AW: VC envelopes

[Haible Juergen]

Hi Joachim (and diy list),

I have build VC controlled envelopes around the LM13700
(see homepage), but I, too, have offset voltages in the
magnitude you described. Thinking of it, I never checked for
CV feedthru, because I only use them for manual voltage control.
(and for the possibillity of parameter storage)
While I can measure the offset voltage, I have no practical problems,
because my VCA has one diode threshold anyway.

Ok, this won't help your problem, but I thought I should mention that
VC envelopes with cheap otas are not ideal, but still quite usable.
(And wouldn't the CV feedthru be filtered by the ENV's time constant?)

Other / better OTAs:
Yamaha have built their VC envelopes in the CS synths around 
a special chip that only containd the env *logic*, plus an external VCA.

As far as I see it, the problem with the 3080 is that the offset voltage
varies with the amplifier bias current, so you can only compensate
for one point. Discrete transistors can be much better here. My
BC550-based VCAs have much less CV feedthru than average 3080's,
so maybe a VCADSR would also profit from a discrete OTA.
But I haven't tried it.

With "a la Oberheim" you mean the CEM3310 ? 
Excellent solution, but a lot of transistors involved.
Stupid question: where would you still need the 3080 then?

Then there are the Korg VCADSRs. Separate discrete circuits
for charging and discharging the capacitor. Different variations
in the Trident and in the PS3200. Especially the PS3200 circuit
must have been designed as an efficient low parts count solution,
as they needed one for each key. Never checked if the Korg
VCADSRs have exponential slopes or not.

Special Award for "most different" VCADSR solution goes to Roland's
JP-4. Variable resistors are emulated by analogue switches and smoothing
capacitors, and each switch is controlled by a little expo VCO.

JH.

****************************************************************************
***************
 
	>Hi,

> As I don't have much experience with voltage controlled envelope
> generators, I thought I'd ask the list for some advice.
> 
> I recently built a CA3080-based ADSR, and discovered that it does
> have the problems that were discussed in Curits' newsletter in the
> early '80s, i.e. offsets of the magnitude of 100 mV, and substantial
> CV feedthrough.
> 
> Is there any easy way to avoid these effects using modern-day
> op-amps, etc?
> 
> Or perhaps the multiplication is best carried out with a expo
> converter (a la Oberheim), and the 3080 could be used for current
> polarity switching only?
> 
> Ideas welcome!
> 
	>-joachim