A solid-state power amplifier’s sonic character is almost entirely defined by the intervening physical and emotional filters that separate the listener’s aural perception from the amplifier’s output. Notable intervening screens include the loudspeaker system’s crossover network, the fidelity and character of the drivers , the acoustical nuance of the listening environs, and the shifting vagaries of your own personal mood. Most especially, the final sound will be the result of preconceived expectation; i.e., what your mind says you will hear. These compound and complex factors will cloud the aural linkage between you and the output of that new power amplifier. Any impression or conclusion that you extract will forever remain the product of this imprecise and subjective exposure. In essence, this is why listening is not an accurate or reliable way to appraise audio equipment. Do seek more rational options.
(1) Check for published reviews on the product. Focus on the factual test data + photos, and the related technical commentary. Skip over the author’s tiresome aural assessment; it’s pretentious nonsense. The Audioholics website is generally more useful than most because they excel on the tech stuff and minimize such fluff as “has expansive soundstaging, explosive dynamics, and exceptional transparency”, or claims that “it produces natural transient attacks, a generous, almost tube-like sustain, and take-your-breath-away decays that produce the sensation of floating on a cloud.” (These latter phrases were as extracted from an on-line Stereophile product review.)
(2) Examine a photo of the amplifier’s back panel; it’s often available on the maker’s website, or in a published review. (Interior photos sometimes turn up as well. They can be informative too, but only if you already know a lot about what you see.) The back panel photo will give you a good grasp of the various input/output options and the connector detail, plus such features as the turn-on options, provisions for line level input trimming, bridged mono operation, and an understanding of how the line cord connection is implemented, and how the cord is dressed. This latter information is useful if you intend to substitute a replacement line cord; refer footnote.*
Briefly consider price, then size, then appearance. Eliminate the Dan D’Agostino ilk first. Gaudy products with preposterous prices are obsession traps. Don’t get snared; just move on. Review product size next. Nothing needs technical analysis if it won’t fit within the space allotted. Ditto if it exhibits a pretentious blue-glow-bloat and says McIntosh. Mac’s products just don’t fit my concept of contemporary design; they look more like archived relics from the 1939-’40 NY World’s Fair. Of course, you might disagree, but do reject, up front, all of the equipment that you feel presents undesired esthetics. Appearance matters—especially if it sports a prominent display panel.
(4) Last, conduct a patient technical assessment of the pertinent performance specifications. Some of the more critical parameters that apply to power amplifiers follow, with comment about how the specification can affect operation. The primary objective is to select an amplifier that will perform in a manner that’s fully compatible and complementary when it’s integrated with your existing components, so you should also be well acquainted with their operating characteristics as well; review those specs too. Lots of this detail will become more apparent as we get into the individual parameters, so let’s begin now with…
INPUT IMPEDANCE (Zin): Whatever the value, assure that the noted Zin is consistent with what fits your need. Explanation: In audio electronics, effective (lossless) signal transfer requires that a low source (output) impedance (Zout) should feed into a much higher load (input) impedance (Zin). Of course, these low/high terms are relative, so their absolute value is flexible. If a preamp exhibits an output impedance of 50 to 100 Ω, that’s low, so almost any load impedance ≥ 10kΩ is then high enough. But if the Zout climbs as much as 2.5kΩ (as with a passive preamp using a 10kΩ attenuator), you’ll then want the load to be some 30kΩ to 50kΩ or higher. In general, when considering a power amplifier, don’t settle for anything with Zin < 30kΩ because you’d start to limit your source options. And yes, some hi-end power amplifiers actually exhibit Zin as low as 10kΩ.
