Hafler 9300 Power Amplifier Review

The Hafler Model 9300, a basic stereo power amplifier, is rated to deliver 150 watts per channel into 8-ohm loads with less than 0.03 percent total harmonic distortion (THD) from 20 to 20,000 Hz. Its 4-ohm rating is 225 watts into 4 ohms.

The front panel contains only a rocker-type power switch with a green pilot light. The heat-sink fins for the output transistors extend out from the sides of the chassis and wrap around the back.

The rear panel is almost as simple, with heavy-duty gold-plated multiway binding posts for the speaker outputs (the two hot terminals are used for bridged operation) and gold-plated phono-type input jacks. The heavy-duty power cord plugs into a recessed socket on the rear. There is also a rear mono/stereo pushbutton switch that converts the left- and right-channel amplifiers into one bridged mono amplifier rated at 450 watts into a single 8-ohm load.

The Hafler 9300 features the company’s patented TransNova (trans-conductance nodal voltage amplifier) circuit, which employs grounded-source MOSFET output transistors that are said to provide a considerable voltage gain compared with conventional transistor output stages. Hafler says the advantages of this design include wide bandwidth with low distortion and noise. The MOSFET output transistors (six per channel) are also immune to thermal runaway, eliminating the need for protective circuits to guard the output devices against destruction from excessive current.

The massive power transformer has three separate secondaries for the two output stages and the low-level portions of the amplifier. A useful feature is the Soft Turn On/Off system, which monitors the regulated low-voltage power supply. When the amplifier is turned on, its circuits are disabled until the system determines that voltages are normal; then it activates the amplifier through a small MOSFET switch in its input stage. When the amplifier is switched off, the system rapidly cuts off the amplifier circuits before the power-supply voltages begin to collapse. The result is a complete elimination of thumps and noises from the speakers when the amplifier is turned on or off.

The Hafler 9300 is a compact, solidly built amplifier with 1/2-inch-thick, removable mounting feet. The case is extensively perforated with ventilating slits and, like the heat sinks and front panel, is finished in black. An optional rack-mount version has a silver-finish 19-inch front panel slotted for an EIA-standard rack or installation in a cabinet. The amplifier weighs 36 pounds.

During preconditioning at one-third rated power for an hour, the Hafler 9300 became quite warm, though not too hot to touch. In later listening tests we found that it ran nearly as hot as it did during bench testing, suggesting that its operating conditions may have been closer to true Class A (in which the power dissipated in the amplifier is relatively independent of signal level) than to Class B.

The amplifier exceeded its continuous power ratings by a substantial margin and was able to deliver almost 400 watts into 4 ohms in the dynamic-power test. Noise was very low, and frequency response was almost perfectly flat across the audio band, dropping off by less than half a decibel at 100 kHz and 2 dB at 200 kHz.

Overall, the Hafler 9300’s measured performance was as close to ideal as we have seen in some time. Most amplifiers using Class AB output-stage biasing show a steady increase in distortion at low levels as power is reduced (especially below 1 watt). The Hafler amp’s distortion, however, was not only low at any level short of clipping but was still decreasing as the power dropped below 1 watt, where it had already fallen to 0.0025 percent into 8 ohms and 0.004 percent into 4 ohms. This may or may not have any relationship to the Model 9300’s sound quality, but it is certainly evidence of careful and thorough circuit design.

The exterior of the amplifier presented a finished appearance, with no sharp corners on the panel or heat sinks. The same thoroughness was apparent in the owner’s manual, which included not only complete electrical and mechanical specifications, drawings of the front and rear panels, and useful suggestions on installation and operation, but also a parts list, a drawing of the main circuit board with part-number ID’s, an electrical schematic diagram, and a functional block diagram.

The manual, one of the most complete and businesslike we have seen for a power amplifier (or any other consumer product, for that matter), is consistent with the performance, workmanship, and attractive design of the Hafler 9300. Clearly, it is a class act, especially among amplifiers in its price range.

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