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Positive Feedback ISSUE 9
october/november 2003


ps audio

HCA-2 amplifier

as reviewed by John Zurek

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Thiel 3.6

Vacuum Tube Logic 2.5 preamplifier,  Conrad-Johnson MF2300A amplifier, and a Lehmann Audio Black Cube phonostage

VPI HW-19 /AudioQuest PT6 /Benz Micro Glider and a Musical Fidelity E-624 24/96 CD player.

John Dunlavy Reference, Acoustic Zen MC2 interconnects, Acoustic Zen Satori speaker cables, and Silver Audio PowerBurst AC cords.

Headroom Supreme headphone amplifier, Sennheiser HD 600 headphones, Argent Room Lenses, VPI HW-16.5 Record Cleaner, Monster 2000 power conditioner, Vibrapods, Sonex panels, and AudioPrism Quiet Line filters.


How transistors were really designed to work.

I've been on the quest for the amplifier for about 10 years. When my addiction solidified in the early 90's, I bought a pair of Martin-Logan Aerius hybrid electrostatic speakers. Back then I was using a Classe-80 power amp paired with an Audio Innovations tube preamp. This combination with the Logans was an eye (ear?) opener. Glorious mid-range and highs, detail, imaging, soundstage, ahhhh…

But (you knew there would be a but didn't you?) the amp/speaker combination wouldn't play very loud without getting harsh, and on most recordings the bass was sorely lacking. Why? The Logans were a reactive load, dipped below 2 ohms, and were not very efficient. The woofer was not ported. I realized my little (but refined) Classe amplifier was not up to the challenge. These speakers were great, but hard to drive (although the problems mentioned were not entirely the amp's fault). My first real case of audiophilia nervosa.

10 years later I own another speaker that is also notoriously hard to drive—the Thiel 3.6. I'm still looking for the right amp. The 3.6's impedance dips below 2 ohms, and with their all-aluminum drivers, have a bit of a hard character when not fed a large quantity of quality watts. I could change speakers, but between the economy pecking away at my funds, and not being able to find a speaker that really betters the Thiels (without spending much more money), I'm keeping them. At least for a while. They're a bit long in the tooth; however, they are truly a classic, and while not strictly state-of-the-art, they speak the truth.1

What was my criteria for a new amp? In the simplest terms I wanted an affordable amp that would let me listen longer (and maybe a little louder). The 3.6s were unforgiving, and fatigue was my enemy. I hadn't yet found the amplifier that was a synergistic match for my speakers. I needed something powerful. But, (uh-oh another but!) I simply cannot afford the solid state likes of a Pass, Levinson, Spectral, Rowland, or Krell, or a monster tube amp like VTL, CJ, Joule Electra, or BAT. Solutions for Thiel speakers are easy if you can throw money at them. My mission (like so many real-world audiophiles out there) was to find the balance between price and performance.

I'd been through several affordable solid state current monsters—McCormack, Adcom, and Conrad-Johnson (all good, but not great with the 3.6s)—and was just about to try some affordable tubes—Rogue and Jolida (neither had enough power in my judgement) when I heard about the PS Audio HCA-2.

Digital Switching Technology

The buzz around audiophile circles was the HCA-2 sounded like a tube amp, but had solid state slam in the nether bass regions. We've heard that claim before haven't we?

Right! I'm down. I believe it. Not a chance!

Was I skeptical about this product? Absolutely. First, it was a new product. Second, PS Audio hadn't made amplifiers in a long time. Third, and most important, when I found out that this amplifier used digital technology—the warning antennas went up! I prefer pure analog sound, thank you very much.

I started by checking out their web site to see if they explained the technology. Shazaam! (Think Gomer Pyle) Not only did I understand what they were talking about, it made a boatload of sense. I actually remembered something from my university days besides how to smoke resistors and op-amps on the test bench. (I think I may also have smoked some components while performing measurements, but I don't remember, there was a lot of smoke around in those days.)

In a nutshell: Tubes were designed to amplify a signal. A vacuum tube is a valve, as our British brothers say. The more you open the valve, the more current comes through. Like a water faucet. Transistors, on the other hand, were originally designed to turn on and off, electronic switches. Not (originally) to amplify a signal. Maybe this is why so many prefer the sound of tube amplifiers. They are doing what they were designed to do, act as a current valve, while solid state amplifiers have been trying to adapt transistors to mimic what tubes do naturally.

Enter the HCA-2 using Pulse Width Modulation. PWM (not to be confused with Pulse Code Modulation—PCM—how a CD is encoded) is simple to understand.

The HCA-2 output uses PWM to switch the current on and off so fast that you hear it as being continuous. It is electronic switch, rather than an analog "valve", connected to the amp's power supply. Like all switches it is either on (1) or off (0). The switch turns on or off in longer (wider) and shorter (narrower) time periods. If the volume of the music is low, the on/off times (pulses) are very short. The louder the music gets the longer the switch stays on. A PS Audio proprietary filter using capacitors and inductors smoothes out the gaps.

