Playing an original pressing of Fritz Reiner and the Chicago Symphony Orchestra's justifiably legendary 1954 stereo recording of Strauss's Also sprach Zarathustra (LP, RCA Living Stereo LSC-1806) produced many rewards: the opening low C on contrabassoon, double basses, and pipe organ, the lush strings, the well-focused solo trumpet pealing out the famous three ascending notes appearing in three-dimensional space, and all the other sonic wonders this recording provides, were more richly and fully drawn than I'd ever heard them, yet with the bite of the brass still fully intact. The sound was richly drawn yet light on its feet and absolutely explosive, the weight of the orchestra's low end reproduced fully and well controlled. And that was just side 1!
Side 2 was mind-bogglingly better than I'd ever heard it, all of the inner instrumental voices clearly revealed. Especially amazing was the return of the trumpet call, backed by delicate, barely audible woodwinds that were now clearly delineated—and, a few minutes later, the triangle, each stroke's attack, sustain, and decay convincingly reproduced with great deliberateness and delicacy. And string pizzicati were perfection.
We interrupt this review for a word about power cords . . .
As I listened to that LP of Reiner's Zarathustra, I realized that the piccolo, clarinets, oboes, bassoons, trumpets, and horns all sounded more recognizably "right" than I'd ever heard them. Each was individually well defined without pulling apart the sound of the entire orchestra, and within and among the various sections there was instrumental dimensionality and separation. Succumbing to temptation, I removed my reference power cords, AudioQuest's Dragons, and replaced them with a pair of stock molded-rubber cords. Though the Hyperions have the fairly rare 20A IEC AC jack, they are not supplied with cords.
I gave the Ypsilon amps some time to again warm up and listened again. Anyone who claims that power cords can't possibly make a difference in the sound is wrong. Especially at this level of sound quality, the wrong cord (and/or speaker cable) can ruin everything. In the past few months I've heard two great systems—one costly, one moderately priced—whose sounds were destroyed, tonally and spatially, by unbearably bright- and hard-sounding speaker cables.
The rubber power cords produced a glaring overall brightness that bleached the harmonic structures of instruments and flattened what had been three to little more than two dimensions. The nuanced orchestral balance the Hyperions produced with the AudioQuest Dragons was messed up, and the Ypsilons' convincing reproductions of attacks, sustains, and decays were smeared.
The best I could say for what I heard with the stock cords was that the opening trumpet call was big and gloriously bright, but incongruously so—it sounded as if it had been added in postproduction.
Under favorable conditions, the Ypsilon Hyperions could put on a spectacularly realistic, impressively well-balanced display of power, grip, depth, and delicacy, and do all of the right things for acoustic music, whether performed by full orchestra or small scale chamber ensemble. You need to hear what they can do with the new 45rpm edition of Duke Ellington's Masterpieces by Ellington (2 LPs, Columbia Masterworks ML 4418/Analogue Productions AAPJ 4418-45).
What about rock?
Some ripe-sounding amps can be wonderful with acoustic music but won't do rock. Once I'd been floored by the Hyperions' reproduction of recordings of acoustic music, I moved on to the Who and homed in on Who's Next, comparing an original UK Track pressing mastered by "Bilbo" (Denis Blackham) with Classic Records' 2005 reissue, mastered by Chris Bellman from the original master tape. In 2005, people complained about this reissue's price: $30. Today, a copy will cost you a few hundred.
The Classic toasts the Track original, as well as MCA's mid-1990s "Heavy Vinyl" edition I was involved with. While Keith Moon's drums in "Baba O'Riley" are disappointing in every edition, "Bargain" is stupendous in every way, especially on the Classic reissue. Through the Hyperions the kick drums on this track had believable texture, tonality, and especially drive, and John Entwistle's bass had great growl, extension, and definition. But what really stood out were the handclaps and, most particularly, the tambourine, which sounded as if it was being played by someone standing in the room. The wet reverb around Roger Daltrey's voice was presented with the same well-defined clarity, transparency, and balance of direct and "reflected" sound as was Trifonov's piano in the Tchaikovsky concerto. This rock album confirmed that one the Hyperion's greatest strengths was its midrange transparency. Its sound was remarkably transparent throughout, but especially from the lower to the upper midrange.
