Originally published March 4th, 2013

Introduction

Whether or not amplifiers sound different is a subject of ongoing controversy. There are sensible, well-informed enthusiasts and magazine reviewers who will swear to their many, obvious differences, differences that are—to them, anyway—almost as obvious and important as the differences between speakers.

There are also just as many well-informed enthusiasts and reviewers who say that there are no meaningful differences in the sound of two properly-functioning, properly-performing amplifiers operating within their undistorted performance envelope. And there is a third contingent that opines that the in-situ application of different amplifiers with different speakers and other associated equipment and connectors can produce differences in the sound even though nothing in the system is misbehaving per se.

We’ll open up the discussion, put forward some factual information, relate a few anecdotal experiences and look forward to your responses.

Rod Elliott of Elliott Sound Products (aka. ESP) has written a great article, called Amplifier Sound and we suggest reading it in its entirety. We've excerpted some important passages here and we’ll use those for the basis of our discussion.

Please also watch our recently added YouTube video where Gene and Hugo discuss amplifiers and debate whether or not can sound different and the reasons behind it.

Do All Amplifiers Sound The Same?



[Elliott]

The sound of an amplifier is one of those ethereal things that seems to defy description. I will attempt to cover the influences I know about, and describe the effects as best I can. This is largely hypothesis on my part, since there are so many influences that, although present and audible, are almost impossible to quantify. Especially in combination, some of the effects will make one amp sound better, and another worse. I doubt that I will be able to even think of all the possibilities, but this article might help some of you a little, at least to decipher some of the possibilities.

I don't claim to have all the answers. This entire topic is subject to considerable interpretation, and I will try very hard to be completely objective.

The Components of Sound

When people talk about the sound of an amplifier, there are many different terms used. For a typical (high quality) amplifier, the sound may be described as "smeared", or having "air" or "authoritative" bass. These terms - although describing a listener's experience - have no direct meaning in electrical terms.

Electrically, we can discuss distortion, frequency response, phase shift, current capability, input/output impedance, and numerous other known phenomena. I don't have any real idea as to how we can directly link these to the common terms used by reviewers and listeners.

Some writers have claimed that all amplifiers actually sound the same, and to some extent (comparing apples with apples) this is "proven" in double-blind listening tests.





[Audioholics]

Several years ago, noted reviewer Tom Nosaine conducted a study for Stereo Review magazine in which he attempted to compare the sound of two systems to a listening group through blind A-B testing. One system was made up of the very latest SOTA components and connectors.

The other system was comprised of very “pedestrian” equipment, such as a 1970s-vintage Heathkit power amplifier, a garden-variety CD player, etc.

Both systems were connected via a switcher to the same pair of speakers, so unit-to-unit speaker differences and speaker placement issues didn’t enter the picture at all. An acoustically-transparent but visually-opaque drape was drawn in front of the systems and the room lights were dimmed to assure no visual distraction. Nosaine was well known for his adherence to the scientific process and his rabid attention to detail, so the scenario was well executed.

His results showed that as long as neither system was pushed into distortion there was no statistically-significant preference shown for either system by the group, using a very extensive and varied selection of program material. His listening panel included novice listeners, mild enthusiasts and self-professed “golden eared” audiophiles.

It made no difference. Whether novice or golden ear, no listener or group of listeners scored any better at identifying which system was which. Supporters of the “no difference in amps’ sound” side were quick to say, “See? Told ya so.” Believers in the “amps’ sound is easily identifiable” camp were just as quick to point out the myriad aspects and conditions of the test that were faulty in their view and thus rendered Nosaine’s conclusions inaccurate. So, in the end, his test settled nothing.

[Elliott]

There are some differences that cannot be readily explained. An amp that is deemed "identical" to another in a test situation, may sound completely different in a normal listening environment. It is these differences that are the hardest to deal with, since we do not always measure some of the things that can have a big influence on the sound.

For example: It is rare that testing is done on an amplifier's clipping performance - how the amp recovers from a brief transient overload. I have stated that a hi-fi amplifier should never clip in normal usage - nice try, but it IS going to happen, and it is more common than we might think. Use a good clipping indicator on the amp, and this can be eliminated, but at what cost? It might be necessary to reduce the volume (and SPL) to a level that is much lower than you are used to, to eliminate a problem that you were unaware existed.

