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AT6012 Amppeteraczel | 15 October, 2007 10:21
12-Channel 6-Zone Power Amplifier Amplifier Technologies, Inc., 19528 Ventura Boulevard, #318, Tarzana, CA 91356. Voice: (818) 343-4777. Fax: (818) 343-7444. E-mail: amptech@ix.netcom.com. Web: www.ati-amp.com. AT6012 twelve-channel six-zone power amplifier, $1995.00 (discounts sometimes available). Tested sample owned by The Audio Critic.
As I have explained a number of times before, I am no longer interested in reviewing conventional (viz. me-too) audio electronics. Now, it could be argued that a 12-channel power amplifier capable of nearly 1000 watts total output is not exactly conventional, but my immediate reason for getting involved with the AT6012 was to power my newly upgraded Orion++ speaker system with a single amplifier. The Orion++, as distinct from the basic, plain-vanilla Orion, is ideally driven from 12 clean power-amplifier channels, and the AT6012, which clips at 80 watts per channel into 8Ω, appears to be the most powerful 12-channel beast available. I could have used several amplifiers, of course, but matching the gains of all 12 channels is a great inconvenience that way. (The Orion++, which adds rearward-firing tweeters and two subwoofers to the original Orion, will be reviewed here presently.) The Design ATI’s basic amplifier design is not new. I reviewed the six-channel AT1506 in Issue No. 27 (downloadable from this website) more than six years ago, and I see no significant differences in topology between the two models, except of course for the intended power output per channel, which is approximately halved in the 12-channel model. Morris Kessler’s classic circuitry appears to be timeless; I refer the reader to the older review for details. The AT6012 is divided into six two-channel modules, each of which can be assigned to a different “zone” and independently gain-controlled via a special three-wire connection, six in all. I was not interested in this feature, since I was powering 12 drivers simultaneously through 12 channels at full gain. Each module is fully protected by a special circuit against overloads and shorts, automatically resetting itself when the excessive input or the short is removed. As for the 12 output terminals, they are spaced for dual banana plugs, thank goodness, unlike those crazy European speaker terminals. That alone is worth the price of admission to users of elaborate wire harnesses. The Measurements I must hasten to confess that I did not measure all 12 channels. Two random channels, simultaneously driven, were all I had the patience for. Presumably, the remaining ten channels are similar. I began with our specialty, the PowerCube test, which is best performed with two simultaneously driven channels, although the readout shows only one channel. As I’ve explained many 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 points, at 1% THD, 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. Fig. 1 shows the PowerCube of one channel of the AT6012. It is a typical result for an amplifier that is perfectly stable and adequately power-supplied into 8Ω and 4Ω loads but not quite so perfect, although still pretty good, into 2Ω and 1Ω loads. The output at 1% distortion into 8Ω/0° is 26.1 volts (85 watts); into 4Ω/0° it is 23.4 volts (137 watts); into lower impedances it drops off sharply. The very slightly rising output into all reactive loads of 8Ω and 4Ω is as it should be; into reactive loads of 2Ω and 1Ω there is some unevenness but nothing major. The drop-off in output voltage into the lower impedances is of course inevitable in an amplifier without a gigantic power supply. All in all, a decent but not a brilliant PowerCube.
Fig. 1: PowerCube of one channel with two channels driven. The three axes are output in volts, impedance in ohms, and phase angle in degrees. Fig. 2 shows the standard distortion vs. power output measurement, without which no amplifier can be properly characterized. The six curves represent THD+N in two channels with test tones of 20 Hz (bottom), 1 kHz (middle), and 6 kHz (top), into a resistive load of 8Ω. The 6 kHz test tone was the highest that could yield a meaningful result with the 20 kHz measurement filter used (harmonics of 12 kHz and 18 kHz). The curves are largely, but not entirely, noise-dominated; maximum distortion at any power level above ½ watt with any input frequency in any channel is –60 dB (0.1%), which is not particularly low but certainly inaudible. The average distortion at typical levels is at least 12 dB lower (0.025% or thereabouts), and at 20 Hz in one channel the distortion dips to a spectacular low of –94 dB (0.002%) in the 7- to 11-watt region. Clipping at all frequencies occurs at 80 watts into 8Ω, which as I said appears to be a record for 12-channel power amplifiers. I did not measure the distortion into 4Ω because the PowerCube clearly shows that the results would scale to the 8Ω measurements.
Fig. 2: THD+N vs. power into 8Ω in two channels with inputs of 20 Hz (blue & green), 1 kHz (magenta & cyan), and 6 kHz (cyan & black). I could have made more window-dressing measurements à la Stereophile, but I believe that the above sufficiently characterizes the amplifier, especially since it is basically the same design as the abovementioned AT1506. The Sound I am absolutely convinced that one channel of the AT6012 connected to a passively crossed-over loudspeaker would sound exactly the same as one channel of any other well-designed power amplifier. My reasons for that conviction have been explained over and over again on this website, so I won’t repeat myself here. In my case, however, the AT6012 was used to drive the multiplicity of electrodynamic transducers in the Linkwitz Lab Orion++ system and was fed from two electronic crossover/processors. I can say that magic happened, but it wasn’t due to the amplifier. The latter was merely doing its job of being a transparent conduit for the signals. The magic will be discussed in a forthcoming Orion++ review. Conclusion I would have liked to use a more “interesting” amplifier to drive the Orion++, maybe one of those newfangled Class T designs (which would have saved some money, too), but I searched the Internet in vain for a 12-channel model with adequate power. The conservatively designed and clearly reliable AT6012 proved to be the best choice for the job, a clean class AB amplifier with lots of power at a decent price. I couldn’t be happier.
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