The 2011 Richard C. Heyser Memorial Lecture: "Where Did the Negative Frequencies Go?" Case Study 4: Amplifiers

Case Study 4: Amplifiers
To many audio engineers, the amplifier is a solved problem. Static distortion and noise levels can be restricted to well below the threshold of human hearing at all audible frequencies and at all power levels short of clipping. Yet the darned things continue to surprise by sounding different—perhaps only slightly different, and sometimes for trivial reasons, such as too high an output impedance. But over the years I have been measuring amplifiers, some things have fallen out of the cloud of measured data: factors that are shared by amplifiers that sell well to audiophiles.

First, in a post–Peak Oil world, the high efficiency of class-D amplifiers is very tempting. Yet the paradox is that a class-D amplifier that measures as well in every respect as a linear amplifier of the same power tends to be as large and as heavy!

Second, if your design's small-signal measurements are relatively stable despite changes in the output current—that is, it offers the same THD+noise percentage into 4 ohms as into 2 ohms for the same voltage—people will prefer it to an amplifier whose THD is proportional to its output current.

Third, a wide open-loop bandwidth seems preferable to a low bandwidth, perhaps simply because you can use less overall negative feedback.

Fourth, if you as a designer can use loop negative feedback to linearize the open-loop behavior, you should err on the side of too little feedback rather than too much. If the result is a linear increase in second-harmonic distortion with increasing output power, and provided you don't also introduce too much intermodulation, listeners will like the sound of your amplifier.

Fifth, given that even short lengths of speaker cables have finite impedances, there seems little point in maximizing your amplifier's damping factor.

These last three points are all related, of course. Perhaps Harold Black's negative feedback is something that, like a spice, is best used in moderation; that the more linear the circuit is without loop feedback, the more it behaves in a manner consonant with the brain's need to construct internal models. And yes, this is conjecture.

But again I'm reminded of Richard Heyser, who decades ago showed a colleague of mine a box that measured superbly on continuous tones: it had suitably low levels of harmonic and intermodulation distortion, a flat frequency response, would pass a squarewave intact, and, with pure tones, would even pass an input/output nulling test with flying colors. Yet if you played music through it, it sounded terrible. The late Peter Walker possessed the rare ability to reduce a problem to a succinct expression of its essentials. When talking about amplifier design, expressed to me his opinion that it was all "Ohm's Law and common sense," something that has stuck in my mind ever since and has proved to be true. Peter suggested a similar black box to me. Again, it passed every steady-state test of goodness, yet its effect on a music signal was immediately noticeable, even objectionable.

The Heyser box was an amplifier with a series relay controlled by a side chain that analyzed for symmetry. With symmetrical signals—test tones—the relay would stay closed. With asymmetrical signals—music—it would be continually opening and closing, if only momentarily. The Walker box was an amplifier whose gain varied with signal level; in other words, it was a compressor or expander. A steady-state measurement using a repetitive waveform allows the unit to stabilize its gain, and it thus acts as any other "perfect" amplifier. With music, however, you hear the aberration in its response.

Both Heyser and Walker mentioned the multidimensional nature of audio-component performance. However, when you make a measurement on an amplifier, you have to limit those dimensions to just the two, or possibly three, mandated by your test. The very act of making the test procedures practicable has changed the situation so much that the results may not be applicable to real-life use. Perhaps, therefore, the real issue with amplifiers is that they are designed and tested in isolation, but are actually used as part of a complex system consisting of arbitrary cables and loudspeakers on one end and arbitrary sources on the other. (Note that difference testing, where the output under actual conditions of use is compared with the input, would be very revealing. As of yet I have had no results worth publishing with this technique, though the tools are now available.)

So an amplifier's absolute performance can't be considered in isolation. You have to consider its interactions with the source component, the loudspeakers, and the cables connecting them.

First, one of my bugbears measuring amplifiers, particularly if they have single-ended inputs: The first thing I always do is to try all the different possible ground arrangements, to get the lowest noise. I try floating the Audio Precision's output ground, and/or its input ground. With a stereo amplifier, I try floating just one channel rather than both. I float the amplifier's AC cord (with care). With some components, changing a ground connection can increase the level of hum and RF noise by a factor of 10. The lowest noise may not be achieved with a typical coaxial cable. It may be necessary to run a separate ground reference wire and connect the shield to just one rather than both chassis. The system's noise level may well change, depending on whether the cable's shield is connected to the source component's ground or the load component's.

So when that amplifier is used in an owner's system, there is no knowing what the noise level of that system is. When he reports that changing the amplifier to another model or even changing a cable made an audible difference, he may just be lowering or increasing his system's noise level.

