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
GeorgeHolland's picture

No you may address me as George or Mr Holland. Georie is an attempt at making fun but then again that's about all you know to do anyway.

Please ask a relevant question or just shut it.

The lamb would first have to prove they can hear a difference with a dbt

The shepard can buy whatever he wants.

The big bad wolf tried to convince both that the expensive boutique amp was the one to buy

They told him to shove it and bought a less expensive but well built amp and they all lived happily ever after except for Mr Big Bad who soon folded his shop due to no sales.

ChrisS's picture

So Georgie,

Let's make this question relevant....

Let's say we're testing two amplifiers with two listeners. The first listener is an 18 year old young lady who is trained in classical piano at a Grade 10 music conservatory level. She can hear that Amp A has an excellent range 16hz-40khz through the test system, but even though Amp B doesn't have the same range, she likes the "sound" of it better.  The second listener is the shepherd boy who's grown up now. He's 54 years old, likes big band jazz, but his hearing has been damaged by working with heavy machinery without hearing protection. He can't hear a difference between the two amps.

 

Which amplifier should the ex-shepherd buy?

GeorgeHolland's picture

The 18 year old can hear to 40KHz?  Were her parents bats?

"Like" doesn't have anything to do with blind testing. You don't pick which one you "like" you see if you can tell WHICH amp is playing. You don't even know how a dbt test is run I can see already.

 Who cares which amp they buy?  Maybe the people selling them do but that is completly irrevelant to dbt. You have no clue as to what blind testing is all about.

ChrisS's picture

Georgie,

Are you a real person or a computer generated figment from JRusskie's russian clone of an old IBM PC? Do you know anyone with normal hearing? Do you know how real people shop or do you isolate yourself in the sanctuary of your closed mind and order everything on-line after reading extensive reviews in Consumer Reports?

ChrisS's picture

You call those proper Double Blind studies?

GeorgeHolland's picture

Oh so you are an expert on double or single blind studies. The ABX system is a proven dbt way to do things. Just because the results have you so upset, you claim the people doing the testing are doing it wrong? Laughable. Tell me some more jokes.

ChrisS's picture

...Swinging down the street so fancy-free..."

 

In fact, Georgie, my major(s) for my undergraduate degree were in Developmental Psychology (including Perception) and Statistics (including Research Methodology). So yes, the set up and methodology shown in those links are crap and the results are laughable...

ChrisS's picture

Get thee to a local college and enroll in a first year research methodology course. Have fun learning!

GeorgeHolland's picture

Either address me as George or STFU you stupid little boy. I think you majored in being a twit and smart ass. Who can take anything you say as serious? Grow the fuck up already. You act out like a lil boy with the IQ of a rock.

Go tell the people who make the ABX test system what you just said and see how they laugh you out of the room. You bring nothing to this discussion other than what you don't agree with , with zero facts to back up your claims. Come on show us all how the ABX test methods aren't any good or why the blind testing done in the othe link was faulty. Better yet tell Harman Kardon that their blind testing techniques are faulty and not worth doing.

ChrisS's picture

Georgie,

You must be running out of neurons if you don't trust your own eyes and ears. Yes, the facts are out there.

JohnnyR's picture

.is what SBT and DBT are all about. Using your eyes to test audio products? Well yes I can SEE that YOU would have to look so you would know which one is "better".

ChrisS's picture

Whose ears? Why?

John Atkinson's picture

GeorgeHolland wrote:
I find it sad that Stereophile keeps saying that DBT or even SBT are not a valid way to test those claims.

Please do not put words in my mouth. That is not what I have said. What I _have_ written is that to design a blind test that limits the variables to just that which you are interrsted in and that produces valid results when there is a small but real audible difference is complicated and time-consuming. The literature is full of poorly designed and performed blind tests that have been proclaimed by audio skeptics as "proving" that there are no audible differences. Such people demonstrate both their ignorance of the Scientfiic Method and their unquesitoning faith in "Scientism."

John Atkinson

Editor, Stereophile

GeorgeHolland's picture

JohnnyR was right more EXCUSES.

It's pretty simple Mr Atkinson but then having the will or gumption to put a dbt into the line of testing is the first thing you have to have,

You obviously don't have that or just don't care so it's a moot point anyways.

JohnnyR's picture

.nor ever will when money is involved.

dalethorn's picture

I would hate to see magazines and websites like Stereophile become intimidated by naysayers who demand "proof" of everything they say, in advance or after the fact. There's a lot of that in mainstream media, due no doubt to controversial topics and false information being fed to reporters. But come on, people - this isn't a mainstream news outfit reporting life or death stories. We have here an incredibly rich article full of facts that can be researched and questioned with references that are well established over time. Instead we have people questioning the author's motives or his pursuit of truth? I think people who are looking for "The truth" should be looking in a religious forum, not a hi-fi forum. There's very little you can "prove" on these topics - the value here is the very informed opinion that costs you nothing.

Ariel Bitran's picture

nice comment Dale. as an attendee of this lecture, i can tell you it was surely enlightening.

GeorgeHolland's picture

I think you have it all backwards there friend.....

