September 2002PS Audio HCA-2 Stereo Amplifier
by Tom Lyle
A little explanation of this HCA-2s hybrid class-A digital output stage is necessary, but I strongly suggest you visit PS Audio's website for a much more comprehensive discussion. The folks at PS Audio believe that if you treat each of the two independent sections of a power amplifier -- the input and output stages -- as separate and unrelated, and then optimize each without regard to the other, it doesnt really matter what technology you use to achieve the highest performance.
So the HCA-2 has two distinct parts with unique topologies: the voltage-gain input stage and digital output stage. The voltage-gain stage is a zero-feedback, JFET input stage -- a simple discrete analog circuit with, according to PS Audio, "astounding common-mode rejection, low distortion, fully differential balanced inputs" and is, "in short, a perfect voltage-gain stage." A digital output stage is not the same as a digital-to-analog converter, which is used in the digital front-end of an audio system. Basically, instead of the output devices moving current up and down to match the musical signal at the input to the amplifier, a digital power amp like the HCA-2 has only on or off states at its output.
It is for this reason that this current-gain stage runs with virtually no heat. It is a digital output stage known as SDAT (Super Digital Amplification Technology) and unaffected by the loudspeaker load it is driving because it does not have the problem of varying impedances affecting the amplifiers performance. According to PS Audio, problems common to other digital amplifiers, such as a hard, bright, or sterile sound, or problems sounding right on different speakers, are eliminated when using SDAT technology.PS Audio claims that the HCA-2 is nearly 95% efficient (also why it generates almost no heat). This means that almost everything the amplifiers power supply delivers to the amp is used to produce music. Traditionally, a smaller power supply is used when requirements for power are lower due to the efficiency of the design. But PS Audio uses a large, custom-wound toroidal transformer that was designed for a typical class-A/B 150Wpc amplifier. PS Audio claims that this is one of the reasons why the HCA-2 sounds "so big, so alive."
The HCA-2 can communicate with any other piece of PS Audio equipment via the included PS bus. This bus can be linked to other PS Audio equipment through a standard CAT-5 communication link (which looks very similar to a standard telephone cord). It can turn all connected equipment off or on, among other features. The HCA-2 also has a DC trigger for A/V use.
PS Audio put great emphasis on HCA-2's chassis. The company worked with designer Alex Rasmussen to create a sleek-looking cabinet that was reportedly inspired by classic architecture and, believe it or not, the Audi TT. The car, according to Rasmussen, "was most inspirational in the HCA-2s front-panel power/standby button detail" and will also appear on some of PS Audios future products. The only other feature on the front panel that is visible once the power is turned on is a lighted PS Audio logo, whose brightness can be adjusted with a bottom-mounted control. The rear panel sports an IEC AC receptacle and both single-ended RCA and balanced XLR inputs. The speaker outputs are WBT five-way binding posts. A nice feature is that AC input is via a built-in high-current Ultimate Outlet, which PS Audio sells separately for $399.
The HCA-2 delivers 150Wpc into 8 ohms, 225Wpc into 4 ohms, and 275Wpc into 2 ohms. It measures 17"W x 4"H x 15"D and weighs 30 pounds. The HCA-2 ships with a 14-gauge shielded power cord, but PS Audio recommends their own Mini Lab or Lab Cable "to further enhance the performance" of the amp. I used the stock cord, but I was able to plug it into a PS Audio P-600 Power Plant.
Expectations and sound
In their literature, PS Audio states that the sound of the HCA-2 combines the best of both tubes and solid-state. If I had a nickel for every time Ive read this about a piece of audio gear I could have bought a house with the cash by now. Even so, my first impressions of this amplifier, even before it was fully broken in, were very positive. It had tight, tuneful bass, a lush midrange, and sparkling and grainless treble. It wasnt quite a combination of tubes and solid state; it just sounded like a very good solid-state amp.
One of the first LPs that I spin when just about any new piece of equipment graces my system is Miles Davis Kind of Blue [Columbia/Classic Records CS-8163]. I know this choice of music seems like an audiophile cliché, but everyone should have a reference for setting up and testing, and this album is as good as any -- and better than most. Indisputable talent occupies the midrange, namely Miles Davis on trumpet and John Coltrane on tenor saxophone. These instruments had very natural presence with very few aberrant peaks or dips in frequency via the HCA-2, or any other artificial flotsam that I could discern. When it came to midrange reproduction, this amp just got out of the way, and endowed the signal with pure gain, so the speakers could then do their job.
After some extended listening, I noticed a slight increase in the amount of upper-midrange energy, but it was hardly a huge defect. Yet this ever so slightly bumped-up region may have been reason for a characteristic that did sound somewhat tube-like. The upper mids ended up sounding bouncy; that is, they displayed a slightly rounded transient response. This made certain instruments, especially percussion or others that veered toward the treble, had a rather animated timbre. The horns on Kind of Blue were affected by this character, but in a good way. I felt as if somehow this enhanced the sense of the musicians being connected to their music. When playing Kind of Blue, the sound was what high-end audio is all about -- that the pursuit of being aurally transported back in time was somehow closer to fruition.
But I don't want to give the impression that this amp leans toward euphonic sound. It doesnt. Its just that this slight anomaly in a narrow band in the upper midrange added to the amp's positive characteristics. In fact, in my listening notes I wrote that other than this quirk, the amp sounded exceptionally fast, and quick transient response was noticeable throughout the frequency range. Tautness was also heard in the images, which were notably stable. The resulting soundstage wasnt as wide as Ive come to expect (especially with more expensive amps), but it was good nonetheless.
