[sdiy] Compensating output level for Q

Nicholas Keller niroke at tampabay.rr.com
Sat Jun 1 14:23:54 CEST 2013


  Thank you for the very detailed response.  This circuit is from the Octave Cat SRM, so that schematic is not my original design.  The reason it says "Revised by N Keller" is because there were errors in it compared to the actual PCB circuit.  Several components were missing, filter CV scale section was different, Q control pot was backwards, etc.   So I'm working with what I've got here and trying to make a mod that anyone with a nearby RadioShack can perform.  For me personally, I don't have a problem adding inverters, or perhaps even electronic switches.  So I might end up showing a simple way and a harder way when I finally write up the results and instructions.  

That link you included is a wonderful resource!   I might take a few hints from that, although the CEM3320 has on-chip resonance, so it's not quite the same circuit.  Also I'm not trying to toss out the original character of the Cat SRM.  I'd like it to still sound like a Cat, but have the option of other filter modes and a more usable resonance, as right now it is very difficult to adjust the Q while playing live or recording because of the jump in output level.  I'm afraid that the Hi-Q/Low-Fc bass it produces will be sacrificed once the Q is tamed.  So an ideal solution might allow me to turn off/bypass whatever control mod I install when I want the extreme low-end.

I'm going to try the diode clipper between the Q pot and R166 first.  

My first idea for creating BP was to cascade HP into LP, but when I learned that BP could be tapped right from the circuit, I decided to try that instead.  However, with the difference in level between the BP and the other tap points, perhaps it is easier to just make a cascade BP and avoid having to install op-amps to balance all the signals going to the VCA.  Also, I really didn't notice that much difference between the 12dB and 24dB modes, so perhaps trying to provide all modes is not necessary.  I will consider limiting the mode options to fewer modes.  I realize I will still need to invert something for the HP4 Q to work. I was hoping to avoid a 3pole rotary switch, but I might need to use one after all (input to second stage, selected output to VCA, inverter bypass)


Sent from the future

On Jun 1, 2013, at 3:09 AM, Andrew Simper <andy at cytomic.com> wrote:

> 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,
> Andy
> --
> 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 synth-diy.org
>> 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.

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