Adcom GFA-2 Power Amplifier Review

The Adcom GFA-2 stereo power amplifier features a stable, low-noise, low-distortion “front end” using differential FET amplifiers. The output stages of the GFA-2 use high-speed bipolar transistors, and entirely separate power supplies for the two channels enable each channel to deliver its maximum power regardless of the status of the other.

The output transistors of the GFA-2 are fully protected against damage from short-circuits or overheating. The protective system disconnects the speakers by means of a relay, simultaneously changing a front-pan-el indicator light from green to amber. The same system delays the speaker connection for about 4 seconds each time the amplifier is turned on to allow internal transients to die away.

The Adcom GFA-2 is rated to deliver 100 watts per channel to 8-ohm loads from 20 and 20,000 Hz with no more than 0.05 per cent total harmonic distortion. Its rated signal-to-noise ratio is 95 dB, and the input sensitivity is 97 millivolts (both figures relative to a 1-watt output in accordance with EIA Standard RS-490).

The GFA-2 is 19 inches wide, 14 inches deep, and 5-1/4 inches high, and its front panel is slotted for rack mounting and fitted with handles for easier moving or lifting. It is finished entirely in black, with several small LED front-panel indicators and a large square pushbutton power switch. In addition to the protection light, there is a red power pilot light and two yellow peak lights that flash when the output voltage approaches the amplifier’s clipping level. On the rear of the amplifier there are insulated spring-loaded speaker connectors, phono-type input jacks, and two line-fuse holders (each power supply is separately fused). The GFA-2 weighs 29 pounds. Price: $360.

Laboratory Measurements

The exterior of the Adcom GFA-2 did not become excessively warm during its one hour of preconditioning and subsequent high-power testing. With both channels driving 8-ohm loads at 1,000 Hz, the outputs clipped at 120 watts per channel for an IHF clipping-headroom rating of 0.79 dB. The GFA-2 is not specifically rated for 4- or 2-ohm operation, but the clipping outputs into those impedances were respectively 177 and 95 watts per channel.

The dynamic-headroom test showed a maximum output of 150 watts into 8 ohms for a dynamic-headroom rating of 1.76 dB, 200 watts into 4 ohms, and 118 watts into 2 ohms. The peak lights flashed at an output equivalent to 85 watts into 8 ohms. The 1,000-Hz distortion was extremely low under any load condition and at all power outputs up to the clipping point. Driving 8-ohm loads, the distortion was about 0.001 per cent at 1 watt or less and typically less than 0.002 per cent up to 100 watts. With 4-ohm loads it was between 0.0025 and 0.0035 per cent up to more than 100 watts, and even with 2-ohm loads the distortion was typically under 0.005 per cent below the clipping point.

The frequency response of the GFA-2 was flat from 5 to 20,000 Hz (within 0.2 dB) and rolled off to —3 dB at 240 kHz. Its rise time was 1.5 microseconds. The slew factor was 2, a relatively low figure but quite adequate. The high-frequency linearity was good, as shown by the intermodulation-distortion tests with equal-amplitude input signals at 18 and 19 kHz. With a peak output equal to that of a 100-watt sine-wave signal, the second-order distortion at 1,000 Hz was —82 dB, and we could find no trace of third-order products at 17 and 20 kHz. The input level required for a reference output of 1 watt was 96 millivolts, and the A-weighted noise was a barely measurable 90 dB below 1 watt. The amplifier was stable with simulated loudspeaker (reactive) loads and when driving an IHF reactive load.

Comment.

Not only did the Adcom GFA-2 easily surpass its key specifications, but it proved to be rugged enough to withstand our full test sequence without damage or even a blown fuse. The protection relay was tripped only when we shorted the outputs or drove the amplifier into overload at ultrasonic frequencies. We never succeeded in heating it enough to operate the thermal protection system.

Adcom cautions against inserting a phono plug into the inputs of an amplifier while it is turned on. Aside from the risk of damage to one’s speakers, there is an additional hazard to the output transistors, which can be destroyed by excessive r.f. input. The manufacturer indicates that this may be one of the very few ways in which the amplifier can be damaged. We did not put it to the test deliberately, but on the bench we took no special precautions such as shutting off the power when changing input cables. The amplifier survived unscathed.

In a music system the Adcom GFA-2 performed just about as we would have expected. Given the extremely low levels of noise and distortion in its output, one would hardly expect the GFA-2 to add any sonic character to the program, and it did not.

Perhaps the single most important factor that distinguishes this amplifier from all the others is its unusually attractive price. Most 100-watt amplifiers sell for a far higher price than this one. Therefore, in addition to being a top-quality product with state-of-the-art performance, fully protected against most common operating hazards, the Adcom GFA-2 is a genuine bargain in market.

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