Second, here is a block diagram of an amplifier, something with which all engineers will be familiar:

It has an input on the left and an output on the right. Here is a similar diagram, this time of a feedback amplifier:

Again, it has the input on the left and the output on the right. But now there is a second input: the output terminals are the input to the negative-feedback loop. It can be argued that the cable connecting the amplifier to the speakers is actually an antenna. At audio frequencies, that antenna is connected to very low impedances, so why would this matter? But think about this: the loudspeaker may have low impedance at audio frequencies, but this may well not be so at radio frequencies. These days, we all are immersed in a bath of RF radiation—in my basement listening room, I can pick up not only our own but several of our neighbors' WiFi networks—and it might be possible that at frequencies at which it best behaves as an antenna, the cable will inject RF energy into the amplifier's feedback loop. Even a few millivolts of RF can drive a feedback amplifier into slew-rate limiting. Martin Colloms in the UK published work showing that audiophile speaker cables varied by a large degree in their efficiency as RF antennas. Some "audiophile" cables use a weave to reduce RF pickup; others use an RC network; others don't do anything. Perhaps that may be one reason cables might sound different in different systems and locations. The effect is arbitrary and therefore unpredictable. But there might be something there.

Unless your listening room or studio is enclosed in a Faraday cage, therefore, whether or not cables make a difference in the sound quality—and any difference can be a degradation as easily as an improvement, of course—is as much a function of the system as of the cable. I am beginning to believe that when listeners report wires and amplifiers as having sonic signatures, they are actually responding to small, perhaps subliminally perceived differences in their system's noisefloor, which may not always be sufficiently low in level nor truly random in nature to ensure audible transparency.

Other than that, I will pass over the thorny topic of signal cables having an effect on sound quality that is due to anything other than the usual electrical parameters of resistance, inductance, and capacitance. We could easily be here all night discussing that subject. I won't say any more about cables except to point out that, as with light beer, gasoline, and tobacco, the brand differentiation of cables is achieved primarily through advertising. That doesn't mean that there aren't also differences in sound quality, only that, as with mass-market beer, those differences can be relatively small. But does "small" necessarily equate with "inaudible" or "unimportant"?

Incidentally, this is why judging a cable's value for money by comparing its retail price with its bill of materials is misleading, as the large cost of advertising needs to be factored in. And what if there were no advertising? Decades ago—and my apologies for not remembering which brand it was—a cigarette brand decided that they could make a lot more money if they drastically cut back on their ad budget. (This was at a time when cigarette advertising was ubiquitous.) Without ad support, their market share collapsed!

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Comments
JohnnyR's picture
Sure is a Lot of............

......blind testing mentioned by a person that doesn't believe in doing them. Aren't they "too difficult" according to yourself? Wait......nevermind just don't answer that because I know it will end up with you Mr Atkinson making some EXCUSE as to why you can't do blind tests for the magazine when it comes to reviewing products. ZzzzzzzzzzZzzzzz.

John Atkinson's picture
Paul Simon to the white courtesy telephone, please

JohnnyR wrote:
Sure is a lot of blind testing mentioned by a person that doesn't believe in doing them.

A man sees what he wants to see and disregards the rest, eh? In the 15,000 words of this preprint, there are just a few paragraphs devoted to a discussion of blind testing.

JohnnyR wrote:
Aren't they "too difficult" according to yourself?

As I say, I took part in my first blind test 35 years ago and since then have been involved in well over a 100 such tests, as listener, proctor, or organizer. My opinion on their efficacy and how difficult it is to get valid results and not false negatives - ie reporting that no difference could be heard when a small but real audible difference exists - was formed as the result of that experience.

John Atkinson

Editor, Stereophile

GeorgeHolland's picture
In other words JohnnyR, Mr

In other words JohnnyR, Mr Atkinson DID find excuses not to use blind testing but found them worthwhile enough to mention them in his Heyser speech funnily enough.

JohnnyR's picture
Atkinson's Career..........

A. Join a band as a second rate bass player

B. Fail to get a record contract

C.Pretend he's an electrical engineer

D.Get an offer to ruin.....ermm I meant run Stereophile

E. Proceed to ruin........ermmmmm excuse me......run Stereophile like Hitler would.

F. Spend his time talking online instead of actually doing any real work.

G. Make excuss about his busy schedule and why he can't do DBTs

H Profit!!!

Regadude's picture
Johnny's career...

Johnny makes plywood boxes in his mother's basement, and then claims he is a speaker expert.

Trolls audio websites. 

 

That is all.

