Perreaux PMF 2150B Power Amplifier Review

Perreaux Sound Limited of Napier, New Zealand, is a small manufacturer whose high standards of workmanship and performance quality are exemplified by the PMF 2150B power amplifier. By any standard, it is an imposing amplifier, rated, according to FTC regulations, to deliver 200 watts per channel to 8-ohm loads from 20 to 20,000 Hz with no more than 0.009 per cent harmonic or intermodulation distortion. It also carries a 4-ohm output rating of 400 watts per channel.

The output stages of the PMF 2150B employ power MOSFET’s (six per channel) that operate in class-AB mode, not pure class-A as implied in the somewhat confusing brochure. Other than d.c.-power-supply fuses, no current-limiting circuits, relays, or other devices are used to protect the output transistors. Unlike bipolar transistors, MOSFET’s are inherently immune to thermal “runaway” and thus do not require the protection circuits associated with bipolar transistors. They also have outstanding high-frequency power capability, low distortion (requiring less overall negative feedback), and high reliability. As used in the PMF 2150B, they can deliver tremendous signal currents to low-impedance loads, currents limited ultimately only by the power supply. Because of the thermal stability of the power MOSFET's, the Perreaux PMF 2150B needs no cooling fan. The considerable heat developed by the high-power circuits is dispersed through the large heat sinks and radiating fins on both sides of the amplifier.

The PMF 2150B is a massive unit, with panel dimensions of 19 by 7-1/2 inches and a depth of 15-1/2 inches. The plastic handles on its rear surface and metal handles on the front panel allow it to be set down on any of its sides without damaging it or marring its finish. Its 48-pound weight makes the handles especially useful features. The 1/4-inch-thick satin-aluminum-finished front panel of the PMF 2150B supports the large power transformer. This places the center of gravity of the amplifier near the front panel, which simplifies lifting or moving it. The output transistors for each channel are mounted on a heavy aluminum plate bolted to the heat-sink structure. The other circuits are assembled on military-grade fiberglass boards. The top and bottom plates are black vinyl-clad metal, and the heat sinks on the sides are black-anodized.

The front panel of the PMF 2150B contains only a large square power button and a small red pilot light. On the rear apron (finished like the front panel) are two pairs of five-way binding posts for the speaker outputs, phono jacks for the inputs, a line-fuse holder, and a knob-operated switch marked stereo and bridge. In the latter position, the two channels are connected to form a mono 600-watt amplifier (for 8-ohm loads only), with the speaker connected across the two “hot” output terminals and only the left-channel input driven. The suggested retail price of the Perreaux PMF 2150B is $1,495.

Laboratory Measurements

Although the Perreaux PMF 2150B is an exceptionally rugged amplifier, we were advised by the manufacturer of certain precautions to be observed in testing it. These concerned a phase-correcting resistance/capacitance network across its output terminals (most amplifiers have similar circuit elements). If the amplifier is driven to full power at frequencies well above the audio range, the resistance can be burned out, impairing operating stability. Therefore, we were warned not to drive it to full power at ultrasonic frequencies for more than a couple of seconds. The principal effect of this restriction was to require a slight modification of the slew-factor measurement, a test in which we have damaged a few other amplifiers with the same susceptibility whose manufacturers did not see fit to warn us.

After the PMF 2150B had driven 8-ohm loads at 1,000 Hz for 1 hour at 67 watts (one-third rated power), its heat sinks were quite hot to the touch (a skin contact longer than about 10 seconds was painful). In subsequent testing, they became even hotter, uncomfortable to touch for more than a second or two. No part other than the heat-sink fins became more than mildly warm, however, so the amplifier should present no thermal hazard to any user. (A more serious hazard was posed by the sharp corners on the front-panel rack-mounting slots and a few on the heat-sink fins. These left us with various cuts and scratches from routine handling of the amplifier. Of course, this also need not be a problem in normal use, since the amplifier will presumably be installed and not moved about afterwards.)

The output waveform clipped at 319 watts per channel (1,000 Hz, 8 ohms), for a clipping-headroom rating of 2 dB. With 4-ohm loads, the clipping power was an impressive 473 watts per channel, corresponding to a 0.73-dB clipping headroom. When we tried to measure the 2-ohm clipping power (driving only one channel to prevent blowing the amplifier’s line fuse), the internal 5-ampere d.c.-power-supply fuses blew long before the output clipped. The 20-millisecond tone bursts of the dynamic-power test did permit us to judge the current capabilities of the PMF 2150B, however, and they were truly impressive. The outputs clipped at 340 watts (8 ohms), 680 watts (4 ohms), and 960 watts per channel (2 ohms). The PMF 2150B behaved like a true constant-voltage source for the higher impedances, yielding dynamic-headroom ratings of 2.3 dB for 8- and 4-ohm loads.

At 1,000 Hz, with both channels driving 8-ohm loads, the distortion was less than 0.002 per cent from below 1 watt to more than 300 watts output. With 4-ohm loads, the distortion was about 0.004 per cent over the full power range up to about 450 watts per channel. No measurements were made with 2-ohm loads because of the power-sup-ply fuse limitation. Across the audio frequency range, the distortion at or below rated power was less than 0.002 per cent from 40 to 2,000 Hz and did not exceed 0.008 per cent from 20 to 20,000 Hz. The IHF IM distortion, with 18- and 19-kHz inputs driving the amplifier to the equivalent of a 200-watt sine-wave output, was nearly unmeasurable. The second-order distortion at 1,000 Hz was undetectable at our measurement floor of —110 dB (0.0003 per cent), and the third-order distortion at 17 kHz was —97 dB (0.0014 per cent). The PMF 2150B showed no signs of instability with complex reactive simulated-speaker loads.

A 100-millivolt input drove the amplifier to a reference output of 1 watt, and the A-weighted output noise was 88 dB below 1 watt. The frequency response of the PMF 2150B was flat from 20 to 20,000 Hz, down 0.5 dB at 5 Hz and 77 kHz, and down 3 dB at 210 kHz. Consistent with the last measurement was the amplifier’s measured rise time of 1.5 microseconds. The slew factor, measured with the tone-burst signal used for the dynamic-headroom test, was greater than our measurement limit of 25.

Comment

The Perreaux PMF 2150B proved to be one of the huskiest amplifiers we have tested or used over the years. A few others could claim a higher rated power output, but in most cases a price was paid for this in output-current limitations, protective systems that could influence the performance of the amplifier with “difficult” loads, or the need for audible cooling fans.

The PMF 2150B is a no-compromise brute of an amplifier, almost certainly indestructible in normal use. Very rough treatment is likely to blow out its power-supply fuses, which can be replaced without difficulty after removing the top cover (an operation that is just difficult enough to discourage unnecessary abuse of the amplifier). There was no evidence of “thumps” or other transients when switching the amplifier either on or off.

We found this product’s basic honesty and simplicity a refreshing change from the current features-oriented school of consumer product design. Nothing is visible on its panel to warn a user when it is being driven to the almost unreasonably high power outputs of which it is capable. It is, therefore, incumbent upon the owner of a Perreaux PMF 2150B to use it with speakers that can absorb its power without damage (except perhaps to the listener’s ears).

The Perreaux PMF 2150B is hardly an inexpensive amplifier. If you do not need all of its rather exceptional performance capabilities, it is possible to buy about as many watts for less money. Still, it is not at all overpriced when you consider that no effort has been spared to make it do everything that is claimed for it. This sort of “no-holds-barred” approach to doing a first-rate design and manufacturing job is uncommon in all walks of life these days, and the result is an audio component of which both the manufacturer and the lucky owners of the PMF 2150B can justly be proud.

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