Spectral X-Contamination: Problems in Op-Amp Chips Postscript August 2020

Postscript August 2020

As a band plays a lazy jazz-blues in the corner, and everyone has gotten refreshed with eine kaltes Bier, the professor's central-European audience are warned that a small spectral leakage across multi-channel ICs is developed into a far greater and wider kind of audioland X-contamination, as outlined below.

Another threat is the Ring of 3, an op-amp "state variable" topology used in posh equalizers and crossovers. As with the innards of NFB amplifier stages, while low distortion on the outside lures use, voltage waveforms inside the ring aren't pretty. They're triangular, so nasty harmonics predominate, so any leakage is a no-no. Yet as the R-o-3 uses four op-amps, engineers and accountants will demand quad(ruple) ICs, ensuring that the R-o-3 perverts music. One can see why live-show engineers who use 1/3rd octave graphic equalizers, have preferred the sonics of instruments (like EMO) that employ inductors.

As for ICs, Texas Instruments, the world's longest-lived transistor maker, is also one of the analog IC makers left in the US. Great companies with initials BB, LTC & AD have been subsumed. Decades back now, Walt Jung came up with splitting op-amp stages, separating the output part. This cuts out the thermal distortion and harmonic contamination, but doubles a load of costs.

Symmetry—A Brutalist Architecture
It's routine for audio since the 1920s, thro' to 2020, to employ circuits that are variously termed bridged, balanced, differential, push-pull, and symmetrical.

The signal is split: It flows through opposing halves, that are presumed to be exact complements. This arises all over, from vinyl disc front-ends, thro' high-power amplifiers' driver and output stages. These arrangements can also be nested, most often in output stages. One symmetrically driven output is pitted against another, speaker in the middle. The second drive is inverted, so voltage and current swings are doubled, and power delivery is potentially quadrupled. Oft used where serious oomph is sought for bass and sub. With analog amplifiers, such bridging, where two channels become one, is often assignable. Across 30 years, sonic opinions of bridging with otherwise fine stereo amplifiers, have been lackluster.

Power-heads can even bridge a bridged amplifier—an Amcron specialty. (Amcron used to be called Crown and Crown amplifiers were last deemed "high-end" in the mid 1970s, and this source of industrial muscle dissolved some years ago. "Go figure.")

There are also balanced input stages. Sat in just part of the path, the noise/EMI rejection may be more important. Then balanced outputs, that aren't needed in homes (footnote 1) and when active, are miniature bridged amplifiers. In a common, low and nasty circuit that's became standard, positive feedback (zut Alors!) is even added to risk sub-stable operation, resonances and abnormal harmonics. Marlowe frowns, and slips the catch on his Colt.

Meantime, symmetrical discrete circuitry was popularized in the USA in the early 1970s (footnote 2) and a decade later promoted to drive Lateral MOSFETs by Erno Borbely. It's neat on paper. The same sort of mirror-image topology/architecture still appears across US products, and unimaginative copies from the orient.

A study of national circuits is informative. In other places, symmetrical circuits are rarely met. They appear profligate to engineers, and eyebrow-raising to French audio factory accountants. Quelle Horreur!

Same goes for the circuits inside US-designed op-amps—most are un-symmetric. Symmetrical appears smart—a dangerous, hubris-attracting adjective. Mistaken ideas couldn't be clearer now.

First, symmetrical circuits need parts of opposite gender. For example, PNP and NPN bipolar transistors, or N- and P-channel FETs. But, these genders are in no way exact opposites, any more than with humans. There are complicated differences. For example, PNP parts depend on slightly differently-behaving "holes", not electrons, to transact business (footnote 3).

Some astute makers (footnote 4) have mastered ways to inter-weave NPN and PNP bipolar transistors and symmetrize them in triples. But however neat, symmetry remains inherently damaging to musicality. There are no such pairings for tubes. Sounds good. But in the '30s, designers still managed to create a form of bridge: push-pull.

Reasons to be careful: With symmetrical circuits, any approach towards overload (clipping), will generate mainly odd harmonics. Just what the circuit does not reject. "Voll Petard und Hoist!" exclaims the German-speaking Swiss host, mangling Shakespeare.

