Archive of the former Yahoo!Groups mailing list: MOTM

The reason a mixer is taking so long is the quest for the perfect op-amp.
This is an op amp
this is suitable for both audio mixing AND DC (control voltages). The DC is
the issue.

When you design a DC op amp (like the MXL1013s used as DC mixers in all the
modules) you
trade off speed (called 'Slew Rate' on the data sheet) with DC drift. One of
the main contributing
factors to past modular drift (Moog being the worst, just because they were
first) are the mixers. They
were OK for audio, but ∗horrible∗ for DC.

You would think with today's technology a good audio op amp with low drift
is available. NOT SO! In fact,
the trend is the opposite: there are insanely DC accurate op amps that have
a max frequency of 15HZ
(yep, 15 HERTZ). Then you have insane audio parts with 0.0001% THD and 95dB
SNR that drift like a mother
(because they assume AC coupling of the output).

So, I have toyed with at least 4 different ways to go, including a 100%
discrete cascode JFET design. But
what I'll have to do is just 'bite the bullet' and pick the best compromise
part.

Paul S.