Integrated Amplifiers

While certainly more powerful than the average 300B-based SE tube amp – a clear goal for designer David Shaw – the ST30SE is still not blessed with a prodigious output and remains better suited to high sensitivity (90dB+ re. 2.83V/1m) loudspeakers. Distortion increases with power so the ST30SE's maximum output rather depends on our definition of 'hi-fi' – 17W into 8/4ohm (via the 8 and 4ohm taps) at 1% THD up to 30W into 8/4ohm at 2% THD. There is only a marginal increase to 32W with a further relaxation to 3% THD, but a reduction in output to 17-18W into 8/4ohm loads and just 10W/6W into 2/1ohm loads (via the 4ohm tap) under dynamic conditions [see Graph 1, below].

Power output in Triode mode is typically ~60% of that in Ultralinear, the ST30SE achieving 2x22W into 8/4ohm loads at 2% THD. In Ultralinear guise, and with 'Low gain/moderate feedback' selected, the output impedance is 0.85-1.17ohm (20Hz-20kHz), but this falls to 0.47-0.54ohm in Triode mode, improving LF damping. Into 'flat' non-reactive 8 and 4ohm loads the response reaches down to 6Hz and out to 50kHz (–3dB points) but there will be greater variation in the amp/speaker system response in Ultralinear mode because of its higher source impedance. Distortion is slightly lower at low power (1-5W) in Ultralinear mode at 0.2-0.5%, but in Triode mode it is more consistent at 0.58-0.62% (all at 1kHz/8ohm). Versus frequency, at 10W/8ohm, the THD trends are very similar indeed [see Graph 2, below]. Finally, there is about 1dB difference in gain between Triode and Ultralinear (+29dB vs. +30dB) but nearly 2dB advantage in A-wtd S/N at 85.1dB vs. 83.4dB, respectively, with any residual hum/flicker noise less than –64dBV. PM