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Until recently, the largest Audio Research power amplifier was the D-250, a massive, fan-cooled stereo chassis offering 250Wpc into a well-matched load. With its 16 output tubes, plus many others besides, designing and manufacturing the D250 represented an act of considerable courage on the part of ARC! Now in Mk.II form, the D-250 stands as one of the great high-power amplifiers.

Traditionally, ARC power amplifiers were stereo units, but the virtues of monoblock operation were also explored in a version of the D115 called the M100. Ultimately this proved to…

Design and Technology: How has the output power been raised to 300W with the established configuration of eight push-pull 6550 output…

However, my first reaction to the M300 was one of disappointment, having set it up on the 8 ohm…

On the 4 ohm setting the M300 spoke with undisguised authority, breathing an articulate, lively dynamic quality into the reproduction of recorded music and providing abundant depth where the…

Description: Hybrid mono power amplifier. Rated power: 300W, 16Hz-20kHz into 2, 4, or 8 ohms, at below 0.8% THD. Power bandwidth: 10Hz-60kHz. THD: 0.05% at 1W output. Clipping: 330W at 1kHz, nominal. Input impedance: 60k ohms. Feedback: 20dB. Slew rate: 25V/us. Rise time: 3us. Sensitivity: 0.9V in for 300W out. Damping factor: About 30. Hum & noise: Less than 2mV RMS; 90dB below full output. Power consumption: 130W on standby, 450W quiescent (no signal), 900W at rated output power.
Dimensions: 19" W by 10½" H by 16" D. Weight: 110 lbs.
Price: $9800/…

While many lab tests were easily passed, the M300 did exhibit some odd quirks which suggested that there is still room for further development and improvement in future years.

8 ohm tap: Covering the basics first, the power testing commenced (after two hours' warm-up) with 8 ohm loads on the 8 ohm tap. Rated at a continuous 300W or 24.5dBW, this amplifier raised an extra 1dB heading toward an exact power of 400W into the precise load impedance (7.7 ohms)—some power indeed! In terms of a marginal clipping criterion, the power bandwidth was excellent,…

Editor: Thanks for the opportunity for us to comment on the early draft of Martin and Gordon's M300 review. We were unable to reach you by phone, so here are a few comments on technical errors.

First, we are naturally pleased and grateful for your thorough and complimentary review, as well as your suggestions for improvement.

1. Our published THD rating at 1W midband is 0.005%, not 0.05%.

2. The 2.2% THD at 20kHz, 1W, is much higher than typical. This is probably due to an unbalanced capacitive load from a single-ended input on a 'scope or…

Some additional listening to a pair of these big amplifiers, which I reviewed in Vol.10 No.9, revealed that they do have one relatively minor weakness: their low end is not quite as detailed or tightly controlled as that of some big solid-state amps (like the Krells and Thresholds). Otherwise, their sound continued to enchant me until...I had turned the volume down while changing a record, and when I next touched the knob, a static discharge produced a very small click through the speakers,…

"What are you saying?" I sputtered. "That being an audiophile and being a music lover are mutually exclusive? How can a speaker designed for someone who loves music not be a speaker designed for an audiophile?"

His cryptic reply: "Let me know what you think after you listen."

Until the Beethoven Concert Grands arrived in…