Audio Research M300 monoblock power amplifier Manufacturer's Comment
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 analyzer connected to the balanced output of the M300. We find it necessary to use a true balanced differential input on measuring equipment to get valid readings of the M300's true capabilities at high frequencies. This could also account for the higher order harmonics observed at 1kHz, 1W.
3. The description of the "cascode" circuit in footnote 1 is not totally correct. Each of the two active devices does, in fact, amplify the signal (each has a voltage gain greater than 1). The cascode circuit was developed by Wallman of MIT Radiation Labs in 1948 for its unique low-noise properties. Of the nine possible ways to interconnect two tubes (each a grounded-cathode, grounded-grid, or grounded-plate configuration, or the solid-state equivalent), the grounded-cathode-grounded-grid direct-coupled version was selected and dubbed the "cascode." A discussion of the cascode can be found in Vacuum Tube Amplifiers by Valley & Wallman, McGraw-Hill 1948.—Richard Larson, Audio Research Corporation, Minneapolis, MN