Fans of female vocals ought to check out Eva Cassidy. All of her CDs are good listening, but my favorite is Live at Blues Alley [Blix 10046], the only CD that was released during her all-too-short life. Even though the recording was most likely executed on a shoestring budget, it is excellent. Only a touch of reverb is added to Cassidy's voice on these standards; the rest of her five-piece band is recorded as they played. Through the HCA-2, the entire CD sounded very good, but very noticeable was the highest treble, which sounded simply fantastic. I hesitate to use the term "attractive" because the treble hardly called attention to itself, but it had such a grain-free and unfatiguing personality. It was just there. Ride cymbals had just the right amount of sizzle. Plus, even though this isnt the most challenging recording that has passed through my system, it was fairly obvious that its separation of instruments was just about perfect. In spite of this, there was no real highlighting of instruments that made the recording sound overly detailed. I guess that is the greatest compliment I can pay to a component labeled "digital." There was still that slight rise in the upper midrange that I spoke of, and this time it manifested itself as a bit of glare, but this might be a bit of exaggeration. Still, this CD sounded great.
The bass foundation laid down by the PS Audio HCA-2 was impressive. I suppose my impression of the bass was a little skewed because I was so surprised that such bass could emanate from such a relatively petite amplifier. The dimensions of the HCA-2's cabinet werent at all what I expected, and while its relatively large toroidal transformer adds some weight, the HCA-2's 30 pounds aren't going to tax any audiophiles floors or equipment racks. Just the same, the deep bass coming from this little amp was pounding.
However, even more impressive was its midbass response. Most audiophiles might not realize how important this area of an audio components sound is. For it is in this region that the fundamental tones of all instruments that contain any bass information reside. Dont get me wrong -- ultra-deep bass response is very enjoyable, and the deep-bass overtones of any instrument are extremely important. But the key to pitch specificity and stability lies in this midbass area. The lowest note of a piano is 27.5Hz, and I doubt there is one high-end amplifier on the market that cant reproduce the frequency of this note and lower. But it is the clarity of the bass frequencies above this where the HCA-2 really excels. Detail in the bass came not only from the slightly higher frequencies, but from the detail contained within the bass note itself.
With the Gladiator soundtrack [Polygram 467094], the bass is what I think makes this CD so popular with so many audiophiles (although Im also drawn to Lisa Gerards otherworldly voice). The HCA-2 doesnt just get the bass right; it communicates more of the information within the bass that the composer meant the listener to hear. It wasnt the kind of detailed low end that many were in awe of when CD first hit the streets, but a more organic depth coming from the pumped-up orchestra.
One of the last recordings I played through the HCA-2 before I started writing this review was of Prokofievs Alexander Nevsky, with Yuri Temirkanov conducting the St. Petersburg Philharmonic [RCA 09026-61926-2]. This is a reproduction of the original complete score for the Eisenstein film. The cantata for this Soviet film is what is usually played, so this complete version is a revelation. I wanted to test HCA-2 with some of the most complex music I could find, and this recording of a large orchestra and chorus was perfect. Could this amp sort out this thorny masterpiece and at the same time retain its composure while doing so? Easily. The HCA-2 passed with flying colors, delineating the details of the score while at the same time demonstrating all of the positive sonic characteristics I outline above.
But what if you spend twice the money?
Of course, there is no guarantee that spending more money will buy you a better amplifier. But my reference Krell KAV-250a sold for $3000 before it was recently discontinued (its replacement, the KAV-2250, sells for $4000) and is, at least in my system, a benchmark for solid-state amplification at a somewhat affordable price. Ive compared it to countless other amplifiers, and it has always showed itself well.
After the PS Audio HCA-2 was in my system for quite a while, I put the Krell amp back in. What first struck me was that the KAV-250a had a much more seamless presentation. When I listened critically, I didn't have to pay as much attention to analysis of individual qualities. From the lowest lows to the highest highs, from the depth of the soundstage to the images within the soundstage, there was more of a sense of completeness in Krell amp's sound. This gave the impression that the bass went deeper and the highs were more extended, that the midrange seemed more transparent as well. However, the PS Audio HCA-2 held its own, especially when compared to the memory of all the amplifiers I've heard that are within its price range. Spending twice the amount of money on an amplifier might not produce sound that's twice as good, but in this case there is definitely an audible difference.
To make a long story short
My first thought when finally sitting down to do some serious listening to the PS Audio HCA-2 was to have an open mind. Im not a digiphobe; I have a huge CD collection. But when faced with the choice between analog or digital sources, the majority of the time Ill choose analog. Wow, was I pleasantly surprised by the HCA-2, which did not sound anything like what I have come to recognize as digital.
So dont let the fact that the HCA-2 is a digital switching amplifier cause you any concern. This is a fine amp that uses a new technology to perform the age-old task of increasing the gain to the speakers, and it does quite a good job of it. Its frequency response is first-rate, with an extended, grain-free treble, lucid midrange, and whomping bass. What the very inventive folks at PS Audio deliver via their HCA-2 is a muscle amp in a small, good-looking package. What makes it even better is that the advanced technology that PS Audio uses to deliver the power makes the amp run with almost no heat and makes it compatible with many different types of speakers. Combined with its small footprint, these things make HCA-2 a good match in just about any system.
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