Secondly when symmetrical and push-pull halves are rejoined, even (2,4,6) harmonics made within, are cancelled. Overall, odd harmonics are built-up and evens diminished. This is even brazenly stated in textbooks.

As circuitry inside op-amps and many discrete signal stages outside of high-end fashion circles, isn't obsessively symmetrical, harmonics are permitted to be a mix of even and odd.

"So circuit symmetry is a rather localized viewpoint phenomena" summarized the professor.

...As Long as it's Groovin'
No less immense yet ignored, is that odd harmonics (footnote 5) inherently generate nasty intermodulation (IM) products. Even at low levels, such additions to the noise floor aren't stochastic. In the grunge is a chorus of ugliness built from inharmonic intervals. Unsurprisingly it masks detail. Whenever there's less IM, lo, new details are heard. And when there are fewer odd harmonics, stage-by-stage, sonics are bound to be less harsh, brittle and fatiguing.

Over half-a-century ago, effects of IM were categorized as "acoustic roughness" (footnote 6). Tests exist (footnote 7) to show the IM products' degree of "infill." The fact no one appears to be using such, demonstrates the low state of audio test.

Late Realizations
Alice thought this was very peculiar and counterintuitive. Throughout nature, symmetry was the diamond standard in beauty. Though with human faces, almost-symmetry being even more alluring. In electronics, symmetry is no less elegant. The counterintuitive exception is handling music signals. So there are fundamental conflicts, that listeners, not authorities, have uncovered. "And on Hammond B3" said the compére in a lazy southern drawl, "the great-test in show-biz'nezz toooo day." With a jolt, Alice realized it was The Toad (footnote 8) disguised with a turban, sunglasses, and mask. All would be safe, now the B3 pedals were being thrashed "à la Dennerlein." Some weasely men in suits at the back began to look uncomfortable.

Some types of balancing can be used with a high benefit ratio, such as dual polarity supplies. Red flags are balanced and bridged outputs; and symmetrical stages relying on "complementary" transistors. A simple corollary is cruder electronics—the worlds of Zen and SET—which generates larger harmonics, and some are odd, but mainly all low-order, and higher ones taper swiftly, as in the nature. This isn't to say SE is all. Far from. But the merit is clear. Hi-Fi is after all, for music. Corollary: Electronics Engineering is in the dock for low musical intelligence, and future analog topology designers must meld linearity with natural musical harmonic structures. Otherwise, Toads, with vast popular support, are known to vigorously evict weasels.—Ben Duncan

Footnote 1: For over 30 years, hi-fi-land has shown some mental difficulty grasping that any unbalanced or floating source interfaces just fine with a true balanced input. Balancing at the sender is only relevant to large systems, long cables, many channels, and extensive toxic/noisy environments such as TV studios, venues, stadia, and hotels. Stay safe, stay away, keep your outputs unbalanced.

Footnote 2: In about 1997,Walt Jung kindly mailed me Xerox-copies of DIY amplifier projects with high symmetry, appearing in early 1970s Audio—a magazine that was not made available nor advertised in England (unlike The Audio Amateur). These advanced and elegant-looking topologies took over a decade to percolate to UK and Europe. If they ever did. These circuits depended on a complementary transistor illusion that transistor makers sold, but didn't fix.

Footnote 3: In the late 1960s, I was working in a research lab and used a massive electromagnet to measure charge mobility—electrons or the absence of electrons, called "holes"—in semiconductors.—John Atkinson

Footnote 4: Bryston in Canada—see the author's High Performance Audio Power Amplifiers (1996, Newnes), section 4.3.4, Triple sub-topology. Many other topologies (including symmetrical ones) are also shown and analyzed.

Footnote 5: Cyril Bateman, Electronics World, series on ultra-low distortion measurement of components and circuits, 2002-3.

Footnote 6: P. Tharma (Mullard Application Labs), Transistor Audio Amplifiers, Iliffe, London 1971, Section 9.4, Acoustic Roughness (around 1967). Wigan's work on harmonic weighting (BBC, 1960) is also cited.

