On Power Amplifier Listening Trials…

A common opinion among qualified audio experts (restated here by me) is that…
All modern audio power amplifiers that present high input impedance (≥ 20 kΩ), very low output impedance (≤ 0.2 Ω), flat power response (20 Hz–20 kHz ± 0.5 dB at rated output), ultra-low distortion (≤ 0.1% THD at rated output), and a virtually inaudible noise floor will sound exactly the same when operated at matched (± 0.25 dB) levels, not clipped, and properly connected to the same load.

Do appreciate that amplifiers meeting the above specifications all sound the same because they share a single common attribute: They are highly accurate. A power amplifier that is accurate is one that faithfully replicates the original input source signal without making any change, loss, or addition. Given this measured assurance of accuracy, why do so many hi-end audiophiles conduct listening trials to detect the audible difference between power amplifiers? Well, here’s my opinion on…

Why listening trials persist, and grief to avoid when conducting a listening trial…

1—Irrelevance: To an overwhelming extent, today’s audiophiles are non-technical subjectivists*. They have no interest (and little understanding) of the measurements that define audio performance, and they don’t expressly seek accuracy. The audiophiles’ primary goal is to identify equipment that renders euphonic sound; i.e., sound that’s subjectively pleasing, and vacuum tube power amps provide more range of choice. This accrues because…
(a) THD at rated output runs > an order of magnitude higher for a tube-type power amp than on a solid-state equivalent, and that difference might be audible.
(b) And because transformer-coupled tube amps can’t approach the optimal low impedance drive of solid-state power amps, where Zout is typically ≤ 0.1Ω. The tube amp’s higher Zout will directly interact with the shifting Zin presented by the loudspeaker/crossover load because both values will then be of similar order. This will color the sound; the speaker’s sonic signature will differ slightly from its natural voice. The result could be deemed either pleasing or displeasing—a subjective option. Of course, the change will reflect degraded source accuracy, but accurate sound was never the audiophile objective.

2—Expectation: Do appreciate that it’s instinctive to assume that different amplifiers will sound different. Indeed, for a great many decades (from the 1949 origin of “hi-fi” until the early 1980s) most power amplifiers did sound different, and their sound was materially affected by load impedance. Of course, few of those power amplifiers met the stringent specifications cited in the opening paragraph, so they were not highly accurate amplifiers, although some might approach that goal when paired with a favorable load. When sampling amplifiers today, some listeners still expect to hear such differences, and some are certain to fulfill their expectation, regardless of the sound. Aural perception is a fickle and fleeting sensation, prone to uncertainty and subject to human frailty; be wary.

3—DC Offset Orphans: Since power amplifier comparisons imply the probability of high output operation, it’s helpful to know (for sure!) that your samples exhibit only nominal DC offset. So measure the amps’ initial DC offsets before running a listening trial. Don’t evaluate some random lemon with anomalous excessive offset. Be certain that your intended samples truly represent the breed. (Note: This need for offset screening applies only to direct-coupled power amps; not to transformer-coupled amps.)

4—Marginal Mavericks: If a comparison trial involves a power amplifier that is just marginally accurate, that margin might prove audible. This generally involves an amplifier with slightly out-of-spec power response, and the character of the aberration might be perceived by a sensitive listener. Lots of audiophiles express personal preference for a particular sort of biased sound (warm/detailed/liquid/analytic, et al.) that marginally differs from source reality. (In many cases they might do well to reassess their speakers’ crossover settings, rather than change their power amp.) Of course, a “maverick” is an outcast, it’s generally not representative of the breed, so another sample of the same model might not sound the same. Instrumented measurement is a useful way to isolate marginal mavericks, and testing power response (frequency response at high power output) is easily done, with basic test equipment.

5—Output Level Accuracy: All listening comparisons must be accomplished at precisely identical output levels. This is best assured by actually measuring the respective outputs at some appropriate (400-800 Hz) fixed test frequency, using a microphone or sound level meter, mounted on a stand, and fixed precisely at the intended listening position. Simply keeping the main volume dial at the same position will not assure identical output because different amplifiers exhibit different voltage gains. Most modern power amps have internal voltage gains that fall between +23 dB and +29 dB, and some amps have rear panel trim pots to independently adjust the left/right input balance. So, (a) review the technical aspects and check the gain spec; (b) inspect and adjust sample amplifiers as needed; then (c) listen as desired, but be aware that you’ll learn very little if the amplifiers are accurate (see opening).

