CEM3360 and another OB-8 mod

[Haible_Juergen#Tel2743]

> 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).

These "high fidelity" VCAs have one great disadvantage: They have
extremely unwieldy linear control inputs.

I did some modifications on my OB-8's final VCA section last night.
I wanted to get rid of the various thresholds that interact with each other
in the final VCAs'control paths. This problem is most prominent when
you try to use a volume pedal on the OB-8. There is a remarkable
range at the low end, where the Volume pedal has no effect, and then
it suddenly "jumps in". Ok, there are volume pedals with a second
knob to adjust some bias level, but the bad thing with the OB-8 is that
the threshold is not constant, but depends on the master volume
knob on the front panel to a high degree.
I thought I had located the source of all this trouble in the simple
one-transistor voltage-to-current-converters that drive the 3080's
(which in turn attenuate the programmable volume CV, before it goes
to the 3360 final VCAs ...).
So I put those PNP transistors into a feedback loop of a LF351 each,
but the threshold was still there. Then I found that there is another
threshold built into the 3360s themselves. And this is a really bad
thing.
Hey, these 3360's start to conduct at approx.250mV (with a *large*
tolerance, according to the data sheet!), and they reach their final
gain at 2V !! Couldn't believe it, but it's true. These things have a
very good audio quality for sure, but the linear control input is crap.

I ended up putting another bias network with a trimpot on each voice
board, and I got most of the threshold compensated. It's far better
than before now, but an optimal adjustment is not possible if you
want to go down at 0 without a notable threshold. The reason is
partly the (not compensated) offset voltage of the 3080 that drives
the 3360. But there was the point for me to stop.
Well, now, the final VCA is surely *not* the best section of my Obie !!

I think this behaviour would make it very hard to use the 3360
as a main VCA in a synth voice.

JH.