Solid State Power Amp Reviews

I performed a full suite of measurements on one of the Rotel Michi M8s (serial no 256-9511006) using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). Because I don't have a test load with sufficient thermal capacity to handle the Michi amplifier's very high power, I wasn't able to perform my usual preconditioning of operating it at 1/8 power for 30 minutes, let alone the more demanding 1/3 power for 60 minutes. But with its fans and massive heatsinks, the M8 remained cool throughout the testing. The heatsink temperature was 80.0°F/26.7°C during the low-power testing and had risen by just 10°F when I examined the maximum power capability.

The Michi M8's voltage gain at 1kHz into 8 ohms was 29.6dB for the balanced input and 33.7dB for the unbalanced input. Both inputs inverted absolute polarity. The single-ended input impedance is specified as 12.5k ohms. I measured 12.75k ohms at low and middle frequencies, decreasing inconsequentially to 12k ohms at the top of the audioband. The balanced input impedance, specified as 100k ohms, was 96k ohms at 20Hz and 1kHz and 89k ohms at 20kHz. The output impedance (including the series resistance of 6' of speaker cable) was low, at 0.09 ohm at 20Hz and 1kHz, rising slightly to 0.2 ohm at 20kHz.

The gray trace in fig.1 shows the Michi M8's small-signal frequency response into our standard simulated loudspeaker. The variations in response were a negligible ±0.05dB. Into resistive loads (blue, magenta, and red traces), the amplifier rolled off above the audioband, with the –3dB frequency dependent on the load. The fastest rolloff was with 2 ohms (red trace), where the response was down by 3dB at 58kHz. The Michi M8's reproduction of a 10kHz squarewave into 8 ohms (fig.2) featured short risetimes and no overshoot or ringing.

The unweighted, wideband signal/noise ratio (ref. 1W into 8 ohms and measured with the unbalanced input shorted to ground) was good, measuring 71.7dB. This ratio improved to an excellent 94.6dB when the measurement was restricted to the audioband and to 97.9dB when A-weighted. Spectral analysis of the low-frequency noise floor while the Michi M8 drove a 1kHz tone at 1W into 8 ohms (fig.3) revealed that although some AC-related spuriae were present, these all lay at or below –100dB.

The Michi M8's maximum continuous power is specified as 1080W into 8 ohms (30.33dBW) and 1800W into 4 ohms (29.5dBW). With our usual definition of clipping (when the THD+noise reaches 1%), I measured clipping powers of 1020W into 8 ohms (30.1dBW, fig.4) and 1500W into 4 ohms (28.75dBW, fig.5). These are slightly lower than the specified powers, but it isn't possible to hold the wall voltage constant during the tests. Rotel warns in the M8's manual that 4 ohms is the recommended minimum load impedance. I did think about examining how much power was available into 2 ohms once I had finished the testing, but when I reexamined how much power could be delivered into 4 ohms before doing so, I tripped the circuit breaker. Hint taken.

Fig.6 plots the percentage of THD+noise into 8 and 4 ohms at 28.3V (equivalent to 100W into 8 ohms and 200W into 4 ohms), a level at which I could be sure I was looking at distortion rather than noise. The THD was very low at low and middle frequencies, especially into 8 ohms (blue trace), but rose in the top two audio octaves.

The distortion signature into 8 ohms was predominantly low-order harmonics (fig.7), with the second and third harmonics equal in level at –112dB (0.0003%). When I repeated the spectral analysis at the same voltage into 4 ohms (fig.8), the third harmonic was now the highest in level at a still-low –97dB (0.0015%). Despite the decreasing linearity at high frequencies seen in fig.6, the Michi M8 performed well when driving an equal mix of 19 and 20kHz tones at 200W into 4 ohms (fig.9). Both the 1kHz difference product and the higher-order products at 18 and 21kHz lay at –97dB from the peak signal level (0.0015%).

The Rotel Michi M8 is one of the most powerful amplifiers I have had on the test bench. It delivered excellent measured performance.—John Atkinson