Op Amps: Myths & Facts

INTRO: The op amp is a fundamental building block of audio gear. Op amps are widely used and, not surprisingly, they’re a popular topic among audiophiles. So what are the myths and facts?

MYTH: MOST OP AMPS SOUND DIFFERENT - There’s a general perception that op amps sound different. Many gear manufactures tout op amp brands and part numbers in their marketing literature. The $3 OPA2134 is supposed to sound much better than the $1 NE5532 and the $10 AD8610 is supposed to sound better still. But do they? The chip companies don’t help the perception implying some of their op amps offer better sound. But used properly, in a typical audio application, I’ll challenge anyone to a listening test and bet they won’t be able to tell the above three op amps apart. The only catch is it would be a blind test and the listener won’t know which op amp is which.

HISTORY: The Operational Amplifier (op amp) was invented in the 40’s. Bell Labs filed a patent in 1941 and many consider the first practical op amp to be the vacuum tube K2-W invented in 1952 by George Philbrick. Texas Instruments invented the integrated circuit in 1958 which paved the way for Bob Widlar at Fairchild inventing the uA702 solid state monolithic op amp in 1963. But it wasn’t until the uA741, released in 1968, that op amps became relatively inexpensive and started on the road to ubiquity. And they didn’t find their way into much consumer audio gear until the late 70’s and early 80’s.

MYTH: DISCRETE IS BETTER - For audio use the op amp’s main competition is a fully discrete amplifier made out of transistors, resistors, etc. Which is better? It turns out, for nearly all applications for which IC op amps are suitable, they easily outperform discrete designs in the following areas:

• Better Performance – It’s very difficult to match the overall performance of even the  inexpensive 5532 op amp with a discrete circuit. The discrete circuit is at a disadvantage in many areas including component matching, bias stability, and the need to use off-the-shelf components (every “component” in an IC op-amp can be custom tailored and optimized to its task).
• Simplicity – To even come close to the performance of an IC op amp many more components are required. You need differential pairs, multiple stages, current mirrors, constant current sources, bias circuits, protection circuits, etc. You end up with dozens or even hundreds of components to try and match a single dual op amp IC in a little 8 pin package.
• CMRR/PSRR – Common Mode Rejection Ratio is how well an amplifier can reject unwanted noise.  Because their internal components are so well matched it’s easy for op amps to achieve excellent CMRR and PSRR (Power Supply Rejection Ratio) performance. This helps their real world performance in audio applications because they can reject noise on the power supply, and inputs, much better than most any discrete circuit.
• High Open Loop Gain – Op amps typically have higher open loop gain. This allows more feedback which in turn lowers distortion. There’s another audiophile myth high feedback is somehow bad but that’s the topic of another article. Look up Bruno Putzeys recent article on the topic. He pretty much busts all the feedback myths wide open with real science. He even explains how so many people got off track. Trying to get comparable open loop gain in a discrete design typically creates challenging stability issues.
• Repeatability – Op amps have tightly controlled specifications and detailed information available on their performance. They’re typically individually tested when they’re made so you know exactly what you’re getting. You can buy a TI 5532 today and an On Semi 5532 a year from now, and both will perform very similarly. High performance discrete circuits often require matched or hand picked components to achieve their best performance. This makes them difficult to reproduce and their real world performance is more of an unknown and sometimes requires detailed testing of each implementation (something few DIYers can do). Discrete circuits also typically cannot hold as tight of performance over a wide temperature range.
• Massive R&D – The big semi conductor companies compete against each other for design wins. They spend serious money trying to out perform each other and have million dollar labs full of advanced equipment. They design the ICs right down to the properties of each transistor and have at their disposal types of internal components that are not even available as discrete parts. It’s impossible to match all their capabilities with a discrete design.
• Built In Protection – Many op amps have at least current limiting and some also have other forms of protection like thermal shutdown. This makes them more robust than a discrete circuit unless similar circuitry is added to the discrete design making it even more costly and complex.
• Ease Of Use – Op amps are very well characterized and typically well supported by the manufactures with detailed specs, performance graphs, and even application notes and sometimes reference designs. Following their guidelines usually results in predictable performance. They’re also typically easier to “glue” together when using a proper bipolar power supply as the outputs are referenced to zero volts and can be directly coupled to the next stage. The discrete designer is largely starting from scratch and has far more hurdles to clear.
• Lower Power – An op amp, because of all the advanced techniques available to IC designers, can operate its output stage in Class-B with vanishingly low levels of distortion. While discrete designs are often forced to use much more power hungry Class-A to even come close. Overall, an op amp typically needs substantially less power than a typical discrete equivalent. This is a huge advantage for battery powered gear or if you need lots of amplifier stages.
• Lower Cost – You can get amazing performance for under a $1 with an op amp. You can’t even come close with discrete designs. In fact, discrete designs often end up significantly compromised to limit their cost and complexity to reasonable levels. So you end up comparing a highly optimized IC against a compromised discrete circuit that still costs a lot more and performs worse.

