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
bernardperu's picture
Why would you read Stereophile anyway?

JhonnyR

 

If Stereophile has the principle of not doing DBT and for you doing so is a matter of principle why oh earth would you even care to read Stereophile? Your statement above suggests you have very extensive knowledge of the content of Stereophile's articles. So you keep reading stereophile's reviews knowing they are irremediably flawed?? How perverse is this?

 

The real question lies on your double blind tested hatred. 

 

Whatever your answer is I will have to assume that your hatred is a broken record and it will just go on.

GeorgeHolland's picture
He probably reads them as I

He probably reads them as I do for the laughs.smiley

JohnnyR's picture
You Didn't Comment..........

......on your error filled post above so why should I comment on your assumptions? If you wish to worship Fearless Leader then by all means bow down and do so. I myself prefer to ask questions and be skeptical about his "methods" and the reason "why" he doesn't test ALL items under review.

John Atkinson's picture
You're Welcome

Andreasmaaan wrote:
It's a pity that some proponents of DBT as the only valid methodology have used the comments thread here to launch personal attacks against JA.

Indeed. I am not sure why I have become a lightning rod for these audio skeptics. Perhaps it is because it is a matter of religious belief on their part - having had extensive experience of blind testing, I rejected it and thus became a heretic in their eyes. Or perhaps it is just our courtesy in allowing them the space on this website to express themselves.

Andreasmaaan wrote:
JA does measure every piece of gear his reviewers review - a practice which ensures that the opinions of the reviewers are grounded in objective data, or otherwise as the case may be.

As my reviewers do not see the measurements until after they have written the review, I take my hat off to them.

Andreasmaaan wrote:
To cut what is risking becoming a lengthy expression of indignation short: thank-you JA for a wise and thought-provoking read.

You're welcome and my thanks to everyone else who appreciated the preprint. It was an honor to have been invited by the Audio Engineering Society to give this lecture - it's not often that you get the opportunity to look back over a 4-decade career!

A Happy New Year to everyone who surfs this site.

John Atkinson

Editor, Stereophile

JohnnyR's picture
Ahhhhhhhhhh Typical Atkinson Answers

You ignore the errors and just plow along blissfully gobbling up the unwarranted praise.

"JA does measure every piece of gear his reviewers review"

As I pointed out above, along with other errors on the part of Andreasmaaan,you do NOT test every piece of gear your reviewers review, but then you pretended that you do in your response. Only certain items are tested by you. Please get your own facts straight if that's even possible for you to do so.

Regadude's picture
Johnny brand speakers

Johnny, I wish you a great new year 2013, in which you sell 10 times more of your home made speakers than in 2012. 

Since you sold zero of your speakers in 2012, you need to sell 10 X 0=0 speakers in 2013 to meet this objective. Maybe you can hire George Holland as your director of marketing and sales... 

Get cracking, you have only got 364 days left to sell your quota...

JohnnyR's picture
Happy 2013 Raghead

[flames deleted by John Atkinson]

Unlike yourself Raghead, I will be DESIGNING and BUILDING my own speakers and learning. You do know that word "learning"? Hmmmm maybe you stopped after age 10. You on the other hand will be on here *yawning* and acting the fool and spending your money on Stereophile reviewed crapola.yes

Regadude's picture
So only your speakers are good?

So according to you Johnny, everything Stereophile reviews is crapola?! There is not one good piece of equipment that has been reviewed by Stereophile? Hundreds of reviews, not one good product?

But you, in your basement, build quality speakers... 

I have challenged you before (as well as George) to name your gear. What kind of gear merits the Johnny seal of approval? 

As always, you always ignore my challenge (George chickened out to). It seems you can only complain, but cannot even provide a list of your gear. 

I guess ever seeing pictures of your "famous" speakers is also out of the question. 

All talk and no action. 

Regadude's picture
Johnny the pussy

So Johnny, where are your speakers pics and specs?

