Amcron D-75 amplifier Lab Report

Back in the day this multi-role amplifier was renowned for its rugged, purposeful minimalism. Its 1U rack mount form factor was a nod to all the pro/on-the-road users, as were the balanced XLR input connections, but the green/red LED indicators were even more specialised: the former flashing in synchrony with the input signal, the latter flashing when the output THD exceeded ~0.05%. In practice the red IOC (Input/Output Comparator) does indeed light when the D-75 breaches its 0.05% distortion rating, or just as the output stage begins to clip (at 41W/1kHz/8ohm in this instance). The green ‘signal’ LED, meanwhile, starts glowing dimly with a 50mW/8ohm output from the D-75 and increases in brightness as the power delivery increases.


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


Distortion versus extended frequency at 10W/8ohm (left channel, black; right channel, red)

Talking of which, Amcron/Crown rated its workhorse D-75 amplifier at 35W/8ohm and 55W/4ohm while, on the bench, it achieved closer to 2x45W/8ohm and 2x63W/4ohm with 67W and 119W possible into the same loads under dynamic conditions [see Graph 1], albeit limited to 85W/2ohm and 38W/1ohm (6.5A maximum current at <1% THD). Distortion is very well managed, and while Amcron’s ‘<0.001% from 20Hz-400Hz’ is no idle boast this was only achieved on the right channel here [red trace, Graph 2] at 0.00025-0.0093% from 20Hz-20kHz at 10W/8ohm. The 0.0014-0.085% recorded on the left channel [black trace] will undoubtedly be a function of age. A-wtd noise, too, was slightly better on the right channel (89.3dB vs. 86.6dB re. 0dBW), though both are ‘above average’. Finally, the supremely low 0.007ohm output impedance (20Hz-2kHz) is impressive and the response usefully extended at 6Hz-48kHz ±1dB into 8ohm. PM

Power output (<1% THD, 8/4ohm): 45W / 63W
Dynamic power (<1% THD, 8/4/2/1ohm): 67W / 119W / 85W / 38W
Output imp. (20Hz–20kHz/100kHz): 0.007–0.110ohm / 1.135ohm
Freq. resp. (20Hz–20kHz/100kHz, 0dBW): –0.2dB to –0.23dB / –2.90dB
Input sensitivity (for 0dBW/35W): 137mV / 830mV
A-wtd S/N ratio (re. 0dBW/35W): 89.3dB / 104.7dB
Distortion (20Hz-20kHz, 10W/8ohm): 0.00025–0.0093%
Power consumption (Idle/rated output): 11W / 135W
Dimensions (WHD) / Weight: 483x45x223mm / 4.5kg