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Notes on Audio Op-Amps
After I built several simple DIY headphone amps
around the OPA132/134 family of op-amps, I
decided to start trying different ones to see how
they would affect the sound.
Although there are hundreds of families of
op-amps on the market today, not all are suitable
for DIY headphone amps. We can eliminate power
op-amps, super-high-speed op-amps, high-voltage
op-amps, low-frequency op-amps, and low-spec
general-purpose op-amps. Audio requires a fairly
high slew rate to maintain low distortion, flat
frequency response from 20 Hz to 20,000 Hz, and decent output current if the chip is
to drive headphones directly. A low supply voltage requirement is a plus for portable
applications. We also ignore chopper-stabilized and CMOS op-amps as they're
generally unsuited to audio. This eliminates the vast majority of op-amps on the
market.
This article documents my testing process, and the results for many quality op-amps,
plus some jellybeans:
jellybean adj. /jel-ee-been/ 1. Of electronic parts: cheap, generic, available from
multiple manufacturers. Buy 'em by the bag and use 'em by the handful.
Pejorative.
Test Method
The first test is a clipping test, to find the op-amp's minimum useful voltage with a
given output voltage and load. Only a few datasheets give you enough information to
get this number without testing for it. To do the test, I run a 1 kHz tone through the
amp while it is driving a 33 Ω dummy load with the volume set to give 0.5 V output.
Then I repeat the test for 2.0 V into 330 Ω. The amp is powered from a variable bench
power supply, which I start off at a rather high voltage and then I lower it until I
observe clipping by seeing harmonics appear on a spectrum analyzer.
I chose these test voltages to be about 6 dB higher than "loud enough" with my
benchmark headphones, the Grado SR series (33 Ω numbers) and the Sennheiser
HD-580/600 (330 Ω numbers). I chose to test at these higher-than-necessary voltages
to account for peaks in an otherwise quiet recording, and for the occasional "rock out"
session. But, beware that voltage requirements don't always go down as headphone
impedance goes down. For instance, the 120 Ω AKG K501 and the 64 Ω HD-570
require about as much voltage as the 300 Ω Sennheiser HD-580. These lower-
impedance phones would have higher clipping points than the HD-580s because the
Notes on Audio Op-Amps http://tangentsoft.net/audio/opamps.html
1 de 15 27/1/2012 00:45
lower impedance means the op-amp has to provide more current at these voltages.
The second test is a listening test. I look for more-or-less objective things like
perceived distortion level and sonic signature. I also give you my subjective value
judgement on the overall experience. I try to make it clear which are the objective
aspects to the sound and which are just my opinion.
Test System
Most frequently I use a stock Music Hall CD25 CD player and Sennheiser HD-570
headphones. I chose this player for the tests because it's the best one I have, so I'm
giving the op-amp the best chance to sound good. The heapdhones, on the other hand,
were selected because they're a bit on the bright side so they're quite revealing. Also,
they're only 64 Ω, yet they require uncommonly high voltage. This makes them a good
worst-case test for finding sonic weaknesses. When I'm looking to test detail, accuracy,
and pleasantness, I switch to my Sennheiser HD-600s with the Cardas cable upgrade.
The test amplifier is a close variant of the CMoy pocket amp on a professional PCB.
The major difference between it and a stock CMoy is that I buffer the virtual ground
with a BUF634, and I use larger input and rail capacitors than originally specified.
(0.22 µF and 470 µF, respectively.) I use Brown Dog adapters to adapt chips that I
cannot get in dual DIP-8 versions to work with the board.
(You might recognize this amp as the forerunner of the MINT amp.)
I use this design rather than another heapdhone amp because it is very nearly the
simplest headphone amp design that could possibly work, while not making the tested
chips look overly bad, or masking too many weaknesses.
The adjustable bench supply is the B+K Precision 1710. It lets me dial in any voltage
from 0 to 30V, with good accuracy. I've tested its voltage quality, and it's quieter than
some linear supplies I've used.