6—Termination Variables: In the course of disconnecting and reconnecting the speaker load to the various amplifiers in a listening trial (and repeating that cycle numerous times—often as quickly as possible), it’s quite likely that there might be some variation in the uniformity of the termination. Indeed, sometimes even different connector types are used (see photo**), with different results. It’s really quite easy, in the course of this repetitive attach/detach process, to introduce some contact inequality, and that variable could cause aural disparity when conducting high power comparisons. To assure optimum signal transfer it’s vital to carefully inspect, clean, reseat, and tighten all terminations, and assure that all parts are in optimum visual alignment every time that they’re mated.

7—Room Acoustics & Comb Filtering: Aural comparison trials require that all acoustic environs be uniform. This includes a precisely fixed position within the listening space. It also means that the occupancy and every furnishing detail must be exactly the same. Even a minor change can materially affect the acoustic reflections and standing wave patterns, and these disruptions, however slight, can alter the sound. Despite every effort to comply, the small listening areas typical in most private homes can still yield appreciable (6 dB) variation over distances as small as four inches and frequencies as low as 200 Hz. This is the unfortunate consequence of comb filtering. (Refer p.101-104 of “The Audio Expert”, by Ethan Winer [Routledge, 2nd edition, 2018], ISBN 978-0-415-78884-7.)

8—Sight Before Sound: In a previous paper (refer “On Evaluating Audio Equipment…”), I cited a significant study (http://www.pnas.org/content/110/36/14580), conducted in 2013, describing the finding that visual cues convey far more impact than any audible evidence. In sum, your eyes will implant a more vivid and persistent impression than anything that you hear, and what you see will determine what you think you hear. As a consequence, any serious listening trial should be administered under blind test conditions. If you know which amplifier is playing, you will be unable to render valid aural judgement. The statistics in support of this finding are persuasive, and the evidence is undeniable. Listening trial choices that were formed prior to observing this guidance should be ignored.

9—Confirmation Bias: When you’re comparing your old power amplifier against some new and costly wonder that you’ve got on temporary loan, it’s very difficult to admit that there might be no audible difference. And in the event that you’ve already purchased that prized new model, I’d say that it’s not possible to declare that both amps sound the same. We’re not robots.

BG (November 16, 2019)

*Among audiophile organizations, the sole exception appears to be the Boston Audio Society, where most members comprehend and respect the value of technical analysis in defining product performance.

**The discrete banana plugs that are shown in the photo are a bit unusual. They have an internal free-floating slug that achieves uniform compression against the (side entry) wire without transferring any twisting strain; see…
A  TIP:  If your amplifier’s rear clearance (to wall) is especially tight, you can shorten these banana plugs (by 5mm) by discarding the knurled end posts and substituting flat point metric set screws (use M8–1.0 x 8mm) to retain the wires.

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Meet the Staff

Meet the Staff
John J. Puccio, Editor, Publisher, Reviewer

Understand, I'm just an everyday guy reacting to something I love. And I've been doing it for a very long time, my appreciation for classical music starting with the musical excerpts on The Big John and Sparkie radio show in the early Fifties and the purchase of my first recording, The 101 Strings Play the Classics, around 1956. In the late Sixties I began teaching high school English and Film Studies as well as becoming interested in hi-fi, my audio ambitions graduating me from a pair of AR-3 speakers to the Fulton J's recommended by The Stereophile's J. Gordon Holt. In the early Seventies, I began writing for a number of audio magazines, including Audio Excellence, Audio Forum, The Boston Audio Society Speaker, The American Record Guide, and from 1976 until 2008, The $ensible Sound, for which I served as Classical Music Editor.

