[sdiy] OTA performance (was SSM chip reissue)

[Richie Burnett]

The few analogue SVFs that I've built from OTAs & op-amps always seemed to 
prefer oscillating at the HF end of the scale.  I'd always put this down to 
additional phase-lag around the loop caused by the OTAs and their op-amp 
buffers not having infinite bandwidth (instant transient response).

If you take a look at this schematic of a state-variable filter from 
Roland's Jupiter 6, you can see small capacitors C107/108 that form a 
phase-lead network in the feedback path, presumably in an attempt to 
compensate for excess phase-lag at HF and even out the resonance over the 
audio spectrum:

http://www.florian-anwander.de/roland_filters/JP6.jpg

-Richie,


-----Original Message----- 
From: Mattias Rickardsson
Sent: Thursday, April 27, 2017 4:03 PM
To: Richie Burnett
Cc: SDIY List
Subject: Re: [sdiy] OTA performance (was SSM chip reissue)

On 27 April 2017 at 16:57, Richie Burnett <rburnett at richieburnett.co.uk> 
wrote:
>> But if the gains match well at 0dB, pretty well at -20dB, and not so well
>> by the time you're down at -80dB, then you'd expect resonance to drop off 
>> at
>> lower frequencies.
>
> Or it could do the opposite, depending which of the two integrator gains 
> is
> lowest/highest!  Like in that paper you referenced about varying SVF gain 
> by
> manipulating the integrator gains.

True!
This is intriguing. Does this indicate that the loss of resonance
towards the bottom end is actually caused by another effect than the
gain mismatch? Possibly capacitor leakage (e.g., through zeners) or
something else?

/mr

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