[sdiy] Yet more questions: best VC drive approach

Quincas Moreira quincas at gmail.com
Fri Nov 9 04:18:31 CET 2018

I would use a Vactrol with the variable resistor in the negative feedback
path of a saturating opamp circuit...

On Thu, Nov 8, 2018 at 6:46 PM Tom Wiltshire <tom at electricdruid.net> wrote:

> 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 -
> http://www.frequencycentral.co.uk/?page_id=1484
> )
> HTH,
> Tom
> ==================
>        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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Quincas Moreira
Synth Diy Guy
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