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
ChrisS's picture
Who's on First?...

Who's doing the listening in your tests, JRusskie? Do you know any 18 year old musicians? Oh, of course not...But you probably keep company with a bunch of construction guys (lucky you!) with damaged hearing. They should all find that there's no difference between any products.

ChrisS's picture
What's on Second? The Man of La Mancha...

Once again, comrade JRusskie, you are on your Quixotic journey down that twisty, winding path for "truth"... Being an upstanding citizen of the former-USSR, you should know about "truth", right?

DBT is SCIENCE, and SCIENCE is ALWAYS RIGHT, especially during the heyday of the Union of Soviet Socialist Republics and you've been hanging on to this idea since the dissolution of the USSR. Hanging on to something you really want to be TRUE.

Comrade JRusskie, just because you want it, doesn't make it so.

Ariel Bitran's picture
Hiya Johnny

just wanted to let you know i haven't forgotten about you.

i've been south of the equator spending time with my father and brother, but now that I'm back in the Stereophile office, i'll answer your question in full a little later.

peace out homeslice.

be nice.

Regadude's picture
Asking Johnny...

Asking Johnny to be nice... Wow, you are very optimistic Ariel! 

GeorgeHolland's picture
Ariel, how about doing your

Ariel, how about doing your job and deleting post and start placing a 30 day BAN on Regadude and ChrisS for spamming this whole article day after day with nothing less than pure drivel and taunts? I've seen kids frums better moderated than this fiasco but it's already turned into a child's forum the way they act. You would think they had some dignity but we haven't seen any yet.

Ariel Bitran's picture
My Policy

I believe the internet is pretty much open range, so I'm happy to just watch folks speak their minds using whatever manner to express themselves.

one thing that bothers me is when a member is unneccessarily rude to someone who is being respectful or even innocently posting unknowing of the snark they may receive. in those instances, I say something so new users are not discouraged to continue posting.

On the other hand, if you're being provocative with your initial comment, I can only expect others to react to your provocation. 

******

The other policy I encourage and have stated many times is as follows 

"don't feed the trolls"

******

Finally, we do delete comments from ANY member we feel is totally off-topic and insulting to an individual member, as we want the conversation to continue and bring light to the subject  at hand rather than turn into a name-calling game. That being said, I'd like to delete Regadude's comment above since it just makes fun of Johnny. Also, I probably shouldn't have typed be nice. i dont know what i was thinking. I think i just wanted a rhyme in there, and also to include a general suggestion of positivity, but deleting RegaDude's comment would delete your reply, and this is a conversation I'd like to keep around.

Regadude's picture
What?

What did I do? In response to your comment of "be nice" to Johnny, I merely joked that you were asking a lot. A lot, because Johnny is often not nice. He is that way, that is a fact. I have no responsibility for his behavior. I just pointed it out.

Don't shoot the messenger!

ChrisS's picture
"...and made them cry"

What is reliability?

Think, Georgie, think.

Ariel Bitran's picture
So I read

the two links provided earlier giving examples of some DBTs.

I found the matrixhifi test to be ignorable: who is the sample? how did they select these people? how are they representative of a population of listeners as a whole? in order to gather significance from these these tests, the first and most important step is determining your sample, sample size, and how you select your sample. this just seems like a bunch of friends having fun. also, since there were multiple components being switched at the same time, system synergies could have been the cause of the weaker sounding more 'hi-fi' system. maybe those components weren't right for each other, but the cheaper system just sounded better. at least in ABX, they only changed one piece at a time

what i found interesting in the ABX test was the user's ability to control the change of system component themselves. this helps eliminate the idea that the listener might feel like they are getting 'duped' or constantly searching/guessing for the difference.

Also regarding the ABX method, the # of times a difference was heard was 33. the # of times no difference was heard: 29. Interestingly, cables were the least discernable. 

DBT is time consuming and for signnificant results you need a large sample size (to represent a large population of listeners). With a small sample size, as in both of these tests, you risk a greatly flawed hypothesis and will lack confidence in your results. 

I don't want to whip out my textbooks, b/c i have other stuff to do, but 17 listeners is not nearly large enough of a sample size to even represent a population of 70,000 Stereophile readers (for example). Then we run into an even bigger problem of "who" is selected-->ie what type of sample you are trying to represent.

