Hitachi HA-4700 Integrated Amplifier Review

The Hitachi HA-4700 integrated amplifier is rated to deliver 50 watts per channel to 8-ohm loads from 20 to 20,000 Hz with no more than 0.02 per cent total harmonic distortion. It employs a “super-linear” output circuit similar to those used in some other amplifiers to reduce crossover distortion without sacrificing the efficiency of class-AB amplification. The bias on the output transistors is varied dynamically according to the signal level to shift their operating characteristics from class-A at low levels to class-AB at higher outputs.

The HA-4700’s phono preamplifier can be switched for use with either moving-magnet (MM) or moving-coil (MC) cartridges. The program source is selected by a row of flat buttons with colored lights below them showing which has been selected. The inputs are identified as phono, aux, tuner, and tape (the last can be operated independently of the others for listening to a tape recorder’s playback regardless of the regular program source).

On the same horizontal line with the in-put-source lights are two horizontal rows of LEDs that display the instantaneous power output of each channel (based on 8-ohm loads). The lights are calibrated from 0.04 to 100 watts (— 31 to + 3 dB referred to the rated 50 watts output). Green LEDs are used up to 50 watts, red ones at higher power levels.

Below the power indicators are three knobs, protruding only slightly from the panel, for the bass, treble, and balance controls. There is a large volume knob at the right of the panel, and all the other controls are pushbuttons. The illuminated power switch and the headphone jack are at the left of the panel near the two speaker switches. Below the input selectors are narrow rectangular buttons for the tone defeat, SUBSONIC FIL (filter), LOUDNESS, TAPE copy (from deck 1 to deck 2 only), tape selector (for playback from either deck when the input tape button is engaged), and phono selector for MM or MC phono cartridges.

On the rear of the Hitachi HA-4700 are the usual phono-jack inputs and outputs, insulated spring connectors for the speaker outputs, and three a.c. outlets, two of them switched. The amplifier is finished in satin silver and measures 17-1/8 inches wide, 12-7/8 inches deep, and 3-1/4 inches high. Weight is about 13-3/4 pounds. Price: $299.95.

Laboratory Measurements

After an hour’s operation at one-third rated power the top of the HA-4700 (over the enclosed power-transistor heat-dissipating fins) was uncomfortably hot to the touch, but it never became more than mildly warm in normal operation.

When we drove both channels at 1,000 Hz into 8-ohm loads, the output waveform clipped at 60.5 watts for an IHF clipping-headroom rating of 0.83 dB. The amplifier is not rated for use with lower load impedances, but we measured clipping outputs into 4 and 2 ohms of 54.8 and 25.2 watts, respectively. The IHF dynamic headroom (8 ohms) was a very good 2.58 dB, corresponding to a maximum short-term output of 90.6 watts. Into 4 and 2 ohms the dynamic power output was 60 and 29.7 watts, respectively.

For a reference output of 1 watt into 8 ohms, the HA-4700 required an input into the aux jack of 24 millivolts (mV), 0.32 mV into the phono (MM) jack. The respective A-weighted noise levels were —83 and —81 dB referred to 1 watt (both very good readings for an integrated amplifier). The phono-preamplifier stage overloaded at a high 230-mV input at 1,000 Hz, and the equivalent overload limits at 20 and 20,000 Hz were 250 and 152 mV. The phono-input impedance was 46,000 ohms in parallel with 250 picofarads. Although we made no measurements with the MC input, its gain is about seventeen times higher than the MM input ( + 24 dB), and it terminates the cartridge in 100 ohms.

The harmonic distortion of the Hitachi HA-4700 was extremely low. With 8-ohm loads, the distortion at 1,000 Hz was between 0.004 and 0.008 per cent from 1 to 50 watts output, reaching the rated 0.02 per cent at 60 watts just as clipping was beginning. The performance into 4-ohm loads was generally similar, with distortion readings falling from 0.03 per cent at 1 watt to well under 0.01 per cent between 20 and 50 watts and rising to 0.08 per cent at 55 watts. The amplifier’s output-current limitations prevented it from delivering much power to 2-ohm loads, and its distortion was in the range of 0.015 to 0.04 per cent for outputs up to 25 watts. The two-tone intermodulation-distortion measurement with equal-amplitude signals at 19,000 and 20,000 Hz revealed no detectable second-order distortion (1,000 Hz) and a barely detectable third-order product (18,000 Hz) at —92 dB referred to the rated 50-watt output of the amplifier. The slew factor was greater than 25, and the amplifier rise time was 4 microseconds. The overload-recovery time (from a 10-dB overdrive condition) was a negligible 5 microseconds, and the amplifier was stable with simulated complex reactive speaker loads.

The distortion at rated power output was no more than 0.002 to 0.008 per cent over most of the audible frequency range, reaching a maximum of 0.015 per cent at 20,000 Hz and falling to a nearly unmeasurable 0.0007 per cent from 3,000 to 7,000 Hz. At half and one-tenth power the shape of the distortion curve was similar, with only slightly higher readings.

The tone controls had a sliding bass-turn-over frequency (from about 100 to 500 Hz) and treble curves hinged at 2,000 to 3,000 Hz. The maximum bass boost occurred in the range from 50 to 100 Hz. The loudness compensation boosted both low and high frequencies, but only moderately. The “subsonic” filter appeared to have only a 6-dB-per-octave slope, with its — 3-dB response at 60 Hz and — 10 dB at 20 Hz. The RIAA phono equalization was accurate within ±0.25 dB from 20 to 20,000 Hz, and the inductance of most cartridges caused no more than a 0.5-dB rise in the output between 10,000 and 20,000 Hz.

Comment

The Hitachi HA-4700, although appearing with relatively little fanfare or publicity, has most of the features that have been seen of late in the top amplifiers of many other manufacturers. It is a compact, handsomely styled, and versatile amplifier that sounds as good as it measures. No manner of sonic flaws or operating “glitches” showed up in our testing or listening to the amplifier, and we came away with a thoroughly positive impression of its performance and overall design.

The HA-4700 has a very comprehensive protection system that manages to be completely unobtrusive as well as effective. A short circuit across the outputs will shut down the amplifier if it is driven hard, and the power must be switched off for a few seconds to restore it to service. Although a d.c.-offset voltage at the ouput will also silence the amplifier, it recovers from that automatically when the fault is removed. There is also a thermal cut-out that shuts the HA-4700 off if it becomes too hot. None of these protective systems operated during our tests, in spite of their severity, but it was reassuring to know they were there and waiting if need for them arose.

The schematic of the amplifier revealed a feature whose presence we would not have suspected from our measurements. A major problem of modern hi-fi systems is their susceptibility to interference from strong radio-frequency fields from nearby transmitters. Often these signals enter through the phono leads, overloading the low-level stages and superimposing the modulation of the r.f. signal on the regular program (or even completely obliterating the program). External filters are a possible treatment for this condition, but their success is uncertain, and there is also the possibility of degrading the amplifier’s frequency response by an improper cartridge termination. Hitachi has built an L-C low-pass filter into the HA-4700 at the phono input. We calculate that the filter has a cut-off frequency of about 1.5 MHz, which should help in cases of interference from nearby CB or amateur-radio stations, although it probably will not do much if one is unfortunate enough to live in the shadow of a powerful AM broadcast station. So far as we can see, the only sacrifice made for this feature is the relatively high 250-pF input capacitance of the phono section, but this should not cause problems with most combinations of record player and cartridge.

The Hitachi HA-4700 is, therefore, a thoroughly up-to-date, very high-quality, moderate-power integrated amplifier selling at a most modest price. It is an excellent value in every way.

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