On Rational Selection--Part II…

By Bryan Geyer

In Part I, I listed the most important specifications that should be reviewed when selecting an appropriate power amplifier. These truly critical parameters included…

            …size and appearance.

            …input impedance (Zin).

            …output impedance (Zout).

            …power output capability.

            …voltage gain; i.e., input sensitivity.

Now let’s look at some other specs that merit attention. These aren’t quite so vital as the essential considerations that were listed in Part I, where appropriate compatibility was the main objective. Instead, these specs reflect basic quality-related issues, such as…

SIGNAL-TO-NOISE (s/n) RATIO: Freedom from hum will be highly dependent on power supply design and grounding issues. Freedom from noise will generally trace to circuit design and component selection variables. It’s desirable to exhibit a wide spread (e.g., a difference of 100dB or more) between the value of a given output signal (the modern reference is 1Vrms) and the residual detected when there’s no input signal (input shorted). This represents the amplifier’s innate signal-to-noise (s/n) ratio. It’s commonly measured through a filter (it’s “A-weighted”) to emphasize audible frequencies rather than ambient hum or response beyond the audible spectrum. If the consequent noise reading is 10µVrms, the related s/n would be -100dB relative to a 1Vrms reference. A fine quality solid-state power amplifier of current design will meet -110dB (re 1Vrms) s/n ratio. A 90dB s/n ratio is certainly not state-of-the-art, but it’s relatively acceptable. A s/n ratio that dips to -80dB (0.1mVrms re. 1Vrms) means that you’ll start to hear some annoying background hiss from your loudspeakers, dependent on their efficiency. Of course, all vacuum tube power amplifiers produce poor signal-to-noise ratios—it’s an inherent limitation.

QUIESCENT DC OFFSET VOLTAGE: Well, it’s just nuts to neglect checking your power amplifier for DC offset. It’s easily measured with almost any DC voltmeter, and it’s vital that offset be minimized. Do it! Check this column for background info.: https://classicalcandor.blogspot.com/2019/10/on-power-amplifier-dc-offset.html

FREQUENCY RESPONSE, DISTORTION, & DAMPING FACTOR: These specs were once of vital concern in expressing the efficacy of a power amplifier. That’s no longer the case today because virtually every solid-state power amplifier exhibits near-perfection with respect to its innate frequency response and distortion performance. Ditto for damping factor, as that parameter is directly related to a solid-state amplifier’s ultra-low output impedance; refer Part I of this paper. As a consequence, you can safely skip these three specifications when ranking the merits of solid-state power amplifiers. Any divergence will be too small to justify analysis. Conversely, there are operating characteristics that you should research, such as…

INPUT OPTIONS: If it’s your intent to apply balanced interconnects, make certain that the required XLR sockets are provided. I don’t personally recommend the use of XLR balanced interconnects in a normal home stereo installation (refer “On Noise, Coax, and Control” at https://classicalcandor.blogspot.com/2020/08/on-noise-coax-and-control.html); however, you might want that option. Balanced in/out connections are of great benefit when operating in noisy environs, but typically present no measurable advantage in benign home environs. XLR plugs and sockets derive from an archaic vintage (see https://classicalcandor.blogspot.com/2019/10/on-equipment-interface-options.html), so they’re grossly oversized and generally awkward to accommodate.

Most modern power amplifiers now offer both XLR (balanced) and RCA-type (unbalanced) inputs, although some older units might lack the XLR option. Confirm (view back panel photo) that you’ll get what you want. To the best of my knowledge, there’s just one power amplifier on the commercial market that provides XLR balanced stereo inputs but NO unbalanced (RCA-type) stereo inputs, and that’s the Benchmark AHB2. They offer short RCA-to-XLR adapter cables to accommodate users that require unbalanced input compatibility.

POWER LINE PROVISIONS: Verify that any power amplifier of interest will be compatible with the power line provisions available at the site where the amplifier is to be installed. This implies more than merely confirming that the AC supply voltage and power line frequency are compatible—it should also encompass the AC current drain (AC line load) compatibility. Refer “On Assuring Adequate AC Power” at https://classicalcandor.blogspot.com/2020/05/audio-tech-talk.html.

