffer that puts out the supply to a transistor would also allow to change drive level.
> What do you think? How it this typically done? I just bought a Novation Peak, and am impressed with it's three stages of overdrive although it suffers from noise issues due to the amounts of again at hand. It makes me wonder how I would design such a stage myself.
> Rutger
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