Adcom GFP-1 Preamplifier Review

The Adcom GFP-1 is a compact, no-frills preamplifier/control center whose design stresses low noise and maximum compatibility with other system components. The GFP-1 weighs about 11-1/2 pounds and is suitable for rack mounting: front-panel dimensions are 19 inches by 3 inches and depth is 12-1/2 inches.

The GFP-1 is an ideal companion for the Adcom GFA-1 power amplifier, matching its flat-black exterior finish. A large, square pushbutton power switch at the left of the front panel controls two switched a.c. outlets in the rear (there is also one unswitched outlet). The 800-watt rating of the switched outlets makes them suitable for powering the 200-watt-per-channel GFA-1 or any other high-power amplifier.

The most prominent feature of the front panel of the GFP-1 is the large volume knob, which operates a lightly detented forty-position control. Smaller knobs are used for the bass and treble tone controls, balance control, and input selector. The last operates a switch at the rear of the preamplifier close to the input jacks through a flexible shaft and provides inputs from two magnetic phono cartridges and two high-level sources (tuner and aux).

The other front-panel controls are pushbuttons. Two engage the lo and hi filters (whose infrasonic and ultrasonic cutoff frequencies exclude extraneous noise rather than modify the program content). A mute button drops the gain about 20 dB, and the tone button switches the tone-control circuitry in and out. The loudness button activates the loudness compensation, which follows the criteria of psychoacoustic researchers Robinson and Dadson, providing less extreme low- and high-frequency boost at low volume levels than the earlier Fletcher-Munson curves.

The mono button interconnects the two signal channels, and the remaining three buttons control the tape functions for either one or two decks: monitor connects the playback output of either tape 1 or tape 2 (as selected by the adjacent button) to the amplifier circuits, and dub cross-connects the two tape decks (the playback output of each to the recording input of the other) for dubbing tapes in either direction. The GFP-1 also has a front-panel headphone jack.

The rear panel of the GFP-1, in addition to its signal inputs and outputs, has two pairs of main out jacks and a +48-volt jack for powering other accessories to be made available in the future. The price of the Adcom GFP-1 is $350.

Laboratory Measurements

The maximum rated output voltage of the GFP-1 is a sizable 10 volts, and in our tests it handily exceeded that rating, clipping at 12.3 volts output into a standard IHF load of 10,000 ohms in parallel with 1,000 picofarads (pF). The output noise with the high-level inputs was unmeasurable under our test conditions, being less than the 100-microvolt minimum reading of our meters even when unweighted. The phono-input noise was 0.48 millivolt (mV) unweighted, but it was less than 100 microvolts with A-weighting ( -74 dB referred to the standard output level of 0.5 volt). The preamplifier’s noise ratings are -90 dB (high-level) and -87 dB (phono) with A-weighting; both exceed our measurement capability.

The harmonic distortion was also very low (it is rated at 0.015 per cent). At 1,000 Hz it was only 0.0003 per cent (the instrument residual) at 1 volt, increasing to 0.0016 per cent at 10 volts. At the frequency extremes the distortion was slightly higher, typically 0.0015 to 0.003 per cent for outputs of a few volts at 20 and 15,000 Hz and increasing to 0.003 and 0.008 per cent, respectively, at 10 volts. The GFP-1 can obviously be considered, for all practical purposes, distortionless.

At maximum gain, the high-level sensitivity was 31 mV and the phono sensitivity was 0.5 mV for a 0.5-volt standard output. The phono preamplifier (a low-noise FET stage) overloaded at 300 mV at 1,000 Hz, and the equivalent overload values at 20 and 20,000 Hz were 342 and 262 mV. The phono-input impedance was 47,000 ohms in parallel with 75 pF. The RIAA equalization was accurate within ±0.25 dB from 20 to 20,000 Hz and was completely unaffected by cartridge inductance.

The tone-control curves had a sliding bass-turnover frequency from less than 100 to about 500 Hz, and a family of treble-response curves hinged at 3,000 Hz. The maximum tone-control range was unusually large, with about ±20 dB of bass adjustment available at 30 Hz and ± 15 dB of treble range at 20,000 Hz. Adcom’s literature points out that the tone-control rollover frequencies match those of the RIAA phono equalization curve for better control of phono-playback response. However, the bass turnover of 500 Hz (which is also the RIAA turnover frequency) is realizable only at the maximum control setting, which we doubt would be used in any reasonably good system.

The loudness contours showed a bass boost beginning at about 100 Hz and reaching a maximum of about + 12 dB at volume settings below about —30 dB. The treble boost was very moderate, beginning at 10.000 Hz and reaching only +4 dB at 20.000 Hz. The lo filter, which is supposed to have a 12-dB-per-octave slope and a — 3-dB response at 20 Hz, was down 8 dB at 20 Hz on our sample, and its -3-dB frequency was 40 Hz. The hi filter had a barely detectable effect in the audio range ( -2 dB at 20.000 Hz), but when its button was pressed the overall gain of the preamplifier dropped by 2.5 dB. The listening volume through headphones was excellent even with medium- to high-impedance phones.

Comment

Adcom’s term “Silent Partner" seems wholly appropriate as applied to the GFP-1. Electrically, this is as quiet as any preamplifier we have seen (hum and noise are not only inaudible, but are too low for convenient measurement). There are no switching transients, partly because of a turn-on time delay that mutes the preamplifier outputs with a relay for about 5 seconds after power is applied, allowing the other parts of the system to reach stable operating conditions.

The phono preamplifier section of the GFP-1 is about as close to ideal as any we know of from the standpoints of equalization accuracy, freedom from interaction with cartridge inductance, and overload capability, to say nothing of its very low noise levels. The tone controls, whose characteristics are very good, have a large range of adjustment and should be used with discretion (20 dB of bass boost could overtax almost any amplifier or speaker system). Although we have rarely been satisfied with the effects of loudness compensation unless separate control of level and frequency response are provided, we must say that the Adcom system is remarkably unobtrusive. There is little or no evidence that it is in use unless the program contains appreciable low-frequency energy, in which case switching the loudness compensation in and out makes a modest but clearly audible change in the overall program balance.

The Adcom GFP-1, like its companion power amplifier the GFA-1, is compact and unobtrusive. It is a “Silent Partner” in a visual sense as well as sonically, doing its job without unnecessary frills or gimmicks. At its modest price, this is one of the better preamplifier values on today’s market.

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