Footnote 7: Belcher, Alan, "A New Distortion Measurement Using Comb Filtering," Wireless World, May 1978. Also BBC engineering report, Dec 1976.

Footnote 8: The anthropomorphic Mr. Toad (as vast landowner, stern industrial magnate, and extreme hi-fi fanatic) has appeared in the author's articles since 1984, and in wider literature about English society for over a century, a more recent example being The Wind in the Pylons of 1993.

CG's picture

Very interesting reading.

There's far too much to comment upon here. Not the time or place... (Not to correct, but to ask for expansion. More of this would be great!)

One comment I will make. As Mr. Duncan pointed out, IM products tend to rise and fall over real time as the relative phases of the contributing signal components change. Other effects, such as thermal changes, also create different distortion patterns over time. This is why averaging the spectrum can be misleading.

If you look at the distortion spectrum over time with a more or less real time spectrum analyzer, you can see these peaks changing. The distortion products when averaged look like bumps in the noise floor. But, if you zoom in, they're more like fields of grass swaying in the wind. This isn't unique to audio systems - you can see the same thing with various RF systems as well.

What you really hear in an audio system are the peaks of each distortion product as they change over time, not so much the averaged signal power. Listening to plain old noise is a constant change of spectral content and levels. You can hear that.

One immediate burning question - why do these design guys disappear into the aether so often?

John Atkinson's picture
CG wrote:
One immediate burning question - why do these design guys disappear into the aether so often?

From an email I received from Ben Duncan this afternoon: "I am gathering quite a list of Electronic Engineers who have been broken by the mental ordeal of designing and putting perfectionist products into production."

John Atkinson
Technical Editor, Stereophile

CG's picture

And, I'm not even in the audio biz.

But, the glory, heart felt appreciation of the customers, and the money makes up for it. Oh - wait...

Bogolu Haranath's picture

.... and all those 'electronic engineers who have been broken by the mental ordeal', are now listening to and designing, single ended, class-A, zero feedback, triode tube amps :-) .......

tonykaz's picture

I made an attempt at selling perfectionist audio products.

The perfectionist aspect is the consumer, not the Product.

Audiophiles are neurotic & psychotic.

Perfection is a moving target that Guru types review and approve ( this month ).

The headphone world seems to have an accessible and stable True-North relating to Sound Quality ( perhaps because there isn't a $50,000 Headphone Amplifier just being released )

Tony in Venice

Ortofan's picture

... for the mill:





John Atkinson's picture
Ortofan wrote:

From that article: "At gains of 5X and higher nothing could beat the $0.65 NE5532 except by a few dB in noise performance."

The NE5532 is the dual version of the NE5534, an op-amp that, as described by Ben Duncan on this article's "The Pink Floyd Connection" page, started life as the high-audio-quality, metal-can Mullard TDA1034. I first encountered the NE5534 in the phono stage of the Meridian 101 preamplifier and used it in a microphone preamplifier circuit I designed in the early 1980s.

Also from that article: "The older TL072, despite being used in thousands of audio devices, is way out of its league against newer op amps."

The Quad 34 preamp (or was it the 44) used TL072s. I asked Peter Walker why, when even then there were better-performing op-amp chips available? He felt that saving his customers money outweighed any measured advantage of higher-performance chips.

John Atkinson
Technical Editor, Stereophile

CG's picture
What most people measure is but a subset of the actual operating performance of devices. That's true whether it's an opamp or an amplifier in a snazzy box. There are practical limits of what a for-profit company can and will do. Really dedicated amateurs might measure more if they had the resources, which they almost always do not.

In addition, those for-profit companies have budgets for every bill-of-material used in each product. They all cut costs where they think they can, unless they can find a good reason not to. Usually, that comes down to the marketing of a product and what the market is willing to pay.

That's not a knock on these companies. For every potential customer who might say, "That's great! Exactly what I want! The price is OK, too. Sold!", there's probably 873 who will consider the price first and foremost. (That could go either direction - Inexpensive might be a negative in many cases.) I don't envy the decisions and trade-offs they have to make.

