Opinions: On op amp replacement

[jhaible]

> I find the greatest reductions in circuit (audio) noise often come from
> replacing Carbon Comp. resistors with Metal Film. The metal films are
> usually 1%, which makes summing and diff amps happier at the same time.
> Also, faster op-amps somtimes do weird things. I built a phase shifter
with
> quad op amps (LM324) and replaced them with the TL084 (it oscillated at a
> near ultrasonic frequency). It drove my mother's dog nuts (but she
couldn't
> hear it). Made me want to not fix it (I hated that Dog...... :-)

I can confirm that. Phasers are aparently quite sensitive. No HF
attenuation
in a chain of all pass filters, opamps driving capacitive loads with
variable
resistors in series ...

Built a VACTROL-based 12-stage with TL072 and ran into problems. Works
fine with TL062's and 6 stages, though.

Replaced the dual opamps in my HiFli Clone with NE5532's and ran into
stability problems at extreme modulation settings.

Sometimes 741's and 1458's are not *that* bad !

BTW, I think one can avoid slew rate distortion on slow opamps by using
very small feedback (or load) resistors. They would clip the signal with
the
output current limit feature of the opamp, which should sound much more
pleasant than intermodulation, or than getting stuck at the supply rails.
Note the words "think" and "should" - I have not really made tests so far.
It's
just some speculation about existing designs.

JH.