The dummy load is a project box with stereo pairs of 33 Ω and 330 Ω power resistors,
a switch to select between them, and a headphone cable coming out.
Translating the Listening Test Results to Other Amps
Notes on Audio Op-Amps http://tangentsoft.net/audio/opamps.html
2 de 15 27/1/2012 00:45
Although I favor more complex designs in amps I listen to for enjoyment, I stand by
the choice to test chips by making them drive the headphones directly. It gives the
clearest possible picture of the op-amp's sound, since there are so few components in
the amp to mess with the sound.
When translating these results to the sound you'd get with the chip in a different
amplifier, you will have to adjust for any components in the amp that change the sonic
signature. For instance, the BUF634 output buffers used in the MINT and PIMETA
headphone amplifiers add the characteristic Burr-Brown laid-back quality to the
sound, so a chip that's overly aggressive with your headphones when in a CMoy amp
might sound quite nice in a PIMETA. For the PPA, the transparency of the amp tends
to make op-amps show everything I hear in these CMoy-based tests, plus any little
faults the chip has that I can't hear in this more veiled CMoy amp. If I say a chip is a
little harsh in a CMoy, it'll probably be unlistenable in a PPA.
If you have another amp than the ones I talk about on this site, please don't email me
asking how I think the chip will sound in that amplifier. If I don't talk about it here, I
probably don't have the experience or interest to talk constructively about it with you.
You're better off searching or posting on Headwize or Head-Fi asking for others that
have the amp about their experience. Failing that, try some of the chips I review here
and see how what I report coincides with what you hear. You can probably use that
experience to shift my reported results to match the other chips up with what you will
hear.
Translating the Clipping Test Results to Other Amps
These clipping numbers only apply to circuits where the op-amp is driving
headphones directly. If the op-amp drives something else in your application, you'll
need to do your own clipping tests to get useful numbers. For instance, in a
headphone amp with an output buffer, the clipping behavior of the amp will most
likely be dominated by the clipping behavior of the output stage, not the op-amp.
Conventions
Where I give power supply voltages below, I'm talking about the rail-to-rail voltage.
That is, when I say "10V" I mean that V+ is 10V above V-, not that I'm using a +/-10V
supply.
Where I give signal voltages, I mean RMS values unless I specifically indicate that I'm
talking about peak-to-peak (p-p) voltages.
All op-amp prices are for single quantities in US dollars, from Digi-Key. I don't
promise to keep these prices up to date, so just use them as a guide.
Op-amp reviews are sorted first by the company that makes it. (If it's now generic, the
name of the original creator is used, but the review title contains "(various vendors)"
Notes on Audio Op-Amps http://tangentsoft.net/audio/opamps.html
3 de 15 27/1/2012 00:45
to distinguish it.) Burr-Brown chips are separated from TI chips, for historical
reasons. Within each manufacturer group, chips are sorted by part number. This
sorting takes into account the chip maker's part numbering scheme. For instance,
consider the Burr-Brown OPA2107: it is only available as a dual, but if there were a
single-channel version, it would be called the OPA107, so it sorts above the OPA132.
Analog Devices AD744
Cost, single: $4.52 V , 0.5V into 33 Ω: 6.8V
Cost, dual: n/a V , 2.0V into 330 Ω: 9.2V
No detailed review done yet. Sorry.
Analog Devices AD823
Cost, single: n/a V , 0.5V into 33 Ω: 4.3V
Cost, dual: $5.00 V , 2.0V into 330 Ω: 6.1V
Spec-wise, this chip is somewhere in between the OPA134/132 and the OPA604, with
one notable exception: it will tolerate single-supply operation down to 3V. It's also a
"rail-to-rail" design, which ideally means it doesn't have any headroom requirements
between the supply voltage and the output voltage. In practice, output loading and
other things mean a practical chip can't go quite to the rails. This chip does a lot better
than any of the above chips, at any rate. This makes the AD823 ideal for battery-
driven amps.
