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

[Matthew S. Padden]

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.

	The basic problem is that there seems to be very little range in the front 
panel Q control. Although the construction notes say that the cct won't 
self-oscillate at maximum Q, the range I'm getting hardly makes any audible 
difference. If I wire the filter output back to the input, I get MUCH more Q 
(about what I'd expect to get from the front panel control), and a second 
out-to-in link pushes the filter into oscillation.

	(All component references relate to the schematic in ETI, July 1980, p86)

	The resonance in the 80-7 circuit is governed by an LM13600 OTA (IC3), which 
sits between the BP output and the noninverting input of the external opamp. 
The current control for IC3 is supplied via R39, from an inverting summer 
based around IC4b. This mixes a panel control and external voltage control, 
along with a fixed -15V connection via R35.
	I measured the current flowing through R39 at maximum and minimum resonance 
(front panel setting, no out-to-in links):

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.

I tried another LM13600; no change. I tried increasing the gain of the 
resonance control summer, by increasing the feedback resistor R37 from 100k to 
1M5; this caused Q control to be lost completely, so I refitted the 100k 
resistor.

So, I'm still confused. The questions I have are:

1) Do the bias currents look about right?
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?

Please help! Thanks in advance.
--
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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