Herron Audio M150 monoblock power amplifier Manufacturer's Comment
Editor: I want to thank Michael Fremer and John Atkinson for their time spent evaluating the M150 amplifier. I also want to thank Michael particularly for his warm hospitality and a fun afternoon during my visit. He has an amazing record collection.
I was disappointed to see by John's measurements that the idle current of the M150 review samples had been set slightly on the low side at the factory, resulting in the crossover notch shown in fig.4. This would have taken away from the transparency of the units.
The higher price tags of many of today's fine speakers led us to investigate a more extensive approach to speaker protection than is commonly done. This circuitry is refined and does not activate while listening to music, but may engage when a stylus is accidentally dropped on a record at a high volume level or a piece of selected source equipment is first turned on. We have seen it trip when the input interconnect RCA cable is partially pulled out of the M150 input (a loss of the input ground connection). The M150 will pass music, but may inhibit testing with signals that are not suitable for speakers.
I knew before the review that conventional testing with the protection circuits enabled would be a frustrating experience, and John, I apologize to you for that. Any test or operating condition where there is a DC signal component at the input, or if the amplifier is driven to hard clipping, may temporarily engage the speaker-protection (mute) circuitry. The amplifier also has extensive self-protection in the form of volt-amp current limiting, saturation prevention, short-circuit protection, and temperature compensation, which keeps all internal devices in their specified safe operating area under all normal use conditions. We have been producing the M150 for nearly a year and so far have had no failures, instability, or protection problems reported. The M150 will remain stable even with a 1181F film capacitor connected directly across its output.
A lower-gain power amplifier will typically produce a lower overall system noise floor (less hiss), especially when used with a vacuum-tube line stage. The nominal 24dB gain of the M150 was selected for that reason. For best performance, the M150 is recommended for nominal speaker loads of 4 ohms and above.—Keith Herron, President, Herron Audio