HDTT High Resolution Files

By Bill Heck

Record company vaults are filled with analog tapes of classical music performances. As with performances found on contemporary digital recordings, many are nice enough, but only vaguely worthy of preservation for posterity, while some others are eminently forgettable. But then there are those performances that are exceptional, that deserve special treatment to bring the performances to current listeners in the best sound possible. For High Definition Tape Transfers (HDTT), that means providing the best possible transfers of classic performances from the best analog tape they can find. These transfers are then made available in several formats, from CD to high resolution downloadable files.

John Puccio has already reviewed a number of performances on CDs from HDTT. But HDTT offers most of their transfers in multiple formats, ranging from CD resolution (PCM encoding, 16 bit / 44.1 KHz sampling rate) to PCM 24/352.8 and DSD 256. (For the uninitiated, with PCM files the first number, e.g. 16, refers to the number of digital bits per sample; the second, e.g., 44.1, refers to the number of thousands of samples per second. John does not have the equipment to play downloaded high-resolution files, i.e., greater than CD resolution, so he suggested that I try a few HDTT transfers in one of the high-res formats. My NAD C 658 can handle up to 24/192 resolution, so I went with that level.)

For those who have the capability to play high-res files, the process for HDTT files is quite simple. You order a recording, specifying the resolution that you would like; once you order, you receive a link via email that allows you to download the file. Note that high resolution files are large, and the higher the resolution, the larger the files; for example, the three 24/192 files that I downloaded each were around 1.7 GB. All this means that you need a decent internet connection so that the download doesn’t take forever, and you need plenty of disk space. Once you have downloaded the files, you can play them from your PC (if you connect the PC to your audio system), or copy them to a flash drive (to plug into the USB port of your DAC), or copy them to another drive (that you connect to your network where it can be read by your DAC). The main thing is that you need a DAC that can handle files of the resolution that you chose. If you have no idea what any of this means, you presumably don’t have a DAC/preamp to play high-resolution files.

Now, back to those performances that are worth preserving.

The major labels have reissued loads of classic – and not so classic – performances. Some of the reissues have been done with attention to detail, resulting in good to great sounding CDs; others perhaps not so much. However, in many cases, reissues from the early days of CDs have gone out of print, making those performances difficult to find at all.

Enter HDTT with the goal of providing better versions of a recorded performances from decades ago. On the surface, that sounds simple, but it must be a difficult business. To start with, you need to find early-generation tapes from which you can transfer and digitize the performances. You would think that “finding” would be simple, but alas, it may not be: in the worst case, the best tapes may have been destroyed; in other cases, the ravages of time may have damaged them even to the point of unplayability. In other cases, the desired tapes may be…well, somewhere, but not to be found among the thousands upon thousands of poorly inventoried reels. On the assumption that you indeed have access to an early generation tape, you need to have the equipment not just to play it any old way, but to play it in such a way as to recover its full musical potential. If you were starting with high resolution digital copies, that would be easy; with old analog tapes, it can be quite a chore.

All this means that the final product may indeed be the best possible, but may still involve compromises. On the other hand, with luck, the results can be amazingly good. Let’s review a couple of examples.

Andres Segovia: The Unique Art of Andres Segovia (HDTT9328)
In his day, Andres Segovia was the acknowledged master of the classical guitar, perhaps single-handedly (or perhaps we should say two-handedly) making the guitar respectable as a classical concert instrument. A Spaniard himself, many of Segovia’s performances are of works by Spanish composers, and his style remained what we might think of as “Spanish” throughout his very long career. But Segovia was hardly a one-trick pony: for example, he transcribed works by a number of composers, most notably J S Bach. (By the way, if you have never heard transcriptions of any of Bach’s music for guitar, be aware the some pieces can be revelatory, uncovering aspects of the music that are more difficult to hear when the music is played on the instruments for which it was written originally.) Post-Segovia, a number of great classical guitarists have arisen, but Segovia’s work still is well worth hearing.

At least in the modern period, Segovia recorded for Decca. Decca was quite well-regarded in the early period of stereo recording for their “ffrr” (Full Frequency Range Recording) technique.[i] So how did this set turn out?

For this particular album, comparison to a commercially available CD is complicated by the fact that the CD version is no longer available; I could not find even a used copy in a quick search. Discogs does list four used copies of the LP for sale at prices from $5 – 10, but as I no longer have a turntable, that was a non-starter, and anyway this review is supposed to compare the file to a readily available CD. To make things more perplexing, the album in question is not to be confused with the two-CD set released by DG called “The Art of Andres Segovia.” Nor should you confuse it with the 8 (?) CD series previously issued on MCA, then on DG, then on IDIS called “The Legendary Andres Segovia”. So far as I can tell, Segovia’s many recordings have been chopped up and repackaged in various combinations for decades. Sadly, most of these recordings have fallen out of their respective catalogs. Moreover, there is the distinct possibility that some, many, or all of the Decca master tapes of Segovia’s recordings were destroyed in the Universal Studios fire of 2008.