I've heard repeatedly that H/K does have a successful DBT model. I'm sure it takes them years to perform each experiment, and it is wildly expensive and time consuming. You need a large sample size for any of this stuff to matter, not a few dudes in a basement.

GeorgeHolland's picture
More excuses from

More excuses from Stereophile?  Why am I not surprised. You scoff and blow off any attempts at doing a DBT by the people I linked to yet when it comes to "reviewing" cables and amps it's suddenly okay to accept a sighted biased review as being true?  Laughable sir but then I suppose you being an emploee have to toe the line. So be it.

I give up. Go on and trust your sighted "tests" while ignoring a DBT or even a SBT. I know your boss won't let you do any. He "knows" it all. *eyeroll*

Regadude's picture
More of the same...

Hey George! Ariel provided you with a more than satisfactory answer. So why are you still complaining? You never heard the expression "agree to disagree"?

Ariel is right; sample size is crucial. Anyone who has a basic knowledge of statistics understands this. 

JohnnyR's picture
Yawn

George was talking about simply doing a DBT or SBT among friends like he linked to. Let's say you and 5 pals think the "Humungo" amp  blows away every other amp you have listened to. So you set up a simple DBT or even a SBT to see if you can RELIABLY pick which amp is which WITHOUT sighted bias. In other words using your EARS???? I have read on the forums here how much people rely upon "What I heard" yet don't trust a simple test to prove that they can.

So lets say the "Humungo" amp is just simply "liquid, lifts 400 veils and makes the back ground blacker", what ever silly terms you wish to use, If so then you should be able to pick it out 100% of the time doing a DBT. If not and lets say you only choose right 50% of the time then that's proof you were only guessing and couldn't tell which was which. So much for saying sample size is critical. If no one can pick the "Humungo" amp from the other one then where are your rationalisations for saying it's better?

I can tell though that none of you have even bothered to try a SBT or DBT so it's all a waste of time speaking to the peanut gallery. I love how much money you both have WASTED on pricey products that only look good and sound the same. Funny as hell, go on spending YOUR money I love ityes

ChrisS's picture
Everyone stand back!

JRusskie just beat himself with his own thinking...

Regadude's picture
Money spent?

 

Johnny wote:

"I can tell though that none of you have even bothered to try a SBT or DBT so it's all a waste of time speaking to the peanut gallery. I love how much money you both have WASTED on pricey products that only look good and sound the same. Funny as hell, go on spending YOUR money I love ityes"

How do you know what I've bought with my money? I will go on spending my money (but not on your plywood speakers you make in your basement), that's what it's made for!

 

JohnnyR's picture
Thnak You For Proving The Old Saying...........

.a fool and his money are soon parted.

Regadude's picture
Well...

Well I am not that much of a fool with my money. I have not yet bought any of your JohnnyR brand speakers! 

ChrisS's picture
"When the boys..."

"...came out to play,

Georgie Porgie ran away!"

Delusions of absolute truths cloaked in "science" aren't much to hide behind, eh Georgie...

I hear our Man of La Mancha, JRusskie, is looking for his Sancho Panza to accompany him on his journey in search for TRUTH.

Happy trails!

ChrisS's picture
Call Me Maybe...

Georgie, Look at all those scientists backing you up!

If you take a college level research methodology course, Georgie, you may not appear so laughable. At this point, there appears to be very little knowledge in what you say about reviewing audio products.

GeorgeHolland's picture
Here is how you review audio

Here is how you review audio products........ looks in Stereophile and buys whatever the product of the month is. Spends too much money but doesn't care. Goes online and talks like a 5 year old and spews insults and THINKS he's smart. Case closed.

Regadude's picture
Name one product...

George, name ONE SINGLE product that I have bought. Just one. You and Johnny have this little fantasy in which you think you know everything and everyone. 

How about you list your gear? If you are so good and knowledgeable about buying audio products, do share with us the products that are George Holland worthy. If you actually have any audio gear...

GeorgeHolland's picture
ZzzzzzzzzZZzzzzzzzzzzzzz

ZzzzzzzzzZZzzzzzzzzzzzzz sorry but conversing with the likes of you is boring and makes my brain hurt considering the amount of BS you spew,

ChrisS's picture
The cure

Take one course of college-level research methodology and call us in the morning...

rl1856's picture
Objectivist- Subjectivist

Do you like what you are hearing ?  If no, move on until you do.  If yes, then shut up and relax.  This is a hobby focused on the enjoyment of the creative output of artists.  It is not about how many proverbial angels can dance on the head of a pin.

