Myst TMA3 amplifier Lab Report

Based on the classic Hitachi MOSFET power amp application circuit, driven here by two cascaded differential stages running in Class A, Myst's TMA3 meets its 2x35/8ohm specification by delivering 2x40W/8ohm and 2x55W/4ohm. Each channel has a pair of 2SJ82/2SK226 Hitachi MOSFETs that are mounted on limited heatsinking – the external fins exceeding 55°C at continuous power levels over 5W/8ohm, so good ventilation is mandatory. Protection is afforded by a 2.5A anti-surge fuse in line with each channel's output but the TMA3's dynamic power output still stretches to 47W, 77W, 101W and 103W into 8, 4, 2 and 1ohm loads, respectively, or ~10A for a period of 10msec at <1% THD [see Graph 1].


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, grey shadow; right channel, red)

Myst offers two levels of gain via either the tuner/tape/aux input and via the dedicated CD input, the latter bypassing the active preamp stage to offer an overall gain of +29.6dB. One effect is to optimise the performance of the ALPs volume control which, even in our 30+ year-old sample, offered a channel balance within 0.1dB over the top 40dB of its dynamic range. Noise, too, is still in tune with many contemporary amplifiers, yielding an A-wtd S/N ratio of 84dB (re. 0dBW). Distortion, however, brings some 'colour' to the TMA3 for while it remains fairly consistent with power output (0.04-0.06% from 1-35W at 1kHz), it does increase quite smartly with increasing frequency. The minimum of 0.005%/20Hz rises to 0.05%/1kHz and then up to 0.5%/20kHz and 1.2%/40kHz [see Graph 2]. The response also shows a low bass roll-off of –1.3dB/20Hz but with a slight rise at very high/ultrasonic frequencies of +0.2dB/20kHz – a trend that's exaggerated into lower impedance loads. PM

Power output (<1% THD, 8/4ohm): 40W / 55W
Dynamic power (<1% THD, 8/4/2/1ohm): 47W / 77W / 101W / 103W
Output imp. (20Hz–20kHz/100kHz): 0.073–0.115ohm / 0.56ohm
Freq. resp. (20Hz–20kHz/100kHz, 0dBW): –1.3dB to +0.15dB / –1.1dB
Input sensitivity (for 0dBW/35W): 93mV / 560mV
A-wtd S/N ratio (re. 0dBW/35W): 84.2dB / 99.6dB
Distortion (20Hz-20kHz, 10W/8ohm): 0.005–0.48%
Power consumption (Idle/rated output): 14W / 128W
Dimensions (WHD) / Weight: 432x55x220mm / 4.5kg