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Guaranteed or your money back!

I'd called all the other amp manufacturers I was considering (we won't mention them here) and when I mentioned the 3.6s, they all sounded a little shaky. Several just flat-out said "we don't recommend Thiel speakers with our amp". Chickens.

The chap I spoke to at PS Audio was very, very confident about the HCA, but didn't sound like a used car salesman. I told him about my speaker's most unfriendly impedance and sensitivity. The HCA-2 was rated at just 150 watts. I also told him I'd previously used 240-watt monsters with minimal success. He laughed and said I was comparing apples with oranges. The HCA-2 was not like conventional amplifiers, and the load (resistive or reactive) just didn't matter. Further, he said the HCA-2 would not only put a strangle hold on the Thiels, but he was absolutely sure I would be happy with the combination.

My interest was piqued. When I found out about the 30-day in-home trial (you pay the shipping return, $22 if you decide not to keep it) I just had to try this amp. I gave them the sacred Visa number and expiration date.

I wish more high-end companies would adapt this trial period thing. I've experienced more and more audio shops refusing to let customers audition components at home. That translates to: No $ale.

Listening, Listening, and more Listening

It finally arrived. I knew the HCA-2 needed at least 100 hours to break-in. 200 would be better. But I couldn't resist. I had to get just a peek at the initial out-of-the-box character was. I set it up and fed the CD player a disc. My first impression? Smooth—oh so smooth. The Thiels never sounded like this before. Tall and wide soundstage. Bass was lacking.

After removing the stock power cable and switching to my trusty Silver Audio PowerBurst, I put my Musical Fidelity CD player on Random/Repeat. Then, I tried to convince myself to just forget about it for a few days and let it cook.

My first real listening session came 4 days later. Did the HCA-2 really sound that smooth 100 hours ago? Yes, and even more so now. Excitement. I'm stuffing a lot of reference disks in the CD player now, listening to all those cuts I use to audition new components. Mids are natural—the first good sign. I plunk in "Jack of Speed" from Steely Dan's Two Against Nature. There's the bass guitar slam I've been missing! Cymbals sound right. It boogies.

I spent some time the next day repositioning the speakers, and adjusting the Argent Room Lenses. After a while I was able to achieve an unprecedented spatial presentation in my room. The soundstage was indeed huge.

wilson.jpg (14997 bytes)In goes Cassandra Wilson's New Moon Daughter. The bad news: Something wrong… the muted trumpet solo on "Love is Blindness" should be stage left, but it is definitely stage right. Unless I'm really mistaken the channels are reversed. Yes. No doubt. Checking the specs doesn't mention anything about this. The good news: Harmonics are rich and luscious—not unlike like the best Swiss chocolate, leaving a delicious lingering taste you won't soon forget.

I'll make a note to call PS Audio later2—right now I want to keep listening. I reversed the inputs from my VTL preamp and pressed on.

After a while I just couldn't help but notice the width of details. The sound is really, really panoramic... and... is coming from just about everywhere but the speakers. I'm hearing… so much detail from all the instruments and voices that are not center stage, or even close to center.

I always assumed that these sources (on many recordings) were always recorded like this, i.e., as you move away from the center of the soundstage, the supporting instruments or voices don't have the detail the important stuff in the center does. The HCA-2 has changed that forever, the resolution of detail is now equally vivid throughout the spacious soundstage.

One of the few things that didn't change was front-to-rear depth. The soundstage was wider and taller, but not deeper, although the depth was decent to being with.

Over the next few weeks (I had three more to decide whether to keep the HCA-2), I kept the amp playing to determine if there would be any further break in changes.

What did I hear? The lower bass regions became more forceful and pronounced on some discs.

I decided to use my low bass evaluator recording "Temple Caves", track seven from Mickey Hart's Planet Drum. The track includes a drum—I assume this is the "earth drum" listed in the credits—that is the lowest drum frequency I've ever heard. When reproduced properly it has the effect of energizing a room—you feel it more than you hear it. I think the frequency is about 30 Hz or maybe slightly less. (The Thiels normally go down 30 Hz, or 27 Hz at –1.5 dB.3) Usually you need a really good sub or a real full-range speaker coupled with high-quality solid-state watts to do this track justice. The HCA-2 was up to the task! Darn near perfect, not too heavy (as I've heard with some subwoofers), not too lean—as I've heard with other amps driving these speakers in my system. No worries.

taj.jpg (17877 bytes)Mid Bass. The HCA-2 seems to have a controversy in this area. I've heard arguments on both sides, i.e., some (including PS audio) claim it has lots of mid-bass energy. Others say it's mid bass is lean. My take? For the first time since I've owned the Thiels the mid-bass was right. On Taj Mahal's Senior Blues the title cut begins with a dotted-eight sixteenth note ostinato on the bass guitar that continues through most of the tune. It really drew me in to the music like never before, and I've listened to this tune many times. In the middle the bass stops, and the drums now take over the bass line, a quarter-note triplet figure in stop-time, with the drums almost perfectly pitched to match the song's key signature. The bass guitar then takes over, playing the same figure, same pitches. Wow! I never understood this dialog before. This tune took on a whole new meaning. Gotta love it.

