Herron Audio M150 monoblock power amplifier Measurements part 2

However, when I examined the distortion content, I was surprised to see—superimposed over what appears to be basically a third harmonic (fig.4)—what looks like crossover distortion. (The spikes in this 'scope trace occur at exactly the zero-crossing points of the sinewave.) Yes, the measured level in this graph is very low (0.003%), but crossover distortion tends to be more audible than its level would imply, due to its high-order content and the fact that it tends to dominate at low signal levels. At high powers (fig.5), it tends to drop into insignificance. Intermodulation distortion was very low, the 1kHz difference component with the very demanding 19+20kHz test signal remaining at -90dB (0.003%), even just below the onset of visible clipping (fig.6).

Fig.4 Herron M150, 1kHz waveform at 12W into 8 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.5 Herron M150, spectrum of 50Hz sinewave, DC-1kHz, at 110W into 4 ohms (linear frequency scale).

Fig.6 Herron M150, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 120W into 4 ohms (linear frequency scale).

Herron does not recommend the M150 for driving loads of less than 4 ohms, and I found that the amplifier's aggressive protection circuitry would not let me measure its continuous clipping power. I always test this at the end of the test program in case something breaks, and the sequence of test signals to which I had been subjecting the amplifier may have had something to do with what happened. The level had reached 100W into 4 ohms when a relay clicked and the amplifier turned off its output. The THD level was 0.003%, meaning that this power level was well below the actual clipping point. I also found that rather than ramping up an input signal, if I suddenly turned on an input signal that, while high in level, was not large enough to cause the amplifier to clip, also caused the M150 to turn off the output. However, looking at the oscilloscope as this happened revealed a burst of ultrasonic oscillation just prior to the amplifier shutting down, which might suggest that the design is only marginally stable.

Evidently the low-duty-cycle 1kHz toneburst used by the Miller Amplifier Profiler—10 cycles on, 400 cycles off—does not trigger this behavior, so I was able to test the M150's power delivery with this signal. This also suggests that with typical instantaneous musical transients, which rarely persist for more than one cycle, the apparent instability will not have time to develop. Under these test conditions, the M150 proved capable of delivering 240W into 8 ohms (23.8dBW) at 1% THD, as demonstrated by the black trace in fig.7. The clipping power increased to 429W into 4 ohms (red trace), though this was accompanied by an increase in distortion. Confirming Herron's recommendation not to use the amplifier into impedances below 4 ohms, the burst output power dropped to 177W into 2 ohms (blue trace) and 80W into 1 ohm (green).

Fig.7 Herron M150, distortion (%) vs 1kHz burst output power into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green).

Given the excellent engineering evidenced in the Herron line of preamplifiers, I was left puzzled by the M150. Yes, it does have very low distortion into higher impedances. But in my opinion, the presence of what appears to be crossover distortion, the current-restricted power delivery into low impedances, the possible presence of ultrasonic instability, and its aggressive protection circuit are going to make the amplifier very critical of source components and matching loudspeakers if it is be the basis of a sonically satisfying system.—John Atkinson

Herron Audio
12685 Dorsett Road, #138
St. Louis, MO 63043
(314) 434-5416