…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.
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)