OUTPUT IMPEDANCE (Zout): Low is desirable; higher Zout can breed issues. But how low/high? Well, it depends on what you’ve got. If it’s a solid state preamp, its implicit Zout of 50 to 100Ω is always fine. A vacuum tube preamp (typical Zout ≈ 400 to 500Ω) might be low enough too, if the ensuing load impedance is high enough (10kΩ) to prevent any significant interactive loss. A load impedance (Zin) that’s some 10X to 20X the source impedance (Zout) is a good guideline, so a really low Zout provides more freedom with respect to what might later constitute an acceptable load. When this ratio can’t be conveniently satisfied, an active unity gain buffer (UGB) stage can be inserted. The UGB is designed to provide a relatively high Zin (say ≥ 75kΩ), and a very low (50Ω) Zout, so that it can serve as an idealized “brick wall” to isolate some interim stage that has a higher-than-optimal output impedance; e.g., a passive preamp using a 25kΩ attenuator (worst case Zout ≈ 6.3kΩ).
https://www.mouser.com/datasheet/2/58/BPS-DAT-(ST6U)-Datasheet-1282874.pdf) are readily available, at reasonable cost, from Mouser Electronics (https://www.mouser.com). And here…https://goldpt.com/enclosures.html…are two very elegant mini-box enclosures** that have internal support rails compatible with mounting the designated ST6U (1/16 inch thick) stripboard stock. Cutting that glass/epoxy (tougher than phenolic resin) stripboard to the desired size is best done with a portable Milwaukee M12-series saw (https://www.homedepot.com/p/Milwaukee-M12-FUEL-12-Volt-3-in-Lithium-Ion-Brushless-Cordless-Cut-Off-Saw-Tool-Only-2522-20/305663849), and the related abrasive cut-off wheel (https://www.homedepot.com/p/Milwaukee-3-in-Metal-Cut-Off-Wheel-3-Pack-49-94-3000/306599198).
BACK TO OUTPUT IMPEDANCE: Power amplifiers represent a dramatic example of the implicit advantage of low output impedance. A solid-state power amplifier will exhibit a Zout that’s naturally very close to zero; i.e. it’s generally ≤ 0.1Ω. That value will assure an ideal damping factor (≥ 100) and virtually no response-altering interaction with the loudspeaker load. As a result, the sonic character of the output will be shaped almost exclusively by the loudspeaker components alone, without related nuance traceable to the driving amplifier. Conversely, a vacuum tube power amp will exhibit a much higher Zout (> 20X higher) because its transformer-linked load interface is incapable of better coupling. As a result, the tube amplifier will directly interact with its speaker load (the Zout and Zin values will be of similar order) to create a unique “sonic signature” that can vary from the speaker’s natural voice. The higher Zout will similarly degrade the tube amp’s potential damping factor. (NOTE: Benchmark Media Systems offers a white paper concerning the importance of high damping factor; refer https://benchmarkmedia.com/blogs/application_notes/audio-myth-damping-factor-isnt-much-of-a-factor.)
POWER OUTPUT: This is the highlight specification for power amplifiers. First, verify that the stated watts are measured according to the industry standard code; namely that they reflect Volts rms (root mean square), not peak volts, and that a specific load is noted (e.g., 8Ω), and that the bandwidth extends from 20Hz to 20kHz. Obviously, more Watts = more power, and more power output is always good. The power output rating should also be listed for 4Ω loads too, and the closer that 4Ω rating gets to = 2X whatever power was listed for 8Ω, the better. Amps that can’t come within ≥ 85% of doubling their 8 Ohm power rating when full output is applied across a 4 Ohm load might be a bit current-limited; maybe too wimpy to meet the peak dynamic transients in Respighi’s Pines of Rome when it’s played at sound pressure levels approaching 90dB (C-scale weighted).
Always assure that your amplifier’s full power output capability is at least some +2dB (1.6X) to +3dB (2X) > the rated power that your loudspeaker system can safely tolerate. This will assure that the loudspeakers are never exposed to a clipped input signal when driven to levels that are within their rated maximum power limit. Outside of physical abuse, nothing can be more potentially injurious to a loudspeaker system than consistently clipped drive signals, and nothing sounds worse than peak level clipping.
