[sdiy] Compensating output level for Q

[Tom Wiltshire]

Interesting filter design, that. What is it exactly? Comparing it with the SSM2040 data sheet, it looks most like a couple of 2-pole SVFs stuck end to end, with feedback round the whole lot - a bit like the Jupiter 6. Or am I completely wrong?

Tom

On 30 May 2013, at 22:27, Nicholas Keller <niroke at tampabay.rr.com> wrote:

> 
> Richie,
> 
>  It's the deafening squeal one.  The synth is an Octave Cat SRM with an SSM2040 based filter.  Here is the schematic:
> 
> http://i.imgur.com/R028I6l.jpg
> 
> I see D30 and D31 in the SH-101 schematics, so I could try that.  Looks easy enough.  I don't know what an equivalent diode is to that: 1s2473.  I'll look it up online if you don't have a suggestion.  The 101 has other parts between the Q wiper and the diodes though, TR26 and TR27, etc.  I don't know what those do.... http://manuals.fdiskc.com/flat/Roland%20SH101%20Service%20Manual.pdf page 9
> 
> If you hadn't guessed, I'm rather uneducated in regards to electronics.  I've built a few things, modded a few things, but I still don't "get it".
> I appreciate your help.  
> 
> nick
> 
> 
> 
> 
> 
> -----Original Message-----
> From: Richie Burnett [mailto:rburnett at richieburnett.co.uk] 
> Sent: Thursday, May 30, 2013 4:39 PM
> To: Nicholas Keller; synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] Compensating output level for Q
> 
> 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
>> 
>> 
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