With its generous, powerful, well-controlled bottom end, its extraordinary midband transparency, its high-frequency delicacy and airiness, its ideal attacks, sustains, and decays at all frequencies, and its richly drawn harmonic palette and dynamic authority, the Ypsilon Hyperion amplifier did justice to every genre—from the string-driven subtleties of Willie Watson's Folk Singer, Vol.2 (LP, Acony 174LP), to Ogilala, the edgy new album by confessional solo rocker William Patrick Corgan (LP, Martha's Music 538321011), produced by Rick Rubin and mixed by Jan Erik Kongshaug, best known for his engineering and mixing for ECM —to, of course, the grand orchestral recordings cited, and many others.
Some amplifiers that sound as lush and beautiful as this one are simply not useful as reviewing tools. But I found the Hyperion as reliable in that regard as the most "analytical" amplifiers I've used, while giving me far more musical and sonic pleasure from both analog and digital sources.
Rolling tubes and preamps
I used three different input tubes in the Ypsilon Hyperions: two pairs of 6H30s and one pair of 5687s. Unlike with the Aelius amplifiers, in which different tubes produced profoundly different sounds, here the differences were more subtle. The stock Sovtek 6H30 Pi's sounded great, while a set of Balanced Audio Technology 6H30pDRs (NOS) notably improved the already impressive bottom end, widened and somewhat deepened the already finely drawn soundstage, and improved instrumental focus.
Both the darTZeel NHB-18S and the even more costly Ypsilon PST-100 Mk.II Silver Edition preamplifiers are impressively transparent and quiet. I was able to use the Ypsilon in passive mode, which made it essentially invisible. (When, in July 2011, John Atkinson measured the PST-100 Mk.II Silver in passive mode, it was flat from 10Hz to 200kHz!) Equally impressive was the transparency of the active darTZeel. Either would make a great mate for the Hyperions—though if I were buying, I'd opt for a preamp and monoblocks of the same brand.
Conclusions
Ypsilon Electronics' Hyperion is a powerful, cannily designed, exquisitely voiced monoblock power amplifier. Inside and out, its construction quality is as impressive as it should be for $93,000/pair. Because of its tubes, its distortion spec of 0.7% at 100W into 8 ohms will probably freak out the measurement fetishists, to whom I say: Just listen to it, and keep in mind that the tube microphones used in the making of many of your favorite recordings probably measured similarly.
The Hyperion strikes the ideal balance between tube-amp richness and flow and solid-state quiet, authority, and dynamic swagger. And it does this without making you conscious of each technology's contribution to the whole—something I felt that Ypsilon's far less expensive Aelius didn't manage quite as well.
When I first heard, and then bought, the Ypsilon VPS-100 phono preamplifier, I concluded that its designer, Demetris Baklavas, was some kind of genius. His PST-100 preamp didn't shake that conclusion, but I was somewhat disappointed with the original Aelius. With the Hyperion, I must again characterize Baklavas as an electronics design genius whose name deserves mention alongside the more familiar names you probably know. He's seriously underrated, and deserving of greater acclamation.
When you make your living by listening to audio gear, at some point you inevitably suffer burnout. While I can't live for long without hearing music in my listening room, there are times when I crave silence to avoid such a burnout. But during the time the Hyperions were here, I think I did more listening for pleasure alone than I've done in years.
If you've got the money, I wholeheartedly recommend the Ypsilon Hyperion. However it measures, it's among the handful of the best-sounding power amplifiers I've heard, and it's the most musically enjoyable of the lot.
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COMMENTS
Now I've seen everything- A tube rectifier in a SS behemoth. Tube rectifiers are loved by guitarists for the sag they exhibit, something I assume would not be welcomed in a SS audio amp. Feel free to correct me if I'm wrong, and explain to me why.