VOLTAGE GAIN: This is an important parameter that’s often overlooked. Power amplifiers typically exhibit different voltage gains. They generally vary between ~ +23dB and +30dB, with most of them ranging between +26dB and +29dB. This is an intrinsic design parameter, not the consequence of random variance, so it’s an important user consideration, and it deserves your attention.† The actual gain, expressed in dB, may not appear as a listed spec, but you can readily derive it from the published “input sensitivity” spec. Just do the math: P = EE/R, where P is the power in Watts, E is the AC signal in Vrms†† (EE = E-squared), and R is the load resistance in Ohms. A power amplifier that requires 1.0Vrms to produce 100 Watts into an 8Ω load has a voltage gain = 28.28X; otherwise expressed as +29dB.*†
INPUT SENSITIVITY: Is simply a measure of the input voltage needed to drive a power amplifier to a specified output level (generally, but not necessarily, to full output). It’s precisely the same parameter as voltage gain (above); it’s just restated after somebody else does the math. The “input sensitivity” is = 1.0Vrms when it drives the same amplifier to 100 Watts output across that 8Ω load.
The term “input sensitivity” brings to mind a fundamental concern: You want to assure that your power amplifier has sufficient internal gain that source signals, at their highest levels, are sufficient to drive your power amplifier to full output when your volume control is at zero cut (volume full up). Of course, if you’re using an active preamplifier between your source and the power amplifier, then copious extra gain—likely some +8dB to +12dB, and maybe even more—is available, with virtually all of it unneeded. However, in the event that you’ve modernized your system to eliminate that superfluous high level gain stage, then do assure that your CD player’s output, when driven at the maximum standard 0dB record level, is sufficient to push your power amplifier to full output when volume is set at zero cut. Let’s take an example: My own CD player (a discontinued Onkyo C-7000R) puts out ≈ 1.98Vrms (let’s say 2Vrms) across my 20kΩ volume control when driven at the industry standard 0dB maximum recording level (at 1kHz), and the channels match to within 0.2dB. I’m confident that your CD player’s output will be very close to same. (A good quality CD test disc with 0dB record levels at 1kHz [actually 1.001kHz for some obscure reason] is easily purchased. Mine is “Denon Audio Technical CD” #38C39-7147, and the 0dB test tones are on bands #18 [left] and #19 [right]. This is a professional grade test disc from 1984; it’s now scarce, but sometimes pops up on Amazon. Equivalents are commonly available. Boston Audio Society member DB Systems lists a compendium of test CDs at http://www.bostonaudiosociety.org/db_systems.htm.)
My power amplifier (a Parasound A23, circa 2014, now superseded by A23+) has a voltage gain = +29dB, and the specified full output is 125 Watts/channel into an 8Ω load. As a result, I can be certain that a CD source signal of 2.0Vrms will be more than sufficient to drive my amp to full output. How do I know that? Well, 125 Watts into an 8Ω load means that there’s 31.6 Volts of source signal impressed across that load. Subtracting 29dB of gain (0.035 x 31.6Vrms) tells me that the input need be only 1.1Vrms to drive my amp to full output. So I’d have a full 0.9 Volt margin relative to the potential 2.0Vrms input. (The volume control serves to prevent input > 1.1Vrms, hence avoid clipping.) OK, but what if my power amp had only +23dB of voltage gain instead of +29dB? Well, with volume then turned full up (zero cut), that would mean that the amp would need 0.071 x 31.6Vrms = 2.2Vrms input to reach 125 Watts into 8 Ohms. With just 2.0Vrms of drive from the CD player I’d be close, but still some -0.9dB short of the 2.2Vrms minimum required to produce 125 Watts across 8 Ohms.
The implication is clear: Pay attention to a power amplifier’s voltage gain specification. Be certain that your power amp can always reach its fully rated output (regardless of what that output might be) when your CD player hits the highest signal level that the CD recording process permits; i.e., the 0dB record level. Here is a summary of the recommended amplifier gain consistent with stated output levels, assuming an 8Ω load and a CD player that puts out ~ 2.0Vrms when fed at the standard 0dB maximum record level.
+26dB minimum amplifier voltage gain when rated output is ≤ 200 Watts.
+27dB minimum amplifier voltage gain when rated output is ≤ 250 Watts.
+28dB minimum amplifier voltage gain when rated output is ≤ 300 Watts.
+28.5dB minimum amplifier voltage gain when rated output is ≤ 350 Watts.