In your imagination.... [flame deleted by JA]

GeorgeHolland's picture
Johnny never offered to show

Johnny never offered to show you anything other than his backside which I think is approriate considering the childish actions of yourself.What does it matter what our audio components are? Anything we mention would be ridiculed by yourself no doubt. Sorry but I'm not playing your silly game. Good God grow up and act like a man for once will you? angry

John Atkinson's picture
Getting Facts Straight

JohnnyR wrote:
As I pointed out above . . . you do NOT test every piece of gear your reviewers review, but then you pretended that you do in your response. Only certain items are tested by you. Please get your own facts straight if that's even possible for you to do so.

Out of morbid curiosity, I looked at the past 15 issues of Stereophile - we published 98 full reviews and follow-up reviews. Of those 98 reviews, 83 were accompanied with a full set of measurements, or 84.5%. So even if not every review includes measurements, the vast majority do, which supports Andreasmaaan's point.

I have explained to you before, in other comment threads, why we do not publish measurements of analog playback components, which is due to lack of resources. I have also explained to you why we do not publish measurements of tweaks, which is that it it is difficult to determine exactly what to measure. I don't see any reason to reopen those discussions, but that you continue to be fixated on these two issues and continue to raise them is sad.

And, as I have warned you, JohnnyR, I am continuing to delete comments from you, and others, that are nothing more than flames aimed at other readers.

John Atkinson

Editor, Stereophile

JohnnyR's picture
Get Out Your Dictionary

"Reviews" means just that, any item reviewed by your staff. If you think "tweaks" don't qualify then they should be labled as "Opinions" not a review.angle All of those full reviews you speak of were what speakers, amps, preamps, cd transposrts and the like?

Yeah I know why you don't test tweaks, that would involve you using those horrible SBT or DBT. That's a really nice system you have set up. "We don't believe in DBTs or do them or can't afford to do them"......"We don't know what to test on the tweaks"..........add the two up and you get "We don't have to show that tweaks actually do anything other than what our "reviewer" subjectivly alludes to in their sighted biased listening using their golden ears"......EXCUSES Mr Atkinson plain and simple. no

Once more I ask you to read and comment about this link:

http://forums.audioholics.com/forums/loudspeakers/83412-diy-loudspeakers...

You seem to think DIY can't do anything right in your previous post in answer to Raghead. Insulting intelligent people that actually build better speakers than the status quo can produce is beyond being pompus in your case.THAT is what's SAD.

Don't you have some real work to be doing instead of being on the forums? I thought that was Ariel's job. Let him earn his pay.

ChrisS's picture
Circle Game

JRusskie,

Look up "fixation" and "perseveration" in your dictionary. "Absolute truths" regarding audio products seems to exist only in your basement and in your head.

John Atkinson's picture
What?

JohnnyR wrote:
John Atkinson wrote:
I looked at the past 15 issues of Stereophile - we published 98 full reviews and follow-up reviews. Of those 98 reviews, 83 were accompanied with a full set of measurements, or 84.5%. So even if not every review includes measurements, the vast majority do, which supports Andreasmaaan's point.

"Reviews" means just that, any item reviewed by your staff. If you think "tweaks" don't qualify then they should be labled as "Opinions" not a review.angle

I am not sure what you mean. You were objecting to Andreasmaan's statement that Stereophile accompanies its reviews with measurements as not being true. I offered the analysis above to show that he was correct.

JohnnyR wrote:
Once more I ask you to read and comment about this link:

http://forums.audioholics.com/forums/loudspeakers/83412-diy-loudspeakers...

You need to consult with your fellow traveler GeorgeHolland, who criticized me about linking to other sites :-)

JohnnyR wrote:
You seem to think DIY can't do anything right in your previous post in answer to [Regadude.] Insulting intelligent people that actually build better speakers than the status quo can produce is beyond being pompus in your case.THAT is what's SAD.

