AW: Opinions: On op amp replacement

[Haible Juergen]

	>There is one thing to look out for: The circuits from the 70s and
80s where
	>the 741 is used thoughout, usually have lots of FET buffers in
them, where
	>low leakage is needed. It would pay to change the circuit as well,
since
	>the FET-resistor buffer has a nonlinearity and offset drift! (I
have the
	>formant, and the SEM-1a and even the TB303 in mind here). When
changing the
	>circuits to FET-opamps then these buffers become obsolete.

There are applications where matched FETs and a bipolar opamp are 
actually connected to form a FET input opamp - I'd say it's save to replace
it here. (If the common mode range is not violated, that is.)
Been there, done that, in my version of the ARP Quadra phaser. (schemos
at my homepage)

There are other circuits, like the FET buffer in the SEM-1a, where you have
a source follower and an active load built from a second FET. That's another
method to compensate for offset voltage (and drift) to some extend. And this
circuit has one big advantage: it's faster than a general purpose opamp like
the TL07x can be. I won't say that it hurts much to replace it in the SEM
VCO.
In fact, the first version of my SEM VCO clones were built with JFET opamps.
But I changed them later to the discrete version. I'm too lazy to do valid
A/B
comparisons in such cases, but I wanted to point out that the discrete
solution
has some theoretical benefits, too. BTW, in an integrator / schmitt trigger
loop
such as a VCO, a little offset drift in the comparator section doesn't hurt
at all.
The thresholds are set in the schmitt trigger. My asumption is that Oberheim
has used the active load because of linear slopes, and not offset drift in
the first
place.

JH.