Sugden A21 Series 2 amplifier Lab Report

Lab Report

When our reviewer, Tim Jarman, suggests the A21 Series 2 'gets unusually hot in use' he is not joking – I measured 58°C on the rear heatsinking. At idle, the amplifier draws 70W from the wall and when delivering its rated 2x12W/8ohm it's still drawing 70W, illustrating just how many watts are being dumped into those little sinks. This is indeed a true class A amplifier. Rated at 12W/8ohm it achieves 2x13.5W/8ohm and 2x10W/4ohm at 1% THD, a performance that improves under dynamic conditions to 15W/8ohm and 19W/4ohm albeit with distortion relaxed to 2% [see Graph 1]. Maximum current is a mere 2.4A (2% THD into 1ohm) so this amplifier is best partnered with very sensitive, if not 8ohm nominal impedance, loudspeakers.


Dynamic power output versus distortion (up to 2% 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)

Back in the day, Sugden specified this amplifier's THD at '0.25% for 10W and 0.01% at ½W' – figures that are not unrealistic across midrange frequencies. In practice, the A21 Series 2 achieves <0.25% THD from 100Hz-10kHz at 10W/8ohm with a minimum of 0.11% at 2-3kHz [black trace, Graph 2] although this necessarily increases through low bass (1.7%/20Hz) and high treble (0.8%/20kHz) frequencies. Distortion also falls at lower output and, indeed, is ~0.005% at 0.5W, or <0.02% at 1W/8ohm from 100Hz-10kHz. The ±1dB 30Hz-20kHz frequency response was closer to 18Hz-16kHz with our sample, the 2nd order 7kHz HF filter yielding –3dB/5.5kHz and –18dB/20kHz here. Of course, the moderate 0.335ohm output impedance (rising to 1.1ohm/30Hz) will also have some impact on system response depending on your choice of loudspeaker. Noise is not inconsiderable either, with hum at –54dBV and white noise pushing the A-wtd S/N back to 72.5dB (re. 0dBW). PM

Power output (<1% THD, 8/4ohm): 13.5W / 10W
Dynamic power (<2% THD, 8/4/2/1ohm): 15W / 19W / 12W / 6W
Output impedance (20Hz–20kHz): 1.10 to 0.33ohm
Freq. resp. (20Hz–20kHz/100kHz, 0dBW): –0.6dB to –1.5dB / –13.6dB
Input sensitivity (for 0dBW/12W): 58mV / 205mV
A-wtd S/N ratio (re. 0dBW/12W): 72.5dB / 94.1dB
Distortion (20Hz-20kHz, 10W/8ohm): 0.11–1.7%
Power consumption (Idle/rated output): 70W / 70W
Dimensions (WHD) / Weight: 394x127x229mm / 10kg