Hafler XL-600 Power Amplifier Review

The XL-600 is the most powerful amplifier to bear the Hafler name. This formidable unit, which is rated to deliver 305 watts per channel into 8 ohms from 20 to 20,000 Hz with no more than 0.05 percent total harmonic distortion (thd), is not current-limited. Its rated output at the clipping point is 360 watts per channel into 8 ohms, 500 watts into 4 ohms, 750 watts into 2 ohms, and 900 watts into 1 ohm! It also has a bridged mono mode in which it can deliver 1,000 watts into 8 ohms or 1,500 watts into 4 ohms (at the clipping point).

The XL-600 was designed to use some 30 dB less overall negative feedback than previous Hafler amplifiers, which contributes to its low transient intermodulation distortion (tim) and high stability. In accord with Hafler’s “Excelinear” concept, phase shift in each channel has been kept to a minimum. Each output stage has eight power MOS-fet’s (metal-oxide-semiconductor field-effect transistors) capable of driving the most difficult speaker loads. Except for the single large power transformer, the two channels are completely separate, and the total filter capacitance of 72,000 microfarads gives the amplifier a powerful low-end capability. A small, separate power supply operates the low-level stages.

The output stages are constructed about a square, chimney-like heatsink structure through which a fan blows cooling air. The air enters the cabinet through openings on the sides and exhausts through the rear apron. The fan’s speed is variable and responds automatically to the cooling requirements of the amplifier. The outputs are protected by a heavy-duty relay that connects them after a 3-second turn-on delay to prevent transients from reaching the speakers. If any significant DC component appears at the speaker outputs or if the heat-sink temperature becomes excessive, the relay also disconnects the speakers while allowing the fan to continue cooling the amplifier.

The rear apron of the XL-600 contains the gold-plated input jacks, four multiway binding-post speaker terminals (with a speaker-fuse holder for each channel), and a slide switch that converts the amplifier to bridged mono operation. The front panel, fitted with heavy-duty handles, has a rocker-type power switch and a clearly visible red pilot light. The XL-600 weighs 51 pounds and measures 19 inches wide, 13 inches deep (plus 2 inches for the handles), and 7 inches high (plus half an inch for the feet). Price: $1,195.

Lab Tests

The cooling fan of the XL-600 turned slowly and inaudibly in normal operation, although during the 1-hour preconditioning period and other high-power operation it sped up and could be heard clearly. It was never as noisy as most other amplifier cooling fans we have heard, however, and it should not be audible in a home environment.

Our clipping-level power measurements clearly established the credentials of the XL-600. Driving two-channel loads of 8 and 4 ohms at 1,000 Hz, the amplifier clipped at outputs of 372 and 680 watts, respectively. For the 2-ohm measurement, we loaded only one channel with 2 ohms, leaving the other to drive 4 ohms. The 2-ohm clipping power measured an impressive 900 watts. The dynamic power, as expected, was even greater, with readings of 478, 900, and 1,300 watts into respective loads of 8, 4, and 2 ohms.

The amplifier’s distortion was typically about 0.01 percent at most power outputs and frequencies, but it increased to about 0.03 or 0.04 percent at the upper audio frequencies. The amplifier required an input of 0.14 volt for a reference 1-watt output, and its A-weighted noise level was — 89 dB referred to 1 watt. The frequency response varied only ±0.02 dB from 10 to 30,000 Hz. Phase shift was truly negligible, varying ±0.1 degree from 20 to 7.000 Hz and reaching 0.8 degree at 20.000 Hz.

Comments

The Hafler XL-600 lived up to the claims made for it in full measure. Not only did our bench measurements confirm its specifications, but it proved itself to be as “bulletproof” an amplifier as one could wish.

Testing a large, powerful amplifier such as the XL-600 often produces unwelcome surprises, such as blown-out power transistors or even damage to one’s speakers. When we blew out a 10-ampere speaker fuse at one point, we continued the test with the only available substitute, a 15-ampere fuse (these fuses protect only the speakers; the amplifier is separately fused in its DC power circuits). Although this inadvisable procedure enabled us (barely!) to make our 2-ohm clipping-level power measurement, that channel of the amplifier shut down, having blown one of its power-supply fuses. After a fuse replacement, however, the amplifier appeared to be as good as new.

Otherwise, there were no surprises in our use of the XL-600. Its fan was quite inaudible in normal use, and its exterior never became more than faintly warm. As claimed, it came on (and shut off) with not a trace of a thump. The speakers we used had reasonable impedance characteristics and probably would not tax the abilities of any good amplifier, but we have no doubt that the XL-600 would be able to drive any speaker to its limits without approaching its own limits in any respect.

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