So what did I use for a CD to compare with the HDTT version of this album? Absent this exact album, I searched my few Segovia disks and found that Volume 7 of the aforementioned “Legendary” series on MCA began with the Milan Pavanas (Pavanes), and that the series of six pavanas comprise tracks 6 and 7 on the HDTT file. From what I can tell, the same original recordings were sources in both cases, so I used them as my standard of comparison.

In the discussion above, I mentioned some potential compromises because of imperfect sources. That concern applies here: the very first thing that I noticed in listening to this file was the result of one of the vagaries of the process. I cued up Milan’s Pavana 1 on the HDTT file and immediately heard a tape issue: the first note’s on pitch, then a weird “wow” in which the pitch dips, as if the tape had been damaged – stretched? – then a return to normal. The episode lasts less than a second, but my goodness, what a sound to open the listening session!

Listening to the HDTT version immediately suggests that this is an old recording. First, there’s a slight resonance that lends an “aw” quality to the sound. I won’t make too much of this: while noticeable, the resonance is not too distracting – but it is there. In addition, lower notes from Segovia’s guitar sound considerably duller than the higher ones – but this is only partly, and perhaps not at all, an issue with the recording. One needs to remember that Segovia was a master instrumentalist and colorist: he is using this thumb to play those lower notes in counterpoint to the higher ones played with fingers and, in some cases, perhaps struck with fingernails, thus creating different voicings for different parts. (This is especially noticeable on tracks of music by Bach later on the album, as the music is more contrapuntal in nature.) One also needs to take account of the characteristics of Segovia’s instrument, the type of strings that he would have used, and his own technique. All in all, the more I listened, the more comfortable I felt that I was hearing tonal balance fairly close to what I would have heard in person, notwithstanding that resonance mentioned earlier.

The Decca recording engineers brought the microphones rather close; the tape technology available at the time probably made that advisable, if only to keep the signal to noise ratio high. The close mic placement leaves one wishing for a little more air around the sound, but the apparent size of the guitar remains natural. Decca’s approach is pretty consistent in the works throughout the album, even though the recordings may have been made at different times, so we don’t suddenly and jarringly jump from performance space to space. Meanwhile, the good news about the upfront presentation is that we can hear the subtle details of Segovia’s performance – and Segovia provides delectable details aplenty.

Turning to the Pavanas as they appear on the MCA disk, the most obvious point of contrast is that these reissues “civilize” the original recordings. Tonal balances are more tipped up, which at first sounds more natural, and there seems to be an attempt to put some distance between Segovia’s guitar and the listener. However, there are downsides to these changes. First, the MCA versions lean toward the dreaded “eight-feet-wide guitar” effect. Second, the de-emphasis of the bass end robs the lower registers of their power, thus obscuring the harmonic structure that should be present. Third, the enhanced treble emphasizes finger noises. All guitarists, classical and otherwise, produce finger noises – squeaks, if you will – as their fingers move on the strings of the instrument. In the HDTT transfers, these noises are audible but subdued, simply a natural part of the background. On the MCA disk, however, these same noises are far more obvious, to the point of being obnoxious. In fact, the finger noises were such as to suggest steel strings on the guitar. (I hope that no classical guitarists reading that last sentence fainted in horror….)

Overall, then, it is clear to me that the HDTT transfers provide a better listening experience. No, they are not perfect – but that brings us to the issue of availability. Simply finding this album – and many other Segovia recordings – is a challenge. If you are a classical guitar fan, and you want to hear the father of modern classical playing (as you should), and you don’t want to spend your time haunting second-hand record stores, just get the HDTT transfer and enjoy the closest thing to the original that we’re ever likely to have.

Bartok: Music for Strings, Percussion, and Celesta. Reiner/Chicago

In the review above, I had to elaborate on details and close comparisons to elucidate the virtues of the HDTT transfer of the Segovia album. Now I get to relax: my job is way easier with the recording here. The performance is universally recognized as great, perhaps definitive. As we shall see, it’s also an easy call in favor of the HDTT transfer.

In keeping with my “easy job,” I do not have much to say about the Reiner/Chicago performance, not because the performance is unworthy of description, but because everything that I might write already has been written, and likely written multiple times. Suffice it to say that Bartok and Reiner were friends who surely communicated deeply about this music, and it showed in the performance. Moreover, the RCA recording team, with producer Richard Mohr and the legendary engineer Lewis Layton, was at the top of its game in this era, producing recordings that even today are considered masterpieces. That praise applies especially to the recordings of the Chicago Symphony under Fritz Reiner, and this recording, made in 1958, is perhaps one of their best.