These minimum gains assure that a 2.0V input signal will drive the power amplifier to the cited output. More gain is fine, but only up to a point. Hold amplifier gain to +31dB or +32dB tops. If your gain was any higher you’d then be forced to unduly retard your volume control setting (to the 9 o’clock arc) when listening at more moderate levels, and that’s not good either. Indeed, that would replicate the penalty that you accept when retaining a conventional active preamp. The preamp’s high level boost of some +8dB to +12dB is grossly excessive, and that unwanted excess prevents good volume management. Dump that archaic active preamp! Instead, get a passive equivalent, and buy (or build) a better volume attenuator than the $15 Alps RK271-series dual pot that’s inside most of the hi-end active preamps.
My own DIY 20kΩ stereo volume control uses a stepped 24 position double-deck switch (Goldpoint type V24C-2, see https://goldpt.com/compare.html), with 23 fixed ±0.1% tolerance metal-film (low noise) 1/4 Watt resistors per channel (46 total), all carefully hand-soldered in position. It provides precisely -2dB-per-step attenuation over 70% of its full -60dB span, and its calibrated ±0.1% accuracy assures far better channel tracking than the crummy 2dB or 3dB ∆ that’s specified for premium Alps pots.
That’s quite enough for this session. In the next paper we’ll continue with this same subject, and discuss some additional parameters that you should review when perusing power amplifier specifications.
BG (September 16, 2020)
*There are often good reasons to seek a replacement line cord. You might need a shorter or longer cord, or one with an angled (90˚) socket, to reduce the rear clearance. And you might choose to utilize a 14 AWG line cord instead of the supplied 16 AWG cord. (Don’t consider 8-10-12 AWG cordage. There is no electrical benefit, and fatter gauge power cords become difficult to route.) USA market amplifiers utilize a molded SJT-type cord, with a Nema 5-15P plug and a IEC320 C13 socket (C13L or C13R if it’s angled). Simply order a replacement (molded, SJT grade, 14 AWG) cord of the length that you want, with socket and plug as desired. This site-- https://www.pchcables.com/expocoandad.html--offers many existing stocked options at very reasonable prices, and the quality is excellent. This site…https://www.stayonline.com/cordbuilder/molded-cord-configurator.asp…will custom-build your cord to your own personal specifications. Just fill in the options on the form; pop-up photos will guide you in making your selection.
Ignore all chatter about ultra-costly replacement line cords that allegedly improve the sound of an amplifier. That fable traces to a pervasive human weakness that we know as confirmation bias (https://en.wikipedia.org/wiki/Confirmation_bias). The effect is persuasive, and groupthink-infused (https://en.wikipedia.org/wiki/Groupthink) zombies on sponsored websites keep this promotion pumped—as do the retail dealers. Upgraded line cords are among the most profitable of all products stocked. Sales are good, and returns (sometimes not permitted until a mandatory “burn-in” cycle has elapsed) are easily resold. Here is an array…https://www.thecableco.com/cables/power-cables.html?limit=all…of 318 assorted line cords for sale; prices range from ~ $100 to $12,000 each. Of course, science says that the mythical emperor still has no clothes. A substitute line cord will produce no audible change, at least not until some other user says “Hey, can’t you hear that tighter definition in the mids?”
**The Goldpoint #EN1-C enclosure size is quite sufficient if you’re content with using standard metallized polyester film input and output coupling capacitors. However, if you’re going to use premium Vishay MKP 1848 series polypropylene coupling capacitors (they’re big!) you’d need the slightly wider Goldpoint EN2-C enclosure.
†When comparing different power amplifiers in a listening trial, don’t expect to freeze the volume control at some fixed position and assume that you’re thereby providing a fair comparison. The amp with the higher gain will always win, even when decidedly inferior. A difference of just 1dB is enough to sway all votes.
††Use of the designator “E” to represent voltage stems from the classic tutorial use of the term “electromotive force” to describe the voltage function. It’s akin to pressure when expressed as a plumbing analogy.
*†Do keep a “Decibel Table” handy, and review how to read it. Here’s an on-line pdf copy of the “Handbook of Electronics Tables and Formulas”, 6th edition (1988); refer decibel table, pp. 32 to 36…
Especially note the scaling difference between voltage gain or loss and power gain or loss. A voltage gain of 2X = +6dB, whereas a power gain of 2X = +3dB.