ChrisS's picture
JRusskie's trajectory ->

JRusskie seems to have become a failed audio (and DBT) expert in one easy step...

 

A. Failed...

dalethorn's picture
Hitler? I don't know whether

Hitler? I don't know whether to laugh or cry.

Ariel Bitran's picture
Cry

also, brings both of these to mind:

http://en.wikipedia.org/wiki/Reductio_ad_Hitlerum

http://en.wikipedia.org/wiki/Godwin's_law

ChrisS's picture
Whiff...

There it is, again...

ChrisS's picture
Poof...

I think a synapse just exploded...

dalethorn's picture
Blind testing

I asked a question of Harman Testing Lab in the Computer Audiophile forum, and post #11 is the answer, which I thought was perfect. i.e., let the testee control the switch, so whatever adjustment time they need they can accomodate.

http://www.computeraudiophile.com/f12-headphones-and-speakers/behind-harman-testing-lab-14112/

Other observation: The comments in this Stereophile article about the noise floating free of the recording due apparently to having a higher playback bandwith - that made my day.

On R.C. Heyser: His description of the 1971 Sylmar quake, hearing a low-frequency "tone" prior to the shaking, got him started on comparing earthquakes and woofers and low-frequency playback in rooms, and he got beat up on a lot for those writings. But having been involved with both, those observations of his were like gold in the bank. Saved me lots of money.

Regadude's picture
SRH940 vs Senn 800HD

Aren't you the dude who "tested" the Shure SRH 940 headphones and claimed they were almost as good as the 800HDs from Sennheiser?

I own the SRH940s. I was somewhat willing to believe you, but when I saw that your source was an Ipod, I just flicked your review in the trash.

You chose a 200$ source to test 325$ and 1500$ headphones?  And you believe your results mean something?

 

no

dalethorn's picture
My review of the 940, agreed

My review of the 940, agreed upon by more than 95 percent of respondents, was vetted with specific music tracks listed in the review and subsequent comments, using ipods alone, ipods with analog amps, and desktop DACs and amps. Some of the naysayers are either impatient or just grumpy and disagreeable, which the comments in this Stereophile article perfectly illustrate.

Regadude's picture
Touchy are we?

How nice of you to call people who question your methodology grumpy, disagreable, etc. I question your methodology because it makes no sense to test 325$ headphones with an ipod. It makes even less sense to test 1500$ headphones with an ipod...

I actually listened to the Shure SRH940 and HD800 today, using my ipod Nano. You are correct, they both sound almost the same. They both sound like shit!

I know from experience that these headphones really shine with a good source, and especially, proper amplification. As good as the Shure is (I own them), it is no match for the HD800 when both are hooked up to decent equipment.

Your test is like testing the acceleration of a Ferrari and a Prius in your 12' driveway. You won't get much better results from the Ferrari in such a limited distance.

As for the 95% who agree with you, what gear did they use (ipod)? Were those 95% SRH940 owners and fanboys? 

dalethorn's picture
Saying "they both sound like

Saying "they both sound like shit" when one of those is a revered $1500 headphone of great pedigree and popularity among actual audiophiles is telling of your judgement. I would suggest taking a deep breath, pick a headphone and some music tracks you like, then tune out the matrix and just enjoy your music. You'll be less grumpy that way.

Regadude's picture
Your covering your arse

Don't try and hide behind my words! They both sound like shit through an ipod! What a weasel you are. I did not say the 800HD sounded like shit... BUT IT SOUNDED LIKE SHIT THROUGH AN IPOD. You rigged your test, Dale. 

Again, you use insults...

 

dalethorn's picture
A long time ago one person on

A long time ago one person on a big audio site challenged people like you to be more specific instead of throwing words like s**t around. There were just two persons who offered test tracks with which to compare those headphones, but everyone else (like you) just threw out expletives, and a couple members even made serious threats. Now I have over 30 years experience comparing headphones like the Stax and Sennheiser series, and quite a bit of time invested with the Audio Engineering Society as well as reading Stereophile magazine, so even if nobody in the world were to agree with my conclusions, I'm certainly qualified to test and evaluate these devices. I'd suggest you refrain from purely ranting with words like s**t and offer specific testable facts, or just go away.

This article may help you:

http://dalethorn.com/Headphone_Ipod_Versus_Amp.txt

dalethorn's picture
BTW, and speaking of Prius

BTW, and speaking of Prius and Ferrari, you may be one of the better off who don't mind paying $1500 for a headphone and then shelling out $2000 for a headphone amp. But let's pretend for a moment that you have to pinch pennies to get those luxurious goods, so when you finally have them you really appreciate what you have. In this latter case, were you to stumble across an ipod touch or iphone of recent manufacture, with the new Earpods ($29 purchased variety only) being driven by the Dirac DSP player, and had an opportunity to listen at length, it might make you ill to realize what you got for your $3500.

