Confused! How do OTAs control resonance in DG80-7 VCF?

[Colin Fraser]

>----------
>From: 	Matthew S. Padden[SMTP:m.s.padden at hud.ac.uk]
>Sent: 	05 August 1997 12:11pm
>To: 	synth-diy at horus.sara.nl
>Subject: 	Confused! How do OTAs control resonance in DG80-7 VCF?
>
>Hi..
>
>
>	I've been testing a newly-finished Digisound 80-7 VCSVF, based around the 
>SSM2040 filter chip. It works fine in the cutoff and response departments,
>but 
>there seems to be a problem with the resonance circuit. I tried solving it 
>last night, but ended up very confused (and tired!), so maybe someone can 
>solve my dilemma.

snip..

>Minimum Q: current through R39 = 0.92mA (flowing into OTA amp bias input)
>Maximum Q: current through R39 = 0.63mA (          ditto                )
>
>So, it seems that as the front panel Q control is turned up, the control 
>current to the OTAs goes DOWN. Is this correct? I thought control current had
>to go UP to increase gain on an OTA.

snip..2

>So, I'm still confused. The questions I have are:
>
>1) Do the bias currents look about right?

Not to me...

>2) In order to INCREASE the gain of an OTA, should the control current 
>increase or decrease?
>3) Am I missing something really really obvious?
>

The state variable filter operates on a different principle to a
'normal' vcf.
In a four pole filter with resonance, each pole can be created with a
unity gain integrator based around an ota -> cap -> buffer with feedback
from the buffer to the ota. Each pole is inherently stable. To increase
Q, feedback is applied from the output of the last pole to the input of
the first - increasing feedback increases Q.

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.

Without seeing the schematic you're working from I can't offer much more
help.

Colin f