What Ben Duncan's article highlights to me is that in general, we really don't fully understand what makes a sound system sound good. There are products with relatively high distortion compared to others that are beloved based on listening. Others that would seem to be great based on the traditional measurements, but don't sound as good. If you dig deeply enough, you can usually discover why one product sounds more appealing than another. It might not be obvious, and the conclusion will almost certainly be contentious. But, you can often find it.

So, who is going to do that? They don't give out many grants for hifi performance research these days. Magazine reviewers and their editors? I don't think they have the resources. One resource being time. I also don't envy the reviewers or editors for the choices they need to make...

Ortofan's picture

... to sing the praises of the 5532/5534. His 4-part article in EE Times, entitled “Op amps in small signal audio design”, includes a survey of various devices.
He does suggest, though, that the LM4562 might finally be an improved replacement for the 5532.
Likewise, the OPA2134 is suggested as an upgrade for the TL072.

The QUAD 44 preamp used many TL071 op amps – the single version of the TL072.

One advantage of the single 5534 is that the compensation pins are brought out, which enables the possibility of bypassing the input stage for applications where lower input bias current is needed.
An old Siliconix FET databook includes an application note illustrating such a configuration. The normal input pins are connected to the negative supply rail and the bipolar input stage is replaced with a cascode connected pair of 2N5912 dual FETs. The balance and compensation pins then become inputs to the second stage differential transistor pair.
While the 2N5912 no longer seems to be readily available, the Linear Systems LS5911 is a substitute.

One wonders if it ever occurred to Peter Walker to offer a “deluxe” version of his preamp with better performance, from the use of higher performance devices, at a commensurately higher price?
He might have responded that the existing design already performed as a “straight wire with gain” and asked what more than that did you need?

invaderzim's picture

"This has left the world with a legacy of smeared, mangled recordings"

It is so sad that many of us will never know what these performers really sounded like. The true performers from when our equipment just wasn't up to the task of capturing their sound.
Paintings can be restored, manuscripts can be repaired and duplicated but we can't clean or recreate the sound and have it be correct.

CG's picture

Perhaps - some day.

A bigger concern might be the care, or lack thereof, the keepers of the master tapes have shown over the decades. A lot has been thrown out and a lot has been lost to fire. There's some indications that the recording companies have invested a bit more to keeping the recordings - the ones they make their money from, which is crazy if you think about it - stored better. But, an incredible amount has been plain lost forever. I'm sure that has been the case with written documents and other forms of art, but the record companies haven't even been willing to protect their own assets.

CG's picture

I wrote a considered reply to a comment above. Nothing remotely derogatory, inflammatory, or impolite. No bad words.

Gone! It was saved, as shown by the blank text box.

Oh well. This may be a message from the cosmos that posting comments on the internet is not for me.

John Atkinson's picture
CG wrote:
I wrote a considered reply to a comment above. Nothing remotely derogatory, inflammatory, or impolite. No bad words. Gone! It was saved, as shown by the blank text box.

I don't know why the text disappeared but I inserted it from the email I was sent alerting me that there had been a response to one of my postings.

John Atkinson
Technical Editor, Stereophile

CG's picture


I still think the cosmos is trying to tell me something though...

tonykaz's picture

it happens, from time to time.

You can go back a page, in your computer, to retrieve it and re-submit.

It only happens in Stereophile Comments.

Tony in Venice

sdunbar's picture

As an Applications Engineer for Texas Instruments, I can tell you that with advancements in process (smaller die) and packaging (smaller packaged devices) there is very little reason to use dual or quad packaged op-amps in pro-audio applications.

Yes, one can still save some money using dual/quad packages, but the need to use these packages to save space is relaxed.

The OPA1641 that is now already 10 years old and available in a small VSSOP package could be a good start. Other very capable devices are in smaller packages, such as SOT-23. There are literally 100's of devices to choose from, and there is no need to use exactly the same op-amp throughout an entire signal chain. Part of the discretion of design is to use the right part in the right place. Some areas maybe you can cut corners if you know what you're doing, others you cannot.