The mid-range and high frequencies seemed to change very little from my first impression. Smooth highs, with plenty of detail, natural mids.

It took me a while to get used to the change in dynamics that this amplifier portrayed. I was able to listen to most recordings at a much lower level and get great satisfaction. But when I cranked it up… I had Alan Hovhaness' "Mt. St. Helen's Symphony" on, the third movement "Volcano". This movement portrays—you guessed it—the Mt St. Helen's eruption in an explosive- programmatic sketch. When the percussion make its entrance at 1:44—how do I say this—it scared the living hell out of me! I jumped up and almost fell out of the listening chair4. The sheer emotion of the eruption sent adrenaline pulsing through me. I think Hovhaness would have approved.

After a few weeks of playing with the speaker placement and toe-in, and moving the Room Lenses to maximize the soundstage, I had my audio buddy/bass player Scott take a listen. He is one fussy dude, and has never been a big fan of the Thiels. He closed his eyes and listened to the entire "Here Comes the Sun" side of my Abbey Road Special Edition on vinyl. I'm standing in back of the listening chair, watching his left hand finger (almost) every note that McCartney is playing. He obviously knows this record very well. "Damn. This is the best I've ever heard these speakers sound. I'm hearing so much I've just never heard before. Some of the bass parts are different than I thought!?"

Criteria more than satisfied?

How did it stack up to my original criteria? Well sailor, the HCA–2 will love you (let you listen) a long, long time! And—the HCA-2 drove the 3.6s to louder levels than I've ever heard before turning harsh. Icing on the cake included detail, resolution, accurate and slammin' bass, detail, rich harmonics, an absolutely awesome soundstage, and did I mention detail without fatigue?

Every disk I played, whether digital or vinyl sounded not only better in almost every way, but much easier to listen to. The music was conveyed with genuine emotion.

Nits? The soundstage depth was average. The speaker wire terminals were not the highest quality. And, I hate to say this, I wish the HCA was bigger. I miss the looks of my heavy C-J and/or McCormack. They filled up the amp stand. Cooling fins everywhere. 2 person lift. Funny isn't it: Those muscle amps were huge and heavy, yet the puny little HCA pumps waay more power into my very hard to drive speakers. I guess we all have our little neuroses!

This amplifier embodies the antithesis of transistor sound. Silky smooth, with microscopic detail, but none of the blurring that can occur with tubes. Bass that only solid state or the finest of tube amps could touch. I've never felt so involved in the music in this particular room before using this component.

The Right Tool for the Job

Let's get something straight. This is a special product. A completely new design. Not something that's been around the block, had some new parts, wire, or a bit of circuitry changed. This is a new technology that I believe will become the basis of the future of solid state amplification.

Want an amplifier that is easy to listen to, an amp with gobs of detail, smooth highs, most natural mids, slamming bass, and most of all accuracy? An amp that sounds at home with every type of music? Have low sensitivity speakers, first order crossovers, planars, or electrostatics? Don't want to take out a second mortgage to drive your system? Well then, let the transistors in the PS Audio HCA-2 do what they were designed to do. Switch.

You owe it to yourself to try this amp. Is it perfect? No. Am I going to pay the shipping price to send it back? Not a chance. The HCA-2 performs like one that costs much, much more. This is a very unique component, one I predict that will be much imitated. One of the very best amplifiers I've heard at any price. John Zurek

1. I saw amplifier manufacturer VAC pair the 10 year-old 3.6's with one of their products at recent (2003) hi-fi show. I think this says something about a speaker that's more that hasn't seen a design change in 10 years!

2. When I called PS Audio re the channels being switched they were aware that one lot of HCA-2s were shipped with the inputs inverted. Since the PS Audio factory is about 90 miles of Colorado highway up the road I decided to go to Boulder and let them fix it while I visited my favorite vinyl shop. I returned with a correctly configured amp, Miles, Monk, Chambers Brothers, Sibelius, Bach, and Little Feat.

3. Using the Stereophile Test 2 CD I played the lower warble tones, and got usable bass down to 25Hz - 2Hz below the stated frequency response of the 3.6.

4. I decided to compare the percussion entrance on my headphone rig. Almost nothing sounds as good as the Sennheiser 600s with a good amplifier. Well—through the 'phones the sound just couldn't compare, no matter how loud I played it. The sheer force the HCA-2 generated could not be approached.

PS Audio HCA-2 amplifier
Retail: $1695

PS Audio
web address: