PS Audio HCA-2 power amplifier Page 3

"The amount of current required to switch a MOSFET device is minuscule relative to a bipolar, and their switching speed is superior as well," said Paul. "The HCA-2 switches on and off in a series of pulses. If you have a very quiet passage of music, if there is very little musical signal, the pulses are still happening, but they're on for a tiny period of time. When the musical signal gets louder, then these little pulses get longer. The longer each pulse stays on, the louder the music gets. So by changing the width of the pulse in response to the music, we end up generating the musical signal."

"Just like pulse-width modulation in a CD player?"

"Just like pulse-code modulation in a CD player," Paul corrected. "Only the code is the varying widths."

The HCA-2's pulse-width modulation is "superlinearized" as a gain block by incorporating the amplifier's output filter stage in the audio feedback loop. According to Paul, doing so overcomes the problem of varying load response.

"In any digital amplification scheme, you need to filter the ultrasonic switching frequencies out of the audioband before the audio signal reaches the speaker. But a loudspeaker presents a varying load to the amplifier, not a constant load, or a steady impedance. This changes the response of the output filter. Our SDAT filter's response remains unchanged for any load impedance on any speaker, so the sound of the HCA-2 is virtually the same under all speaker load conditions—for instance, with your reference Quad ESL-989 electrostatics."

Like other electrostatic designs, the Quad presents a "reactive" load, meaning that its impedance varies widely. The SDAT filter is designed, in effect, not to react, even to a "reactive" load.

The HCA-2 consists of two stages. "The input voltage gain stage and the output current gain stage perform best when they're absolutely isolated and independent from each other," Paul explained. "Our input stage could be attached to almost anything that produces power."

The same input stage is being used for PSA's more conventional Classic 250 Reference Standard power amplifier, scheduled for release this fall.

"Could you have used tubes in the input stage?"

"Absolutely, you could have tubes in the front," Paul replied. "The input stage is completely analog. I considered using tubes, but tubes give off heat, and part of the idea is to have this amplifier run cool.

"I have a fundamental dislike, not of the sound of tubes, which I love, but for the unreliability of tubes and their microphonics. The minute you turn them on, they begin to die. They are inherently musical, but, like I said, they're funky devices."

Who knows? A cottage industry of tweakers might emerge, retrofitting tubes to the input stage of PS Audio's HCA-2 power amplifier. I can imagine the reclusive Stan Warren producing his custom version of the HCA-2 and calling it a "PS-S Audio." But so far, Stan Warren has been no greater fan of glassware than Paul McGowan.

"J-FETs have much the same sound as tubes," Paul reassured me. "By the way, the input stage uses all discrete components. We don't just throw a couple of op-amps in there."

The HCA-2 is fully balanced from input to output. Both balanced XLR and single-ended RCA input jacks are provided. There is just one pair of speaker binding posts, but of very high quality, and two RJ45 jacks for connecting the HCA-2 to other PSA components, such as the forthcoming 8.0 preamplifier.

"The common-mode rejection on its inputs is far beyond almost anything else on the market," said Paul. "The common-mode rejection of the HCA-2 borders on 80dB, which is almost perfect."

"That assumes you use balanced connectors."

"But I have found that an amplifier that uses a simple method to achieve common-mode rejection sounds far more open and far more musical—tubelike, if you will—even with a single-ended connection. I'm not sure why. This is why I always have no feedback in the input stages of my amplifiers." A small amount of feedback is, however, applied to the HCA-2's output stage.

The HCA-2 is an attractive product, inside and out. A mere 4.5" high, 17" wide, and 14" deep, the amp is compact and attractive. Not a heatsink in sight! (Class-D operation requires only the smallest of heatsinks, like those found in a preamp or CD player.) It's lightweight, too, at 35 lbs. This is what I need—an amp I can easily lift and move from room to room. The amp ran so cool at my place, even in July, that it didn't get warm.

I was intrigued most of all by what was inside.

"It looks like most of the amplifier is dedicated to cleaning up the AC power," I observed.

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