EICO ST-84 stereo preamplifier
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Taken from High Fidelity Magazine, September 1962

AT A GLANCE: The new EICO ST-84 is an attractive, versatile, easy-to-use dual-channel preamplifier and audio control unit that can be mated with any basic amplifier and can accept all standard program sources. It is available factory-built at $89.95, or in kit form for $59.95. Performance of the kit-built version, assembled and tested at United States Testing Company, Inc., was found to be generally excellent with effectively no distortion, though some discrepancies were measured in the unit's tape head equalization characteristic. Manufacturer: Electronic Instrument Co., Inc. (EICO), 33-00 Northern Blvd., Long Island City 1, N.Y.

The appearance of the ST-84 is characteristic of the "new look" that has been designed for EICO components, being at once handsome and functional, with the front panel controls arranged for maximum convenience. At the extreme left is a 7-position input selector with positions for three pairs of high-level inputs and four pairs of low-level inputs (tape head, two magnetic phono cartridges, and microphone). To the right of this control is a 7-position mode selector providing the usual variety of monophonic and stereo operating conditions. Then come the balance and volume controls, with concentric bass and concentric treble controls at the right end of the panel. Above these control knobs are slide switches for tape head equalization (positions are provided for both the 3¾ ips and 7½ ips tape speeds), a tape monitor function, loudness contour, rumble filter, and scratch filter. On the rear of the preamplifier there are seven pairs of input jacks and two pairs of output jacks (to amplifier and tape recorder), as well as one switched and one unswitched AC convenience outlet.

USTC found the performance of the ST-84 to be excellent in several respects. Its frequency response is flat within plus or minus 0.5 dB from 20 cps to 20 kc. It rolls off to minus 3 dB at 44 kc on the high end and extends to below 5 cps on the low end. The preamplifier introduces no measurable harmonic distortion with signals of up to 3 volts output. IM distortion, at 1-volt output, also was nonmeasurable. In fact, the only distortion measured at all was a negligible 0.1% IM at 3 volts output.

The preamplifier's sensitivity for 1-volt output is 0.168 volts on auxiliary, 1.57 mV on phono, 2.58 mV on microphone, and 0.84 mV on tape head. The signal-to-noise ratio of the preamplifier is 74 dB in the auxiliary position, 53 dB in the phono position, 57 dB in the microphone position, and 47 dB in the tape head position. The "noise" that remained was predominantly hum, but the signal-to-noise ratio is good enough so that the hum would be inaudible at normal listening levels.

The phono equalization characteristic (RIAA) was excellent above 100 cps and remained good down to 30 cps where the error was only 2 dB. Tape equalization (for signals direct from a tape head—that is to say, a tape playback deck that lacked its own preamplifier) was not as good, although the equalization for the 7½ ips tape speed was generally excellent above 400 cps. At 100 cps, the equalization had a 4.5-dB drop, and at 40 cps the error increased to 8 dB in comparison to the NAB standard. The equalization for the 334-ips tape speed remained fairly close to the EIA standard for this speed above 400 cps, but fell off below this frequency to minus 9 dB at 40 cps.

The tone controls provided 11 dB of bass boost and cut at 100 cps and approximately 16 dB of treble boost and cut at 10 kc. The rumble filter characteristic was considered relatively poor, in that it attenuated a large portion of the bass response in the musical program. With the rumble filter on, the 100-cps response is cut 5 dB, the 50-cps response is cut 8.5 dB, and the 30-cps response is cut 11.8 dB. The scratch filter also might have a more desirable shape; it attenuates signals at the approximate rate of 5 dB per octave above 3 kc. Thus, while these filters will remove noise in program material, they also may attenuate a fair amount of the program itself. In any case, noise filters—even on costlier units— rarely are completely satisfactory and with good program material really should be left off.

The loudness contour in the ST-84 is effective, but operates in a somewhat unusual way. Instead of providing a boost to the low frequencies, as is generally done, it attenuates the mid- and high-frequencies (about 20 dB at 1 kc). Thus, to obtain the desired end result of using a loudness contour (reasonably full response at low listening levels), the level control should be readjusted by the listener.

Aside from the unit's questionable tape equalization characteristic (which, of course, would be of concern only to those who would use it with a preamplifier-less tape deck), the EICO ST-84 is a generally first-rate control unit going at a relatively low price.

How it went together

Construction of the ST-84 was somewhat impeded by the change in position of certain components shown in one drawing from their orientation in a previous drawing. This orientation meant that the leads cut in previous steps now were too short and required extension: where instruction indicated a lead length that turned out to be excessively long, the leads had to be stripped and tinned a second time after the first cutting to size. Too, the multi-operation steps called for at one point caused some small amount of confusion after getting into the swing of single-operation steps; this was solved by checking-off each operation within the steps. In addition, the colour-code indicated for a given resistance value erred, requiring cross-check with the circuit diagram.

Three missing components—a phono jack insulator and two resistors—were quickly obtained from the manufacturer upon request. Two other parts-a 0.025-µfd disc capacitor, and 12AX7 tube—showed up as faulty after the kit had been built and put into operation. These were replaced and the unit performed correctly. But USTC feels that these problems, while they could be solved by an experienced kit builder or technician, could cause consternation and delay to the beginner.

This unit was well packaged for shipping purposes. Individual components were grouped in cartons or envelopes. Assembly time was about twenty-eight hours.