Adcom GFP-1A Preamplifier Review

Adcom’S GFP-1A preamp is a high-quality unit that features a built-in decoder for playing records made with the CX noise-reduction encoding process and separate input selectors for simultaneous listening and recording from different program sources. Each of the two front-panel selector knobs can control two tape decks, two record players, a tuner, and an additional high-level input marked dad/video (“DAD” stands for digital-audio disc). This last can be used for a Compact Disc player, a video-related source such as the audio output of a TV receiver, a video-cassette recorder, a video-disc player, or any other high-level source. The input knob selects one of these program sources for listening, and the recording knob connects one to the tape-recording jacks in the rear of the unit. The recording knob also has two tape-dubbing positions for copying a tape from either tape deck to the other as well as an off setting that completely isolates the tape-output circuits.

Above the phono 1 input jacks on the rear of the GFP-1A is a slide switch to increase the sensitivity of this input from its normal 2.5 millivolts (mV), a value suitable for moving-magnet cartridges, to 0.5 mV while lowering the input impedance; this permits direct hook-up of most moving-coil cartridges. The sensitivity of the phono 2 input is fixed at 2.5 mV, but there is a switch to select an input capacitance of either 47, 150, or 220 picofarads across a fixed value of input resistance.

CX decoding is available only through the phono 2 input. There are separate screwdriver-adjustable controls for each channel on the rear above the capacitance-selection switch, along with two LED indicators marked under and over. A 7-inch, 33-1/3-rpm CX calibration record is supplied with the GFP-1A; it contains 1,000-Hz tones recorded separately for each channel at a 3.54-cm/s (rms) groove velocity. To calibrate the system for the output level of a particular cartridge, thus ensuring proper decoding, the CX button on the front panel is engaged and the calibration record is played while the control for the corresponding channel is turned. The setting is correct when both indicator lights go off.

Other features on the rear panel include the phono jacks for the other signal inputs and outputs—including two sets of tape-recording jacks and two parallel pairs of main-output jacks—and a set of signal-processor input and output jacks, which are normally joined by jumper plugs. Anything connected to the signal-processor jacks is always in the signal path, and they can be used for any signal-processing accessory that would otherwise be connected in a tape-monitor loop, such as an equalizer or a noise-reduction system. There are also three a.c. outlets on the rear of the preamplifier, one of them switched.

The front panel of the GFP-1A is functional and uncomplicated. At the far right is a large volume knob, and next to it are the pushbuttons to engage the CX-decoding circuits and the loudness compensation. In the center of the panel, next to the two selector knobs, is a group of four buttons. One button activates the tone-control circuits (which are normally bypassed), and one parallels the two channels for mono operation. The other two buttons activate the signal filters; one filter attenuates low frequencies for rumble reduction, and the other rolls off ultrasonic frequencies (—3 dB at 25,000 Hz) to prevent slew distortions in power amplifiers prone to such problems. Since neither filter has any audible effect on programs, Adcom recommends that both be left engaged at all times. To the left of these switches are knobs for bass, treble, and balance adjustment, a headphone jack, and a heavy-duty power switch.

The Adcom GFP-1A is finished entirely in black with white markings. It measures 19 inches wide, 3 inches high, and 11-1/2 inches deep exclusive of the front-panel knobs and the rear connectors or jumpers. Weight is approximately 11to pounds. Price: $375.

Laboratory Measurements

No technical specifications were available to us when we tested the Adcom GFP-1A (a complete operating instruction manual was provided, however). The sensitivity of the preamplifier at its maximum volume setting (for the standard 0.5-volt output at 1,000 Hz into a standard IHF load) was 31 millivolts (mV) through the high-level inputs, 0.52 mV for the 2.5-mV phono inputs, and 0.14 mV for the 0.5-mV phono input. The unweighted signal-to-noise ratio (S/N) was 82 dB (high-level), 77 dB (phono, 2.5 mV), and 52 dB (phono, 0.5 mV). With A-weighting, these figures improved to 85, 80.5, and 60 dB, respectively.

The 2.5-mV phono input overloaded at 304 to 332 mV, depending on frequency, and the 0.5-mV input overloaded at 78 to 97 mV. The phono-1 input termination was 38,000 ohms in parallel with 225 picofarads at the 2.5-mV setting (only 360 ohms at the 0.5-mV setting). The phono-2 input resistance was also 38,000 ohms, which is lower than the typical 47,000 ohms but still acceptable for any cartridge, and the actual capacitance was approximately 40, 150, or 225 picofarads depending on the switch setting.

The output waveform clipped at levels of 11.3 to 12.5 volts over the 20- to 20,000-Hz frequency range. The harmonic distortion in the output was a constant 0.0027 per cent at 1,000 Hz for all output levels from 0-1 to 5 volts, reaching 0.022 per cent at 10 volts. At 20 Hz the distortion was between 0.0018 and 0.005 per cent over the full 0.1- to 10-volt range, and at 20,000 Hz it ranged between 0.004 and 0.006 per cent up to 5 volts and reached only 0.0125 per cent at 10 volts.

The frequency response was down 0.1 dB at 20 and 20,000 Hz, reaching —2.2 dB at 5 Hz and -3 dB at 77,000 Hz. The tone-control characteristics were excellent. The bass control had a sliding turnover frequency; at its middle settings it affected the response only below 100 Hz, with a 7- to 9-dB control range at 20 Hz. At its maximum settings, the turnover frequency was about 400 Hz, with a full range of 18 to 20 dB at 20 Hz. The treblecontrol curves were hinged at about 3,000 Hz, with a maximum range of about 14 dB at 20,000 Hz. The midrange response was unaffected even by extreme settings of the tone controls.

The low-cut filter reduced the response by 3.5 dB at 20 Hz, and the ultrasonic-filter response was down 1.5 dB at 20,000 Hz. The loudness compensation moderately boosted both low and high frequencies (below 200 Hz and above 10,000 Hz). The RIAA phono equalization was very accurate, varying only ±0.5 dB from 20 to 20,000 Hz, and it was totally unaffected by cartridge inductance. A spectrum analysis of phono-preamplifier noise with and without the CX decoder in operation showed that the decoder provided a noise reduction of about 20 dB from 1,000 to 5,000 Hz, tapering off slightly to about 12 dB at 20,000 Hz.

Comment

The construction of the GFP-1A is, to put it mildly, robust. The cabinet and chassis are made of steel, which accounts for much of the weight of the unit. With the cover removed you can see that everything is on a single large circuit board, with an open, uncluttered parts layout and a minimum of point-to-point wiring. All of this indicates probable long-term reliability.

There was nothing about the construction or design of the GFP-1A to which we could take exception—even its tone controls were outstanding in their ability to modify a music program without doing violence to it. (We hardly ever use tone controls, finding them to be more of an ill than a cure, but those on the Adcom GFP-1A are happy exceptions to that rule.) We also found that with medium-impedance headphones the volume was as loud as we would ever care to listen to, in striking contrast to the headphone outputs of some other hi-fi components.

We judged the overall effectiveness of the CX decoding system by playing a number of CX-encoded records. The results were excellent. Not only was there no evidence of “pumping” or other irregular operation, there was a virtually total elimination of audible noise. Our past experience with CX decoders, either as accessories or as part of a receiver or preamplifier, has been uniformly satisfactory, and in the GFP-1A the system has certainly been incorporated in a flawless manner.

Both on the test bench and as part of a hi-fi system, the Adcom GFP-1A left nothing to be desired. It has all the flexibility one could ever want combined with smooth, silent, and bug-free operation. An excellent preamplifier at a very reasonable price.

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