Integrated Amplifiers

Look closely at our pictures and you'll see the ST30SE has just one KT150 pentode tube per channel, rather than employing a pair, as is more typical, in push-pull mode. In these more common tube amplifier designs, a phase-splitter circuit is required to drive the 'push' tube and the 'pull' tube, but while Icon Audio's 'Single-Ended' (SE) mode avoids this – and is necessarily Class A in operation – it is unavoidably compromised in available power output. There's also a further trade-off in power output as its pentodes are switched from Ultralinear to Triode mode – the latter effectively turning the pentode into a triode by connecting the tube's screen to its plate.

So while a pair of KT150s can achieve between 80-120W/8ohm in Ultralinear push-pull mode [HFN Mar '14 and Jan '15], Icon Audio is rating its 'one KT150-per-side' ST30SE at just 28W/8ohm in Ultralinear SE and 18W/8ohm in Triode SE mode [see inset Graph]. But even these relatively low outputs are only possible thanks to the prodigious capacity of the KT150 tube: with only slightly more heater current than the older KT88, it offers nearly double the power output. Avoiding the need for a phase-splitter and tube pair matching is a definite plus for SE operation while Triode mode – here at least – also offers a useful halving of output impedance (better bass control) and few dB reduction in noise. There is no appreciable difference in frequency response, gain or distortion between Triode and Ultralinear modes in this amp, however. PM