Why Audio Amplifiers Can Sound Different

This is a topic that is unlikely to be resolved to everyone’s satisfaction anytime soon. And to whatever extent there are real differences in the sound of different amplifiers, it’s very possible that those variations in sound are not from the obvious visual cues that subliminally influence our opinions, like seeing big banks of output devices nestled snugly in a massive cast aluminum heatsink farm. That’s just “got” to sound better than that pedestrian featherweight mass-market receiver, right?

Maybe yes and maybe no.

Do All Amplifiers Sound the Same?

I have had the pleasure and privilege for the past many decades of working alongside some of the most talented and creative people you can imagine in the audio business.  People who routinely reject conventional wisdom as being, well, not-so-wise. People who look at things, whether a design consideration, an engineering approach, a sales/marketing strategy, etc., from a fresh perspective.

I’ve also had the distinct privilege and learning opportunity of working with people who have been singularly unimaginative, predictable and clichéd in their approach to virtually every product development and sales/marketing situation, which has resulted in some of the most spectacular failures imaginable, even when some of those products looked like sure-bet, “can’t-possibly-miss” affairs. They missed. They flopped. They set the company back a mile, or in some cases, absolutely ruined them.

The point is, you learn a ton from observing people at both ends of the spectrum.

Without question, one of the most talented, innovative, practical, intelligent, clever, sane, clear-thinking, analytical, and most importantly, the most uproariously funny person I’ve ever worked with is the Senior Electrical Engineer at Atlantic Technology, Paul Ceurvels.  (By the way, although his primary role was that of EE, Paul happens to be an expert in mechanical engineering, acoustics and materials. This unprecedented wide range of expertise gives Paul an overridingly authoritative engineering perspective to draw upon during the design and evaluation stage that is unequalled by anyone else I’ve encountered. Just don’t ask him about the history of the Phillips screw or he’ll keep you there for an hour.)

I worked with Paul for about a decade in the early 2000s. In addition to doing straight design work, Paul would also evaluate and tweak incoming samples from overseas vendors.

There are virtually no companies that offer powered subwoofers who actually make their own subwoofer amplifiers. Those are provided by overseas (mostly Chinese) vendors. The U.S. brand goes to them and says, “We need a 300-watt plate amp with these features, these knobs and controls and we don’t want to pay more than $X.”

300W BASH plate amplifier. Can we get these with a high pass filter, two channels of PEQ, and a notch filter at 50Hz for $25 apiece???

The vendor sends in a sample for the US brand to evaluate. This is where the U.S. company’s engineering skill really comes into play, because as sent by the vendor, most of these things are unusable. Unusable. They have bad turn-on/turn-off thumps (no muting circuitry). The distortion limiters, if there are any to begin with, don’t work worth a darn. The crossover control? The printed numbers have only a passing relation to the actual frequency. A silk-screened “80Hz” is just as likely to be an actual 40 or 120Hz. The RCA inputs are flimsy and break off the PC board if you look at them the wrong way. Heatsinking? Oh, you wanted heatsinking?

You had to virtually re-design the amplifier, both mechanically and electrically. Every change you made, the overseas vendor would say, “Sure, we can do that. It’ll cost you…” Pretty soon the amp you’d budgeted $X for in the preliminary cost estimate is now $X + $24. Figure the usual cost-to-retail multiplier of 4 or 5, and now your killer $499 sub is $599 or $649. Not so good after all. These are the real-world headaches that drive companies crazy.

See: An Insider's View to Product Development Part I and Part II

Paul was the guy at AT who could re-work a shoddy vendor sample with cost-effective mods and fixes and do it in such a way that it was actually manufacturable by the vendor. He categorized his fixes into two groups: must-haves and luxury mods. Must-haves were just that: without these, the product wouldn’t function up to our brand standards. Luxury mods were in the really-nice-to-have-but-ok-we-can live-without-it-if we-must category.

No one in my long experience was better at this than Paul. No one.

Yet he is so down to earth and unpretentious. Paul would alternate during his lunch hour between downloading data sheets on the latest digital ICs “just for fun to see what those sons of b’s were up to,” to watching old cartoons from the ‘50s and ‘60s. He rode his Honda Gold Wing to work virtually all the time except for the coldest NE winter days. On those days, he’d drive his ‘95 Saturn, which he kept running perfectly as it approached 300k miles by one only-Paul fix after another. “Why not?” he’d say. Why not indeed.

