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Parasound Amppeteraczel | 05 April, 2006 20:00
2-Channel Power Amplifier Parasound Products, Inc., 950 Battery Street, San Francisco, CA 94111. Voice: (415) 397-7100. Fax: (415) 397-0144. E-mail: sales@parasound.com. Web: www.parasound.com. Halo Series A 21 stereo power amplifier, $1995.00. Tested sample on loan from manufacturer.
What? Another stereo power amplifier? Well, this one is a bit more review-worthy than most, even if it has been around for a while. It tries very hard to be “high-end audio” and to a large extent it succeeds, at about $3.85 a watt (into 8Ω), which is not unreasonable these days for an amplifier with pretensions not only in performance but cosmetics as well. This is a very good-looking product, and that plus its high power output (over 250 watts per channel into 8Ω) may be important to audiophiles who could buy a power amplifier of equal sonic capability for less money. Yes, all well-engineered amplifiers sound the same (if not clipped) but not all yield the same satisfaction of ownership. The Design The A 21 circuit was designed by John Curl, who is something of an icon in high-end audio circles; I vaguely remember him espousing some questionable tweako audiophile ideas years ago, but his basic expertise is not in question. David Rich, our former technical editor and circuit guru, muttered something about the open-loop JFET stage in the front end of the amplifier when he saw the circuit schematics; he didn’t think the 20 kHz distortion at full power could possibly be within specs as a consequence—but it is! (After all, it’s the black-box performance that matters.) This is a class AB amplifier; I was rather impressed by the heavy-duty output stage, sporting four pairs (4 × 2 = 8) of high-current bipolar Sanken transistors per channel, by the massive 1kVA toroidal power transformer, and by the amazingly open interior of the unit, consisting mostly of air and causing the cubic footage to swell to quasi-Krell proportions. Quality touches like double-sided printed circuit boards and Allen-head screws for the top cover abound. Even the sheet-metal front panel is shaped to simulate a solid machined piece à la high end. As I said, this is a high-end wannabe and it comes close. It even has both balanced and unbalanced inputs. The Measurements The quickest overview of a power amplifier’s performance is the PowerCube. As I’ve explained a number of times before, the PowerCube test measures the ability of an amplifier to drive widely fluctuating load impedances. As far as I know, The Audio Critic is the only American audio journal to publish PowerCube measurements. The instrument for the test is made in Sweden; it produces repeated 1 kHz tone bursts of 20 ms duration into 20 different complex load impedances across the amplifier (magnitudes of 8Ω/4Ω/2Ω/1Ω and phase angles of –60°/–30°/0°/+30°/+60°). The graphic output of the instrument shows the 20 data point connected to form a more or less cubelike polyhedron. The test shows up the differences between otherwise similar amplifiers when it comes to real-world loudspeaker loads rather than just resistances. The PowerCube of the Parasound A 21 is shown in Fig. 1. It shows that the amplifier is utterly stable into all loads, regardless of impedance, but that its maximum dynamic output of 51.6 volts (which comes to 333 watts into 8Ω) is considerably reduced into 1Ω. That’s no big deal; 22.3 volts into 1Ω (which comes to 498 watts) is still far from puny; and, besides, negligibly few loudspeakers dip that low in impedance at any frequency. What’s more, the voltage/wattage into complex impedances is slightly elevated across board, which is the way it should be. All in all, a very decent PowerCube, with only moderate slope between 8Ω and 2Ω.
Fig. 1: PowerCube of one channel with both channels driven. The three axes are output in volts, impedance in ohms, and phase angle in degrees. Fig. 2 shows the frequency response of both channels at 1 watt into 8Ω. It’s ruler-flat from 10 Hz to 10 kHz (down 0.04 dB) and –0.15 dB at 20 kHz. Can’t ask for more.
Fig. 2: Frequency response of both channels at 1 watt into 8Ω. The “king of curves” in power-amp testing is distortion vs. power output. Fig. 3 shows the THD+N of one channel into an 8Ω load, both channels driven, with inputs of 20 Hz, 1 kHz, and 20 kHz. The measurement bandwidth was opened up to 80 kHz for the 20 kHz (red) curve to include the 2nd, 3rd, and 4th harmonics, so more noise was included as well, and therefore you can’t expect this curve to coincide with the other two. As can be seen, the amplifier clips at a little over 250 watts into 8Ω, just as the specs claim, and even at 20 kHz the full-power distortion is only –57 dB (0.14%), which again accords with the <0.2% specs. The specs’ claim of <0.03% (–70.5 dB) distortion at typical listening levels is well exceeded. These are not infinitesimal distortion figures (as boasted by Boulder, Halcro, etc.) but they are more than good enough from the standpoint of audibility. As for THD+N into 4Ω, I did not measure it this time because the PowerCube clearly showed that the 4Ω performance matched the 8Ω performance quite closely in every respect.
Fig. 3: THD+N vs. power of one channel into 8Ω, with both channels driven, at 20 Hz (magenta), 1 kHz (cyan), and 20 kHz (red). I also looked at the FFT spectrum of a 1 kHz tone at full power into 8Ω, shown in Fig. 4. I wish the distortion products consisted almost entirely of 2nd harmonic instead of just about every multiple of the fundamental, but even so the measurement confirms Fig. 3 without tumbling major skeletons out of the closet.
Fig. 4: Spectrum of a 1 kHz tone at 250 watts into 8Ω, one channel. The single-point noise of the A 21, with a measurement bandwidth of 22 Hz to 22 kHz and referenced to clipping level into 8Ω, was –111.1 dB and –113.5 dB, respectively, in the left and right channels. Those are superb figures. Fig. 5 shows the crosstalk (i.e., separation) between the two channels at a level of 1 watt into 8Ω. The curves are somewhat unusual in that they bottom out at 500 Hz instead of reaching their minimum at the lowest end of the spectrum, but –70 dB or better at all frequencies, even the highest, and better than –90 dB at midfrequencies is a very respectable measurement that actually exceeds the specifications.
Fig. 5: Channel separation at 1 watt into 8Ω. Thus we see that the Parasound A 21, while it doesn’t set world records in any of the various measurement categories, proves to be a thoroughly clean amplifier on the lab bench. The Sound Longtime readers of The Audio Critic know the drill that comes at this point: I repeat, for the nth time, that all amplifiers having high input impedance, low output impedance, flat frequency response, low distortion, and low noise floor sound exactly the same when operated at matched levels and not clipped. (Those who are unable to stomach this simple truth, proved over and over again in double-blind listening tests, should stick with Stereophile.) Of course, the Parasound A 21, with its very high output capability, will not clip at levels where most others do and therefore sounds better at those levels. The deep bass profits especially from all those watts, especially through inefficient loudspeakers. But those who expect the heavens to open up and the seraphim blow their trumpets when they turn on the new amplifier—any amplifier—that they just brought home from the store in their hot little hands will be sorely disappointed. Recommendation I know there are some of you out there who just cannot listen to low-priced mass-market audio components (Pioneer, yecch!) no matter how they sound. It goes against your grain and you don’t want to know about it. To you I heartily recommend the Parasound Halo A 21 because it will satisfy your high-end cravings without bankrupting you. You’ll have the high-end cosmetics, the high power, and some cash left in your account. And, yes, it sounds really good, too.
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