[sdiy] Yet more questions: best VC drive approach

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Fri Nov 9 10:51:13 CET 2018

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.


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
>> 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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> Links:
> ------
> [1] http://jacobwatters.com/
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