Regadude's picture
400$

My headphone amp is a whopping 400$ (CI Audio VHP 2).

I do pinch pennies. The VHP 2 and the Shure 940 are mine.The Sennsheiser 800HD I borrowed from a friend.

Stop trying to cloud the issue with non relavant issues.

Regadude's picture
Stereophile test it!

How about we let Stereophile test both headphones! First through an ipod, then with decent equipment!

Both headphones sound similar through an ipod.

Plug them into a receiver, they sound less like shit.

Plug them into a decent headphone amp, and they both sound MUCH BETTER. BUT the Sennsheiser pulls away and wipes the floor with the Shure.

Buy yourself a CI Audio headphone amp (the one I use), or any other decent headphone amp Dale... Before buying a 2500$ DAC to install on an ipod.

DUH...........................

GeorgeHolland's picture
The continuing saga of ChrisS

The continuing saga of ChrisS being a troll and all around noncontributing poster. Classy act there ChrisS.

Mostly what I got from Mr Atkinson's article was the amount of photos of himself rather than Mr Heyser. I suppose a memorial lecture should be all about the presenter and how he sees fit to turn it into reasons why Stereophile does what it does regarding testing and the lack there of.

"Yes, what you think you are hearing might by dismissed as being imagination, but as the ghost of Professor Dumbledore says in Harry Potter and the Deathly Hallows, "Of course it's all happening in your head, Harry Potter, but why on earth should that mean that it is not real?"

Oh I see now. Stereophile's philosophy is based upon a ficticious Harry Potter character who spouts that what we hear is real.I'll be sure to let those people being treated for such voices in their heads that all is well and medication is not necessary.

"Footnote 2: For a long time, I've felt that the difference between an "objectivist" and a "subjectivist" is that the latter has had, at one time in his or her life, a mentor who could show them what to listen for. Raymond was just one of the many from whom I learned what to listen for."

Well if only that was true. Your readers tend to all be subjectivsts who aren't trained what to listen to but blindly ( I made a pun there since blind testing is not wanted at Stereophile) believe whatever they think they hear as the truth when in fact measuremnts will show that either they heard nothing related to the measurements or perhaps it's Dumbledore once more in their heads, make believe or wishful hearing.

All in all I found your presentation 90% about yourself and Stereophile and reasons why you defend subjective listening and gloss over things like cables and doing serous testing and throwing in various quotes and ambiguous stories barely related to the subect you were supposed to be talking about, unless that subject was muddling the understanding of Stereophile and telling everyone your own personal history instead of Mr Heyser's.

"Third, one well-known skeptic sitting in the audience tonight criticized my abstract a few weeks back on the grounds that I am just offering "hypotheses about stuff that might be just to stir the pot, while offering no real explanations."

Class act there Mr Atkinson, speak out and criticise someone you don't like or agree with in the audience who then has no opportunity to speak up and address the audience themselves. Very childish and immature.I already see how you do the same on here. "My way or the highway"

JohnnyR is once more sadly correct, Blind testing is not and wil not be used by Stereophile. Just because you Mr Atkinson, know how to push a few buttons to measure speakers, does not make you even close to the same league as Mr Heyser was. He furthered the cause of objectivity and made Audo magazine the best audio magizine there ever was. The Audio Critic is a very close second. Mr Aczel used to be one of the worst subjectivists around untill he admitted to being such and saw the light and spoke out against foolish subjective reviews. Testing will always be the most important part of audio magazines. Simply letting reviewers spout off about "blacker backgrounds" "lifted veils" or other such nonsense belittles both the reader and the so called reviewer not to mention the product under "test". Truth in testing is what Mr Heyser strove for not some silly review about rainbow foil, magic bowls or pebbles.

John Atkinson's picture
Correct: the lecture was not about Richard Heyser

George Holland wrote:
Mostly what I got from Mr Atkinson's article was the amount of photos of himself rather than Mr Heyser. I suppose a memorial lecture should be all about the presenter and how he sees fit to turn it into reasons why Stereophile does what it does regarding testing and the lack there of. I found your presentation 90% about yourself and Stereophile...throwing in various quotes and ambiguous stories barely related to the subect you were supposed to be talking about, unless that subject was muddling the understanding of Stereophile and telling everyone your own personal history instead of Mr Heyser's.

