[sdiy] Yet more questions: best VC drive approach

Mike Bryant mbryant at futurehorizons.com
Mon Feb 14 12:27:29 CET 2022

Also if you want to get into the actual characteristics of the long tail pair tanh() transfer function, look up the papers by Barrie Gilbert (of Gilbert cell fame).  He discusses how to linearize it but also how to make use of it in some papers.  I recall there is one where he uses parallel pairs with different currents to achieve some interesting transfer functions.

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of rburnett at richieburnett.co.uk
Sent: 14 February 2022 11:15
To: Rutger Vlek
Subject: Re: [sdiy] Yet more questions: best VC drive approach

Hi Rutger,

Wow, that is an old thread.  I've slept since then! ;-)

They are both based around a "long tailed pair," which is a differential amplifier made up of two transistors.  If you read up about this arrangement you will find that there is a tanh() function in its transfer function, that leads to a soft saturation behaviour.  Both the Moog ladder filter and the input stage of a bare OTA exhibit similar
tanh() soft distortion.  Although the exact effect on the cutoff frequency in each type of filter may be subtly different.

There are some good papers out there discussing the non-linearities in the Moog ladder filter arrangement.  Ones by Antti Huovilainen, Tim Stinchcombe and to a lesser extent Tim Stilson are the ones that immediately come to mind.  Some of the stuff in those papers is about making a digital "DSP" model of the filter, but the first bits about how the analogue filter works discusses the non-linear behaviour for large signals.

I don't have a reference for OTA operation immediately at hand but I would have thought the non-linearities for large input signal amplitudes would be discussed in the datasheet, or documented somewhere.  As I said, the first stage is just a long-tailed pair diff amp, so assuming the OTA doesn't have any fancy linearising diodes, it will have a tanh() shape to its transfer function that starts to kick in once the differential input signal amplitude goes over a few tens of millivolts.

Hope this helps...


On 2022-02-12 20:15, Rutger Vlek wrote:
> 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
>>>> 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
>>>>> _______________________________________________
>>>>> Synth-diy mailing list
>>>>> Synth-diy at synth-diy.org
>>>>> http://synth-diy.org/mailman/listinfo/synth-diy
>>> Links:
>>> ------
>>> [1] http://jacobwatters.com/
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