[sdiy] [OT] Analog synths with 2 pole filters

Tom Wiltshire tom at electricdruid.net
Fri Feb 22 17:36:11 CET 2013

On 22 Feb 2013, at 14:00, rburnett at richieburnett.co.uk wrote:

> Op-amps operated as finite-gain blocks will exhibit more phase-lag at high frequencies, so I would expect state-variable filters to be more inclined to self-oscillate at the high end of their cutoff frequency range than the low end.  Given that they are theoretically critically stable when there's no damping feedback, this effect should be quite noticeable.  Can anyone who's built one confirm or dispute this?

This is absolutely what I found with the SSM2164 SVF. It squeals like hell at the top, but getting a reliable oscillation down to the lowest frequencies requires a lot more gain.

Dave Dixon and Olivier Gillet and I all shared notes on this filter, so perhaps they have something to add on this point? -Guys?

For a while I messed around trying to compensate for the phase lag, but (for me) it wasn't worth it. For a given build, you can improve things a little by hand-tuning with little bitty caps here and there, but I don't really regard that type of building as reliable engineering. You should put it together and it should work. If it's any good, you should be able to put it together with cheap 5% resistors and 20% caps and it should still work. So I'm not a fan of fine tuning.

I'm all ears if anyone has better ways of compensating or avoiding the phase lag though. Fancy-pants op-amps?!

> Agreed.  For the "Moog/Roland" 4-pole cascade you stabilise the self-oscillation amplitude by soft-clipping the feedback signal.  As Neil said, this diode clipping to limit sinewave amplitude is crude and introduces distortion.  However, soft-clipping only the part of the output signal that is fed back to the input is neat.  It means that the clipped sinewave gets low-pass filtered four times by the following RC sections before emerginging at the output all nice and smooth again.  This at least removes some of the distortion from the soft-clipping operation in the feedback path.

This is probably why the SH101 filter sounds so damn nice. It distorts quite a lot, but it's smooth, so you hear it as "fat" rather than as "crunchy".


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