OTA offsets

[Haible_Juergen#Tel2743]

>
> appr. +- 1V at the lowpass output. The trimmers are only altering the
> point on the CV-scale where the offset sets in. There must be something
> wrong with my circuit, or ?

I think that ota offset trimming actually works only for one certain
control current (i.e. point on the CV scale). So if you have exact
compensation for one control current, it depends on the specific
chip how good the compensation works for other control currents.
I think this was a reason for several manufacturers to *select*
3080's for their offset behaviour.
A possible method of selection would be (haven't done it
myself yet):
Build a simple VCA witch a socket for the 3080. Don't connect
anything at the input (just the two 200R resistors to gnd, plus
the offset compensation network), and feed a fast envelope
into the control input. Now adjust the offset trimmer so that the
thumping is minimal. This means that you have set some kind
of "average" minimum feedthru over the whole control range.
Then compare several OTAs for different thumping (each with its
own individual optimized offset pot setting).
Just an idea - have *not* tried it.

BTW: According to an article of Doug Curtis, this behaviour of
otas was a reason *not* to use otas in the CEM ADSR chips.
In theory, you could just use a single ota plus control circuitry
to build a VCADSR. You would just switch asymptotes (ota
input voltage) and rates (ota control current). *But* as the offset
voltage depends on the control current, you would get a nasty
glitch when you release a key and the Decay setting is different
from the Release setting.
This is not a problem in a SEM- or Minimoog-style ADS-Envelope
(it works great with a LM13600 for voltage control), but ADSR would
be a problem.
If I find the time, I will redraw my VC-ADS circuit over christmas,
and then send it to Kevin for the www page.
Sorry for sliding a little off topic, but maybe someone found it
interesting nevertheless.

JH.