[sdiy] Yet more questions: best VC drive approach

[Rutger Vlek]

Hi Richie,

I hope you don't mind me bumping up an old thread. I was reading back what
you wrote in 2018 and wondered if you could refer me to more background
information on filter saturation. I'd like to understand what happens in a
ladder filter, and weather something musically similar could also be
recreated in other ways (e.g. in other topologies than a ladder). If you
have an opinion on the latter, please share!

Regards,

Rutger

Op vr 9 nov. 2018 10:51 schreef <rburnett at richieburnett.co.uk>:

> When you over-drive OTA based 1-pole "leaky integrator" stages, you
> actually get a signal dependent shift in the cutoff frequency as the OTA
> saturates, rather than what you would typically describe as "clipping".
> This behaviour is down to the way in which the OTA and filter capacitor
> are wrapped up inside a negative feedback loop.  The behaviour is quite
> like how the cutoff frequency of the Moog ladder filter changes
> dynamically with drive signal level.  It is much more musical that
> simple signal clipping.
>
> -Richie,
>
>
> On 2018-11-09 08:48, Rutger Vlek wrote:
> > Hi Jacob,
> >
> >> This also ensures that the clipping happens in the chip used for the
> >> integrators, and not in the OTA's, which sound bad when overdriven.
> >
> > I presume you refer to the OTA in the VCA that controls the drive
> > level? Or do you mean OTAs inside your integrators? In the latter case
> > I don't understand what you're saying (sorry)...
> >
> > Rutger
> >
> >> JACOB WATTERS
> >> Web & Multimedia Specialist
> >>
> >> JacobWatters.com [1]
> >> Tel: 226-886-3526
> >>
> >> On Thu, Nov 8, 2018 at 3:32 PM 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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> >>> Synth-diy at synth-diy.org
> >>> http://synth-diy.org/mailman/listinfo/synth-diy
> >
> >
> >
> > Links:
> > ------
> > [1] http://jacobwatters.com/
> > _______________________________________________
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>
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