Different amplifiers react in different ways to these momentary overloads, where their overall performance is otherwise almost identical. I have tested IC power amps, and was dismayed by the overload recovery waveform. My faithful old 60W design measures about the same as the IC in some areas, a little better in some, a little worse in others (as one would expect).

Were these two amps compared in a double blind test (avoiding clipping), it is probable that no one would be able to tell the difference. Advance the level so that transients started clipping, and a fence post would be able to hear the difference between them. What terms would describe the sound? I have no idea. The sound might be "smeared" due to the loss of detail during the recovery time of the IC amp. Imaging might suffer as well, since much of the signal that provides directional cues would be lost for periods of time.





[Audioholics]

Yet it’s not just clipping behavior that distinguishes the sound of one amplifier from another. Many people swear that there is a fundamental difference in the overall tonal quality between amps, that, indeed, their ‘color’ or ‘character’ differs from each other.

Here’s another great anecdotal example: Many years ago we were listening to our latest albums on a very high-quality (for that time) system—a top-of-the-line Kenwood integrated amplifier (rated at 60/60 wpc RMS with vanishingly low distortion), AR-3a speakers and a Dual 1249 turntable with a Shure V15 cartridge. Highly-regarded equipment, operating well within its intended performance environment.

My friend had just purchased a Dynaco ST-120 power amplifier (60/60 wpc RMS) and he wanted to make sure it worked properly, so he brought it over. The Kenwood had pre-out/main-in jacks, so we used the Kenwood as the preamp. The speakers, turntable, cartridge, and speaker wires remained the same. The only change in the system was the power amp.

We weren’t looking to “compare” the power amps’ sound, we were only looking to confirm that the Dyna worked.

We played the original system, then we swapped in the Dyna and played the same material.

The Dyna worked, but the difference in sound character was stunning. Stunning. We raised and lowered the volume. The differences persisted at all levels. We reconnected the Kenwood and re-listened. Then we re-connected the Dyna again.

Now, remember, originally we weren’t consciously looking for differences in their sound, but it was so obvious that it just hit the two of us over the head like a ton of bricks. It was so obvious and apparent that we spent the rest of the night listening to all manner of records, first on the Kenwood, then on the Dyna, and over and over.

There might be several explanations for the differences, but they were real, without question.

Oh, just for the record, the Kenwood (a more modern design) was much “tighter” and “more controlled” in the bass, while the Dyna was “flabby” and “loose” by comparison. Night and day.

Distortion

[Elliott]

Technically, distortion is any change that takes place to a signal as it travels from source to destination. If some of the signal "goes missing", this is distortion just as much as when additional harmonics are generated.

We tend to classify distortion in different ways - the imperfect frequency response of an amplifier is not generally referred to as distortion, but it is. Instead, we talk about frequency response, phase shift, and various other parameters, but in reality they are all a form of distortion.

The bottom line is that amplifiers all suffer from some degree of distortion, but if two amplifiers were to be compared that had no distortion at all, they must (by definition) be identical in both measured and perceived sound.

Naturally, there is no such thing as a perfect amplifier, but there are quite a few that come perilously close, at least within the audible frequency range. What I shall attempt to do is look at the differences that do exist, and try to determine what effect these differences have on the perceived "sonic quality" of different amplifiers. I will not be the first to try to unravel this mystery, and I doubt that I will be the last. I also doubt that I will succeed, in the sense that success in this particular area would only be achieved if everyone agreed that I was right - and of that there is not a chance! (However, one lives in hope.)

In this article I use the somewhat outdated term "solid state" to differentiate between valve amps (“tubes”), and those built using bipolar transistors, MOSFETs or other non-vacuum tube devices.

Clipping Distortion

How can one amplifier's clipping distortion sound different from that of another? Most of the hi-fi fraternity will tend to think that clipping is undesirable in any form at any time. While this is undeniably true, many of the amps used in a typical high end setup will be found to be clipping during normal programme sessions. I'm not referring to gross overload - this is quite unmistakable and invariably sounds awful - regardless of the amplifier.

