Hafler DH-500 Power Amplifier Review

The Hafler DH-500 power amplifier is a “big brother” to the company’s widely acclaimed DH-200, embodying many of the same design concepts with a thorough reengineering to deliver high-power performance with high reliability. It is rated to deliver 255 watts per channel to 8-ohm loads from 20 to 20,000 Hz with no more than 0.025 per cent total harmonic distortion.

Like the DH-200, the new DH-500 has no electronic current limiting to protect its output devices (such circuits can often introduce unpleasant distortion “spikes” into the output when they operate). Its output stages use complementary-symmetry power MOSFETs, which are inherently self-protecting against thermal runaway. The low-level stages are also symmetrical, complementary designs using bipolar transistors. The amplifier is completely direct-coupled except for an input-blocking capacitor to exclude any d.c. component that might be present in the incoming signal.

Other design features of the Hafler DH-500 include a low dynamic-output impedance to minimize “interface” distortion resulting from the back-emf generated by speaker voice coils, a wide power bandwidth combined with an input low-pass filter that effectively prevents TIM distortions under any practical operating condition, and inherently low distortion so that large amounts of overall negative feedback are not required to achieve the amplifier’s exceptionally low distortion rating.

The DH-500 is a large, heavy amplifier suitable for rack mounting. It is 19 inches wide, 7-1/4 inches high, and 13 inches deep (including the front-panel handles) and weighs 48 pounds. It is entirely finished in flat black, and its only front-panel feature (besides the identifying logo) is a rocker-type power switch that lights in red to show that the amplifier is on. On the rear apron are heavy-duty five-way binding posts for speaker outputs (on 3/4-inch centers to accept dual banana plugs), the input phono jacks, and holders for the speaker-protection fuses. As supplied, these have a 2-ampere rating, which is suitable for protecting most speakers from excessive drive levels without inhibiting the high-power shortterm peaks that might be required from the amplifier. Hafler also supplies 5-ampere fuses for use when high-power operation is required or if the speaker impedance is much lower than 8 ohms. Since the amplifier has no internal current limiting and is highly resistant to damage from overloads or improper terminations, 10-ampere fuses can be substituted safely for high-power testing. These fuses are only for speaker protection—Hafler states that the DH-500 can drive 20 amperes into a short circuit without damage, although its internal thermal protection will soon shut it off. A d.c.-sensing circuit operates a relay to disconnect the speaker outputs instantly if more than 1.8 volts of d.c. offset is present in the output. This also protects the speakers against high-level transients from input switching, dropped phono pickups, and similar potentially catastrophic events. The relay also provides a 3-second turn-on delay.

In addition to the speaker fuses, an internal 15-ampere line fuse, and four internal 10-ampere d.c. power-supply fuses, the DH-500 is protected against damage by thermal circuit breakers. If the output transistors become too hot, the amplifier shuts down and the red light in the power switch blinks at a 3-Hz rate. Recovery is automatic when the temperature falls to a safe value. The DH-500 is cooled by a three-speed fan that normally runs at a very low speed and is audible only if you are close to it in a quiet room. Higher continuous-power operation will eventually cause the fan to shift to its intermediate speed, and in extreme cases (such as testing under high-power conditions) it will operate at high speed. In such cases, the fan sound is likely to be masked by the volume of the music.

Like other Hafler products, the DH-500 is available either in kit form or factory-wired and tested. The kit assembly is largely mechanical, since the active amplifier circuits are supplied as a pre-assembled and tested module. This roughly cubical cast-metal assembly carries the circuit boards on its sides, with the fan at one open end and heat-dissipating fins extending into the hollow center. The fan pulls air into the DH-500 from the sides, passing it over the fins and exhausting it at the rear of the amplifier. Most of the interior of the DH-500 is devoted to its power-supply components (a massive power transformer and a pair of 20,000-microfarad filter capacitors) and to a modest amount of power-supply and out-put-circuit wiring.

The DH-500 is available for $749.95 (factory-wired) or $599.95 (in kit form) from stereo dealers.

