[sdiy] OTA performance (was SSM chip reissue)

[Neil Johnson]

Hi Tom,

> I can still see a lot of useful applications for the chip even if the gain errors were as bad as I suggested, and I was exaggerating. Not everything *requires* carefully matched gains, although there will be a few things that do.
> More numbers in the datasheet would be very useful, and some idea of what sort of spread is hidden behind that word "typical" would be very useful too.

You won't get those numbers.  If they did print them then they would
have to guarantee those figures, either by testing each part
(expensive) or through a lot of statistical modelling plus caveats.
Not worth it for the target market (audio, not synthesizers).

> You raise an interesting possibility though - it could be that variations in gain matching at large attenuations are one reason why it's harder to make 2164-based SVFs resonate at the low end than at the high end. Previously when this has been discussed, phase lag has been proposed as the likely culprit. 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.

Phase lag would affect the top-end due to something called "Q
enhancement".  At the low frequency end the currents in the
integrators are extremely small, so I think a combination of poles
diverging due to gain mismatch, together with the effects of bias
currents and other imperfections, are more likely to result in
problems down there.

> Incidentally, Oberheim managed to make an SVF with CA3080s in the OB-X, and they're not matched at all (unless they matched them?). The filter is quite highly thought of. Ultimately, it's not always about technical perfection.

Oh I agree, ultimately it is the sound that matters, and imperfections
are part of that (just ask the valve audio crowd, or the guitar
distortion pedal folks).  However it is still good to understand what
a circuit's imperfections are, even if it's to make sure that you
don't lose *that* sound when the circuit is reused in other
synths/modules, or cost-reduced going into production, or revised in
later models, etc.

Cheers,
Neil
-- 
http://www.njohnson.co.uk