AW: VC envelopes

[Tim Cockram]

In message <1BF5E20E0C4DD111BBAB00805FE2D582684839 at nbgm336a.nbgm.siemens
.de>, Haible Juergen <Juergen.Haible at nbgm.siemens.de> writes
>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,

How about using the H11F1 opto fet (available from RS etc)?  A 0-16mA
current varies the ISOLATED fet's resistance between 300M and 200 Ohms.

They are 2-3 ea in one off quantities.  The H11F3 is about half the
price but the minimum fet resistance is circa 470 Ohms.

These devices have been around for a while now.  I must Thank Fred
Mundell (who used them in synth designs in the early eighties) for
introducing me to thes devices.  Unfortunately Fred's designs never made
it to the stores.

Where are you Fred?

Regards

Tim
 

>From memory the Korg PS envelopes were linear (I think) but with a bit
of feedback Viola.
-- 
Tim Cockram