Audio Research M300 monoblock power amplifier
After having proven that vacuum tubes could do some sonic things better than transistors, Audio Research is now endeavoring to show that transistors can do most things better than tubes—at least in the front end of a power amplifier. The M300 monoblock power amp uses FETS from the inputs through the driver stages; only the output devices are tubes. Audio Research describes it as a hybrid amplifier, which of course is just what it is: it's half tube, half solid-state.
But the description is, in one respect, both ambiguous and misleading. The M300 is claimed to use "the same hybrid technology as is used in the SP-11 preamplifier." It doesn't. Yes, both are hybrids, but the nature of that hybridization is completely different. In the SP-11, each stage is hybridized, with a cascode tube/FET arrangement which acts to cancel inherent nonlinearities in both. The M300 is a conventional cascade circuit (footnote 1), wherein no attempt is made to cancel device nonlinearities by combining the complementary characteristics of tubes and FETs. Thus the M300 is more akin, circuit-wise, to the SP9 (reviewed in Vol.10 No.8) than to the SP-11. Sonically, though, the M300 is in a different league from the '9.
Description: A continuing wonder of Audio Research's designs is how they can sound so good with such a complex signal path. The complete antithesis of the view that the fewer active stages the better, ARC's equipment is instead characterized by complexity and sophistication. The simplest push-pull power amplifier requires only three active devices: an input voltage amplifier, a phase inverter, and the output stage (footnote 2). For a high-powered amp, where more voltage swing is needed, add one more stage—a driver—to this lineup, and we end up with a total of four. The M300 has seven: two in the phase inverter, and five in the push-pull signal path. (In push-pull circuits, one complementary pair is considered a single stage.)Phase inversion takes place right at the input, where a drain-coupled FET feeds the FET inverter. From there to the output transformer, everything is push-pull FETs. ARC's unique cross-coupling arrangement is used to ensure near-perfect balance of signal and DC supply parameters in the push-pull circuitry, and the output tubes—two banks each of four paralleled 6550s—are source-coupled from the drivers to provide a low-impedance feed. There are only three interstage coupling capacitors in the entire signal path (the odd one being at the inverter output), and each of these is a bypassed pair—a large-value Wondercap and a small polystyrene bypass.The output circuitry is ARC-exclusive, too. The transformer has three center-tapped primary windings: one for the tube-bank anodes, one for their screens (a conceptual grandchild of the Hafler/Keroes "ultra-linear" circuit), and one for the output-tube cathodes. The output tubes are thus both plate- and cathode-coupled to the transformer, an arrangement which allows for higher coupling efficiency (claimed to be twice as high as plate-only coupling), smaller winding ratios (resulting in fewer turn losses), and reduced DC magnetization of the transformer core (since current flow is opposite through the anode and cathode windings). Also unique to ARC is a dedicated signal drive to the output-tube screens using MOSFETs as driver elements, which operates them precisely in phase with the cathode signals—an ideal but rarely realized condition for pentode-tube operation.There are, incredibly, seven secondary taps. ARC has traditionally drawn the negative feedback in their amps from both ends of the output-transformer secondary winding, to provide feedback signals of opposite polarity which could be applied to both sides of the first push-pull stage. These were then referenced to a secondary tap—usually the 4 ohm one—connected to the chassis ground to ensure the stability of the fed-back voltages. The only disadvantage to this arrangement was that, if you connected those negative (0 ohm) terminals to any pair of devices sharing a common ground, it would mess up part of the amplifier's feedback circuitry, impairing the sound or worse. (You could use the 4 ohm output tap as a common-ground output if you wished, but the impedance matching would then be incorrect.)
The M300, however, has two complete sets of output connections, one for isolated-ground "balanced" loads, the other for common-ground "unbalanced" loads. Another departure from conventional tube-amplifier practice is the M300's elimination of the traditional 16 ohm output tap, which hasn't made sense for years anyway because 16 ohm loudspeakers went out of style about the same time stereo was getting off the ground.The power supply, which represents almost half the 110-lb weight of the M300, is almost as complicated as the audio section. Four MOSFETs, three op-amps, three bipolar transistors, and more zener diode voltage references than I cared to count, are used to provide the maximum possible purity and stability of all operating voltages, and a large bank of electrolytics provides enough storage to deliver, without help from the AC line, about 350W of power for a full second! (footnote 3)The M300's 60k ohm input impedance is high for a solid-state unit but lower than that of most tube units. It is still high enough, however, to provide true output bridging for any preamplifier. The back connection panel has an input level-set adjust, a gold-plated Tiffany input receptacle, a large line-fuse receptacle, and the seven-terminal output barrier strip with heavy-duty 3/8" screws.
Footnote 1: A cascade circuit is a tandem arrangement in which the output from one stage feeds the input to the next. In a cascode circuit, pairs of active devices are combined in what is essentially a single stage in which neither one, alone, is capable of amplifying the signal.
Footnote 2: A push-pull circuit is where opposite-polarity versions of the same signal are amplified simultaneously by different signal paths. The system requires, first, a phase inverter which provides the opposite-polarity signals, and a complementary-polarity subtractor at the output which delivers a difference signal. In so doing, it cancels information which is in-phase common to both signals (sum signals), which—in theory, at least—will be distortion components added by the amplifying devices.
Clearly, the effectiveness of all this will depend on the degree of symmetry between the two halves of the chain. Any differences in signal level or phase will cause incomplete distortion cancellation or imperfect recovery of the difference signal, or both. Much of the complexity of Audio Research's power amps stems from measures taken to ensure that symmetry, as ARC has found that simple but uncorrected push-pull circuitry produces more signal distortion than complex, corrected circuitry.Incidentally, there is widespread belief that push-pull operation cancels only the even-order harmonics and has no effect on odd-order ones. (In fact, I have two textbooks which state that.) This is incorrect. In the ideal push-pull circuit, all introduced harmonics appear at the push-pull outputs as sum signals, and are thus canceled by extraction of the difference component.
Footnote 3: Audio Research rates the capacitor storage at approximately 350 joules of energy. One watt of power is equal to 1 joule per second.