Another VCF Finished

[Joachim Verghese]

On Fri, 12 Apr 1996 gstopp at fibermux.com wrote:

>      I really think that this circuit works so well because the 3080's I'm 
>      using are very good parts. They are of a recent batch from Harris, so 
>      the manufacturing process is probably quite refined by now after all 
>      these years.

But they still need to be selected? :))

I've been thinking about the 3080. Someone mentioned that the PNPs
in the 3080 have low current gain and thus make bad current mirrors.
Maybe this actually adds a positive effect in synth circuits?

I looked at the Memorymoog schematics the other day, and to my surprise
they use those high-fidelity CEM3360 VCAs for VCO mixing and 3080s
for the main VCAs! According to the circuit description, it is also
the 3080 based VCA that's overdriven when turning up the VCO levels
(the filter is not overdriven).

Any thoughts?

>      Also the use of CA3140's as integrator buffers is never a 
>      bad choice, because you know for certain that if there are any 
>      problems in your circuit, it ain't due to buffer errors even with 
>      teeny weeny capacitors.

It would also be nice to try using the 3140s as Miller integrators
instead (cap connected between output and inv. input). Maybe this
configuration would result in less distortion due to 3080 output
compliance error? (-> less interesting sound?)

And using discrete FETs as buffers as in the EN circuit (and Oberheim's
SEM) probably would add some coloration because of the non-linear
response. How would this affect the sound?

Thanks for sharing the experience.

-joachim