DISCRETE ADVANTAGES: There are a few circumstances where discrete designs generally make sense. One is a needing output voltages greater than about 8 V RMS. There are some high voltage op amps but the selection is limited and they’re expensive. And the same is true if you need peak currents much over about 300 mA. There are some high current op amps, but they can suffer from thermal issues and are also relatively expensive. Driving speakers, for example, requires both voltages and currents that exceed these ranges. That’s why most high power amps for speakers are discrete designs. There are also some decent “chip amps” but they’re not strictly op amps. They also have thermal limitations and are limited to relatively modest power outputs. They also tend to have rather invasive protection circuitry due to their thermal limitations.

FIRST GRAIN OF TRUTH: There are often grains of truth behind many audiophile myths. But, very often, those grains no longer apply, or audiophiles apply them in ways that are entirely invalid. The early op amps, like the 741, were rather substandard for some audio applications. They had poor slew rates and they couldn’t manage much gain while maintaining bandwidth to 20 Khz--let alone do so with low distortion. They also couldn’t drive much of a load. That didn’t stop some manufactures from using early op amps in 70’s and 80’s audio gear.

WAX CYLINDERS: Many audiophiles condemn op amps based on the early examples. By that same logic, their $10,000 vinyl turntables should also be condemned because the Edison Wax Cylinder phonograph was horrible. It could barely reproduce intelligible human speech let alone do a good job with music. Obviously, it’s not fair to judge a technology only by the early examples. But that’s how op amps mostly got their reputation of shame among audiophiles, and it stuck. Op amps didn’t offer serious audio performance at reasonable prices until the 5532 was released around 1985—almost 20 years after the 741. And it took several more years for the 5532 to show up in much consumer gear. (photo: photopedia.com)

SECOND GRAIN OF TRUTH: Some op amps have been judged in the wrong applications. The Cmoy headphone amp is just such an example. Typical op amps are made to drive loads of several thousand ohms or higher. Even the more powerful ones are often only rated down to 600 ohms. But that didn’t stop anyone from using these same op amps to drive headphones in the 16 – 300 ohm range. And, not surprisingly, some of them audibly complain at their mistreatment. They simply weren’t designed for such a low impedance load. If you compare overloaded op amp A against overloaded op amp B you may hear some differences in their highly distorted performance. But that’s only because you’re stepping on their tails and making them screech in pain.

YACA (yet another car analogy): Let’s take a Ferrari and a Lamborghini--both amazing cars with very similar performance on the road. But instead of driving them on pavement as their manufacture’s intended, let’s hitch them up to a plow and try to plow some corn fields. Suddenly the two very similar cars behave very differently. The Lamborghini has all wheel drive and likely does significantly better trying to pull a plow in dirt than the 2 wheel drive Ferrari. But so what? Who tries to plow a field with a sports car? This is analogous to misusing op amps and judging their performance based on an application they were never designed for.

MYTH: MOST OP AMPS ARE COMPATIBLE - Op amps are complex devices. While many have the same pin configuration making it appear you can simply substitute one for another, that’s often not the case. They’re optimized for different input bias values, configurations, gains, feedback circuits, load impedances, quiescent currents, speeds/bandwidths, compensation values, operating voltages, noise trade-offs, etc. But some audiophiles can’t be bothered by, or don’t understand, all those details. So they simply swap them out without changing anything else in the circuit. Many have a favorite op amp or two they like to use in most everything without considering if it’s a good match. It’s like the old quote “when you have only a hammer everything looks like a nail”. But that just doesn’t work for op amps. And, not surprisingly, some op amps do better than others when you ignore their requirements.  Again, this is a lot like the Second Grain Of Truth above. If you use a given op amp incorrectly, it may well sound different. But it’s not the op amp’s fault. The person misusing it is creating the audible differences.

