AW: Re: SEM VCO

[Haible_Juergen#Tel2743]

>      that you are talking about an exact
>     duplication of the Oberheim SEM VCO, with a discrete transistor
>     schmidt trigger and dual FET zero-offset buffer.

Yes. I only used different transistors: BF245, BC550C, BC560C.

>     Did you implement the high-frequency track trim?

Yes.

>     Did you use a 3046 or 3086? Will the 3046 work?

3086 I think (like the original.)

>     Were the problems you encountered due to high frequency oscillations
>     around the reference op-amp in the exponential converter, because you
>     used FET-input op-amps? How did you fix the problems, extra caps?

I don't think there was HF oscillation. The waveform looked as if it
ran into saturation at the upper end, because discharge is very
fast, and the opamp is too slow to stop it. Then the opamp runs outside
its common mode region, which makes things worse.
I tried to use a lower reference voltage for the capacitor (10V),
but this didn't cure the problem.
The funny thing, however: Although the waveforms were *heavily*
distorted (clipped sawtooth ..), the frequency control was still
perfect! I even didn't have to change the HFT settung much when
I went back to the original SEM design ! very strange!

JH.