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

[rburnett at richieburnett.co.uk]

Tom and all,

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

You shouldn't need any expensive op-amps as the actual gain isn't very 
high (but don't use 741's!)  Looking at the typical SVF schematic linked 
below, off the top of my head...

http://www.seekic.com/uploadfile/ic-circuit/200961915642854.gif

you want to compensate for phase-lag introduced into the loop by op-amp 
UA, so i'd guess that introducing a bit of HF phase-lead into the global 
feedback signal coming from the lowpass output would help.  A small 
valued capacitor (few pF or 10's of pF) across R2 in that schematic 
would introduce a transfer function zero into the feedback path and 
contribute some phase-lead at high frequencies.

> 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".

Agreed. The same can probably be said for the moog ladder and TB-303 
ladder.  Unless an analogue filter is designed really badly the output 
shouldn't hard clip as it's going to sound nasty.

-Richie,