[sdiy] PAIA 9730 Resonance Problems

[James Patchell]

The squawking also probably only happens at the higher frequency settings 
as well.  This is caused by what is called "Q" enhancement, which is a 
symptom of the fact that you have "Non-Ideal" frequency responses in your 
integrators due to bandwidth limitations of both the opamp and the 
OTA.  You can help this by adding some phase lead to the OTA input 
attenuator.  Now mind you, I can barely read the designators on that GIF 
file, but you might want to try putting 10pF in parrallel with R26, R37, R6 
and R29 (22K) (assuming I read those number correctly)  These are the 
resistors that form an attenuator with a 220 ohm resistor at the inputs to 
the LM13600....

At 04:36 AM 8/16/2004 +0800, Chris Williams wrote:
>G'day diyers,
>
>I've built the Paia 9700 series a while ago and I am generally pretty 
>happy (I have modded it a bit - mostly physically). One thing that I have 
>had gripes with is the Dual SVF module that suffers from uncontrollable 
>squawking at high resonance (others have experienced it too - see 
>http://www.unease.se/mp3/Unease_P9700s_Bass1.mp3 for an example of the 
>problem). I'm still coming to grips with filter circuit theory, but 
>I  want to have a crack at fixing it :). The resonance feedback path seems 
>different to many other SVF's I've looked at. The schematic is at 
>http://www.paia.com/9730sch.gif. Can anyone give me any advice as to what 
>I should look at changing? Or has anyone had a similar problem?
>
>Cheers,
>
>Chris.
>

         -Jim
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