**Sidebar 3: Measurements**
Before performing any measurements, I ran one of the Performance Centaur Monos (serial no. 1101131901) for an hour at one-third its specified maximum power of 500W into 8 ohms, thermally the worst case for an amplifier with a class-B or -AB output stage. The amplifier was too heavy for me to lift it onto my test bench so it sat on the carpeted floor, with some clearance given by supporting it on Black Diamond cones. By the end of the hour, the top panel was warm, at 105°F (43.5°C), and the side-mounted heatsinks were warmer, at 114.5°F (45.8°C). With the amplifier cold, the THD+N at one-third power was 0.0329%. By the end of the hour, it had dropped to 0.0307%.

I performed a full set of measurements on serial number 1101131901 using *Stereophile*'s loan sample of the top-of-the-line Audio Precision SYS2722 system (see www.ap.com and the January 2008 "As We See It") connected to the Centaur's balanced input. I repeated some of the tests using the unbalanced input, but did not test the amplifier via its Direct input, which is intended for use only with Constellation preamplifiers.

The voltage gain at 1kHz into 8 ohms was a little lower than average, at 25.8dB, but close to the specified 26dB. It was the same for both balanced and unbalanced drive, and both inputs preserved absolute polarity (*ie*, were non-inverting). The Centaur's input impedance is specified as a very high 100k ohms unbalanced and 200k ohms balanced, but my measurements were very much lower: 9.6k ohms unbalanced and 19k ohms balanced.

The output impedance was low for a bridged design, at 0.07 ohm including the cable at low and middle frequencies, rising slightly to just under 0.1 ohm at the top of the audioband. As a result, the modification of the Centaur's frequency response by the interaction between this source impedance and the impedance of our standard simulated loudspeaker (fig.1, gray trace) was less than ±0.1dB. However, this graph reveals that the Centaur's ultrasonic behavior depends on the load impedance. With the highest impedance I tried, 16 ohms (fig.1, green trace), the output rose by 0.8dB at 200kHz. The rise was 0.3dB into 8 ohms (blue), and into 4 ohms (magenta) and 2 ohms (red) the response increasingly rolled off above the audioband. Perhaps correlating with this rise at 200kHz, while the Centaur's reproduction of a 10kHz squarewave had very short risetimes (fig.2), there was also a very small amount of overshoot visible, with one damped cycle of ringing at a very high ultrasonic frequency. The 1kHz squarewave reproduction (not shown) was essentially perfect.

Fig.1 Constellation Performance Centaur, frequency response at 2.83V into: simulated loudspeaker load (gray), 16 ohms (green), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red) (1dB/vertical div.).

Fig.2 Constellation Performance Centaur, small-signal 10kHz squarewave into 8 ohms.

Although MF noted that the Centaur was very quiet, the amplifier's wideband, unweighted signal/noise ratio (ref. 1W into 8 ohms), taken with the balanced input shorted, was slightly disappointing at 72.1dB, though it did improve to 81.2dB when A-weighted. Spectral analysis of the amplifier's low-frequency noise floor while it reproduced a 1kHz tone at 1W into 8 ohms (fig.3, blue trace) indicated that the residual full-wave rectified power-supply component at 120Hz lay at –83dB (0.007%), with the 240Hz harmonic at –87dB. The magnetically sourced component at 60Hz lay at –93dB, and all the other supply-related components lay at or below –95dB. I repeated the spectral analysis with the other sample of the Centaur, S/N 1101131902. The spectrum of this amplifier's noisefloor (fig.3, red trace) was very similar to that of the first sample (blue trace), but the odd harmonics of the 60Hz AC frequency were a little higher in level.

Fig.3 Constellation Performance Centaur, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms, samples 1101130901 (blue) and 1101131902 (red) (linear frequency scale).

Fig.4, which plots the percentage of THD+noise against output power into 8 ohms, indicates that the Centaur slightly exceeds its specified 500W into this load, clipping (defined as when the THD+N reaches 1%) at 520W (27.16dBW). The minimum THD+N is very low, at 0.0033%; the downward slope of the trace below 20W in this graph reveals that the distortion is actually below the noise floor at low powers. (The noise, being constant, becomes an increasing percentage of the signal level as the power is reduced.) The picture was similar into 4 ohms (fig.5), with the clipping power of 830W (26.18dBW) slightly greater than the specified 800W. The minimum THD+N was higher, however, at 0.0061% between 10 and 20W. The distortion was higher still into 2 ohms (fig.6), with the amplifier clipping at 1210W (24.8dBW), which is 0.8dB above the specified 1kW into this load. However, although I switched off the signal generator as soon as the Centaur clipped into 2 ohms, the amplifier wouldn't pass a signal after this test. Obviously, I had broken something. (The possibility of damage with this very stressful test is why I leave it until the end of testing.)

Fig.4 Constellation Performance Centaur, distortion (%) *vs* 1kHz continuous output power into 8 ohms.

Fig.5 Constellation Performance Centaur, distortion (%) *vs* 1kHz continuous output power into 4 ohms.

Fig.6 Constellation Performance Centaur, distortion (%) *vs* 1kHz continuous output power into 2 ohms.

Before this test, I plotted how the THD+N varied with frequency at a level, 18V, where I could be sure that I was looking at actual distortion rather than noise. The results are shown in fig.7. The THD rises slightly at the top of the audioband, but is very low into both 8 ohms (blue trace) and 4 ohms (magenta). The amplifier is less comfortable with 2 ohms (red), however. Again, to be sure I was looking at distortion rather than noise, I examined the waveform of the THD+N residue at a high level, 50W into 8 ohms (fig.8). The distortion at this level was 0.005% and was predominantly third harmonic in nature. Even at high power, 200W, into 4 ohms (fig.9), the subjectively innocuous third harmonic remained the highest in level, at –72dB (0.03%), with the second harmonic at –90dB (0.003%) and all other harmonics and power-supply–related components at or below –100dB. Intermodulation distortion at high power into 4 ohms was also extremely low (fig.10).

Fig.7 Constellation Performance Centaur, THD+N (%) *vs* frequency at 18V into: 8 ohms (blue), 4 ohms (magenta), 2 ohms (red).

Fig.8 Constellation Performance Centaur, 1kHz waveform at 50W into 8 ohms, 0.005% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.9 Constellation Performance Centaur, spectrum of 50Hz sinewave, DC–1kHz, at 200W into 4 ohms (linear frequency scale).

Fig.10 Constellation Performance Centaur, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 200W peak into 8 ohms (linear frequency scale).

Constellation Audio's Performance Centaur Mono is a powerhouse of an amplifier capable of delivering very high power with very low distortion into 4 and 8 ohms. While it was less comfortable with 2 ohms, it will have no problem driving low-impedance speakers with aplomb. Although I broke it with sustained delivery exceeding 1kW into 2 ohms (footnote 1), there isn't a speaker made that would not be the first to expire at this power level! Color me impressed.—**John Atkinson**

Footnote 1: When he received the amplifier back after the review had been published, Peter Madnick let me know that an internal fuse had blown, that the amplifier wasn't damaged.