EICO ST-70 integrated stereo amplifier
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Vintage test report


Taken from 'High Fidelity' magazine, February 1963

AT A GLANCE: The EICO ST-70 is an integrated stereo amplifier, offering preamp-control facilities and dual power output channels on one chassis. It is available as a kit for $99.95 or factory-wired for $149.95. A metal cover is included. The unit reported on here by United States Testing Company, Inc., was built from a kit. In USTC's view, the ST-70 is well designed, easy to build, and offers exceptionally good performance for a combination amplifier. Manufacturer: Electronic Instrument Company, Inc., 3300 Northern Blvd., Long Island City 1, N. Y.

N DETAIL: As with the EICO ST-84 preamplifier reported on earlier (September 1962), the ST-70 features the handsome, functional styling characteristic of EICO's recent products. The front panel is neatly arranged for maximum convenience in use, and contains a row of knobs for the main operating controls and a series of slide-switches for auxiliary controls. The knobs, from left to right, are: a seven-position input selector; a six-position stereo/mono mode selector; channel balance; volume; and concentric bass and treble controls. The switches, above the knobs, control: tape head equalization (for both 7½ - and 3¾ ips speeds); tape monitor; loudness contour; rumble filter; scratch filter; balance check (which cuts in a special null-type circuit that aids in equalizing the gain of the two amplifying channels); and speaker-phasing.

At the rear of the amplifier, input jacks are provided for magnetic phono cartridges (a 47K-ohm input as well as a l00K-ohm input), tape head (1 megohm input), FM tuner, AM tuner, multiplex adapter, and tape recorder (all 500K-ohm inputs). Tape recorder output jacks are provided as well as 4, 8, and 16-ohm speaker taps for each channel. There are also a centre channel speaker output tap and two AC convenience outlets (one switched, one unswitched).

The amplifier, which contains a total of twelve tubes, has - in each channel - a two-stage 12AX7 low- level preamplifier with feedback equalization; a 12DW7/ 7247 dual-triode serving as first audio amplifier and furnishing gain for the feedback tone control circuit; 1/2-12AX7 as a second audio amplifier; a 6SN7GTB driver and phase inverter; and two 7591 push-pull output stages. Additional negative feedback is used from the secondary of the output transformer back to the second audio amplifier stage.

The chassis contains potentiometers and test jacks for adjusting the DC bias and balance in each channel's output stages, but in order to do this the kit builder must have available a fairly accurate DC vacuum tube voltmeter (VTVM) or a volt-ohmmeter (VOM). The power supply uses a GZ34 rectifier tube, and supplies AC voltage to the filaments of all tubes.

In USTC's tests, the amplifier provided 28.5 watts output at 1 kc before clipping occurred on the left channel, at which level the harmonic distortion was a very low 0.21%. The right channel clipped at 27.8 watts with 0.25% THD. When both channels were operating simultaneously, the left channel clipped at 22.8 watts with 0.6% THD.

The amplifier's power bandwidth was measured at 0.5% THD up to 10 kc and at the clipping point above that frequency. In all, it extended from 23 cps to above 100 kc, which is excellent. Harmonic distortion remained under 0.6% from 36 cps to 20 kc at full power (28.5 watts), reaching 1% at 31 cps. At half power, the THD remained under 0.3% from 30 cps to 20 kc, reaching 0.9% at 20 cps.

The frequency response of the ST-70 was measured as flat within +0 and -1 db from 5 cps to 47 kc, and was down only 3 db at 100 kc. The amplifier's IM distortion was very low, remaining under 0.6% up to 27 watts and under 1.5% up to 30 watts. These all are outstanding figures for a combination amplifier in the price class of the ST-70.

Equally commendable was the excellent square- wave response of the ST-70. As shown in the photos, its 10kc transient characteristic was fine, approaching that found in very good basic power amplifiers. The 50cps waveform showed a flat response with relatively minor phase distortion.

The RIAA equalization characteristic was very good, being flat within ± 1 db from 35 cps to 20 kc. Tape head equalization was not as accurate, inasmuch as the curves for both tape speeds showed significant departures from the standard equalization at these tape speeds. The sensitivity of the amplifier (for 28.5 watts output) was 450 mV on the high level inputs, 4 mV on the magnetic phono inputs, L4 mV on the tape head input with 3%-ips equalization, and 2.17 mV on the tape head input with 7'/i-ips equalization. The amplifier's signal- to-noise ratio was 53 db on phono, 43 db on tape head, and 76 db on the high level inputs. The amplifier had a damping factor of 6.2, which is about average for its class.

The treble controls provided approximately 17 db of boost or cut at 10 kc, while the bass controls provided 10.5 db of boost or cut at 100 cps. The scratch filter operated at the approximate rate of 9 db per octave above 3 kc, while the rumble filter cut at the approximate rate of 9 db per octave below 70 cps - both desirable characteristics from a musical standpoint.

The EICO ST-70, in sum, provides high quality performance at a modest cost. Its only limitation is its tape head equalization characteristics and the signal- to-noise ratio in the tape head position but this would be of concern only to those using a tape deck that lacked its own preamplifiers.

How It Went Together
The kit for the ST-70 contained all parts needed for building the amplifier. The construction is fairly simple, requiring the usual hand tools and no high degree of manual dexterity. The instruction manual is occasionally marred by a minor error or a misprint regarding the length of leads; these are not serious and could be resolved by the average kit builder. In general, the ST-70 was a pleasure to build, and the discovery that the kit-built version performs so admirably was even more of a pleasure. Total construction time was sixteen hours.