Herron Audio M150 monoblock power amplifier
By Michael Fremer   •   March, 2001 Keith Herron plopped himself down in my listening chair and smiled, clearly pleased with the sound of my system now that his M150 monoblock power amplifiers had been substituted for my Musical Fidelity Nu-Vista 300. He began to tell me why.

I stopped him. While I appreciated his enthusiasm, I explained why it's a no-win situation to suggest to a reviewer what he's going to hear. The more a manufacturer tries to plant ideas in a reviewer's head, the more the reviewer feels trapped, and the more likely he is to break away and hear something else.

Herron agreed and bit his tongue, but not before telling me he wanted me to hear his complete system: Herron M150 amps, upgraded "A" versions of the Herron VTSP-1 tube preamp and Herron VTPH-1 phono preamp that I reviewed in March, 2000, all connected with the Herron cables he was about to market. I agreed—but only after first listening to the M150s with the Hovland and Ayre K-1x preamps and the Audio Research Reference phono section, using familiar cables.

Much research tells us that what we see can sometimes affect what we hear, and I kept that in mind while reviewing the M150. While some amplifiers demand shelf or floor space for their looks, that can't be said for the homely M150—its folded sheet-metal chassis is about as plain as it gets. In that department, the dramatic-looking, $16,000/pair Audio Research VTM200 that I reviewed in January would be a hard act to follow, but even compared to the similarly priced Nu-Vista 300 ($5495), the $5895/pair M150 pales. That says a great deal about what Keith Herron thinks he's accomplished and for whom he's accomplished it.

Herron, a music-lover and a no-nonsense electrical engineer with a background in professional audio—in the late 1980s and up until 1992, he was director of research and development at SLM (St. Louis Music) Electronics, who make Crate and Ampeg gear. He was also involved in designing a bar-code documentation system for Chrysler that determines what parts and accessories go on which minivan chassis as it rolls down the assembly line.

For his own line of electronics, instead of concentrating on frills and fancy faceplates, Herron has put his money into R&D, and into parts and design quality—he's convinced that there's a ready and willing marketplace for his "musical accuracy first" designs. As I wrote last March, in my review of his preamp and phono section, Herron approaches the marketplace in a cautious, stealthy manner, working from the ground up to grab the ears of audiophiles.

Inside the M150
The M150 is a fully complementary symmetrical bipolar design, and if that means nothing to you, don't worry about it. What's more important is how Herron described it to me in plain English.

In most tube and solid-state designs, beginning with the input stage, each stage drives the next until there's enough voltage gain to drive the output stage, which provides the main current gain and drives the loudspeakers. According to Herron, solid-state output devices are usually the slowest and tend to have poor linearity at very small signal levels. In other words, they "go to sleep" at the very low levels where music "lives."

Herron showed me in a diagram how he "seamlessly" routes each stage directly to the output stage, as opposed to just stacking the stages atop each other. Along with a unique output-stage configuration, his design is said to eliminate the usual need for high output-stage (or continuous class-A) idle currents, which generate lots of heat. The unique circuit design is one reason these amps are so compact—and, as advertised, they run incredibly cool. Even rocking at moderately high volumes, they remained literally cool to the touch. I'm playing a test pressing of Classic Records' reissue LP of Led Zeppelin IV as I write this (such torture!), and there's almost no heat coming from the M150s.

There are no DC-blocking coupling capacitors in the forward direction of the circuit. For speaker protection, the M150 uses automatic low-level direct-current offset cancellation, plus high-level DC shutdown. Herron believes fanatically in low distortion, and claims that proper stage-to-stage impedance matching ensures extraordinarily low distortion. Combined with what he described to me as "successive stage to output confluence," the claimed result is "a seamless combination of micro/nano-resolution from forward stages and robust controlled power and authority from successive stages."