Laboratory Measurements

The test sample of the DH-500 was constructed from a kit by a Stereo Review staff member. He reports that it took an unhurried eight hours to assemble, no problems were experienced, and it worked perfectly from the start.

As expected, during the preconditioning period of one hour at one-third power the fan operated at its high speed. However, the exterior of the amplifier remained surprisingly cool, barely warm to the touch during this and all subsequent testing. Even the exhaust air was only mildly warm under these worst-case test conditions. During normal operation, with the fan operating at its low speed, the acoustic noise level, measured 1 meter in front of the panel, was 35 dBA— about as quiet as one can expect for a fan-cooled amplifier.

With both channels driving 8-ohm loads at 1,000 Hz, the outputs clipped at 325 watts per channel, corresponding to an IHF clipping-headroom rating of 1.05 dB. The DH-500 does not have a 4-ohm power rating, but we measured 452 watts per channel into 4 ohms and 392 watts into 2 ohms at the clipping point. The dynamic-power output with a 20-millisecond tone burst of 1,000 Hz was impressive, as might have been expected from the DH-500’s kinship to the DH-200, an amplifier noted for its short-term power capability. Into 8 ohms, the output clipped at 434 watts for an IHF dynamic-headroom rating of 2.31 dB. The dynamic output into 4 and 2 ohms was 792 and 653 watts per channel, respectively. An input signal of 0.145 volt drove the amplifier to a reference output of 1 watt (corresponding to 2.35 volts for rated power). The A-weighted output noise was 89 dB below 1 watt, essentially as rated.

The distortion ratings of the Hafler DH-500 proved to be as conservative as every other aspect of its design and performance. At 1,000 Hz (8-ohm loads) the distortion was less than 0.0004 per cent from 1 watt to 10 watts output, increasing gradually to 0.0009 per cent at 200 watts and 0.00135 per cent at rated power. With 4-ohm loads, the behavior was much the same, with the distortion rising from 0.00025 per cent at 1 watt to 0.0015 per cent at 250 watts and 0.0021 per cent at 400 watts. Even driving 2-ohm loads did not degrade the amplifier’s performance, with the distortion being in the vicinity of 0.002 per cent up to 200 watts. However, the amplifier heated up so rapidly when driving 2 ohms at high power levels (this was near the end of a long series of high-power tests) that the thermal cutoff shut it down after a few seconds of operation at outputs exceeding 200 watts, preventing us from measuring the distortion under these conditions.

With both channels driving 8-ohm loads, the distortion at rated power was between 0.001 and 0.0025 per cent from 20 to 4,000 Hz, reaching 0.017 per cent at 20,000 Hz. The distortion characteristic at half power and at one-tenth power (the latter being more than adequate for most home listening) was roughly similar except for readings well below 0.001 per cent in the midrange.

The measured IHF slew factor was 5, due to the appearance of waveform distortion at 100 kHz when we drove the amplifier with a “rated-power” input signal. It was stable with a complex reactive load, simulating a loudspeaker impedance characteristic. The frequency response at 1 watt was flat from 10 to 10,000 Hz, dropping to —0.1 dB at 20 kHz and -2.5 dB at 100 kHz.

Comment

Our experience with testing the Hafler DH-500 confirmed that it is truly a “bulletproof' amplifier hardly likely to be damaged by anything that might be applied to its input or connected across its output. It withstood the most brutal treatment we could apply without damage (or even a blown fuse), and it was perhaps the coolest amplifier we have had the pleasure of using. Amplifiers of this power rating usually become rather hot, especially under test conditions, but one would be hard put to tell that the DH-500 had been running merely by touching its exterior.

As for its sound, if it had any special sound quality we did not detect it. This is as it should be, of course. We recalled our testing of the DH-200, which was one of the first amplifiers to reach our measurement “floor” of 0.0003 per cent harmonic distortion. The DH-500 matched that and even surpassed it by a small amount.

The David Hafler Company seems to be following in its own footsteps, so to speak, with yet another truly state-of-the-art product that is easy to build and offers an exceptional value for the money. As a bonus to a tester, it is even light enough to lift and move around, a much-appreciated characteristic that is not shared by many amplifiers of its power rating.

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