Now I've seen everything- A tube rectifier in a SS behemoth. Tube rectifiers are loved by guitarists for the sag they exhibit, something I assume would not be welcomed in a SS audio amp. Feel free to correct me if I'm wrong, and explain to me why.
To the best of my knowledge, the tube rectifier is used in the power supply for the front-end circuit, not for the output stage.
John Atkinson
Editor, Stereophile
..given that a 6CA4 rectifier is capable of a "whopping" .15 amp of current. But it still begs the question why. A solid state rectifier would have worked perfectly here, and without the vagaries created by tube wear. Given the overall design and resultant performance of this amplifier, the "why" about the tube rectifier is dwarfed by other, far more serious "why's" about other parts of the circuit. The circuit of this amp resembles a design by Fisher from the early 1960's, using a dual power pentode to drive an interstage transformer, followed by a set of germanium output transistors. It too performed rather poorly, but it was only used in Fisher's stereo consoles where highest performance wasn't required.
Hi John,
your comment of concern over the linear rise in distortion with output power set me thinking. A simple non-linear transfer function generates harmonics that increase proportionately more than the fundamental increases as the input is raised, i.e. for every dB increase in the input (or output below compression) the second harmonic distortion (2HD) will increase by 2dB, the third harmonic distortion (3HD) by 3db etc (for as many terms that describe the non-linearity). Plotting these you get a slope of 1x for the fundamental, 2x for the second harmonic & 3x for the third harmonic when plotted in dB (or on log-log) axes. If you plot the difference between the fundamental and the harmonic you get a 1x slope for 2HD (2x-1x) and a 2x slope for 3HD (3x-1x). This leads to the "intercept point" linearity metric used in radio design. Applying that to your distortion vs power plots, which are not in dB but are on log-log axes: a decade change in output power is 10dB and the THD percentage represents the difference between harmonic and fundamental with a decade change being 20dB (assuming the measurement system is reporting the harmonic/fundamental voltage ratio, not the power ratio). Fig 3. fits a 1x slope perfectly meaning THD is dominated by 2HD (as proved by Fig.7).
Looking at a couple of other examples, your measurements of the Lamm M1.2 Reference also fit a perfect 1x slope. The Audionet Max trends to a perfect 2x slope at higher powers (at least into 8 ohms) so 3HD must dominate. The Pass Labs XA60.8 has a 1.25x straight line suggesting 2HD & 3HD are dominating but do not behave quite as theory suggests possibly due to moderate global feedback. And then there are many amplifiers with weird and inexplicable shapes to the THD vs. output power plot that may indicate transfer functions that change with power level, cross-over distortion &/or high levels of global feedback.
I think "concern" is appropriate for the absolute level of the distortion from the Ypsilon but I find the linear slope itself comforting in that the amplifier behaves with a simple second order transfer function. Perhaps such a characteristic is akin to natural sounds and our brains expect the levels of harmonics to follow the 1x, 2x etc slopes as volume changes.
Perhaps you could have the Audio Precision system plot individual harmonic levels instead of THD to see if there is a better correlation to subjective preference than a % THD number.
Best Regards
13DoW
... lead you to conclude about the performance of Benchmark AHB2 amplifier?
https://www.stereophile.com/content/benchmark-media-systems-ahb2-power-amplifier-measurements
A simple non-linear transfer function generates harmonics that increase proportionately more than the fundamental increases as the input is raised, i.e. for every dB increase in the input (or output below compression) the second harmonic distortion (2HD) will increase by 2dB, the third harmonic distortion (3HD) by 3db etc (for as many terms that describe the non-linearity).
If the transfer function is simple, please write it down and show how you derived it. For many of us, it is easier to follow equations than prose. Thanks in advance.
...you may wish to kindly explain, how you can even talk about a transfer function (defined for linear systems) in the context of distortion, which is a nonlinear phenomenon.
Innovative thinker you are, dear sir.