TURN-ON and TURN-OFF PROVISIONS: While it’s always possible to turn your power amplifier on/off manually, via a switch that’s wired in series with the power supply, there are potentially more convenient options to consider. Here are some popular automatic control options…

(1)  Turn on/off via a low voltage (12-15V) DC control signal: This is a popular turn-on option. It’s commonly offered on many modern power amplifiers and on other peripheral components.

(2)  Turn on/off via signal sensing: The amp senses an audio input, then turns on. The turn on sensitivity setting is generally variable, via a back-panel switch or pot, so that background noise won’t be sufficient to falsely trigger operation. The off function is generally based on a time delay; i.e., when no input signal is detected for a specified period of time (e.g., 5 minutes) the amplifier then turns off.

In either case, it’s helpful to have a power amplifier that connects the input signal path only after a momentary (relay-controlled) sequential delay. That permits the initial DC power-up surge to “settle out” first, so that there’s no audible turn-on “thump”; i.e, no sound output until the amplifier’s internal DC power supply has stabilized.

IN THE FUTURE: As you consider these issues, do bear in mind that the days of a stand-alone stereo power amplifier look limited. The current design trend is to fully integrate the power amplifier with the loudspeaker system. Future audio systems will reflect that preference, especially when listeners learn to appreciate that…

…it’s more accurate to use active—rather than passive—crossover networks to feed the drivers in a speaker system with their intended passbands.

…low bass frequencies (those < 60 Hz) can be more accurately produced by separate self-powered subwoofers than by an integrated driver that must also handle all of the mid-bass frequencies (to 600 Hz) as well. The best top quality speaker systems will no longer include multiple large diameter woofers because we’ve learned that the classic all-in-one-box approach is an inferior and outmoded way to implement a full spectrum sound system.

BG (February 2021)

No comments:

Post a Comment

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 Jon 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 simpleminded, 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 Arcam CDS50 CSD/SACD CD player, Goldpoint SA4 Passive Preamp, Legacy Audio PowerBloc2 amplifier, and a pair of Legacy Audio Focus SE loudspeakers. 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 cell phone. 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.

William (Bill) Heck, Contributing Reviewer

Among my early childhood memories are those of listening to my mother playing records (some even 78 rpm ones!) of both classical music and jazz tunes. I suppose that her love of music was transmitted genetically, and my interest was sustained by years of playing in rock bands – until I realized that this was no way to make a living. The interest in classical music was rekindled in grad school when the university FM station serving as background music for studying happened to play the Brahms First Symphony. As the work came to an end, it struck me forcibly that this was the most beautiful thing I had ever heard, and from that point on, I never looked back. This revelation was to the detriment of my studies, as I subsequently spent way too much time simply listening, but music has remained a significant part of my life. These days, although I still can tell a trumpet from a bassoon and a quarter note from a treble clef, I have to admit that I remain a nonexpert. But I do love music in general and classical music in particular, and I enjoy sharing both information and opinions about it.

The audiophile bug bit about the same time that I returned to that classical music. I’ve gone through plenty of equipment, brands from Audio Research to Yamaha, and the best of it has opened new audio insights. Along the way, I reviewed components, and occasionally recordings, for The $ensible Sound magazine. Recently I’ve rebuilt--I prefer to say reinvigorated--my audio system, with a Sangean FM HD tuner and (for the moment) an ancient Toshiba multi-format disk player serving as a transport, both feeding a NAD C 658 streaming preamp/DAC, which in turn connects to a Legacy Powerbloc2 amplifier driving my trusty Waveform Mach Solo speakers, supplemented by a Hsu Research ULS 15 Mk II subwoofer.

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

Contact Information

Readers with polite, courteous, helpful letters may send them to classicalcandor@gmail.com

Readers with impolite, discourteous, bitchy, whining, complaining, nasty, mean-spirited, unhelpful letters may send them to classicalcandor@recycle.bin.

"Their Master's Voice" by Michael Sowa

"Their Master's Voice" by Michael Sowa