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Design How-To

Op amps in small-signal audio design
- Part 1: Op amp history, properties

Douglas
Self

5/25/2011 04:51 PM EDT
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Audio design has for many years relied
on a very small number of op-amp types; the TL072 and the 5532
dominated the audio small-signal scene for many years. The TL072,
with its JFET inputs, was used wherever its negligible input bias
currents and low cost were important.

For a long time the 5534/5532 was much more expensive than the
TL072, so the latter was used wherever feasible in an audio system,
despite its inferior noise, distortion, and load-driving
capabilities. The 5534 was reserved for critical parts of the
circuitry. Although it took many years, the price of the 5534 is now
down to the point where you need a very good reason to choose any
other type of op-amp for audio work.

The TL072 and the 5532 are dual op-amps; the single equivalents
are TL071 and 5534. Dual op-amps are used almost universally, as the
package containing two is usually cheaper than the package
containing one, simply because it is more popular.

There are, however, other op-amps, some of which can be useful,
and a selected range is covered here.

A Very Brief History of Op-Amps
The op-amp is
today thought of as quintessentially a differential amplifier,
responding to the difference of the input voltages while (hopefully)
ignoring any common-mode component. The history of differential
amplifiers goes back to that great man Alan Blumlein, and his 1936
patent [1] for a pair of valves with their cathodes connected to
ground through a common resistor.

However, the first valve-based operational amplifiers, i.e. those
intended to be capable of performing a mathematical operation, were
in fact not differential at all, having only one input. That had to
be an inverting input, of course, so you could apply negative
feedback.

The first op-amp to get real exposure in the UK was the Fairchild
uA709, designed by the renowned Bob Widlar and introduced in 1965.
It was a rather awkward item that required quite complicated
external compensation and was devoid of output short-circuit
protection. One slip of the probe and an expensive IC was gone. It
was prone to latch-up with high common-mode voltages and did not
like capacitive loads. I for one found all this most discouraging,
and gave up on the 709 pretty quickly. If you're going to quit, do
it early, I say.

The arrival of the LM741 was a considerable relief. To my mind,
it was the first really practical op-amp, and it was suddenly
possible to build quite complex circuitry with a good chance of it
being stable, doing what it should do, and not blowing up at the
first shadow of an excuse. I have given some details of it in this
chapter for purely historical reasons. There is also an interesting
example of how to apply the LM741 appropriately in Chapter 17.

The first IC op-amps opened up a huge new area of electronic
applications, but after the initial enthusiasm for anything new, the
audio market greeted these devices with less than enthusiasm. There
were good reasons for this. The LM741 worked reliably; the snag with
using it for audio was the leisurely slew rate of 0.5 V/µs, which
made full output at 20 kHz impossible.

For a period of at least 5 years, roughly from 1972 to 1977, the
only way to obtain good performance in a preamp was to stick with
discrete transistor Class-A circuitry, and this became recognized as
a mark of high quality. The advent of the TL072 and the 5532 changed
this situation completely, but there is still marketing cachet to be
gained from a discrete design. An excellent and detailed history of
operational amplifiers can be found in Ref. [2].

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re: Op amps in small-signal audio design - Part
1: Op amp history, properties
Nic
Cohen   7/27/2012 8:20:41 AM

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Does
that work that well in comparison? Regards Nic
www.kdweb.co.uk

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re: Op amps in small-signal audio design - Part
1: Op amp history, properties
sinsinsin49
6/4/2011 4:00:07 PM

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I
agree with the opinion that "performance levels of the TL072/82 and
the 5534 to be sub standards when compared to the current generation
of high performance OP AMPs. My top three devices for serious analog
audio work are LME49990, OPA1611 and OPA211. LME49990 is the only
one capable of true 24-bit analog performance when powered with +/-
15 V supply (S(N = 159 dB,THD = 146 dB). As the THD of LT1028 is 96
dB, it is "only" good for 16-bit systems. NE5534A is even less
linear, THD = 93 dB.

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re: Op amps in small-signal audio design - Part
1: Op amp history, properties
Laser
Man   6/2/2011 2:02:26 PM

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I
will never buy another book authored by Douglas Self. I have one of
his books on audio circuits - they are simply reprints of his
articles from a magazine and they contain no design details at all.
Poor material for engineers.