Sonically, this chip is slightly more impactful than the OPA134/132 in the bass area,
and rather more detailed. With aggressive-sounding headphones, this chip will
probably sound too aggressive, unless other parts of the system compensate for it.
The main disadvantage of this chip is that it has a much lower output current than is
typical: 15mA for the AD823 vs. around 40mA for most op-amps. While few
headphones actually require more than 15mA of continuous current, all headphones
seem to perform better when given a "reserve" of current far above their nominal
requirements. I suspect it has to do with how much you make the amplifier strain,
which affects its performance. This chip will perform adequately even with
low-impedance headphones, but suboptimally. Output current isn't a consideration at
all in buffered headphone amps, of course.
Bottom Line: Seriously consider this chip over the OPA132/134. Especially consider
it it you aren't happy with the laid-back Burr-Brown sound and don't mind spending a
bit more on the chip.
Other Info:
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Notes on Audio Op-Amps http://tangentsoft.net/audio/opamps.html
4 de 15 27/1/2012 00:45
www.head-fi.org/forums/showthread.php?s=&threadid=703
headwize.com/ubb/showpage5.php?fnum=3&tid=2374
Analog Devices AD825
Cost, single: $3.76 V , 0.5V into 33 Ω: 4.7V
Cost, dual: n/a V , 2.0V into 330 Ω: 8.6V
No detailed review done yet. Sorry.
Analog Devices AD843
Cost, single: $8.56 (AD843JN) V , 0.5V into 33 Ω: 8.2V
Cost, dual: n/a V , 2.0V into 330 Ω: 12.2V
When I went into this test, I was hoping to find a chip to dethrone the OPA627 which
is expensive and requires a lot of voltage. The 627 also has the characteristically
mellow Burr-Brown sound, which is not always a good thing. Analog Devices chips
tend to be a little snappier and more aggressive, which can help balance some
systems. I wanted a chip that would fit all of these criteria while still maintaining the
OPA627's incredible level of resolution and clarity. The AD843 doesn't completely fit
these criteria, but it does come close.
What this chip gets right: First, the AD843 is definitely cheaper. Like the 627, the
cheaper grade is fine for audio, so a pair of 843s is about half the cost of a pair of
627s. Second, this chip does have that Analog snap and verve.
The downside is that the AD843 requires more voltage than the OPA627. And like the
627, the sound gets very nasty very quickly when it starts clipping.
The AD843 seems to trade smoothness for resolution relative to the OPA627. In some
cases the more detailed OPA627 might be preferrable, and in others the smoother
AD843 could be helpful. I'm torn on what to make of this real difference. The OPA627
isn't hyper-revealing, and the AD843 isn't over-smooth. The OPA627's detail seems
genuine; it isn't grain or overemphasized real detail. I don't mind the way the AD843
ignores these details, but at the same time I don't resent the OPA627 for presenting
them. The OPA627 can be accurate to a fault, if your recording has unpleasant detail
in it that another op-amp would ignore or deemphasize. If you have flawed
recordings, you may prefer the pleasant lie told by the AD843.
Bottom Line: This is a serious contender with the Burr-Brown OPA627 for the title
of "best op-amp I've ever heard". I view these two chips as rough equals; they're both
in the same audio class, but each chip has strengths lacked by the other. Taste and
system matching will be the largest factors in choosing one over the other.
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Notes on Audio Op-Amps http://tangentsoft.net/audio/opamps.html
5 de 15 27/1/2012 00:45
Other Info:
headwize.com/ubb/showpage5.php?fnum=3&tid=1920
Analog Devices AD845
Cost, single: $6.40 (AD845JN) V , 0.5V into 33 Ω: 8.7V
Cost, dual: n/a V , 2.0V into 330 Ω: 9.0V
This is a very interesting chip. Sonically, you can call this a smoother, less detailed
AD843. It's almost tube-like, while retaining the Analog devices snap. Compare the
Burr-Brown sound which is relaxed...mellow...slower. The chip has a lot more
resolution compared to lesser chips like the OPA132/134 and the AD823, but it
doesn't quite match that of high-end chips like the AD843 and OPA627.