This is one of the great characters of our industry. Looks-wise, if you can imagine “Henry Kloss-on-a-bike,” you wouldn’t be too far off.

So with that as a background, it’s not really any surprise that Paul has developed an intriguing, remarkably accurate and reliable way to predict an amplifier’s sound character based on a simple test that he developed.

In Paul’s own words:

Thanks for the opportunity to write on a subject I find interesting. At the end, see if you can guess which model amps I’m talking about.

Actually, this is a longer story than you'd think, dating back to the early 1980s.  My employer had made a big splash with an inexpensive low-powered integrated amp from a small offbeat company, and they wanted to bring out a larger version.  After all, can’t leave well enough alone; gotta increase those sales, right? However, the golden-eared  honchos said prototypes of the new one sounded too forward, verging on harshness.

I was hired just a short while previous to this, and my responsibilities included checking performance of new prototypes developed at the main lab located elsewhere.

Both of these amps had user bypassable sub- and supersonic filters in their main amps, and the geekophiles ears blamed these filters, although the amps measured identically in the audible range (same ± 0.25 dB, 20–20k) whether the filters were on or off.  This time, they were right (amazing!), as it worked out.

We had long been aware that tone control functionality could be quickly checked by observing square-wave behavior (leading edge spike means treble boost, "flat-top" bulged up means bass boost, opposites mean roll-off).  Part of my testing on the new amp included this 1kHz square-wave check.

I noticed a slight, rounded overshoot, maybe 4-5%, in the filtered output that was absent from the bypassed signal; this got me curious.  I asked the Head Engineer about this, but he saw no problem, as the amp measured fine for him.  He had no use for square-waves.

Marantz's 2013 HDAM module versus the 2014 update on the scope.

Notice the purple and green traces are a perfect square wave while the blue trace exhibits overshoot.

I had a fellow working with me who had spent time at AR (among other labs), and he mentioned that they were developing one of the first mainframe-based FFT analyzers, just around the corner.  The AR guy in charge of this project (Bob Berkowitz, I think) was kind enough to run our amp through his Frankensteinian lashup, and we found that the filtered signal showed a 2dB rise across the 20-20k band when given an impulse signal, while the bypass mode's FFT was flat.  Nowadays, a rise as gradual as 2dB across the entire 10-octave range would be called "spectral tilt.”

The filter was redesigned a bit, the overshoot was greatly reduced, and everyone was happy.  The Head Engineer wasn't totally convinced, but he went along with it anyway.

Since then, we've never failed to make this check, and on rare occasion, we have degraded sine-wave performance by maybe 0.2–0.3dB at the very top end in order to get the overshoot right.  From time to time, a certain amount of overshoot is left in, just to make the sound more, um, “interesting.”  And no, I'll not say how much.  However, whatever we do in this regard is both very careful and intentional. So far, nobody has called us on it for having “bad”-sounding amps. Actually, just the opposite.

During the past several years, I've worked on a few subwoofer amps, most of these have had user-adjustable low-pass filters.  Like most small companies, we don't design them from scratch, but instead we take a manufacturer's submitted sample and tweak it; sometimes a little, sometimes a lot.  Initially, these were tested with sine waves only because they weren't considered as audiophile material.

One such amp measured great—flat up to the lowpass cutoff frequency, then a sharp rolloff to prevent voice leakage.  But when we listened to it, the overshoot from the filter made it sound as if there were a hand-held school bell being rung somewhere ("Good morning, children!").  So now, sub amps are given the same square wave test, but at 50 and 100Hz also.  Multi-stage filters are tested individually.

You see, the rounded overshoot itself approximates half of a sine wave, whose relation to the input signal is that it occurs at every half-cycle.  If the overshoot's shape is that of something within the audible range, it can be easily heard as such; if it's outside the audible range, it is perceived by some listeners as a generalized unpleasant presence.  When it’s bad enough, it becomes "grit" riding on the music—you know what I mean.

Now of course, it isn't only misaligned filters which cause overshoot; some amp designs just do it all by themselves, particularly when pushed extremely hard.  But they can generally be fixed also, with a bit of judicious tweaking.  There are a number of good amp designers who could (and do) write volumes about this, so I won't.  My point is that this is a matter that is easily checked, which we do as a matter of course now. And so, we can rectify any problems.

