opamp selection

[brad sanders]

The 4/93 issue of "The Audio Amateur" had an article on grading the
opamps commonly available at the time. Since this is at least ten
years newer than Jung's article, I thought I'd pass along a synopsis,
and a bit of contextual info:

Back when Jung did the first AUDIO article on this, the 5534 seemed
about the best bipolar input opamp available. The TL071 was about the
best FET input. The LF devices all use feedforward compensation and so
are considered generally crap for audio (this isn't to say they
weren't widely used here: the '80s vintage 100WPCH pro amp I use for
my subs uses an LF357 in each channel...)

The TAA article performed THD analysis by passing a 1KHz sine through
a DUT, then performing an FFT of the output as applied across a 575
Ohm load (500 Ohms to ground, 75 Ohm series output resistor from the
opamp). ONLY opamps that would work as a "drop in" replacement were
tested, and they had to be unity gain stable (meaning no comp caps
HAVE to be used).

Listed chronologically, those tested were:

	FET input	bipolar input
	
	TL071		NE5534
	LF351		HA2525
	LT1056		OP27
	AD744		LT1007
	OP42		HA5221
	AD845		TLE2027
	LT1122		AD797
	OPA604

Now, remember: even way back in the '70s, Walt Jung recommended NOT
using the LF series in audio apps. So, at the top of the list above we
have roughly the two "best" opamps available in 1978. At the bottom,
we may have two "best" available in 1993.

There are two forms of HD: odd, and even. I won't dwell on this since
we're all supposed to be "musicians" (or technically bent poseurs,
like myself) but basically ODD order HD is what gives instruments
"edge" - like when Myles pops out a really strong note. Even order
gives "body" - like a piano or guitar.

Of these two, neither in this app is really prefereable over the
other, I guess; if you're using a basic triangle wave you probably
don't mind a little even HD, and with a radical pulse, saw, or square
you might not mind a little added ODD order HD.

But in either case, you want the order of the products LOW. By the
time you get to fifth or seventh harmonics, distortion can start
getting really hairy. Again, this can be useful if you're Trent
Reznor, but it's just not something you want to have zero control
over. So, you use opamps that have very little of these.

In order of "strong signatures" we have:

	odd order	even order

	LF351		LF351*
	TL071		TL071
	HA2525		LT1007
	LT1056		OP42
	OP42		HA2525

* The LF351 has strong HD components out past the 11th harmonic. This 
  is also a good general indicator you're going to get very high IMD.

The AD845 is considered a generally excellent device, and you'll find
them - and the AD797 - in some of the most expensive audio gear in the
world. (None of these are good, however, if you have philisophical
objections to paying three bucks or more for an opamp).

Another great opamp I've used quite a bit is the AD711/712/714. This
is a bifet "upgrade" to the TL series - and it sounds excellent. This
is what I use in my "tweaked" Dolby surround decoder.

Generally, 'tho, I like to "play" - I'll sample opamps from all over
the place. I also rarely use single opamps in new designs; this helps
spread the cost out significantly because quads usually cost only a
few cents more than singles.

It's NEVER going to be as easy as throwing a dart a a dartboard - but
I do hope this helps. If nothing else, remember ALL the above are
fairly excellent (except the LF things) and roughly interchangable.
Use a FET input to replace a FET input unless you're absolutely SURE
of the application, and use the chronological chart as an indicator of
relative age toward obsolescence. 

Remember, 'tho: the TL071 has sold bazillions. They still make 741s -
and I doubt they'll quit making the TL0xx series anytime soon...


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