MYTH: OP AMP UPGRADES ARE USUALLY WORTHWHILE - The forums are full of posts from people who buy some perfectly nice piece of audio gear, open it up, discover the manufacture used a “cheap 5532” op amp, and they promptly pop in something much more expensive and exotic to “improve the sound”. These are usually people who lack the test equipment to have any idea if their op amp swaps help, hurt, or are just a waste of money. They simply use their ears, in highly biased sighted listening, and draw all sorts of erroneous conclusions. I know of op amp swaps causing potentially harmful oscillation. Someone might hear ultrasonic artifacts from the oscillation as “newfound detail” when, in reality, they have unwittingly created a radio transmitter. And this isn’t as rare as you might think because of the “faster is better” mentality. (photo: photozou.jp)

YACA: Some of the car magazines have done objective tests of tire upgrades, shock upgrades, oversized wheels, etc. When they compare say a stock BMW to the same car with one or more of the “upgrades”, the factory set up usually posts the best overall performance, lap times, etc. While those massive 20 inch rims with ultra low profile tires might look cool they often perform worse than what the car came with. That’s because the manufactures, especially for performance-oriented cars, optimize the tires and suspension carefully. They understand all the trade offs better than anyone. And the same is often true with op amps and audio gear. In their attempt to “upgrade” the owner is often messing up a carefully engineered design and making it worse.

MYTH: GEAR COMES WITH LOUSY OP AMPS – Believe it or not, you can usually trust the bigger manufactures of audio gear to use an op amp that’s up to getting the job done. Why? Because for one thing such op amps are surprisingly inexpensive. So there’s little reason for a manufacture not to use an ideal op amp. For another they likely have $50,000+ worth of test instrumentation and can precisely measure differences between op amps. They also designed the circuit the op amp is in, so they better know what requirements matter most. But it seems a lot of audiophiles only see an inexpensive op amp on a circuit board that needs replacing. They may not understand the rest of the circuit, have any way to know if they’re creating new problems, etc.

OP AMP STABILITY: Stability is critical for proper op amp operation. The gear makers tend to specify relatively stable op amps with sane bandwidths. So they don’t have to take many precautions in their circuits to keep such op amps happy. But when Joe Audiophile whips out his soldering iron and replaces the factory op amp with some high strung exotic part, what happens? The new part often requires much more attention to PC board layout, better power supply decoupling, different compensation, perhaps a more stable power supply, etc. In other words, without updating the rest of design (perhaps including even the PC board), the op amp may be at least somewhat unstable or otherwise unhappy. And because most instability is at ultrasonic or RF frequencies, the person doing the swap may not even know they just took a giant step backwards. Even RMAA can’t “see” ultrasonic and RF problems.

THE EASY LIFE: Most op amps in audio gear are not used in stressful ways. Many are simply buffers which means they don’t even have any voltage gain. And even in something like a headphone amp, the gains are relatively modest. And modern audio op amps are virtually never stressed for slew rate. This means the theoretical advantages of many expensive parts are completely useless in these “easy” audio applications. It’s like upgrading your lawnmower with a 400 horsepower V8 when the original 5 horsepower engine cuts the grass just fine.

THE WRONG LIFE: A lot of expensive op amps, including many favored by certain audiophiles, are optimized for entirely different applications—like precise DC performance, video use, etc. Using these for audio is like trying to plow a field with a Ferrari or drive a race car to work. The Ferrari’s main assets are useless in a corn field. For example, the OPA690 op amp used in the AMB Mini3 was never designed for audio use and has relatively horrible audio performance. If you stray too far from what the op amp is designed for it may well sound worse than a much cheaper part designed for audio use—like the 5532.

FOLLOW THE CHAIN: If you follow most music up the signal chain you’ll almost always find it’s already been through dozens, or sometimes even hundreds of op amps. Those big giant mixing consoles you see in the recording studios? Yup, most are filled with hundreds of op amps. Same story for the EQ, compressors, vocal processors, mic preamps, A/D converters, and much more. And do you think they’re all $10 Analog Devices or Burr Brown parts? Nope. They’re mostly inexpensive 5532s in the better gear, and even cheaper parts in the less expensive stuff. Sure music is increasingly produced in the digital domain, but I’ll bet before your music ever touches your headphone gear it’s already been through at least several inexpensive op amps. (photo credit Dennis AB) (photo: photopedia.com)

COMMON SENSE: If much of our most loved music has already been through dozens or hundreds of cheap op amps, is it reasonable to think that one more such op amp is going to make much difference? It’s not and that’s largely because op amps, properly used, are audibly transparent—i.e. you can’t tell they’re even there.

