SEM riddles (was: AW: Opinions: On op amp replacement

[Haible Juergen]

	>And if one connects the gate of the upper fet to the drain of the
lower
	>fet, then you have a nice and very linear inverting amp. The signal
is then
	>fed to the other gate.

I didn't know that - sounds interesting !

	>What I was referring to was the FET buffers in the filter, where
there are
	>a few
	>FET-resistor source followers followed by a 741.  Here I would
simply use a
	>jFET opamp. 

I wouldn't, at least not generally. If you look at the SEM circuit, LP
filter
output, you have a two stage buffer made of a FET and an opamp.
But only the FET (Q40) is inside the filter's feedback loop, not the
opamp (A16). I think that Oberheim did that deliberately, in order to
avoid the delay of the opamp, which would make the Q factor of the
filter less independant of cutoff frequency. A modern JFET-input
may be faster than a 741, but hardly faster than the FET alone.
The downside of Oberheim's circuit is a little nonlinearity from the
simple FET source followers, of course.
But what really puzzles me is that the other 741, the one that buffers
the BP output, is taken *inside* the second feedback loop of the 
filter ! Now why is that ?? And look how the circuit is drawn. This
"smells" like the opamps were added to a working VCF later, to buffer
the outputs (you need very low impedance for the potentiometer that
selects the filter mode !) ... and then he chooses to place the LP
buffer outside the loop, and tries what's best for the BP buffer, and
places it inside the loop.
Ok, that's all speculation of course. In case I am on the right track,
what would be the reason for these decicions ?

	>A second thing is that the dual-FET follower is powered from 18.5V,
so
	>there are no saturation effects when the integrating cap gets
charged to 15V. 
	>(I ran into this problem with my last VCO design....)

Same for me. Learned it the hard way, too (;->).

Many other designs use an auxiliary voltage (like 10V) here, so the
FET can be powered by the "normal" 15V.
(Korg MS synths have the 18V version, too.)

JH.