Herron Audio M150 monoblock power amplifier Followup
You'd really have to be in an audio critic's shoes to understand the embarrassment of reading your own rave review in Stereophile, only to glance down at the "Measurements" Sidebar to find that the product fared so poorly on John Atkinson's test bench that it might very well have been defective. It's like emerging from cold water onto a crowded beach and having your bathing suit come off. Measurements don't tell the whole story, as any vinyl- or tube-loving audiophile will attest, but sometimes the graphs offer incontrovertible evidence that you or the designer—or both—have missed something.
So it was with a great deal of relief that I read JA's measurements of the Herron M150 monoblock power amplifiers$s3 in the March 2001 issue. I tried very hard to like these amps, having enjoyed Keith Herron's vacuum-tube line stage and hybrid MC phono section—not to mention his company, when he delivered them. But the amps' lack of bottom-end weight seemed to leave the music ungrounded and without spatial context. They were ruthlessly revealing and delivered impressive inner detail, but sounded stingy and somewhat mechanical. They also didn't seem as powerful as their rated output suggested they'd be.
JA's measurements revealed an amplifier of very low distortion but some quirks. The distortion doubled as the impedance was halved, and the total harmonic distortion (THD) rose with the frequency, though it was still below 0.05% into 2 ohms above 10kHz. At very low levels there was an unusual anomaly for a modern amplifier design: crossover distortion, audible, according to JA, because of its high-order content and dominance at low levels. Also, the M150 restricted current at low impedances. There was also, according to JA, the "possible presence of ultrasonic instability" and an aggressive protection circuit. While the M150 delivered 240W into 8 ohms (at 1% THD) and 429W into 4 ohms (with higher distortion), the output power sank to 177W into 2 ohms and 80W into 1 ohm. For this reason, Keith Herron recommends using speakers with nominal loads of 4 ohms or above, which means you'll need to know the true impedance curve of any loudspeaker you're considering using with the M150.
In his "Manufacturer's Comment," Herron claimed that the review samples' idle bias current had been accidentally set too low, and that this would indeed have resulted in the measured crossover distortion and an audible reduction in transparency.
Another set of M150s recently arrived, and I spent a few days listening to them using the Sonus Faber Amati Homage and Audio Physic Avanti III speakers. For whatever reason, the sensation that I complained about in the original review—of three groundless images, left, center, and right, hovering like balls—was gone, replaced by more normal soundstaging and imaging, if still on the compact and tidy side.
Compared to my reference Musical Fidelity Nu-Vista 300, the M150 still seemed somewhat lean in the midbass, ever so slightly forward in the upper mids, and slightly dry on top, but not objectionably so—even driving the somewhat lean-sounding Avanti IIIs.
Herron says "bloom" is a distortion artifact. If he's right, the M150 is one of the least-distorting amplifiers you're likely to hear—bloom it doesn't have. Instead, its overall presentation is somewhat analytical, but please don't confuse that with "bright" or "etched," which the M150 definitely is not. What you might lose in bloom and warmth is more than made up for by the M150's resolution of low-level detail—something the original review samples only hinted at.
With the many live recordings I played—Sinatra at the Sands, Mel Tormé and Friends at Marty's, Belafonte at Carnegie Hall, The Weavers' Reunion at Carnegie Hall 1963, etc.—the M150s' ability to spatially delineate the main event from the reverberation was noteworthy. The Nu-Vista 300's presentation was somewhat warmer, with more rounded images, but it tended to lump together the first event with the ensuing reverberation.
Compared to the Nu-Vista, the Herron presented voices and instruments more dryly and two-dimensionally, but with increased clarity and in far greater relief from the ensuing reverberation. This created an intense 3D effect not unlike the old Viewmaster stereo slide viewers kids had in the 1950s, wherein somewhat flat images of fairy-tale characters were set against two-dimensional backgrounds but presented in vivid 3D. In many ways, the Herron M150 reminded me of the sound of early moving-coil cartridges: gobs of impressive detail, highlighted and presented with clinical precision.
The rebiased M150s definitely performed better than the original review pair—by some means or other, my main objection had been addressed. I'm convinced that what I heard in this go-'round was precisely how Keith Herron intended his amplifiers to sound. Still, the M150 is a singular-sounding amplifier that will appeal to some listeners and not at all to others. If you like your presentation lush and relaxed, you probably won't go for it. If you lean toward detail and "event" and demand clarity, a pair of the rather homely-looking M150s might find a home in your system. Just remember: Your choice of associated equipment, especially speakers, will be critical in getting the performance the designer intended.—Michael Fremer