On to the second easy part of my easy job. As with other RCA classics, this performance has been released in a variety of formats and combinations in the CD era. I have a copy from the 1990’s in which the coupling is Bartok’s Concerto for Orchestra. That will serve as my standard of comparison for the HDTT transfer, but I also was able to stream the same performance from a few other albums (thank you Idagio and Qobuz); so far as I could tell in quick comparisons, these latter versions all were identical to the aforementioned CD.

We can go right to bottom line: the HDTT transfer wins. In this case, despite the age of the recording, there are no issues with obnoxious balance or weird tape noises. Granted, at what we might call a “sonic glance,” the HDTT version sounds rather like the CD: sonic qualities such as the overall tonal balance, the perspective on the orchestra, the noise level (very low), and so on initially sound similar. But even a short listen reveals that the HDTT version has more depth, with clearer placement of instruments, than the RCA CD. The result is a more natural, realistic sound.

I could drone on for a while, but to what end? The performance is a must-have for anyone interested in this music, and the HDTT transfer just sounds better than the easily available CD. To put this in perspective: the original RCA recording is so good that the “standard” CD does sound quite lovely when considered on its own. If I had never heard the HDTT transfer, I could have lived happily with the CD. But the fact is that I did hear the HDTT transfer and now I’m spoiled. The differences are, in the great scheme of thing, subtle, but differences there are, and to my ears the HDTT version just sounds better.

What’s the Difference?
The question remains as to what is responsible for the superiority of the two HDTT transfers that I auditioned as compared to the major label CDs. Is it the transfer or the high resolution?

As mentioned earlier, John Puccio has noted the superior sound of several HDTT transfers at CD resolution as compared to major label CDs, thus suggesting that the careful transfers themselves have a significant effect even when playback is at the same resolution. Looking further afield, various sources in audioland sing the praises of high resolution, and the HDTT folks certainly feel that the higher resolution versions sound more like the analog originals. Then again, dissenting voices claim that it is very difficult in practice to hear differences between CD and higher resolutions.

But we’re really asking the wrong question: It’s not whether someone can hear the difference, it’s whether you can hear the difference – given the limitations of your audio system and of your hearing – and, if so, whether it’s worth paying extra for that difference. Fortunately, you can easily answer this question for yourself. Order the Bartok / Reiner / Chicago performance reviewed here in the highest resolution that your system will support, then spend a few extra bucks and buy the HDTT CD of the same performance. (You can even buy the budget CD with no box; you just need it for testing). Listen to them both: take your time, I’ll wait. Can you hear a difference, and does the difference justify the extra expense? Congratulations. You’ve just found the best sound available! No difference, or not enough to justify a higher price? Congratulations. You’ve just found great sound and saved some cash!

BH

[1]. I can’t resist this tidbit: a number of the British Invasion rock bands of the early to mid 60’s, including the Rolling Stones, were on the Decca label in Britain – London Records in the US – with their album covers proudly displaying the ffrr logo. Seeing a thumbnail promo on the paper record sleeve for, say, the Rolling Stones right next to one for Andres Segovia was, to say the least, interesting.

To listen to samples of these HDTT products, here are links to the HDTT Web site:

Segovia:  https://www.highdeftapetransfers.com/products/the-unique-art-of-andres-segovia?_pos=175&_sid=cdeb14039&_ss=r

Bartok:  https://www.highdeftapetransfers.com/products/bartok-music-for-strings-percussion-and-celesta-hungarian-sketches-fritz-reiner-chicago-symphony-pure-dsd?_pos=12&_sid=07452629f&_ss=r

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Handel: Concerti Grossi, Op. 3 (SACD review)

Georg Kallweit, Akademie fur Alte Musik Berlin. Pentatone PTC 5186 776.

By John J. Puccio

If you enjoy music of the Baroque and Classical ages played with elegance and refinement in historically informed performances, you might just like the Akademie fur Alte Musik Berlin, a German, period-instrument ensemble formed in 1982 that plays with a smooth precision and style. While they may not display the abandon of, say, Le Concert des Nations or Europa Galante nor with the sheer joy of the Philharmonia Baroque, the Akademie make up for it with the effortless polish of professionals who know and love their business and aren’t afraid of showing a little restraint in the process.

This is not to suggest, however, that the Akademie fur Alte Musik are in any way dull or commonplace. Far from it, as these performances of Handel’s Op. 3 concertos demonstrate. The concertos are clear, focused, and meticulously presented in interpretations both lively and graceful. It’s a refreshing combination.

