Hitachi HA-7700 amplifier Lab Report

Lab Report

In 1979-80 Hitachi's HA-7700 was promoted as an integrated version of the HCA-7500/HMA-7500 pre/power combo. Although we have only (re)-tested the HMA-6500 [HFN Jan '15] the similarity in power, distortion and other performance indicators leads me to suspect that these claims were well founded. Rated at 2x65W/8ohm, in practice it achieved 2x81W/8ohm and 2x77W/4ohm with sufficient headroom to maintain peaks of 102W and 88W into 8 and 4ohm loads before dropping back to 52W and 29W into 2 and 1ohm loads [see Graph 1]. With a 5.4A maximum current capacity, the HA-7700 is clearly better suited to relatively sensitive speakers that don't dip down to tough impedances. Kept within its 8ohm 'comfort zone' the HA-7700 achieves a remarkably low distortion level – as little as 0.0009% through bass and midrange and just 0.004% at 20kHz, all with very little variation in power output [see Graph 2].


Dynamic power output versus distortion (up to 1% THD) into 8ohm (black trace), 4ohm (red), 2ohm (cyan) and 1ohm (green) speaker loads


Distortion versus frequency at 1W/8ohm (black, 5Hz-40kHz) and 10W/8ohm (red, 20Hz-20kHz)

While these figures are not a million miles off those achieved by the HMA-6500 power amp, the HA-7700 does score with its much reduced 0.08ohm output impedance, increasing to 0.13ohm/20kHz. In DC-coupled mode the frequency response is flat to within ±0.02dB from 1Hz-20kHz, rolling gently away to –3.4dB/100kHz, with slightly greater treble loss into progressively lower impedance loads (–0.35dB/20kHz into 4ohm to –1.3dB/20kHz into 1ohm). The '15Hz Low-Filter' yields a subsonic roll-off of –1dB/28Hz to –3dB/14Hz and –19dB/2Hz – a perfect 'curve' to attenuate turntable rumble or arm/cartridge resonance. Finally, our sample was possibly a little noisier than when new, so the A-wtd S/N of 80dB (re. 0dBW) would have been bettered by 5-10dB in the day. PM

Power output (<1% THD, 8/4ohm): 81W / 77W
Dynamic power (<1% THD, 8/4/2/1ohm): 102W / 88W / 52W / 29W
Output imp. (20Hz–20kHz/100kHz): 0.082 to 0.130ohm
Freq. resp. (20Hz–20kHz/100kHz, 0dBW): +0.0dB to –0.25dB / –3.4dB
Input sensitivity (for 0dBW/65W): 16mV / 130mV
A-wtd S/N ratio (re. 0dBW/65W): 79.8dB / 97.9dB
Distortion (20Hz-20kHz, 10W/8ohm): 0.0009–0.004%
Power consumption (Idle/rated output): 64W / 275W
Dimensions (WHD) / Weight: 435x165x383mm / 16kg