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Design How-To

Op amps in small-signal audio design
- Part 3: Selecting the right op amp

Douglas
Self

7/27/2011 04:04 PM EDT
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[Part
1 reviews a brief history of op amps and then
looks at various op amp properties from a perspective of audio
design. Part
2 looks at distortion in BJT and JFET-input op
amps, and using rail bootstrapping to reduce common-mode
distortion.]

Selecting the Right Op-Amp
In audio work, the
5532 is pre-eminent. It is found in almost every mixing console, and
in a large number of preamplifiers.
Distortion is almost unmeasurably low, even when driving 600 Ω
loads. Noise is very low, and the balance of voltage and current
noise in the input stage is well matched to moving-magnet phono
cartridges; in many applications discrete devices give no
significant advantage. Large-quantity production has brought the
price down to a point where a powerful reason is required to pick
any other device.

The 5532 is not, however, perfect. It suffers common-mode
distortion. It has high bias and offset currents at the inputs, as
an inevitable result of using a bipolar input stage (for low noise)
without any sort of bias-cancellation circuitry.

The 5532 is not in the forefront for DC accuracy, though it's not
actually that bad. The offset voltage spec is 0.5 mV typical, 4 mV
max, compared with 3 mV typical, 6 mV max for the popular TL072. I
have actually used 5532s to replace TL072s when offset voltage was a
problem, but the increased bias current was acceptable.

With horrible inevitability, the very popularity and excellent
technical performance of the 5532 has led to it being criticized by
subjectivists who have contrived to convince themselves that they
can tell op-amps apart by listening to music played through them.
This always makes me laugh, because there is probably no music on
the planet that has not passed through a hundred or more 5532s on
its way to the consumer.

In some applications, such as low-cost mixing consoles,
bipolar-style bias currents are a real nuisance because keeping them
out of EQ pots to prevent scratching noises requires an
inappropriate number of blocking capacitors. There are plenty of
JFET-input op-amps around with negligible bias currents, but there
is no obviously superior device that is the equivalent of the 5532.
The TL072 has been used in this application for many years but its
HF linearity is not first-class and distortion across the band
deteriorates badly as output loading increases.

However, the op-amps in many EQ sections work in the
shunt-feedback configuration with no CM voltage on the inputs, and
this reduces the distortion considerably. When low bias currents are
needed with superior performance then the OPA2134 is often a good
choice, though it is at least four times as expensive as the
TL072.

The question of op-amp selection is examined in much more detail
in the rest of this chapter, where the most popular types are
surveyed.

Op-Amps Surveyed: BJT Input Types
The rest of
this chapter looks at some op-amp types and examines their
performance, with the 5532 the usual basis for comparison. The parts
shown here are not necessarily intended as audio op-amps, though
some, such as the OP275 and the OPA2134, were specifically designed
as such. They have, however, all seen use, in varying numbers, in
audio applications. Bipolar input op-amps are dealt with first.

The LM741 Op-Amp
The LM741 is only
included here for its historical interest; in its day it was a most
significant development and, to my mind, the first really practical
op-amp. It was introduced by Fairchild in 1968 and is considered a
second-generation op-amp, the 709 being first generation.

The LM741 had (and indeed has) effective short-circuit protection
and internal compensation for stability at unity gain, and was much
easier to make work in a real circuit than its predecessors. It was
clear that it was noisy compared with discrete circuitry, and you
sometimes had to keep the output level down if slew limiting was to
be avoided, but with care it was usable in audio.

Probably the last place the LM741 lingered was in the integrators
of state-variable EQ filters, where neither indifferent noise
performance nor poor slewing capability is a serious problem (see
Chapter 10 for more details on this application). The LM741 is a
single op-amp. The dual version is the LM747.

Figure 4.19 shows a region between 100 Hz and 4 kHz where
distortion rises at 6dB/octave. This is the result of the usual
dominant-pole Miller compensation scheme. When slew limiting begins,
the slope increases and THD rises rapidly with frequency.