Voltage-wise, it's a somewhat hungry chip. It's only specified to run down to 9.5V, but
I was able to get it to go a bit lower. In a previous listening test, I was able to take it
down to about 5V. I think what is happening here is that this chip has tolerable
distortion behavior when clipping, so a bit of clipping is tolerated better than with
other chips.
The chip is also a bit of a pig when it comes to current draw. Each chip draws about 10
mA quiescent, which will of course go up in normal operation, and you need two chips
for stereo. Heaven help you should use the chip in a battery-powered Hansen or
CHA47 amp, where you need four chips!
Bottom Line: If you're building a wall-powered amp and want a snappy sound and
either want to save some money or smooth over some detail relative to the AD843,
this is a good chip. However, I think you should make the small step up to the 843
instead. This would be okay in a high-end battery-powered amp where sound quality
is more important than battery life, yet going up to the 843 is too much.
Other Info:
headwize.com/ubb/showpage5.php?fnum=3&tid=1920
Analog Devices AD8065
Cost, single: $4.21 (AD8065) V , 0.5V into 33 Ω: 4.0V
Cost, dual: $5.38 (AD8066) V , 2.0V into 330 Ω: 5.7V
This chip is a good alternative to the AD823 and AD8610 in battery-powered amps. It
draws more current than either of those, but it does run to lower voltages so it may
last nearly as long on a set of batteries in some configurations. As you can see from the
clipping numbers, the chip is running rail-to-rail in the 330 Ω test: 2Vrms is 5.656V
peak-to-peak. You can't get better than that.
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Notes on Audio Op-Amps http://tangentsoft.net/audio/opamps.html
6 de 15 27/1/2012 00:45
This chip kind of splits the difference between the AD8610 sound and the
Burr-Brown sound: not aggressive, but not laid-back, either. It's a bit veiled, which is
expected given the chip's price.
This chip is rare in that it is only rated for a 24V supply. (Absolute maximum is
26.4V.) Another oddity is that it is only available in SOIC versions, so you need to
mount it on a Brown Dog adapter to use it in amps that use DIP chips.
Bottom line: This may be the ideal chip for you if you're running a battery-powered
amp and the AD8610 is too aggressive and the OPA227 too laid-back.
Analog Devices AD8397/AD45048
Cost, single: n/a V , 0.5V into 33 Ω: 4.0V
Cost, dual: $5.38 (AD45058) V , 2.0V into 330 Ω: 6.4V
No detailed review done yet. Sorry.
(According to a well-placed source, the AD8397 and the AD45048 are the same chip,
characterized for different markets. This test was done with an AD45048.)
Analog Devices AD8512
Cost, single: n/a V , 0.5V into 33 Ω: 4.5V
Cost, dual: $3.89 V , 2.0V into 330 Ω: 7.4V
This chip is very similar to the AD823. The main differences are that it has higher
output current and lower supply current. The sound quality is similar, though where
I'd call the AD823 aggressive, I'd call the 8512 a bit harsh. The sound isn't nasty by any
means, just not as euphonic as other chips I've reviewed here.
Bottom line: This chip is best when battery savings are the absolute most important
thing and the 823's low output current is a problem. If you can tolerate higher supply
current or higher voltage requirements, there are better sounding chips.
Analog Devices AD8610/AD8620
Cost, single: $8.00 (AD8610) V , 0.5V into 33 Ω: 5.7V
Cost, dual: $13.33 (AD8620) V , 2.0V into 330 Ω: 7.6V
This is quite possibly the best chip for battery-powered amps, period. Its voltage
tolerance is among the lowest of all the chips I mention here, it has good output
current abililty, it has among the lowest supply current of any chip reviewed here, and
above all it sounds good.
