[sdiy] Compensating output level for Q

Andrew Simper andy at cytomic.com
Sat Jun 1 09:09:22 CEST 2013

Hi Nicholas,

If you tried putting a diode clipper in the feedback path and still
didn't control the resonance enough then I suggest you try boosting
the level into the clipper, then cutting it again after it, this
should keep it more under control but still give you enough gain for
self oscillation.

In the schematic you provided there are a couple of ways to do this.
The best from a clean circuit perspective with minimal alternations
would be to reroute the main output heading to the CA3080 amp, and
take it pre R183, leaving the cap C75 in place to block any slight DC.
Then change the resistor R184 to be a larger value, and place back to
back diodes parallel to it in the feedback loop of the opamp, then
increase the resistor R1166 to remove some of the gain you added with
the opamp. This will clip the signal earlier but still allow self
oscillation, and so keep things more under control.

A better alternative if you really want to keep the resonance really
under control is to place the resonance gain pot prior to the diode
clipper, then you have:

diodeclip (4 * input)

rather than:

4 * (diodeclip (input))

so you can see that for an input of +-1V your signal level will remain
a maximum of +-0.6 V with the first one, but will be around 4 times
higher with the second one. Otherwise it's the same sort of thing, as
above, boost the signal with the opamp while limiting it with diodes,
then cut it a bit heading into the fisrt stage of the filter.

>From the schematic you posted there are two SVF stages, the both have
equal damping of around 10/4.7 = 2.1, so this is equivalent to two
cascaded one pole low pass filters. There doesn't seem to be much
point in using SVFs in this way in this schematic as it stands other
than to use more opamps and have a slightly different core filter
drive, which considering people generally view the drive of an SVF as
harsh sounding is an odd choice. So I am guessing that you probably
want multiple responses, well if you do things start to get more
complicated. Say you want the following responses:

LP2 -> LP2 = LP4
HP2 -> HP2 = HP4
HP2 -> LP2 = BP4

The resonance works fine for the first one, but it won't work for the
second two unless add an extra opamp to flip the high pass output of
the first SVF before sending it to the next SVF:

-HP2 -> HP2 = HP4
-HP2 -> LP2 = BP4

Now the global feedback has the right polarity and you will get self
oscillation again.

Also the frequency response of the high pass has a funny sag in it
just the XPander does when you take the sum of low pass output taps to
form the response. I think all up, if you want different responses you
will be better off using a regular 4 x one pole cascade and switch the
position of the capacitors in the circuit instead. An example of this
can be found at the bottom of
http://www.electricdruid.net/index.php?page=info.cem3320 but there
could be more elegant ways of switching the position of the caps
depending on if you want them mechanical or electronically switched.

All the best,


On 31 May 2013 05:27, 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?
> Nicholas Keller <niroke at tampabay.rr.com> wrote:
>>I have a synth that seriously jumps in volume when its filter oscillates.

More information about the Synth-diy mailing list