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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