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Notes on Audio Op-Amps http://tangentsoft.net/audio/opamps.html
7 de 15 27/1/2012 00:45
What does it sound like? Well, take the AD823, and remove some of the aggressive
harshness. Add a bit of detail and smoothness from the AD843. That's the 8610. It's
not a smooth chip, just not harsh. It's not the most detailed chip, but not heavily
veiled, either.
This chip is rare in that it is only rated for a 26V supply. (Absolute maximum is
27.3V.) Another oddity is that it is only available in SOIC versions, so you need to
mount it on a Brown Dog adapter to use it in amps that use DIP chips.
Bottom line: This is a contender for my favorite chip of all time, especially in
battery-powered amps. When paired with an aggressive or very revealing system, this
chip can be unpleasant. This chip is at its best complementing a smooth, laid-back
system.
Analog Devices OP275
Cost, single: $2.13 V , 0.5V into 33 Ω: 17.0V
Cost, dual: n/a V , 2.0V into 330 Ω: 10.0V
No detailed review done yet. Sorry.
Burr-Brown OPA2107
Cost, single: n/a V , 0.5V into 33 Ω: 10.6V
Cost, dual: $12.25 V , 2.0V into 330 Ω: 10.6V
No detailed review done yet. Sorry.
Burr-Brown OPA132
Cost, single: n/a V , 0.5V into 33 Ω: 5.5V
Cost, dual: $5.40 (OPA2132PA) V , 2.0V into 330 Ω: 8.3V
This is a nice family of op-amps. The sound has the typical Burr-Brown laid-back
nature. It's a bit tubby on the bottom end. This is not an exciting sounding chip, but it
does tend to counteract the flaws in many low-end audio systems, especially portable
ones.
Digi-Key only carries the dual versions (2132) in DIP packages, and there are two
grades, differentiated by whether there's an 'A' in the part name. I've been unable to
find a case where the 2132PA performs worse for audio amplification than the 2132P.
From a look at the datasheet, it looks like the advantages of the non-A grade are in DC
specs, which of course aren't all that important to audio.
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Notes on Audio Op-Amps http://tangentsoft.net/audio/opamps.html
8 de 15 27/1/2012 00:45
Bottom Line: A very good chip to start with. Indeed, you may be so happy with it
that you stop looking at other chips!
Burr-Brown OPA134
Cost, single: n/a V , 0.5V into 33 Ω: 5.7V
Cost, dual: $2.63 V , 2.0V into 330 Ω: 8.4V
This is the audio grade version of the OPA132 family. (That's "audio grade" in the
commercial sense, not the audiophile sense. Read: "lower quality".) Digi-Key only
carries the dual version (2134) in DIP packages.
The OPA134 requires a bit more voltage than the OPA132 does. This won't matter in
circuits that have plenty of voltage, but in a battery powered system a 132 can pay for
itself by letting you run longer on a battery.
I found in earlier testing that the 134 was more likely than a 132 to become unstable in
marginal circuits. Sometimes raising the supply voltage was all it took to make the 134
stable, and other times only swapping in a 132 would fix the problem. If you're
building your own circuit from scratch and you aren't very experienced, the extra cost
of the 132 can pay for itself in a better likelihood of success.
Bottom Line: If your circuit is solid and you have a fairly high supply voltage, the
134 is better than the 132 because it's cheaper and they sound identical to me. The 132
is better for more marginal setups.
Burr-Brown OPA227
Cost, single: n/a V , 0.5V into 33 Ω: 5.4V
Cost, dual: $3.53 (OPA2227PA) V , 2.0V into 330 Ω: 8.2V
This family of opamps sounds very similar to the OPA132 and OPA134 families. The
main difference is that the 227 isn't as tubby on the bottom end as the 132. The 227
will run to slightly lower supply voltages than the 132 family.
Digi-Key only carries the dual versions in DIP packages. The 'A' versions are the right
ones for audio, as the non-A version simply has better DC specs, which is not useful
for audio.
Bottom Line: This is a good alternative to the OPA132. It is arguably even a
significant upgrade.