I think "concern" is appropriate for the absolute level of the distortion from the Ypsilon but I find the linear slope itself comforting in that the amplifier behaves with a simple second order transfer function. Perhaps such a characteristic is akin to natural sounds and our brains expect the levels of harmonics to follow the 1x, 2x etc slopes as volume changes.
THta's a great point. The more complex the transfer function polynomial, the less it resembles how real sounds change with increased spl.
Perhaps you could have the Audio Precision system plot individual harmonic levels instead of THD to see if there is a better correlation to subjective preference than a % THD number.
I do show the harmonic signature of an amplifier's distortion in our reviews, but plotting how the individual harmonics increase with increasing output level might indeed be insightful.
John Atkinson
Editor, Steteophile
The German Stereoplay magazine has this type of measurement graph for amplifiers (done with an AP), they show how the individual harmonics vary with power output until clipping.
One thing rarely –well, never– taken into account is that the harmonic structure of most natural instruments, let alone amplified, significantly changes as volume increases, with higher harmonics becoming dominant at louder levels (http://www2.siba.fi/akustiikka/?id=42&la=en). That's right people, natural instruments can scream as hell. Therefore an ideally linear power amplifier would faithfully reproduce a live session only at the same level it was originally recorded while in the meantime imprinting that fixed harmonic structure to all other volume levels indiscriminately. Of course no one between us cares about the original loudness level (some engineers do though); if it's loud enough, which is usually the case especially on real-time live sessions, then its high-pitched pattern will be projected to all volume levels distorting natural dynamics and giving a distinct shouting quality to every cry or whisper, as unnatural as an adult-size version of a child. Let’s not even mention the usual case where a typical feedback-based amplifier adds its own higher-harmonic distortion signature to the initial scream accumulating insult to injury. A decent 2nd-dominated distortion pattern compensates more or less for the aforementioned discrepancy mellowing down the recorded signal in advance just in case, sometimes unnecessarily so. Hyperion measures actually well given its uncommonly high-powered open-loop hybrid design. Tailored? Maybe – not so bad though considering that the alternative is nothing but some one-size-fits-all attempt to "neutrality", despite whatever audio gurus want us to believe.
... "pleasant" versus "accurate" as David Hafler phrased it 30 years ago:
https://www.stereophile.com/content/manufacturers-comment-0
mrkaic - an amplifier transfer function can be modeled simply by an equation of the form Vout = K1*Vin + K2*Vin^2 + K3*Vin^3 etc ...
If the system is completely linear K2=0 & K3=0 and K1 is the gain.
If K2>0 then there will be some second harmonic and if K3>0 there will be some third harmonic (and so on for any harmonic). When you multiply such an equation by a sinusoidal input (Vin*sinF) you get (Vin*sinF)^2 that becomes a Vin^2*sin2F term using trig identities and similarly (Vin*sinF)^3 becomes a Vin^3*sin3F term. The levels of the harmonics vary with the input squared or cubed etc and when plotted on log vs log axes (or dB vs dB) you get straight lines with slopes of 1x for the fundamental at frequency F and slope of 2x for the second harmonic at frequency 2F etc. Radio engineers use this analysis to generate something called a "third-order intercept point" using the relationship of the slopes to predict distortion for any given power level based on a single metric (at very high frequencies amplifiers do not have enough gain to allow the use of much feedback so you are stuck with their inherent non-linearities). Intercept Point metrics have been proposed for audio in the past though I doubt it would be very useful but I was reminded of the concept when viewing Ypsilon THD plot as it fits the straight line theory nicely. But most of the amplifiers that JA has measured do not.
Ortofan - JA's results for the Benchmark amplifier suggest the harmonics are at or below the minimum measurable levels. Benchmark are an objectivist company so I would expect them to use as much feedback as possible to minimize distortion and am not surprised to see the result. Whether high feedback is good audiophile goal is an on-going debate. Feedback does change a large but simple non-linearity into a much smaller but more complex linearity and most of the THD plots I looked at are difficult to interpret. I do not envy JA trying to add some commentary around each set of results.