Linearity is of course the key to good reproduction. I have yet to see a widely accepted definition of a test for characterizing this parameter. Something based on a comb of multiple, yet non-harmonically related tones and then measuring the residual harmonic and IM distortion and expressed as an RMS value might be a good start. It would be necessary to reduce the tone amplitudes so that the peak/average ratio of the combination is still within the boundaries of the op-amp. But clearly there is plenty to argue about here.

CG's picture

There is this:


A test like this lets you examine the odd and even order IMD products. The fundamentals aren't eliminated, though. That probably could be handled through software trickery.

I'd think that an even more complete test would be to weight the levels of the applied test tones spectrally, so as to simulate some facsimile of the music/sound track/spoken word power distribution. There's higher levels in the lower frequencies than at 20 KHz. Why not shape the test signal? At the analysis end, you could also potentially use A-weighting to approximate the human's ear's sensitivity.

In addition, using some kind of peak hold measurement so that the maximum value in each FFT bin is saved would be desirable. Although averaging helps you look at very low levels because of the randomness of true noise, it also misses distortion peaks that occur when the relative phases of the distortion tones line up to add. You'd hear these peaks, but an averaging scheme would, well, average them them out to be less representative of the reproduced sound.

As for whether it would be widely accepted, I'd doubt it. You can't get anybody to agree on anything. Don't you agree? :8^)

sdunbar's picture

If one knows where the fundamentals are (because the test gear is generating them) and there are no FFT leakage artifacts, it should be simple matter to arithmetically subtract out the fundamentals, leaving the HD and IMD products behind.

Notice how far they have backed-off the magnitude of each tone? No doubt to manage the crest factor/peak-to-average ratio. I would think there are diminishing returns to adding more and more tones; you can argue that more tones better represents real audio, but at the same time you still need to be able to accurately extract the fundamentals and resolve the FFT.

Real audio tends to have lower amplitude content at higher frequencies so using the same amplitude for every tone is probably not the right thing to do, either.

All of this needs to boil down to a single "number" that can be used for comparison purposes. So, RMS of residual.

Surprised that IEEE hasn't standardized something like this decades ago. In a sense, audio is such a commodity now it really SHOULD have industry standards. Since audio is so dependent on linearity, seems like an obvious parameter to quantify.

But your right... it's hopeless.

CG's picture

In the CATV world they've long relied on multi-tone testing to determine the linearity of their systems. That analog testing has been done for at least 40 years. There is CTB - Composite Triple Beat - for odd order IMD products and CSO - Composite Second Order - for even order products. Plus SNR. Three numbers, but people understand what they mean. Today, that's gone away a bit, since the digital TV channels are QAM modulation, so the distortion artifacts are a little different than they were for plain ole analog TV channels. But, the new tests are similar.

In ye olde analog telephone days, the multiple analog channels were frequency division multiplexed so that you could get more than one phone call per line. In fact, they used groups that would let them get hundreds. There, the testing was done with white noise, with some small frequency bands notched out. During testing the noise power in the notched-out bands was measured and compared to the overall power, giving a single performance number called NPR - Noise Power Ratio. IMD of white noise is more white noise, so this gave them a sophisticated way to measure not only the distortion of a group of channels, but also the basic noise level. They pretty much stopped using analog phone channels in the 80's sometime. They were performance testing way before that.

I don't know what the answer is. Perhaps somebody ought to just put a stake in the ground for some sort of test and see how it progresses. The problem then becomes, just what can we hear and what is bad for sound reproduction?

sdunbar's picture

It’s impossible to argue with someone else about THEIR hearing ability. Certainly doesn’t work with my elderly mother-in-law living with us.

I'm the first to admit that my own ears are probably not that attuned. While my own mother had a degree in music and was a piano teacher, I completely squandered that opportunity... nevertheless, I did listen to quite a bit of audio growing up.

Not to be an "HLO", but my general assumption is that if someone/anyone can hear some difference, then that difference can be measured since we can build test equipment that is far more sensitive/perfect than our ears. This article seems to support that assertion.

What’s aggravating is when such a claim is made, but then we’re just expected to take the listener’s word for it- show me on an analyzer, please.

This is further obfuscated when certain distortion products are considered "pleasing" to some people. It's fine if you like the sound of second harmonic distortion, for instance, but from a recording and reproduction perspective, it's still distortion.