Burr-Brown OPA228
Cost, single: $2.36 (OPA228PA) V , 0.5V into 33 Ω: 5.1V
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Notes on Audio Op-Amps http://tangentsoft.net/audio/opamps.html
9 de 15 27/1/2012 00:45
Cost, dual: n/a V , 2.0V into 330 Ω: 8.0V
As the OPA637 is to the OPA627, so the OPA228 is to the OPA227.
Being a faster chip (33 MHz vs. 8 MHz), it wasn't stable in some of my amps, probably
due to minor circuit layout problems. I got it to oscillate outright occasionally, and
other times I just got a kind of "grunge" in the sound.
When I was able to avoid instability, it sounded a bit more analytical than the OPA132.
By comparison, the 132 sounded more "alive". This difference is purely subjective, so
some people may prefer the difference.
Bottom Line: I'm not happy with this chip for audio. If your audio tastes are like
mine, you'll be happier with the OPA227. This chip will run to even lower voltages,
though, so perhaps it's a good idea for battery-powered amps that need to drive
headphones that need fairly high voltages.
Burr-Brown OPA602
Cost, single: $7.60 V , 0.5V into 33 Ω: 10.2V
Cost, dual: n/a V , 2.0V into 330 Ω: 9.6V
No detailed review done yet. Sorry.
Burr-Brown OPA604
Cost, single: $2.36 V , 0.5V into 33 Ω: 8.6V
Cost, dual: $4.28 V , 2.0V into 330 Ω: 11.4V
The specs on this chip are very similar to that of the OPA132/134 family: some specs
are a bit better, and some a bit worse. Like the OPA134, this one is specifically sold
with audio in mind. It's been used in several popular bits of audio gear, especially
near the low end of the audiophile range.
For a headphone amp, the most significant spec difference is that the 604s require
more voltage than the OPA134/132s to sound good. If you're going to use a battery
power supply, you should use two 9V batteries or at least 8 cells for AAs or similar.
This chip has a problem with oscillation, no doubt because it's a "fast" op-amp: 20
MHz, vs. the OPA134/132 family's 8 MHz. In one of my Hansen-type amps configured
with OPA2604APs, I was able to make it oscillate by giving it a supply voltage
anywhere over 18V. In other amps, I was able to give it as much as 34VDC without
causing oscillation. In my main test amp, I didn't have any oscillation problems. It's
hard to pin down a source of blame for this problem. The only solid lesson I have been
able to draw from this so far is that this chip is simply harder to use than the
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Notes on Audio Op-Amps http://tangentsoft.net/audio/opamps.html
10 de 15 27/1/2012 00:45
OPA134/132 family.
Sonically, the OPA604 family is almost identical to the OPA134/132. I did hear slight
differences, but I was at a complete loss when trying to put these differences into
words. Because of the advantages of lower supply voltage and oscillation-free
operation, the OPA134/132s are still more desirable chips for headphone amps.
Bottom Line: Use this only if you can't find an OPA134/132.
Burr-Brown OPA627
Cost, single: $18.38 (OPA627AP) V , 0.5V into 33 Ω: 6.8V
Cost, dual: n/a V , 2.0V into 330 Ω: 9.8V
The first thing I noticed is the cleanliness of the sound. With this chip in the test
amplifier, I heard known problems in a low-end portable source more clearly than
with my reference for this test, the OPA134PA. The 627 also seems to do better on
recordings with room ambience: it reveals details about the acoustic space that the
OPA132/134 chips will hide, making them sound "flat" in comparison. But these two
chips are more alike than different. Both have the characteristic laid-back and dark
Burr-Brown sound, and both are very tolerant, stable chips.
The only remaining differences are that the OPA132/134 family will work well below
9V, whereas the 627's performance falls off a cliff below the clipping points I give
above. There does seem to be a bit of extra low bass impactfulness with the 627. This
seems less to be "extra power" than a removal of some heavier thumpiness in the
132/134 — the 627 seems to have a truer, more refined kind of bass.
