[sdiy] Compensating output level for Q

[Richie Burnett]

Do you want to compensate for the perceived drop in overall volume as the resonance is increased, or compensate for the sudden increase in volume at the instant where the filter commences self-oscillation?

The former is often compensated by feeding a fraction of the input signal into the feedback path around the filter, before the resonance control pot. Essentially this acts to increase the filter drive level as the resonance control is increased and the pass-band gain of the filter naturally decreases. You can compensate some fraction or all of the volume drop depending on how much of the input signal you feed into the hot end of the resonance pot in the feedback path. Some of the Roland Juno filters implement this technique.

Another technique is to take some of the filter output from the hot end of the resonance pot, plus an additional amount from the wiper of the resonance pot. See tb-303 schematic for an example of this gain makeup method.

Compensating for the potentially deafening squeal when the filter goes into self-oscillation is different. The amplitude of sustained self-oscillation is determined by the behaviour of non-linearities in the filter. Eg, hard or soft clipping. If the self-oscillation level is much louder than the oscillator signal you can try driving the filter with a hotter signal, and/or introducing soft clipping into the filters feedback path. Some of Rolands SH synths use back to back diodes in the feedback path around the filter to limit the amplitude of self-oscillation.

You didn't say what model your synth is, or the filter topology. So I hope this info is some help to you.

-Richie,

Sent from my Sony Ericsson Xperia ray

Nicholas Keller <niroke at tampabay.rr.com> wrote:

>I have a synth that seriously jumps in volume when its filter oscillates.  I know that later generation chips had a way of compensating for this change, keeping the level constant regardless of the resonance setting.  Is there a circuit I could add to my synth that would perform this compensation?  I could use an audio limiter perhaps, but I'm concerned about clipping and squaring off the waveform.  I'm wondering if anyone knows how these chips did it, like the SSM2044 and the later CEMs, and if I could build a similar add-on circuit for my synth (and possibly other modules I have, like the Doepfer A-102).   
>
>I have an idea of adding an envelope follower (inverted) and VCA.  Maybe that's how it was originally done.  
>
>Thanks,
>Nick
>
>
>_______________________________________________
>Synth-diy mailing list
>Synth-diy at dropmix.xs4all.nl
>http://dropmix.xs4all.nl/mailman/listinfo/synth-diy