I've only written about leading-edge overshoot, because this can be such an annoying thing.  Of course there's leading-edge undershoot also, and distortions of the square-wave's "flat-top" which represent low-frequency misbehaviors. However, a good designer will generally have already addressed these as part of the sine-wave frequency response testing, and so these should normally be non-issues.

Before I forget, one more thing—ADCs and DACs.  In the early days of digital audio, these overshot and rang horribly, and were primarily responsible for the rotten "CD sound" we all hated.  Thankfully, oversampling and other tricks have reduced this to acceptable (to me) limits.  Please note that I didn't say "perfect", but it's OK by me.  And I still don't love MP3s, but I do listen to them.

In a similar vein, switch-mode amps and their output filters have similar characteristics, and have a tendency to ring.  Higher switching speeds and better matching of the filter to the load has helped a lot.  Perfect, no; better, yes.  If you want a kilowatt in the palm of your hand for pennies a watt, you've got to compromise a little; that's reality.  Sorry folks, no silver bullet. If you want cleaner sound, stick with linear (A/B), at least for now.

Oh, the amps—the NAD 3020 and 3140.

The NAD 3140 stereo amplifier.

Addition by Peter Tribeman of Atlantic Technology (another practical, down-to-earth guy):

Paul is one of the best analytical guys I know in AV electronics and I credit him with isolating and identifying a number of issues with our stuff and of course NAD's when we were there back in the day.

This square wave test is easily demonstrable. Even with my well-worn “senior” hearing, I can detect the differences when an amp is misbehaving in this area. Very few people know about this and even less pay attention to it when they hear about it. The audible differences are not subtle.

I honestly believe that if Gene digs into this with a real world test, and a listening panel confirms it, it could be a breakthrough.

One final story: Years ago at NAD we were dealing with a Japanese OEM who happened to be building both for us and Adcom. The factory had a listening room and we asked what they thought of the NAD Power Envelope design (one of our first models that used it). They said that compared to the Adcom our amp was "sharper—more detailed." The Adcom, to their way of listening, was more "mellow" or smoother. I knew right away what they were hearing: our overshoot with 1k square waves. We had lost the battle with London (NAD corporate headquarters) on that amp, since corporate paid little attention to what Paul was saying.

Going forward, since Paul was now being asked by London to debug all new amps shipped to Boston, I asked him to quietly slip in his modification that would take care of the square-wave issue, innocently buried along with his list of other suggested component value changes. It worked. We never again (at least while Paul and I were at NAD) had an NAD amp that sounded "sharp."

Lonnie Vaughn VP/CTO of Emotiva Audio:

Amplifiers do sound different for a number of reasons.  Paul cites a good one, the square wave response.  On a personal note, I believe the power supply plays a big part in the way an amp sounds.  I get that the power supply accounts for the biggest part of an amplifiers cost.  But so many companies take it down to the bare minimum required to meet the specs that there is no headroom in the system at all and the amp itself just sounds flat as a board.  In these cases, all the designer had to do was put in a few dollars more in storage and it would have made it a completely different unit. 

For more info on this topic, see Trading Amplifier Quality for Features: A New Trend in AV Receivers?

To Hear Or Not To Hear?

So, there you have it—a very interesting take on one aspect of the causes of the audibility differences among amplifiers, as documented by the experiences of two well-known and highly-regarded industry veterans. Another log on the fire, so to speak, but one that’s definitely worth giving full consideration.

Based on these consistent, repeatable, scientifically-valid experiences, it seems logical to conclude that there are some tangible reasons why amplifiers can and do sound different, even when operating well below their distortion limits.  Obviously, the average user is not going to have the opportunity to observe their amplifier’s square wave behavior and see if there is a correlation between its sound character and its square wave overshoot/undershoot performance on an oscilloscope.  Nonetheless, this phenomenon could explain a lot of the reported stark differences in amplifier sound that has previously gone unexplained. Maybe the golden ears at the tweak magazines have really been hearing something all along—they just couldn’t identify it or measure it. Maybe this is it.

What do you think? Have you heard differences in amplifiers?  Share your experiences on our forum.

Acknowledgements

We'd like to thank Paul Ceurvels, Senior Electrical Engineer at Atlantic Technology for his contributions to this article.