Anyway, the German-born Baroque composer George Frideric Handel (1685-1759) wrote his six Concerti Grossi, Op. 3 sometime before 1734, when English music publisher John Walsh assembled them from earlier Handel works. Apparently, Handel himself had no intention when he wrote them of their becoming a “set,” nor did he have any idea beforehand what Walsh was up to with them. But have them we do. What’s more, they aren’t “concertos” in the modern sense at all, but rather they are mostly collections of overtures, fugues, dances, and the like arranged into suites of anywhere from two to five movements each. Handel did not design any of them to highlight individual instruments, either, or even to contrast different sections of the orchestra as do most traditional concerti grossi. But Walsh, wanting to capitalize on the popularity of concerti sets of the day compiled them from whatever bits and pieces of Handel’s music he could find. No matter. Much of it remains charming.

Of course, Handel admirers coming to this music for the first time and expecting to find another set of Water Music suites may be disappointed. There just isn’t the abundance of memorable tunes involved. Instead, we have mainly bits and pieces of Handel kind of thrown together to sell copies of the scores. And, of course, listeners not already familiar with Handel’s music may not care one way or the other because it probably all sounds alike to them anyway. But that is not to say there isn’t a lot to appreciate in the Akademie’s view of things, and I rather enjoyed some of “the bits and pieces.”

Maestro Kallweit and the Akademie play with a refined sensibility, somewhat understated on occasion but lively and brisk at other times. The first of the six concertos makes a good example of this. There are three movements to it: fast, slow, fast. And there is no doubting which is which because the fast movements sparkle with energy, while the slow movement, the Largo, is meaningfully leisurely.

Of the concertos I liked best, No. 2 is probably my favorite. It’s one of the longest at five movements, and each of the sections is clearly distinctive from the rest. Musical scholars have suggested that Handel may have written the whole thing as an overture; who knows. It certainly has all the earmarks of a curtain raiser and may easily be savored as a stand-alone item.

And so it goes. Nos. 3 and 5 both open with Largos, making them a bit different. However, the movements are generally so brief, it’s hard to notice. No. 4 seems the most grandiose, No. 5 the most grave. No. 6, the final concerto, seems almost an afterthought. It has only two movements, and it seems that the publisher simply wanted a concluding sixth work in the set because that was the fashion of the times. In any case, it is distinguished by an organ part in the last movement that foreshadows Handel’s later, more-developed organ concertos.

Producers Renaud Loranger and Karel Bruggeman and engineer Jean-Marie Geijsen recorded the concerti at the Nikodemuskirche, Berlin in May 2019. They made it in hybrid SACD, and I listened in SACD two-channel stereo. There’s more ambient bloom in the auditorium than we normally here, making the relatively small ensemble appear bigger than it is. It is not unpleasant. Otherwise, the sound is smooth and agreeable. You might even be hard pressed to tell it’s a period band and not a modern chamber orchestra because of the polished sheen of the sonics. It’s also a tad close, with only moderate depth. Nevertheless, it’s all quite listenable.

JJP

To listen to a brief excerpt from this album, click below:

On Rational Selection–Part I…

By Bryan Geyer

It’s nice to assess the quality and capability of a new power amplifier in advance of buying it; however, don’t expect to fulfill that research solely by listening. A listening trial will reveal almost nothing about the amplifier’s performance compatibility. You simply won’t hear insufficient voltage gain, or hear the loss that accrues from a poor impedance match. Lots of critical factors can be deficient, or significantly less-than-optimum, without imparting any audible evidence. In truth, it’s not even possible to detect, by ear, whether your test amplifier has sufficient power output capability. (A power amplifier should be capable of providing some +2dB to +3dB more undistorted power output than your loudspeakers can safely handle; refer commentary herein. That value can then be precisely determined by reference to the related product specifications.)

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.

Audio component evaluation is best accomplished by research and inspection rather than by listening. You can pretty well determine everything that you need to know about a given component (let’s assume a power amplifier), by proceeding through the four steps listed below. The data is freely available; you just have to become conversant with how to utilize it effectively. Doing so will certainly require more commitment than passive listening, but it’s likely to be less taxing than interpreting all of the frothy adjectives that you see in a Stereophile product review.

(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.*

(3)  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).

BRIEF DIY ASIDE: If you’re a competent electronics buff with elementary circuit design smarts, you can create and construct your own very capable UGB, just as I and many others have done. Refer my paper headed “On Noise, Coax, and Control” for tutorial guidance on DIY UGB circuit design. The requisite semiconductors, passive components, connectors, and suitable through-hole stripboard stock (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…
https://redcxem.ru/files/Handbook_of_Electronics_Tables_and_Formulas.6-th_ed.1986.pdf.
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.