In all my testing, I've been unable to hear a difference between the OPA627AP and the
OPA627BP. The datasheet says that the differences between the grades are in the DC
specs, so this is not surprising.
Bottom Line: The sonic differences between the OPA627/637 and the OPA132/134
are of the "last 5%" variety, rather than providing a dramatically different sound. If
you like the Burr-Brown sound and can stand to pay 14× as much as for an
OPA2134PA, a pair of OPA627APs is a reasonable investment. I see no reason to pay
extra for the B grade in an audio application.
Other Info:
headwize.com/ubb/showpage5.php?fnum=3&tid=774
headwize.com/ubb/showpage5.php?fnum=3&tid=1865
Burr-Brown OPA637
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11 de 15 27/1/2012 00:45
The OPA637 is simply the "uncompensated" version of the OPA627. This means it has
a higher bandwidth, but that it won't be stable at low gain levels. The datasheet says
that it is minimally stable at a gain of 5, but as with all chips, the higher the gain, the
more stable it becomes. The cost and voltage performance are the same as for the 627.
The higher bandwidth of the 637 results in a somewhat more lively sound than the
627. It still has the overall laid-back Burr-Brown characteristic, though. Given the
choice between these two chips, I use the 637 when I can live within its gain
requirements, but I happily fall back to the 627 otherwise.
Other Info:
headwize.com/ubb/showpage5.php?fnum=3&tid=1197
National Semiconductor LF355
Cost, single: $0.94 V , 0.5V into 33 Ω: 14.9V
Cost, dual: n/a V , 2.0V into 330 Ω: 10.3V
Yet another jellybean chip. The full family is LF355-LF357. Specwise, these are very
similar to the TL071. A related chip is the LF351, discontinued by National but picked
up by makers of generic chips like ST and Fairchild.
Notice that this chip doesn't like to drive low impedances.
National Semiconductor LM6171
Cost, single: $2.83 V , 0.5V into 33 Ω: 6.1V
Cost, dual: $3.80 V , 2.0V into 330 Ω: 9.5V
Digi-Key carries the LM6171BIN single-channel version, and the LM6172IN dual
version in the DIP package.
This is a very high-speed op-amp with bipolar input transistors. Translated, that
means this op-amp is hard to use. However, it has exemplary audio performance for
such a low price, so experienced builders should at least consider using it. If you
decide to give it a shot, you must design the amp around the chip: you cannot just pop
the chip into an existing circuit and expect it work. (For info on what's necessary to
make this chip work, see the companion article, "Working with Cranky Op-Amps".)
I made another test amplifier according to that article's principles in order to test this
chip. To assure myself that the necessary design changes didn't impact the sound, I
tried some of the other chips in it. This brings up an important point: the changes
necessary to make chips like the LM6171 work in a CMoy amp do not prevent less
picky chips from working. If you think you might want to try a chip like this someday,
you might make the design changes from the start so you're free to use most any chip
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in the amp.
Sonically, this chip is significantly more revealing than the OPA132/134 series, and the
bass is a touch more impactful as well.
With a high supply voltage (15V), the difference between the LM6172 and the
OPA134PA is subtle, but real. At lower supply voltages, the LM6172 can greatly
outshine the OPA132/134 series. The clipping numbers above don't tell the full story;
when this chip clips, it does so in a very soft, rounded way, instead of the harsh
clipping you get with other chips. This means you can often run the chip to lower
voltages than my tests indicate by accepting a mild form of distortion.
Bottom Line: This chip is not for tyros, but it's cheap, it performs very well at low
voltages, and at higher voltages it still outshines many other chips in its price class. It
requires more external components to achieve that performance, however.
Other Info:
headwize.com/ubb/showpage5.php?fnum=3&tid=2222
Signetics NE5532 (various vendors)
Cost, single: n/a V , 0.5V into 33 Ω: 5.6V
Cost, dual: $0.80 V , 2.0V into 330 Ω: 8.9V
Another jellybean chip, but bipolar-input so it requires more care in application.