You have misunderstood the nature of the lecture, GeorgeHolland. The Richard C. Heyser Memorial Lectures are not intended to be _about_ Dick Heyser but instead are offered to honor his memory. The invitation for me to be the October 2011 lecturer clearly stated that I was free to choose any subject I felt appropriate; my triple career as a musician/audio engineer/audio editor and the conclusions I have formed as a result of my 4 decades' experience in those careers were felt by the AES Technical Council members to be an eminently suitable subject. 

George Holland wrote:
Class act there Mr Atkinson, speak out and criticise someone you don't like or agree with in the audience who then has no opportunity to speak up and address the audience themselves. Very childish and immature.

The skeptic in question had emailed me before the lecture to let me know he would be in the audience and would take an active part in the anticipated Q&A session. I included this mention in the preprint to give him the necessary opening. As it turned out, he didn't attend, but I saw no reason to delete the point he made.

George Holland wrote:
Just because you Mr Atkinson, know how to push a few buttons to measure speakers, does not make you even close to the same league as Mr Heyser was.

I agree. As you can tell from my discussions of Dick Heyser's writing and thoughts in the preprint, I have an enormous amount of respect for what he achieved. I have never claimed to be his equal, nor would I. But again I must emphasize that the AES Heyser lectures are not intended to be _about_ Dick Heyser. That is your misunderstanding, I am afraid.

John Atkinson

Editor, Stereophile

Rolli666's picture
Trolls do not only live under bridges

I am not sure as to why a person would even want to respond to these posts. Some problems really are better ignored as to invoke the spirit of go away.  I for one appreciated this lecture for what it is thank you.

John Atkinson's picture
You're Welcome

Rolli666 wrote:
I for one appreciated this lecture for what it is thank you.

Appreciate your comment. It is rare that a magazine writer has the opportunity to gather all his thoughts in one place and I owe a debt of thanks to the Audio Engineering Society for inviting me to do so.

John Atkinson

Editor, Stereophile

 

ChrisS's picture
Whizz, sput, sput...

Whoa! There goes another synapse! Now they're both brain dead... I duelled extensively with JRusskie re: blind testing on the Forums and I daresay, he shot himself many times with his own arguments.

Saga? Can't wait for the movie version!

Sorry, Ariel... I'll try not to let it happen again.

Dunadan's picture
The whole is more than the sum of its parts...

...which is a valuable insight, I think, from this article that Mr. Atkinson has generously preprinted. His stories--which, though I am not a longtime reader, I presume also reflect the history of Stereophile--mirror the tensions found in the philosophy of science between "reality" and "perception", and the question of whether our observations are truly disinterested or if our minds shape how and what we see (or hear). And, really, what exactly does a blind test resolve? That the reviewer likes one piece of equipment over another, presumably without factoring in aesthetics or cost? But "liking" one thing better than another is still a subjective judgment, and who's to say YOU would share that judgment? Never mind the problem of translating sounds heard by another into words on the magazine's page and then back again into the sounds heard by the reader when he or she auditions the gear. Or the issue of whether sound quality can be adequately described by a series of measurements, any more than colour can be described strictly in terms of wavelength, or consciousness in terms of vibrating atoms in your brain. That's not to say these things aren't significant, but to say that they add up to the sum of our human experience is absurd.

Thanks for a reflective and enjoyable article (at least in my mind).

GeorgeHolland's picture
The purpose of blind testing

The purpose of blind testing is soley to see if you can hear a difference between two units. After all the subjective rants of "oh this lifted several veils of crud from the liquid sound" or "I heard a definite improvement" I find it sad that Sterophile keeps saying that DBT or even SBT are not a valid way to test those claims. Either you can hear a difference or you can't yet those that claim they do shy away from proving that fact with a blind test. Afaid they might be shown to be wrong is my opinion, so Stereophile allows them to go merrily along in their own self delusions while spending way too much money on sham products.

ChrisS's picture
Do you hear what I hear?

In a properly set up DBT, let's say for two amplifiers, what exactly will the listener be listening for?

 

(And what if the lamb heard a difference, but not the shepherd boy?)

GeorgeHolland's picture
Are you seriously so dense

Are you seriously so dense that you can't figure it out for yourself? Perhaps that's why you depend upon Stereophile to tell you what to buy?

http://home.provide.net/~djcarlst/abx.htm

http://www.matrixhifi.com/contenedor_ppec_eng.htm

Read and learn.

ChrisS's picture
May I Call You Georgie?

So Georgie,

If the lamb can hear that Amp A can make the test system sing in a range of 10hz-60khz, but the shepherd boy can't hear in that range so can't tell the difference between Amp A and Amp B, which amp should the shepherd boy buy?

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