There are subtle differences between the way amplifiers clip, that can make a very great impact on the sound. Valve amps are the most respectable of all, having a "soft" clipping characteristic which is comparatively unobtrusive. Low feedback Class-A amplifiers are next, with slightly more "edge", but otherwise are usually free from any really nasty additions to the overall sound.

Then there are the myriad of Class-AB discrete amps. Most of these (but by no means all) are reasonably well behaved, and while the clipping is "hard" it does not have significant overhang - this is to say that once the output signal is lower than the supply voltage again it just carries on as normal. This is the ideal case - when any amp clips, it should add no more nastiness to the sound than is absolutely necessary. Clipping refers to the fact that when the instantaneous value of output signal attempts to exceed the amplifier's power supply voltage, it simply stops, because it cannot be greater than the supply. We know it must stop, but what is of interest is how it stops, and what the amplifier does in the brief period during and immediately after the clipping has occurred.





Figure 1 - Comparison of Basic Clipping Waveforms

In Figure 1, you can see the different clipping waveforms I am referring to, with "A" being representative of typical push-pull valve amps, "B" is the waveform from a conventional discrete Class-AB solid state amp, and "C" shows the overhang that is typical of some IC power amps as well as quite a few discrete designs. This is a most insidious behaviour for an amp, because while the supply is "stuck" to the power rail, any signal that might have been present in the programme material is lost, and a 100Hz (or 120Hz) component is added if the clipping + "stuck to rail" period lasts long enough. This comes from the power supply, and is only avoidable by using a regulated supply or batteries. Neither of these is cheap to implement, and they are rarely found in amplifier designs.

Although Figure 1 shows the signal as a sinewave for ease of identification, in a real music signal it will be a sharp transient that will clip, and if the amp behaves itself, this will be (or should be) more or less inaudible. Should it stick to the supply rail, the resulting description of the effect is unlikely to accurately describe the actual problem, but describe what it has done to the sound - from that listener's perspective. A simple clipped transient should not be audible in isolation, but will have an overall effect on the sound quality. Again, the description of this is unlikely to indicate that the amp was clipping, and regrettably few amps have clipping indicators so most of the time we simply don't know it is happening.

[Audioholics]

The possible reasons that amplifiers had specific sound characteristics were becoming a popular topic among audiophiles in the early 1970’s. There was a 1973 meeting of the Boston Audio Society that featured a presentation by a test equipment maker (I forget which) that could display the spectral characteristics of an amplifier’s THD. This was pretty heady stuff in the early 1970s. A very popular receiver at the time, the Marantz 2270, was well-known for sounding harsh if pushed too hard. At this BAS meeting, we found out why:

Even though its actual distortion rating was low (probably 0.3% THD @ rated output), the scope/analyzer showed that when it hit its clipping point, the Marantz exhibited large amounts of higher-order distortion products.

As we know, harmonic distortion is the unintended signal products generated by an audio device such as a speaker or amplifier that are whole number multiples of the original signal. For example, if an audio device is tasked with trying to reproduce a 40 Hz signal, and instead produces 40 Hz and a small amount of 80 Hz, the 80 Hz product is called harmonic distortion. Small amounts close in multiples (lower-order) to the original signal are barely audible; larger amounts of distortion in greater multiples away from the original signal (higher-order) are grossly objectionable to the human ear. (Remember the harmonic structure of Western music is based on octaves and 3rds, so 2nd and 3rd-order distortion is harmonically-related to the music in a way that we do not find dissonant or audibly offensive. Lower-order THD has to be pretty significant before we notice it in a negative way.) The sum total of all harmonic distortion products is usually expressed as a percentage of the original signal, or % Total Harmonic Distortion (% THD).

FFT Distortion Analysis of Emotiva UPA-7 Amplifier at Near Clipping - notice the higher level of harmonic distortion products (ie. 2kHz, 3kHz, etc)



On the scope, we could clearly see that the Marantz produced large amounts of 4th-, 5th- and 6th-order harmonics when it was pushed into clipping. A Pioneer receiver there for comparison was better behaved and its distortion was composed mostly of 4th-order and lower components. An AR receiver showed by far the best distortion behavior, with most of its harmonics being very benign-sounding 2nd- and 3rd-order products. Now, finally, there was a concrete, repeatable, tangible explanation for why some amplifiers sounded better than others when pushed into clipping.