Ouverture

As I was saying in the preview, the PS Audio HCA-2 power amp is a sort of premiere down here, being the first amp sporting a digital technology power stage to be actively marketed in the audiophile market sector. The so-called class D amps, one of the hot arguments in the audio field, have some very interesting features, mostly in terms of efficiency and compactness. I have seen, in a consumer audio supermarket-like shop, an impressive Sony system, packing a DVD player and a 5-channel, 60W amp in a compact CD player-sized case.
Efficiency and compactness allow savings on cabinet dimensions and heatsinking, and these savings are brought forward to the consumer, lowering the product's final price. In fact, here we have a 150W per channel power amp, made in the U.S.A., sold at a really competitive USD1700 price.
I am sure the price could be even lower, had PS Audio skimped on build and sound quality; but we are talking about a company for which sound quality is the first concern, and the HCA-2 builds on the previous experiences of the living legend behind the Colorado manufacturer, Paul McGowan.

HCA means what?

HCA is an acronym for Hybrid Class A. We are used, in the audiophile world, at calling hybrid a design which includes one or more solid state and one or more thermionic valve (or tube) circuit stages at the same time. In our case, we have something new and innovative. The hybridisation is between a traditional class A (the CA part of the amp's name) input stage and a digital technology-based power stage. Thanks for asking about the "2": this power amp is the ideal follow-up to a legendary Infinity amplifier, which was named HCA and was a hybrid amp as we were used to know them until today (valve input stage, solid state power stage).

Digital amp with a twist

A small number of Class D amps have already been introduced into the audiophile market, but none of them, as I wrote above, has been actively marketed down here. In the rest of the world there are the well-respected and well-reviewed Spectron and TACT units, but no distributor seems to have noticed in this nick of the woods.
A description of digital amplification technology is beyond the scope of this writing (there are more qualified people than myself to perform such a task); still, some basic elements are necessary to explain the HCA-2 difference.
A standard "analogue" amplifier continuously modulates the incoming signal; a digital amp has two states only, fully on and fully off. Signal reproduction is thus generated by how long the states last: the longer the "on" period, the louder the output signal. This principle is named PWM, Pulse Width Modulation. In this way, the efficiency (the ratio between the energy drawn from the mains and the output power) reaches 90%, which means that only a 10% of power is dissipated as heat.
But there's a host of troubles, the main ones being radio-frequency emissions and the need for an output filter to smooth out the transitions between the on and off states. The radio-frequency emissions problem can be cured; the output filter, though, implies other consequences, the main one being the fact that a filter can be optimised for a specific load only, which makes for power stages whose frequency response is strongly influenced by the loudspeaker system's impedance value and curve (a loudspeaker system is far from being an ideal load).
The HCA-2 solves this problem, according to PS-Audio, by using the patented S-DAT principle. The solution is almost one of those that make you wonder why you didn't think about it by yourself - almost: the output filter is included in a special negative feedback loop; the result of this is an ability of the whole circuit to comply to any load in a virtually instantaneous way. The amount of negative feedback is modest; it is told that the amount was lowered according to the first listening tests. As an outcome, the amp is nothing special on the test bench, but we are told that the sound was much improved. The input stage is a class A intrinsically balanced design. An important part of the chassis hosts power supply components: an oversize power transformer, which was selected by ear - the first test samples sported a smaller transformer, but the bigger one was the better sounding - and the main components of the passive Ultimate Outlet power conditioning filter to clean up the incoming mains.

Description and use

This amp shows a nice look, compact and devoid of any heat-sinking fins. The amp worked without a hitch during my listening tests. You just operate the switch, and the Standby indicator lights up; after fifteen seconds, the Operate indicator and the blue PS Audio logo come to life and the device is ready to perform. The amp is cold even after hours of listening; only the slightest warmth in the back right corner, where I think the input stage is, is noticeable.
There's a Fault indicator also - luckily, I don't know what is needed to switch that one on.

Sound quality

I already said something about this amp's sound quality in the preview. Now I can go further, I think I have gained some more insight on its sound character. The system was my usual one, Sony SCD-777ES SACD/CD player, Scheu turntable equipped with 42cm*8 platter, VPI JMW12 tonearm, Benz/Scheu and Lyra Clavis Evolve cartridges, DACT CT100 phono preamp, Tom Evans Vibe line preamp, Krell FPB200 power amp, B&W Silver Signature loudspeakers, Audiotech, Mana and Target stands and tables, Kimber interconnects, B&W speaker cables, ART, Electrocompaniet and Audio Agile power cords.
The amp's main sound qualities are just about the ones I described: excellent deep bass, a somewhat leanish midbass compared to my reference - which is a challenger for the world champion title in that range -, slight emphasis on sibilants, pleasant sweetness and palpability of the high range and very good harmonic body. Grain is subtle and delicate.
Speed, transient response and micro-dynamics, while not being standard-setters, are satisfactory. It's a very well balanced amp, there is no emphasis on a particular sonic parameter to impress the listener in the first few minutes.
Soundstaging/imaging is somewhat different than what I am used to. The front line is slightly projected towards the listener and I perceive a slight tendency to flatten the farthest entities on the soundstages against the boundary; lateral spread is more limited than with my Krell; height is more realistic with the PS Audio amp, though, and that's a nice result (the price of the Krell is at least five times that of the PS Audio). I told you about focus in my previous writing; it's somewhat more diffuse than what is provided by the pinpoint focusing champs, but know that pin-point images are considered by many to be an hi-fi artefact, and those will find something more to their liking here.
In the preview I pointed out some subtractive colorations. I think it's a slight (I mean slight, this is a review, not poetry: don't over-weigh what I write, please) dynamic discontinuity between the extreme and medium ranges; it's as if the last amount of energy was lacking from the midbass to the mid-high frequencies. It's a kind of authority the Krell provides in spades, but that's one of Dan D'Agostino's trademarks. It's completely possible that this is an amp/loudspeakers/room issue and that it is related with that barely perceivable midbass leanness I was describing above; with more supportive rooms, or more enthusiastic speakers, it won't probably be there - as always, listen for yourself, with your speakers, in your own room.
The noise/hiss which surprised me at first listen and which I have already described is, according to my information, a direct consequence of the low negative feedback amount. I think this noise makes for my perception of a less black silence between stage entities than I am used to; and it's possible (but I can't be certain) that this is the source of a somewhat lower resolution, as if the different recording venues were less easy to discriminate.
Anyway, this is nitpicking. These observations require a direct comparison with a different amp to be perceived. At the end of the day, what I get is the sensation of being in presence of an extrovert amp, one which just never tires of playing music.
The best sound performance is obtained by using a high quality power cord and by plugging it to a wall outlet: resolution, speed and transparency are improved and the midbass is fuller - I think a good connection to the mains helps PS Audio's mains purifying technology get its job done.

Summing up…

Summing up, well, the question stays the same: would I buy it for myself? The answer is: had I decided for this kind of budget for a power amp and weren't I a reviewer with the need for a maximally transparent system (sometimes being a reviewer forces to choose against the maximum musical enjoyment) I would consider this amp very seriously. I think giving this amp a try is something of a must for anyone who would like to understand what is possible to achieve nowadays at this price level. By the way, PS Audio has a nice, liberal in-home trial scheme. Do yourself a favour…