One that I was interested in is the PS Audio BHK Signature power amplifier that has a tube input stage followed by a MOSFET ouptut stage, similar to both the Ypsilon and Lamm power amps. In videos on the PS Audio website both Paul McGowan and designer Bascom King talk at length about how important the input tube is to the sound of the amplifier but its presence doesn't show up in the THD plot in terms of a 1x slope due to second harmonic distortion. The BHK amp does have global feedback that has reduced the input tube's objective footprint to almost nothing so I am surprised that the tube's sonic footprint is still apparent.
ok - I wondered if the harmonic generation from instruments follows the same slope relationship. That would explain why higher order harmonics go up more as the sound gets louder. But, as you suggest, whether it is best to have an amplifier with that same characteristic or have one which produces negligible amounts of distortion?
Regards
13DoW
Thank you for your clear explanation, but I’m still a bit confused.
The way I was taught about the transfer function is as follows: it is the ratio of Laplace transforms of the output and the input. So, voltage (or amplitude, in general) is not even an argument in the transfer function. Rather, the argument is the (complex valued) frequency. Also, transfer functions in this sense cannot be defined for Non-linear systems.
My guess is that we are using the same term for two very different functions.
It's certainly true that, in the context of LTI systems, the transfer function is the Fourier (or Laplace) transform of the impulse response. Of course, there are no physical LTI systems in reality (there are no physical impulses for that matter) so all this formalism is just a very useful and convenient approximation for physical systems that are, over some limited domain, range and time, LTI.
Nonetheless, I agree that transfer function for a non-linear system seems odd to me at first glance. Perhaps, in the pseudo-static case, transfer curve or transfer characteristic is more appropriate?
I agree fully. While there are no real LTI systems “out there”, transfer functions are used when the system is close to linear. What our colleague described, is a sort of input-output correspondence that emphasizes non-linearity. I think we might be using a the same term in two contexts. He appears to be a radio engineer and transfer functions might be defined differently in that field.
If I have used the term "transfer function" confusingly, mea culpa. If "transfer curve" is OK with you then what we are talking about is Vout/Vin measured at one frequency (1kHz). John measured the 1kHz gain as 26.4dB (20.2x) but that was taken for one value of input voltage. If the gain was 20.2x for all values of Vin then the "transfer curve", Vout vs. Vin, would be a straight line with a slope of 20.2x and there would be no distortion. In this case we would have Vout = K1 * Vin with K1= 20.2 and no second-order term because K2= 0.
What actually happens is that the gain is not exactly 20.2x for all values of Vin and the plot of Vout vs Vin is not a straight line but is a very slightly bent line. After a little manipulation of John's Fig.3 I calculated that K2 is actually 0.091 for the Hyperion amplifier. So, Vout = 20.2 * Vin + 0.091 * Vin^2. If you plot this function in a spreadsheet you will see that it looks pretty straight but is perceptibly bent compared to Vout = 20.2 * Vin. That very slight bending is responsible for the second harmonic.
Now I see that you radio engineers use a different terminology. That is cool, but I still don't get your derivation (I know, I'm not a smart person, please have patience with me.).
The horizontal axis in figure 3 is in units of output power. Since output power is Vout^2/R (John uses a load resistor of 8 Ohms in his measurements), you effectively have the following equation:
THD = f(Vout^2), that is, the THD is a function of squared(!) output voltage.
In the first order of approximation, this relationship is equal to THD = gain^2 * Vin^2/R (Since in the first order Vout = gain * Vin). So, I don't understand how you got the linear term in your equation.
Hi mrkaic,
I like to think of myself as an engineer of the world but it is interesting that different fields characterize distortion in different ways even though the non-linearity is the same bent transfer curve. Answering your questions has pushed me to the point where I have to write the whole analysis down for completness! I don't know if you can send PMs on this forum - if you can send me one and I will forward you the analysis when I have written it down. Otherwise I will find a way to make it accessible and post a link here.
Regards
13DoW
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