Figure 4.19: The THD performance of
an LM741 working at a gain of 33, on ±15 V rails, giving 3 and 6
Vrms outputs, with no load. At 6 Vrms, slew distortion exceeds 1%
before 20 kHz is reached; there is visible slew limiting in the
waveform. THD is, however, very low at 100 Hz, due to the high NFB
factor at low frequencies

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re: Op amps in small-signal audio design - Part
3: Selecting the right op amp
Jay
Sinnett   8/10/2011 2:31:15 PM

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In
the circuit description of the 5532, Doug says he doesn't understand
the function of Q14. I am not the designer, but I think he was on
the right track when he referred to clamping. It's plain to see
that Node 3 rides at about 2Vbe above the neg supply (Q8 + Q9). If
node 3 (the collector of Q9) starts to go below 1Vbe, then Q14 turns
on and sucks current out of Node 2, limiting the drive to Q8 + Q9.
In effect, this prevents the collector voltage of Q9 from ever going
below about 1Vbe. In other words, it prevents Q9 from going into
"hard saturation." Hard saturation causes slow recovery time - so
the purpose of Q14 is to keep the circuit recovery time fast
whenever the output stage has approached the negative rail. At
least that's my guess.

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re: Op amps in small-signal audio design - Part
3: Selecting the right op amp
sinsinsin49
8/5/2011 7:07:26 AM

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What
I don't understand is that the modern, real audio opamps: LME49990,
OPA1611, and OPA211 were omitted! They all are superior in
performance compared to those that have been presented. With
properly designed LME49990 based circuitry it is actually possible
to do 24-bit quality analog work. Please
explain.

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re: Op amps in small-signal audio design - Part
3: Selecting the right op amp
kendallcp
8/3/2011 9:50:32 PM

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That's
not quite accurate. You need to multiply the noise spectrum by the
frequency response that it's exposed to, and then rms it up (square
it and integrate over the bandwidth). If the response is a
single-pole low-pass, that 1.57x factor (over sqrt(BW))pops up
automatically from the integration. The methods are equivalent if
the noise density is flat with frequency, but it often
isn't.

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re: Op amps in small-signal audio design - Part
3: Selecting the right op amp
Hughston
8/3/2011 9:35:32 PM

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To
calculate the noise in the audio bandwidth you have to add all the
noise components in an RMS fashion then multiply them by a bandwidth
factor like 1.57 x sqrt (BW). The factor depends upon the slope of
the filtering beyond the 3 dB points. 1.57 in this case is for
single pole filtering. There are app notes for this on the ADI and
Intersil web sites.

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re: Op amps in small-signal audio design - Part
3: Selecting the right op amp
Rene
Prevo   8/3/2011 11:29:01 AM

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The
article talks about low noise, but what are levels of the noise in
the audio range (1Hz to 20000 Hz)? RP

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re: Op amps in small-signal audio design - Part
3: Selecting the right op amp
Dr.
HD   7/29/2011 9:22:37 AM

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doh!
The AD8597, 8599 and 797 parts are of course from Analog Devices.
Note to self: the clue's in the part number!

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re: Op amps in small-signal audio design - Part
3: Selecting the right op amp
Dr.
HD   7/28/2011 11:18:37 PM

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It
isn't true that the LM4562 has no "single" version. The LM4562 is
actually the dual of the LME49710 and is therefore exactly the same
as the LME49720. The LM4562 was released first, and the single
version was then developed. By the time the single was released,
National had changed their numbering scheme; the original plan was
to phase out the LM4562 number and use just LME49720, but the former
had gained too much traction in the market place. Interestingly
enough, the LM4562 is less expensive, in small quantities at least.
National have some other very impressive op amps in the LME series,
most notably the LME49713, which is a current-feedback op amp with
similarly ultra-low distortion, ultra-wide bandwidth, ultra-high
slew rate, lower noise, and higher output current capability (at
least ±93 mA) Other op-amps worth mentioning are the new AD8597
(single) & AD8599 (dual) from TI. These have been released since
Doug's book was published and are recommended by TI over the AD797;
which is very nice of TI given that the AD797 is considerably more
expensive!

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