Improving Escobedo's 9V MS20 Filter??

[nordine]

Hey there guys,
im sure some of you (the more sinth-o-phile ones) are familiar with this neat litte circuit. Well, it delivers, its VERY simple and do whats its intended to do. BUT, it has REAL trouble when it distorts and oscillates. Distortion is HORRID. Other designs feature diodes in the feedback loop (some LEDs, some typical GE or SI diodes); ive tried improving its feedback loop, but i haven't got resuts, ANY IDEAS GUYS?

the circuit:
http://folkurban.com/Site/9VpoweredMS20SallenKeyLPVCF-690.html

[petemoore]

    http://en.wikipedia.org/wiki/Sallen_Key_filter
 
  I too am trying to figure out MS-20 type filters, I have Monotron that I don't want to tear up [has a portion of MS-10 patch capability, nice], wouldn't mind to have an MS-20 alike in a SB version with BP switch/heavier duty jacks etc.
  The linked to wiki examples of S-K filters show no diodes at OA inputs.
S-K / M/S 20 afficianado to chime in.
  That said, a couple things I'd try..the diodes at the input would be clipping if hit with swing>threshold, and the supply voltage 9vdc was design criterion, try at 12vdc to see if increased headroom matters.
  The wiki talks about high gain OA's etc., it'd seem the 9v rail limit could be soon exceeded by signal swing with high gains set on OA's.
 

[Strategy]

In my opinion, one of the really key things about MS-20 is that you have a blend of LP and HP for a kind of strange bandpass filter. It would be amazing to see if this circuit could be expanded into such a configuration (I am not sure how this would be accomplished)

Strategy

[frequencycentral]

I'd certainly think the Escobedo LPF could be modified easily to do bandpass. Take a look at it's two seperate feedback loops - pin 4 > pin 8, pin 13 > pin 9, I'd hazard a guess that if you just had a single loop, pin 4 > pin 9 you could take a bandpass tap from pin 8.

In fact, looking at the state variable filter on the LM13700 data sheet, disconnect the 220k resistor from pin 13 (on the Escobedo lowpass) and connect it to pin 4 instead. Pin 8 will then give you a bandpass output.

[Taylor]

What I've seen in some circuits like this would be like a TS stage - opamp with clippers in the NFB loop - rather than the simple diodes I tried back when I first started messing with this.

But I also think the distortion sometimes comes at the input of the OTA, so a diode clipping stage in front may be needed as well.

Another thing to check out is Tim's Q+D VCF. I have played around with it a lot lately. If you're after real lowpass response, and not so concerned with resonance, it's no good, but for rez sweeps it's really good and ridiculously simple. It distorts in a very pleasing way IMO, and additional diodes can be added at the "T" of the filter to get more headroom.

[earthtonesaudio]

I'm not familiar with this circuit, but if the "bad distortion" occurs when the wiper of the resonance pot is moved up (away from Vr) maybe adding a resistor in between the top of the resonance pot and pin 9, plus a pair of clippers to Vr across the resonance pot, would tame that.

If the distortion is coming from too big of inputs, maybe just increase the 100k input resistors.

I would not put diodes in the feedback loop from output buffer to OTA input.  That seems like it could hurt the OTA.