[sdiy] LAG circuit de-tuning VCO question

[Tim Parkhurst]

>> What op-amps are you using?
>
> TL072s. With the new caps, they seem to be OK, but maybe I should consider
> LT1013s or similar?
> I need to bite the bullet and get a load for CV summing jobs....
>

I think the advantage of using the LT1013 is that is has very a low
offset voltage. Here's what I don't understand about Low Offset op
amps: In a CV application, isn't it a farily easy thing to just add a
trimmer to be able to zero out any offset? Of course, using a low
offset op-amp is probably cheaper and easier than adding a bunch of
trimmers to the CV chain but still, it's possible, to dial it out,
right? Also, Lets say you have a CV summer built from a dual op amp.
The first op amp is wired up as an inverting summer and will introduce
a certain offset voltage. Now the second op amp is wired up as an
inverter follower. Wouldn't that second op amp introduce an equal (and
opposite) offset that would very nearly cancel out the first amp's
offset? Of course, this is assuming that both amps in the package have
identical offset voltages, but I would think a modern dual op amp
would be pretty good in that respect. It should certainly get you
close, right?

Oh, and I know the problem has been solved with the right type of cap,
but adding a guard ring to ground around the 'hot' terminal of the
hold cap is said to help reduce cap leakage. And finally, make sure to
clean off any flux on the circuit, especially around the hold cap.


Tim (right?) Servo
-- 
"Sire, the church of God is an anvil that has worn out many hammers."
- H.L. Hastings