Pre/Power Amplifiers

As we discovered when talking with Peter Kao, MD of Rotel Global, the M8 is not a bridged S5 but a very large, single-ended power amp in its own right. Indeed, the S5 was derived from the M8 (not the other way around) and so both amplifiers have comparable engineering and performance 'traits'. So, just as the S5 exceeded its 2x500W/8ohm and 2x800W/4ohm spec. at 2x670W and 2x1.17kW, the M8 bursts its 1.05kW/8ohm and 1.75kW/4ohm rating by delivering 1.32kW and 2.25kW, respectively. There's plenty of headroom to support dynamic power too, as the M8 stretches out to 1.64kW and 3.13kW into 8/4ohm, but there's precious additional current over the S5 (31A vs. 30A) so output into 2 and 1ohm falls back to 1.79kW and 960W [all <1% THD, see Graph 1 below]. So the M8 will drive 8ohm nominal speakers to higher levels than the S5, but it is no more tolerant of very tough loads.

Again, as we saw with the S5, THD climbs only very slightly with output, from 0.004%/1W to 0.0045%/10W, 0.005%/100W and 0.0055% at the rated 1050W/8ohm (all at 1kHz). THD climbs more markedly with frequency, however, from 0.0004%/20Hz to 0.005%/1kHz and 0.038%/20kHz [all at 10W, but compare THD vs freq with power on Graph 2, below]. This trend is a function of the Michi's limited HF feedback which is also reflected in the output impedance – increasing from 0.025ohm midband to 0.14ohm/20kHz and 1.05ohm/100kHz. The response follows the same pattern as it shelves down to reach –0.1dB/20kHz and –0.6dB/100kHz, the dip magnified into lower loads at –0.55dB/20kHz and –2.8dB/100kHz into 2ohm. Also, just like the S5, noise is low and the A-wtd S/N wide at 90dB (re. 0dBW). PM