Probably better than the TL072 for audio.
No detailed review done yet. Sorry.
Texas Instruments TL07x (variousvendors)
Cost, single: $0.48 (TL071CP) V , 0.5V into 33 Ω: 17.3V
Cost, dual: $0.64 (TL072CP) V , 2.0V into 330 Ω: 12.3V
This is a jellybean chip that was popular for audio back in the 80's. Open up an old CD
player or preamp, and there's a pretty good chance you'll find a TL072.
The 071 is the single version, and the 072 is the dual. There is also a letter or three in
there, which indicates the part's tolerance and sometimes a variation code. In
increasing levels of quality, they are: C, AC, BC. (I'm ignoring the special-purpose
industrial and mil-spec grades.) I can't hear the difference between the grades, so get
the C grade. The price is for TI's version, since they invented the chip.
This family of chips is not as good as the OPA132/4 family. The clipping numbers
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above are informative here: this is one of the few chips I've tested that required more
supply voltage on the 33 Ω test than on the 330 Ω test. Bottom line, this chip does not
like to drive low impedances. I've heard members of this op-amp family distort even
at the chip's full operating voltage of 30V. I'm not talking about minor problems here
— I'm talking about crunchy, ugly, obvious distortion. Therefore, I recommend using
this chip only under duress, and then only if you're willing to give it a fairly high
supply voltage.
Bottom Line: This chip is very common and cheap, but also sonically inferior to all
other chips tested so far. Use it only if you can't find anything better.
Texas Instruments TL08x (variousvendors)
This is a lower-spec version of the TL07x. I can't hear a difference between the two,
but since the TL07x is the same price you might as well get that one. As I said above,
these chips are classic audio jellybeans, mainly useful when the chip doesn't have to
drive a low-impedance load.
The only reason I've bothered to review the 082 here is that you can get them at Radio
Shack. If you need an op-amp in an emergency, it's good to know that you can pay the
moderately obscene sum of $1.99 each to get usable chip without having to wait for
mail order.
Article Revision History
2004.08.15
Rewrote the introductory material, describing the new test methodology.
Re-did all the clipping tests with the new objective method.
Added skeleton reviews of the AD744, AD825, LF355, OP275, OPA2017 and
OPA602.
Added short reviews of the AD8512 and NE5532.
Added full reviews of the AD8610 and AD8065.
Split all of the reviews of similar chips (132/134, 227/228, 627/637, etc.) into
separate reviews, for clarity.
Fixed all the Headwize URLs.
2002.08.17
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Added OPA637 info to the OPA627 review.
2002.05.17
Added the AD843 and AD845 reviews.
2002.05.06
Small improvements. The big one is adding the warning about the AD823's output
current issue.
2002.03.21
Updated the OPA627 review with A vs. B grade results.
2002.03.17
Added the OPA227 and OPA228 reviews.
2002.02.18
Added the OPA627 review.
2002.02.11
Finished the initial LM617x review. And, touched up the rest of the article some.
2002.01.28
Made two new CMoy-type amps solely for the purpose of this test and re-tested all
the chips tested so far in them, then updated the article with the findings. Also,
added info on test conditions, and polished the initial article text some.
Added info on the TL071ACN, TL072ACN, and TL072BCP.
Added info on the Burr-Brown OPA604AP and OPA2604AP.
Added info on the Analog Devices AD823AN.
2002.01.13
Starting with the text from the section "A Few Words About Op-Amps" in the
CMoy tutorial, created the original version of this article. The initial version covers
the full OPA2132/2134 families and the TL082CP. (And of course it generalizes for
these op-amps' extended families until I have an opportunity to try a broader
selection of chips in each line.)
This article is copyright © 2002-2011 by Warren Young, all rights reserved.
Updated Tue Nov 29 2011 13:47 MST Go back to Audiologica Go to my home page
Notes on Audio Op-Amps http://tangentsoft.net/audio/opamps.html
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