Audio Research M300 monoblock power amplifier Page 5
Design and Technology: How has the output power been raised to 300W with the established configuration of eight push-pull 6550 output tubes? William Z. Johnson has used a circuit which he initially considered some time ago, but which required power FETs of a specific quality and rating. Once these had become available, it was possible to complete the research and design the new output stage. ARC's normal output stage involves a multi-wound output transformer where feedback is made at the output winding by including the cathode circuit of the tubes in the transformer circuit. Balanced negative feedback also runs from both positive and negative sides of the secondary to the input stages of the amplifier. To improve the output efficiency, ARC takes the screen grids of the output tetrodes to a regulated supply.
The new refinement provides still greater power and involves the application of a servo or buffer signal to the screens: a controlled audio signal rather than a regulated HT line (fig.2). For each phase, additional windings on the transformer feed the gates of high-voltage power MOSFETs operated from a 600V supply. Operating as a low-output impedance follower, the source terminal goes to all the screen grids (antiphase relative to the anode), these set at a DC level of 330V. The anodes are at 420V, with the 6550 tubes held well within their ratings at 75mA standing bias each. A single bias line is used for each set of four valves, these sharing a single low-value cathode resistor, only present to allow the measurement of the bias current.
Fig.2 Screen buffer principle.
Aside from the two sets of four output tubes working in push-pull class-AB, the rest of the M300 is solid-state, using a total of 16 FETs excluding the power supply. The input is DC-coupled via a 100k potentiometer, with the circuitry wholly symmetrical and incorporating the ARC cross-coupled feature, save for the input phase-inverter stage which feeds the lower half of the circuit. While executed in FETs, the general plan mirrors that of the all-tube circuitry which preceded the M300. The input voltage amplifier is buffered by a source follower leading to the cross-coupled differential, whose output drives the tube grids via another source-follower buffer. Top-quality regulated supplies are extensively used, and incorporate power MOSFET pass elements controlled by IC op-amps.
The amplifier is built to high standards, with excellent workmanship and first-rate components. Only two capacitors are effectively in the signal path, both low-value top-quality polypropylenes, and are bypassed. The only calibration control is that for output-stage bias, while the output tubes are generally supplied in matched sets to improve performance.
The power supplies are substantial, especially the main anode reservoir for the output stage. A valid measurement for the reservoir capacity is the unit for watt-seconds, or Joules (CV2). Typically 50J is accorded to a transistor power amp, with the ARC M100 providing a massive 250J, but the M300 tops the bill at 350J. Reservoir capacity is not the only criterion for good sound by any means, but it can be a good indicator.
ARC gains an advantage in the use of multiple output tubes. A single pair of 6550s might load nicely into a 5k anode-to-anode transformer primary impedance, ratio-matched to an 8 ohm secondary load. That 5k represents a lot of primary turns with consequent leakage inductance and stray capacitance. If four pairs of tubes are used, however, the primary impedance matching will be reduced to 1250 ohms, resulting in a beneficially lower number of turns and a wider bandwidth.
The use of FETs for the low-level and driver circuitry has an advantage over all-tube design in that noise levels are better and microphony is practically eliminated. Likewise, tube balancing and aging effects are minimized while the output tubes are relatively uncritical of bias changes in this design.
Sound Quality: With a fine amplifier such as this, the reviewer may be overwhelmed by a combination of sheer power and performance, to the point where no sensible criticism can be made. With no disrespect intended to ARC, this review is based on personal experience of the c-j Premier 5, the Jadis JA200, Krell KMA-100 II, Mark Levinson No.20, Cello Performance, ARC M100, and Goldmund Mimesis 3. No amplifier is perfect—reference to the performance of a range of comparable designs can help to identify those different weaknesses and strengths.