Not as confused now...(resonance and OTAs)

[Matthew S. Padden]

Hi..

Juergen wrote:

>
>It's hard to tell without the design in front of me, but:
>
>> So, it seems that as the front panel Q control is turned up, the
>> control 
>> current to the OTAs goes DOWN. Is this correct?
>> 
>Most probably yes.
>
>>  I thought control current had 
>> to go UP to increase gain on an OTA.
>> 
>Right. But the OTA gain must be low for high Q, and high for low Q,
>in a SVF.
>Maybe this helps to trace the error.
>

OK. So, I understand that from this that the feedback from the BP output to 
the filter's noninverting input must _decrease_ in order to increase Q? Yes?

>	>Minimum Q: current through R39 = 0.92mA (flowing into OTA ampbias input)
>	>Maximum Q: current through R39 = 0.63mA (          ditto         )
>
>As I said, I don't know the exact circuit, but the range seems to be a
>little small.
>I'd expect max Q: bias current = 0 (or almost), and min Q: bias current
>= 1mA
>or something similar.
>

That seems to fit with what I guessed at above.

And Colin Fraser wrote:

>A state variable is based around a pair of integrators with a 2 feedback
>loops. One loop is from the lowpass output (second integrator) to the
>negative input of the first integrator.
>As each integrator has a phase shift of 90 degrees (180 in total) and
>this is fed back to the negative input, this gives positive feedback
>which will cause the filter to have a high Q.
>The second feedback loop, from the bandpass output (first integrator) is
>applied to the positive input of the first integrator to damp the
>oscillation in the filter.
>So yes - decreasing the OTA current for the bp output feedback decreases
>the damping feedback and increases the overall Q of the filter.

NOW it begins to make sense. So, I'd say that I need to look at the resonance 
control voltage summing amp in order to get the OTA current to the right 
levels.

Thanks! :))

--
Matthew S. Padden
Computer Music Research Group
Room M2/08, Music Dept.
Huddersfield University
Queensgate
Huddersfield
England HD1 3DH
p: +44 1484 422288 x2402
f: +44 1484 472656
e: mattp at mindless.com
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