With respect, you are arguing with the voices in your head here. I haven't written anything about DIY speaker designers not being able to do "anything right." What I have said is that a DIY designer, like yourself, doesn't subject his loudpeakers to the scrutiny of a disinterested marketplace, something professional designers do as a matter of course. So the question whether or not DIY designs are superior to commercial designs isn't put under independent scrutiny. It remains unsupported conjecture, as far as I am concerned.

John Atkinson

Editor, Stereophile

GeorgeHolland's picture
Mr Atkinson, you could 

Mr Atkinson, you could  compare DIY against manufactured speakers but of course you won't. Might upset those that can't make good speakers and I'm not talking about the DIY cheeky

Johnny plainly stated that "tweaks" are reviewed in your magazine and online yet they aren't tested, hence you don't test every component under review. You have a reading comprehension problem? How many components reviewed by Jason Serinus have been tested? uh huh, right.

So now you wuss out about commenting on links? That proves you had nothing to say in the first place about it so you shouldn't have opened your mouth.Cop out time for Mr Atkinson.

The only unsupported conjecture on here are your abilities to print honest reviews. Your ability to avoid testing  alot of audio components is well known by now. Very impressive spinning you do on here.wink

John Atkinson's picture
Reading Comprehension

GeorgeHolland wrote:
Mr Atkinson, you could  compare DIY against manufactured speakers but of course you won't. Might upset those that can't make good speakers and I'm not talking about the DIY cheeky

Back in the early 1990s, Stereophile's Corey Greenberg was one of the judges for a DIY loudpeaker competition in San Francisco. There were 2 winning designs and as part of the prize, those 2 speakers were subject to a full set of measurements in Stereophile, published in the March 1992 issue. The speakers' measured performance was good but not great and certainly didn't embarass professional designers.

GeorgeHolland wrote:
Johnny plainly stated that "tweaks" are reviewed in your magazine and online yet they aren't tested, hence you don't test every component under review. You have a reading comprehension problem?

I don't believe so. JohnnyR was wrong, in that we very rarely publish full reviews of tweak products. Of the 98 reviews published in the magazine January 2012 through March 2013, there was just one review of a "tweak," Robert Deutsch on the HiFi Tuning Supreme Fuses in May 2012. ( I don't count the AudioQuest power strip reviewed by Kal Rubinson's review in December 2012 as Kal didn't make any comment on its sound quality, only on its utllity.)

GeorgeHolland wrote:
How many components reviewed by Jason Serinus have been tested? uh huh, right.

The correct answer is none. But that doesn't support JohnnyR's case either, as Jason's reviews are published on the website Secrets of Home Theater and High Fidelity, which has no connection with Stereophile. I hardly believe I am obliged to support reviews of products in competing publications with measurements in Stereophile.

John Atkinson

Editor, Stereophile

JohnnyR's picture
Wow Nostalgia Time Kids

"Back in the early 1990s, Stereophile's Corey Greenberg was one of the judges for a DIY loudpeaker competition in San Francisco. There were 2 winning designs and as part of the prize, those 2 speakers were subject to a full set of measurements in Stereophile, published in the March 1992 issue. The speakers' measured performance was good but not great and certainly didn't embarass professional designers."

Golly "only" 20 years ago and as we all know, nothing new has happened since then to make speaker design any better (Insert BIG sarcasm face here) cheeky Talk about a lame example Atkinson.

Yeah yeah, "full reviews" whatever, Plenty of "little revews" though all the freakin time and all they are , are OPINIONS but you know what? You let your reviewers get away with murdering the intergrity of the audio world with their stupid banter about  "blacker backgrounds,lifted veils and HUGE improvements in the sound" without them having to justify their claims, Pretty slick there pal.

Oh and Jason Serinus "reviews" as posted about the recent RMAF isn't connected to Stereophile? Give us all a break, Your EXCUSES are even lamer this time than usual.

Yep typical Fearless Leader goobley gook. Did you major in Universisty in the double speak of Orwell's 1984? If not then you are gulity of plagiarizing the concept.

John Atkinson's picture
Not making sense

JohnnyR wrote:
Golly "only" 20 years ago and as we all know, nothing new has happened since then to make speaker design any better...