"I think people who are looking for "The truth" should be looking in a religious forum, not a hi-fi forum. There's very little you can "prove" on these topics - the value here is the very informed opinion that costs you nothing."

Religion is based upon belief and subjectivists cling to their belief, they don't go looking for proof or the scientific method. No you can't prove very much when the reviewers use subjective say so instead of actually measuring the units. Opinons are a dime a dozen or even less than that and worth next to nothing. Just look at all the opinions here.

dalethorn's picture

No, George, you don't get it. You're still stuck in religion, looking for proof of something. Here you get 'information' only, and if you want 'proof' of something, you have to do the work in proving it to yourself. What *you* believe, outside of yourself, is purely opinion. Perhaps all these people looking for truth or proof are just lazy, and trying to intimidate others into doing the work for them. Like bullies.

JohnnyR's picture

......the facts there pal. Your "arguement" is FLAT.  The only "info" Stereophile shows us is what the WANT to show us. Cables, power cords, magic bowls are off the list of even testing them in anyway what-so-ever. The reason? Ohhhhh we really don't know how to test those duhhhhh. How lame an EXCUSE is that?  Stereophile is SUPPOSED to be a magazine for information NOT excuses.

"Perhaps all these people looking for truth or proof are just lazy, and trying to intimidate others into doing the work for them. Like bullies."

 BWAHAHAHAHAH!!! that has to be on my "Top 10 WTF Things of 2012"

 You DIDN'T just say that did you???? So let me get this straight, Stereophiles job as you see it is to just fling out "say so" and it's up to the readers to wade through the muck and mire of those reviews to try and grasp one little bit of truth? Yeah right, you must really love it then because they rarely show any truth at all.

Ariel Bitran's picture

why is it so hard to accept that double-blind listening tests are difficult to achieve as JA has explained in his lecture?

the fact that our existences are commandeered by individual perception based on thousands of variables makes it very easy for me to understand, just as how one person may enjoy spicy foods but not grapefruit or where some may hear too much bass and others not enough. so many VARIABLES!!! culture, upbringing, what sounds you are surrounded by, traffic signals, your genetic structure, your actual physical position when listening. perception is a learned skill that we do not choose to accept, it just happens and it is different for every single person.

i think THESE are the sort of differences between individuals that make DBT difficult: everyone hears differently. there is no absolute sound.

the best example of how an ear and sonic preference can change is in the study of language and sounds. the chinese language has a completely different set of sounds to that of the english language, thus their speaking intonation, laughter, and music reflect their cultural and sonic inclinations. eastern and western and andean and greek and celtic and ... and ... all use completely different scales based on their preferences of sound learned over time through language and their environments.

Thus, i often wonder do hi-fi listeners across the globe prefer different sounding systems based on their installed sonic memory? or is there a constant in terms of preference across the globe? probably not. or even more interestingly, can one find similarities in preferences in sound based on linguistic sounds of an individual region? are the frequencies accented in the german language more easily noticed by a german in his hi-fi? DBTs are a waste of time. instead of focusing why not, it is much more fun to focus on the why.

the heart of all of this lies within JA's question: where do the negative frequencies go? there are aspects to our perception of sound that simply cannot be measured because they are based on individual perception which is different for every single one of us.

GeorgeHolland's picture

If blind testing is so difficult then how did the people that I linked to manage to do so?  Harman Kardon does blind testing at the drop of a hat. Go ask them how they do it so easily. Mr Atkinson's refusal to do so is simply an excuse as to not have to bring up why cables, power cords and other snake oil is indeed snake oil. He can merrily go along his way as he has for years now ignoring such products and letting his reviewers say whatever BS they want about the sham products and not have to worry one bit. He just doesn't care is the bottom line.

ChrisS's picture

Georgie Porgie,

That you cite these links as authoritative sources indicates the level of your understanding of testing methodology.

JohnnyR's picture

......nothing to back up whatever it is you are trying to say but it is amusing.

ChrisS's picture

Research methodology courses are taught in colleges and universities all over the world, even Russia...

Let us know when you and Georgie take one.

JohnnyR's picture

Nothing to cite other than your own wandering silly posts? Thought so.

Regadude's picture

Well little Johnny, at least Chris is A REAL psychologist. He's not a, you know, a hobbyist like yourself...

JohnnyR's picture

........credentials from ChrisSy just say so. Oh lets see I think I'll be a nuclear scientist now just because I say I am on the forums. There now it's a done deal. Besides how he acts out is more like a 3 year old than an adult. Some professional he is and tell us all again just what your expertise is? Trolling perhaps?

Regadude's picture

...credentials from Johnny the hobbyist speaker designer. Let's see some pictures of your Johnny brand speakers! Post some pics, or provide a link to a site where we can see these speakers of yours.

I demand to see these speakers of yours! 

ChrisS's picture

If JRusskie has a misguided and limited understanding of DBT, and....

Georgie has a misguided and limited understanding of DBT, then....

Are JRusskie and Georgie one and the same person?

Has anyone seen them in the same room together? Hmmmm.

Please, one of you (I guess it doesn't matter which...) ask Harman Kardon how they do their DBT's and how they use the results.

Thank you.

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