Today, I'm retired from teaching and use a pair of bi-amped VMPS RM40s loudspeakers for my listening. In addition to writing the Classical Candor blog, I served as the Movie Review Editor for the Web site Movie Metropolis (formerly DVDTown) from 1997-2013. Music and movies. Life couldn't be better.
Karl W. Nehring, Contributing Reviewer

For more than 20 years I was the editor of The $ensible Sound magazine and a regular contributor to its classical review pages. I would not presume to present myself as some sort of expert on music, but I have a deep love for and appreciation of many types of music, "classical" especially, and have listened to thousands of recordings over the years, many of which still line the walls of my listening room (and occasionally spill onto the furniture and floor, much to the chagrin of my long-suffering wife). I have always taken the approach as a reviewer that what I am trying to do is simply to point out to readers that I have come across a recording that I have found of interest, a recording that I think they might appreciate my having pointed out to them. I suppose that sounds a bit simple-minded, but I know I appreciate reading reviews by others that do the same for me -- point out recordings that I think I might enjoy.

For readers who might be wondering about what kind of system I am using to do my listening, I should probably point out that I do a LOT of music listening and employ a variety of means to do so in a variety of environments, as I would imagine many music lovers also do. Starting at the more grandiose end of the scale, the system in which I do my most serious listening comprises an Onkyo C-7030 CD player, Legacy Audio StreamLine preamplifier, Legacy Audio PowerBloc2 amplifier, and a pair of Legacy Audio Focus SE speakers augmented by a Legacy Point One subwoofer. I also do a lot of listening while driving in my 2016 Acura RDX with its nice-sounding ELS Studio sound system through which I play CDs (the ones I especially like I rip to the Acura's hard drive so that I can listen to them whenever I want) or stream music through the system using my LG G7 ThinQ cell phone, which features surprisingly sophisticated audio circuitry. For more casual listening at home when I am not in my listening room, I often stream music through the phone into a Vizio soundbar system that has remarkably nice sound for such a diminutive physical presence. And finally, at the least grandiose end of the scale, I have an Ultimate Ears Wonderboom Bluetooth speaker for those occasions where I am somewhere by myself without a sound system but in desperate need of a musical fix. I just can't imagine life without music and I am humbly grateful for the technology that enables us to enjoy it in so many wonderful ways.
Bryan Geyer, Technical Analyst

I initially embraced classical music in 1954 when I mistuned my car radio and heard the Heifetz recording of Mendelssohn's Violin Concerto. That inspired me to board the new "hi-fi" DIY bandwagon. In 1957 I joined one of the pioneer semiconductor makers and spent the next 32 years marketing transistors and microcircuits to military contractors. Home audio DIY projects remained a personal passion until 1989 when we created our own new photography equipment company. I later (2012) revived my interest in two channel audio when we "downsized" our life and determined that mini-monitors + paired subwoofers were a great way to mate fine music with the space constraints of condo living.

Visitors that view my technical papers on this site may wonder why they appear here, rather than on a site that features audio equipment reviews. My reason is that I tried the latter, and prefer to publish for people who actually want to listen to music; not to equipment. My focus is in describing what's technically beneficial to assure that the sound of the system will accurately replicate the source input signal (i. e. exhibit high accuracy) without inordinate cost and complexity. Conversely, most of the audiophiles of today strive to achieve sound that's euphonic, i.e. be personally satisfying. In essence, audiophiles seek sound that's consistent with their desire; the music is simply a test signal.

Mission Statement

It is the goal of Classical Candor to promote the enjoyment of classical music. Other forms of music come and go--minuets, waltzes, ragtime, blues, jazz, bebop, country-western, rock-'n'-roll, heavy metal, rap, and the rest--but classical music has been around for hundreds of years and will continue to be around for hundreds more. It's no accident that every major city in the world has one or more symphony orchestras.

When I was young, I heard it said that only intellectuals could appreciate classical music, that it required dedicated concentration to appreciate. Nonsense. I'm no intellectual, and I've always loved classical music. Anyone who's ever seen and enjoyed Disney's Fantasia or a Looney Tunes cartoon playing Rossini's William Tell Overture or Liszt's Hungarian Rhapsody No. 2 can attest to the power and joy of classical music, and that's just about everybody.

So, if Classical Candor can expand one's awareness of classical music and bring more joy to one's life, more power to it. It's done its job. --John J. Puccio

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"Their Master's Voice" by Michael Sowa

"Their Master's Voice" by Michael Sowa