Again you are arguing with the voices in your head, JohnnyR. Of course a lot has happened in the past 20 years, especially the advent of low-cost measuring equipment. But you're missing my point, which is, as I wrote earlier today, that a DIY designer doesn't subject his loudpeakers to the scrutiny of a disinterested marketplace, something professional designers do as a matter of course. You have repeatedy claimed that your speakers are as good as if not better than commercial designs. However, unlike engineers like Kevin Voecks, Paul Barton, Richard Vandersteen, Jeff Joseph, etc, your ability to pay your mortgage and feed your family doesn't depend on your skill as a speaker designer. Their's does, and it makes a difference.

Look, if you seriously believe your speaker designs are fully competitve with commercial designs, email me your measurements. I'll get back to you with my interpretation of what they mean, assuming they are as comprehensive as what I publish with every Stereophile speaker review. Put up or shut up.

JohnnyR wrote:
Yeah yeah, "full reviews" whatever, Plenty of "little revews" though all the freakin time and all they are , are OPINIONS but you know what? You let your reviewers get away with murdering the intergrity of the audio world with their stupid banter about  "blacker backgrounds,lifted veils and HUGE improvements in the sound" without them having to justify their claim...

You're not making sense. "Andreasmaaan" was discussing reviews in the magazine being accompanied by measurements. I was discussing reviews in the magazine being accompanied by measurements. You seem to be discussing something that exists only in your mind. And unlike Professor Dumbledore's quote, that doesn't mean it's real :-)

JohnnyR wrote:
Oh and Jason Serinus "reviews" as posted about the recent RMAF isn't connected to Stereophile?

That was a show report. No-one other than yourself equates a magazine's coverage of a show with its formal equipment reviews. Are you seriously suggesting that magazines shouldn't publish show reports without including a full set of measurement data for each room they report on? Really? There isn't an audio magazine or website that does that.

John Atkinson

Editor, Stereophile

JohnnyR's picture
BLAH BLAH BLAH

Go have another pink drink Fearless Leader.

Me let YOU interpet my frequency response? Don't make me LAUGH. You can't even interpept Wilson't ragged ass frequency response laugh

[usual flames deleted by John Atkinson]

John Atkinson's picture
Making my point

JohnnyR wrote:
John Atkinson wrote:

Look, if you seriously believe your speaker designs are fully competitve
with commercial designs, email me your measurements. I'll get back to
you with my interpretation of what they mean, assuming they are as
comprehensive as what I publish with every Stereophile speaker review.

Me let YOU interpet my frequency response? Don't make me LAUGH.

You make my point for me, JohnnyR. It was a serious offer on my part. Professional loudspeaker designers send out their products for review and for the public scrutiny of possible customers. Amateur speaker designers, such as yourself, are often afraid to let anyone else judge their work.

John Atkinson

Editor, Stereophilee

Regadude's picture
There you go

I have been challenging Johnny tp provide pictures and or the specs of his speakers. He has never met my challenge.

Now the editor of Stereophile offers to look over his data. Johnny again makes up excuses to avoid providing any information on his heavenly speakers.

Big talker...

bernardperu's picture
Contamination

Dear Jon,

What should have been an enlightening discussion of your great lecture has been contaminated by ill-conceived comments. Some individuals have posted comments that have created an environment of nastiness. 

I would seriously urge you to start deleting ill-conceived comments from now on, whether they use flamable language or not. Some people are just turned on by the flames, but others aren't. And those others would much rather focus on the joy of engaging in the music and exchanges related to it.

You do not run a state-owned site but a private one that is likely to become a lot less lucrative (in all senses) if such nasty comments are to become visible.

I am not even pointing out names and we all know who I am talking about.

I am just giving you feedback that comes from an avid reader of Stereophile online. 

Finally, the correction of my use of English was mentioned by one of these individuals. Please allow me to say that English is my second language and it is oxidized. Sorry about that. 

Thanks!

 

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