Vintage

Compact though it is, the SB-A10 from Toshiba's 'luxury brand' Aurex puts in a very punchy performance, and all this hailing from an era before the development of hi-fi-quality Class D amplification. Class AB all the way, Aurex rates the SB-A10 at 2x20W/8ohm which it very generously exceeds at 2x30W/8ohm and 2x40W/4ohm. The linear PSU has headroom to sustain 45W, 68W, 88W and 65W into 8, 4, 2 and 1ohm loads, respectively, under dynamic conditions although the amplifier is most linear into the higher 8ohm loading [black trace, Graph 1 below]. So, distortion increases markedly into lower loads (0.009%/10W/8ohm versus 0.09%/10W/4ohm – all at 1kHz) but there's also an uplift in THD at very low frequencies, despite this being a direct-coupled output stage. At just 1W/8ohm, distortion increases from a low 0.008%/1kHz to 0.06%/20Hz and 0.9% at a subsonic 5Hz [see black trace, Graph 2] while offering better control at HF where THD increases marginally to 0.055%/20kHz/1W.

It's tempting to correlate this distortion with the bass/treble tone circuits as these remain in the signal path and, even at the centre/detent position, also play a role in adapting the SB-A10's frequency response. Here there's a steady shelving down below 1kHz to –0.9dB/100Hz before rolling away more rapidly through the bass to –1.4dB/50Hz and –2.2dB/20Hz (and –5.6dB/5Hz). There's also a mild +0.2dB presence band boost followed by a 'sweetened' treble of –0.4dB/20kHz out to –9.0dB at an ultrasonic 100kHz. Noise, meanwhile, is higher than average but this is a white noise that draws the A-wtd S/N down to 81dB (re. 0dBW) rather than hum – in practice the transformer is very well shielded and residual noise is <1mV. PM