New filter schematic

[jh]

>Chris (and others who could answer):
>
>I've seen some circuits using multiple "3080 type" OTAs that share a single
>current limit resistor (R12 in your design), some that use individual
>resistors per OTA and some that use no resistors at all! Is there anything
>magical going on that affects the decision of how to do it? I've read that
>the current control input was VERY unforgiving of mistakes and had a couple
>of bad experiences with shorts, killing the device. 

Well, first of all you cannot damage anything if the resistor is big enough
to limit the current below the max rating of *one* OTA.

Tracking is a different story, however. Normally you want two or more
OTAs tracking their gm if you control their Iabc's from the same source.

The LM13600 / LM13700 / NE5517 are specified for very good tracking
if you're directly connecting pins 1 and 16.

Connecting different packages, or single 3080's might produce undesired
results. I'd say the unit with the lower input resistance will take the bigger
share of the current. Using separate resistors will help at high Iabc currents,
but hardly on the low end of the control range.
Nevertheless this is done, often with separate resistors, sometimes 
(Small Stone) even with direct connection. Maybe the resulting random
pole spread will make each single box sound a little different (?)

JH.