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re: Op amps in small-signal audio design - Part
1: Op amp history, properties
jewilson
6/1/2011 5:26:22 PM

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I
have found the performance levels of the TL072/82 and the 5534 to be
sub standards when compared to the current generation of high
performance OP AMPs like the AD797 and AD8597/AD8599. In addition,
the uA739 was superior in performance to 741 and other early opamps
designs, excluding the 5534. The AD797 is a very low noise, low
distortion operational amplifier ideal for use as a preamplifier.
The low noise of 0.9 nV(root)Hz and low total harmonic distortion of
-120 dB at audio bandwidths give the AD797 the wide dynamic range
necessary for preamps in microphones and mixing consoles.
Furthermore, the AD797's excellent slew rate of 20 V/µs and 110 MHz
gain bandwidth make it highly suitable for low frequency ultrasound
applications. The AD8597/AD8599 are very low noise, low distortion
operational amplifiers ideal for use as preamplifiers. The low noise
of 1.1 nV/vHz and low harmonic distortion of -120 dB (or better) at
audio bandwidths give the AD8597/AD8599 the wide dynamic range
necessary for preamplifiers in audio, medical, and instru-mentation
applications. The excellent slew rate of 14 V/µs and 10 MHz gain
bandwidth make them highly suitable for medical applications. The
low distortion and fast settling time make them ideal for buffering
of high resolution data converters.

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re: Op amps in small-signal audio design - Part
1: Op amp history, properties
rpell2
5/28/2011 3:19:58 PM

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I
first saw this technique ("forced Class A") used in Walt Jung's
"POOGE" modifications for the Philips DAC960 DAC unit in the early
90s. It has since become de rigueur in the DIY/audiophile community
to "bias op amps into Class A", where it seems any op amp in a
circuit is an excuse for an accompanying pull-up/down resistor or
CCS - even op amps driving high-input-impedance buffers:
http://tangentsoft.net/audio/opamp-bias.html

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re: Op amps in small-signal audio design - Part
1: Op amp history, properties
bcarso
5/26/2011 11:58:43 PM

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I
have a tube of "GL358s" which are actually not unity-gain stable! A
very good technician kept reporting that a very-low-frequency twin-t
notch filter (with feedback to greatly increase the Q) was
oscillating, and I assured him that he must have wired something up
wrong! Nope! The plain 358 (or as a quad, the 324) almost always
has severe crossover distortion, except at very low output currents,
and some companies actually treated that strategy of forced class A
as a trade secret! For years I never saw a voltage noise spec, but
recently noticed that it is now listed as
40nV/rtHz.

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re: Op amps in small-signal audio design - Part
1: Op amp history, properties
Bert22306
5/26/2011 7:40:03 PM

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Bcarso,
right you are. The TL082 noise spec is 16 nV/SQR(Hz), and my
resistor combinations at the + and - input terminals created 5.59
nV/SQR(Hz). But in my first attempt, where I used higher value
resistors, the resistor noise was clearly audible when the circuit
was idle. I lowered the value in two stages, and noticed the
deminishing returns of lowering resistor values. If I'd used very
low noise opamps, with noise spec around 3 nV/SQR(Hz), it may not
have been as easy to lower the resistors enough to make a negligible
contribution. Because of interfacing issues with other components.
Just thought I'd mention those two items, load capacitance and
theral noise, because I didn't think the article mentioned them
explicitly.

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re: Op amps in small-signal audio design - Part
1: Op amp history, properties
anon9303122
5/26/2011 7:03:42 PM

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Gasp.
Reminds me of the pre-amp I made using a pair of 741's for my new
Technics turntable to go into my ancient receiver that didn't have
the phono inputs, only line inputs. Ok, this was 1981 vintage,
freshman EE cobbling, but it worked just fine. I saw it in the
basement piles of old "junque" a few weeks ago.

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re: Op amps in small-signal audio design - Part
1: Op amp history, properties
Robotics
Developer   5/26/2011 6:29:34
PM

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Really
nice article with great details and information! Makes me want to
read the book (BSEE - digital guy, but I like analog audiophile
applications). Thanks!!! I always look forward to Doug Self's
stuff..

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re: Op amps in small-signal audio design - Part
1: Op amp history, properties
Rick_Hille
5/26/2011 5:30:18 PM

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I
recall using LM1458's and LM358's extensively in telecom circuits
(tone generators, filters, voice coupling, etc.). Not Hi-Fi, but
low enough noise and distortion to meet telecom requirements of the
time. We encountered significant performance variations between
different manufacturer's devices of the same part number, as well as
some date code dependencies. One trick that still sticks in my mind
was having to add a "forced class A" modification (output pulldown
or pullup resistor) in small signal stages to tame output crossover
distortion from certain makers.

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