Go listen to MUSIC !

hnipen's picture
Thanks John !!!!

Thanks John for a very interesting and exciting presentation, lots of interesting information here and I'm surprised, to say the least, from the lack of positive feedback.

There are many who are skeptical to some of the ways of doing measurements in Stereophile and in some ways I'm one of them too, especially the way speakers are measured so close, large array speakers like the bigger Dunlavy's and some others will not sum up very nicely in this way. We do, however, not live in a perfect world and Stereophile cannot afford an anechoic chamber, so this is probably the best they can do.

I wish John would share more of this kind of information as he has gathered lots of knowledge during a long interesting career at Stereophile and other places.

Go on John :-)

Merry Christmas

Cheers harald

absolutepitch's picture
Heyser lecture

John, thanks for getting this lecture pre-print available for us to read. I have been looking forward to this.

I agree that there is a lot fo information combined into one lecture that anyone would need a lot of time to learn and understand the details. Pardon me for paraphrasing some of your words below.

Regarding the null result of DBTs, your description of the interpretation is in agreement with what I remember from statistics classes. I might add that a statistician would include a probability value or confidence band with the interpretation (something to the effect that 'the null hypothesis of no-difference-detctable is accepted with high probability'), and equally for the case when a difference is detected with high probability. I personally think DBTs should be done for product reviews, but agree that valid DBT's are difficult and time consuming (expensive) to do correctly, as Dr. Toole has shown in his writings.

The example of the 'backwards' impulse being not agreeable to listeners is something I have noticed in reference to digital recording. It's a wave form that does not occur naturally in music production, so reproducing it should sound 'bothersome'.

I also agree with the previous post, that more articles like this would be welcomed, to further highlight how complicated this field really is.

bernardperu's picture
Congratulations, Mr Atkinson!

I have read your essay with great pleasure (all of it!) and I think it is a great example of the Liberal Arts and Science coming together. In the end, it feels like a piece of applied music philosophy, which I find fascinating. It also seems to be free of busines-oriented interests, as your opinion on cables clearly suggests. It is awesome and very unsual to meet an accomplished person who gives priority to his passions and principles over financial interests (as also expressed on your 2012 writing on the CES and Las Vegas). 

I consider myself to be an audiophile that turns off the lights and tries to connect his emotions with the music with a very relaxed mind (this seems to be a category in itself, as the un-relaxed passive listeners who cannot focus on the music on a mid to long term basis tend to be very opinionated). Having said this, I recently purchased a pair of Class D mono amps that can clearly connect me to the music (Hephaestus brand). I have not ever listened to amps which are over 15k. Within similar prices, class D seems to be the better choice (but how relative this can be, Jon!)

I will continue to follow your writings with deep admiration and I thank you for making a difference on my musical experience (which is passed on to my girlfriend and my child). 

 

Bernard

GeorgeHolland's picture
Clueless you are if you think

Clueless you are if you think Mr Atkinson is something special angle

ChrisS's picture
Georgie?

Yoda you are?

Andreasmaaan's picture
Thank-you JA

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. Personally, I found the lecture fascinating and thought-provoking, and I thought that the nuggets of personal history provided a powerful context for the thoughtful opinions expressed. For better or worse, Stereophile doesn't restrict itself to DBTs as their only reviewing tool, but 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. I'm not sure why this approach, coupled with a reliance on an income stream from advertising, seems to place JA in line for so much personal vitriol. If similar attacks were levelled at me in my professional life, I'd be mortified and enraged.

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

JohnnyR's picture
Uhhhhhhhhhhhhh Sorry But You Are..............

WRONG!

"For better or worse, Stereophile doesn't restrict itself to DBTs as their only reviewing tool, but 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"

Do you even READ the reviews? Seriously dude. Stereophile DOESN'T do DBTs at ALL! Atkinson has said repeatedly that they are "too difficult to do". Show me ONE single DBT he has done in a review please. Plus he does NOT "measure every piece of gear his reviewers review". Cables, power cords, record demagnitizers, just to name a few. You obviously have been reading another publication NOT Stereophile.

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