[sdiy] Op amp notes

[Cary Roberts]

>1) in general purpose high-level audio circuits (e.g. EQs, summing
>amps)?

I don't believe in monolithic opamps for critical audio stages.  I prefer
the API 2520, Hardy 990, Forssell JFET992, JLM99V, Quad Eight AM10, UA 1108,
etc.  There was a time not too long ago when I didn't know the difference
and thought a Mackie full of 4560s sounded just fine.  Then again, done
right, designs employing monolithics can still sound fine.  My D&R Orion is
chock full of TL074 and TL072 with a bipolar transistor pair and 5532 for a
mix buss and it still sounds great.

Fred Forssell has the schematic to his original JFET opamp (same as Millenia
still uses) on his website.  You can also find Melcor 1731, QE AM10, and
Jensen 990 (good luck making it work) schematics on the web.


>2) in critical low-level audio circuits (e.g. mic preamps, guitar
>pre-preamps/boosters)?

See above.  SSM2017 weren't bad when available, although these days you
gotta use the SSM2019 or INA217.  Neve/Focusrite used 5534s in various
designs with great success.  I actually have a little 5534 circuit with
follower pair that is a drop in replacement for API2520 and compatible
discretes.  I can't hear much of a difference in EQ circuits but you can
tell with mic pre high gain circuits.

Other than building some Mackie-like circuits with 4560s and some generic
TL0xx buffer circuits I don't have much experience using monolithics for
audio.  I haven't seen the need to spend time tweaking monolithics when
building '70s vintage discrete audio designs are so much more fun.


-Cary