CG's picture

Two things...

One is that being even a virtuoso on an instrument, or many, is no guaranty that you're a good listener. My wife went to a fancy music school, played with and was tutored by several recognizable musicians there, and so on and so on. She tells me that most serious musicians aren't concerned with fidelity when they are listening. They've certainly listened to a lot of lousy sounding music in bad halls, just as they've listened to great sound. For them, they hear the musical intent in their head. They fill in the blanks. So, don't feel badly about that part.

(Fun fact: After getting critical, umm, "compliments" from their teachers, most of the voice majors would go sing to themselves in the porcelain lined rest rooms, just to make themselves feel better. Most everybody sounded glorious in there due to the natural reverb. Not sure if the tuba players did that. The pianists certainly didn't.)

Second thing is that just because it's probably possible that we could measure whatever distortion if we only tried, that's not what's done. Test equipment is good in that it often measures one characteristic at a time. But, to do that, it often trades off sensitivity for some of the bigger picture. A good example is the use of averaging and very narrow FFT bins to be able to capture teeny tiny harmonic products. As you pointed out, that not only doesn't exercise the DUT with regard to crest factor considerations, but the distortion products that are subject to the moving peaks are deliberately ignored. Can your hearing system do that?

sdunbar's picture

Certainly this has been discussed ad nausium elsewhere, but whether or not something "sounds good" or "sounds xxx" where xxx is your favorite audio related adjective, will always be a subjective matter.

For it to be objective, it would have to be time and space invariant, and since we cannot transplant ears, etc... from one person to another, it will remain subjective. Even the most attuned, demonstrably astute, refined listener is still engaging in an experience that by its very nature is subjective, and their descriptions of the experience must then be so as well.

Everyone also has their own tastes that can be tempered only so much, and it just becomes more subjective.

There is nothing wrong with this; that's just how it is. There are things about an audio signal chain that may be objective. Linearity performance, cross channel contamination (hence the article), noise floor, power consumption, etc... all of these things can be objectively compared through experiments that can be reproduced at will.

While nothing can be "perfect" we can certainly compare two specimens to see which is better in any particular dimension. My fear is, and this is probably why so many of these audio electronics engineers have vanished, is that despite desirable and measurable improvements in signal chains, there's still some "audiophile" who just doesn't like how it sounds.

When it become irrational, it's no longer fun for the engineer.

CG's picture

I think that when it gets in the way of making a living, that detracts from the fun in a big way.

Audio is not my livelihood, so that's easy for me to say.

All that said, I think there's still more that could be measured in a way that would help give better definition of what is better. Obviously, we're not there yet.

For me, it's far easier to make adjustments and then measure through listening. Jumping to the end is probably not right, but it's hobby.

Herb Reichert's picture

And bravo (!) Ben Duncan for such a broad, knowledgeable, cultured view of some very complex (and thorny) topics.

I am envious of your skills.


Ortofan's picture

... "Hard-Line Subjectivists"?
By not doing so, is he revealing his own biases?

Worth reading is the referenced letter from Bob Orban (split over two pages):

John Atkinson's picture
Ortofan wrote:
Worth reading is the referenced letter from Bob Orban (split over two pages): https://www.stereophile.com/content/scientists-ivsi-audiophiles-1999-more-letters . . .

Also in the referenced article, a superbly written letter from the late Harvey Rosenberg: www.stereophile.com/content/scientists-ivsi-audiophiles-1999-more-letters-part-4.

John Atkinson
Technical Editor, Stereophile

CG's picture

Dr. Gizmo sure was great!

I'm not sure that digital can't sound great, but an awful lot of recordings that were digitized were done in a hurry, with not much care as to the sound. Or, maybe worse - some, ahh, creative engineer (probably coked up to keep going at 3AM in order to get the releases out quickly) decided to add his or her own special interpretation of the original artists' and producers' work.

Ortofan's picture

... the "Digital Pact" neglected to ensure that the agreement included the recipe for second harmonic sauce.

Maybe Dick Burwen and Bob Carver should collaborate on a device to add transient and background noise into digital recordings to make them sound more like analog.