TRANSPARENCY: If op amps really have a “sound”, as many audiophiles suggest, it would follow when you add op amps to the signal path the sound should change. Two guys named Meyer and Moran conducted a very interesting rigorous study. They played high resolution SACDs on a high end system and sometimes inserted an extra A/D and and D/A into the signal path to “down convert” the high resolution audio to CD quality (16/44) audio. After 500+ trials lasting more than a year, using audiophiles, recording engineers, and students as listeners, they found nobody could tell when the extra A/D and D/A was in the signal path. On top of demonstrating the supposed benefits of SACD are highly questionable they also managed to demonstrate that A/D and D/A converters can be audibly transparent as well. And, as you may have guessed, both the A/D and D/A add several op amps to the signal path. But nobody could tell they were even there. There have been many more blind tests that also demonstrate different op amps (and much more) indeed sound so much alike even audiophiles can’t hear the difference. See the Matrix audio test for another example.

SIGHTED LISTENING: So early op amps sometimes sounded bad and if you misuse an op amp you can make them sound different. But what about all the decent op amps in proper designs? Why do so many claim they sound different? The answer is they use sighted listening. Our brain filters what we hear using other knowledge—like which op amp you’re listening to. It’s an involuntary response that even the most skilled listener cannot “turn off”. Check out this short BBC video on the McGurk Effect for an example of how our brain influences what we hear. And if you find that interesting, there’s a lot more in my Subjective vs Objective Debate article. Basically when someone “upgrades” an op amp they’re often expecting to hear a difference based on the manufacture’s claims, other (similarly biased) subjective listening tests they’ve read about, audiophile myths, etc. So their brain serves up the expected difference much like in the video linked above. But if they let someone else swap the op amps, and they don’t know which is which, the differences always seem to disappear unless there’s another problem. And such problems can be revealed with proper measurements. So if two different op amps both measure reasonably well in a given piece of gear, the evidence strongly suggests they will impossible to tell apart in blind listening tests.

MYTH: FASTER IS BETTER - An often cited reason for expensive op amps, or upgrades, is to get more speed. Sadly, it seems those making these claims don’t understand how “speed” applies to audio. As can be demonstrated with some simple math (and has been verified by Doug Self and many others), any op amp with a slew rate of 3 V/uS or greater is fast enough for nearly any audio application on the planet. And op amps like the 5532 can easily have bandwidths out to 200+ Khz in most applications which results in negligible phase shift or “delays” in the audio band. Using an op amp rated for 20 V/uS or even faster is just asking for other problems and it probably performs worse in other ways (i.e. more noise and distortion). This will be discussed more in the next article.

OP AMP ROLLING: Swapping op amps out, often using a socket so they’re easy to switch, is known as “op amp rolling”. Countless words have been written describing one op amp as having more “depth”, another as having a “blacker background”, etc. The Tangent headphone amp site has a typical list of subjective comments. As explained above, sometimes the differences might be real because some of the op amps are seriously unhappy in that particular configuration. But, more often, it’s just the usual sighted listening bias described above. If the swaps are done blind, and they’re using suitable op amps operated correctly, the alleged differences seem to always disappear.

DISCRETE OP AMPS: There’s another tiny grain of truth here. There are some very expensive discrete op amps that were designed specifically to outperform IC op amps in very specific ways. Their highly specialized benefits, to my knowledge, don’t offer any real world advantages in typical audio use. And audiophiles seem to favor much less rigorously designed discrete replacements for IC op amps. These discrete substitutes typically lack any sort of valid test data to demonstrate their real performance. So how does anyone know they outperform ICs? They mostly use Sighted Listening which is completely invalid (see above). Audio-GD discrete op amps were tested by Samuel Groner using an Audio Precision analyzer and the results were horrible. See The Discrete Is Better Myth.

OP AMP MEASUREMENTS: To try and keep my articles to a more digestible size, I’m splitting this into two parts. This article has covered the history, applications, and myths. The next article will cover the more technical aspects including op amp parameters (what matters on a datasheet for audio) and some actual measurements using my dScope audio analyzer and a number of op amps that were evaluated for the O2 Headphone Amp.