[sdiy] Yet more questions: best VC drive approach

Tom Wiltshire tom at electricdruid.net
Tue Nov 13 01:07:39 CET 2018


The problem might be that you’re overdriving the OTA itself by maxing out its gain. The “nice” distortion from OTAs come from overdriving the differential inputs for a smooth tanh curve distortion. This is what Craing Anderson does in the Device article Terry B. posted. There are two CA3080s used. The first controls the drive level sent to the second. The second is a fixed gain, and is used just for its pleasant input distortion.

Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 9 Nov 2018, at 12:15, Jacob Watters <jacobwatters at gmail.com> wrote:
> 
> Yes in the VCA and not the integrator. I find that OTA's don't sound nice when overdriven. They have a crackly, noisey sound. I prefer to do my overdriving in a 4069, like the wasp filter.
> 
> On Fri, Nov 9, 2018, 3:48 AM Rutger Vlek, <rutgervlek at gmail.com> 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
>> 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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