Integrated Amplifiers

With 6L6/KT66 pentodes replacing the ST40 MkIV's KT88s [HFN May '19], and with smaller output transformers, power output is reduced from 2x55W to 2x28W into 8/4ohm loads at <1% THD, respectively. There's also a commensurate drop in dynamic output from 52W, 52W, 69W and 73W to 32W, 32W, 30W and 22W into 8, 4, 2 and 1ohm loads, respectively [see Graph 1, below]. The ST40 6L6 runs in ultralinear mode only (though triode mode can be ordered from the factory) yielding a total +32dB gain and fairly uniform ~1.8ohm output impedance across the 20Hz-20kHz audioband. Some variation in system response will result from its interaction with the swings in impedance that occur in 'real' loudspeaker loads but into a fixed 8ohm it's flat to within ±0.2dB from 20Hz-20kHz, with –1dB points at 10Hz and 56kHz. There's no buffering after the passive preamp/volume control, however, so there's also some change in HF response with volume position, and also hum and noise which impacts the achievable S/N ratio by some 20dB. The best-case A-wtd S/N ratio is 78.5dB (re. 0dBW) which is still slightly below average and significantly behind the performance of the KT88-equipped ST40 MkIV.

The KT66 tubes fitted here exhibited some slight drift – and feedback is a low 8.5dB – with distortion starting at 0.02/0.075% (L/R) when cold before coalescing at 0.030/0.035% (L/R) after 30 minutes at 1W/8ohm. Then again, Icon Audio accurately predicts the ST40 6L6's warm-up time in the user manual. Once stabilised, distortion increases with level from 0.035%/1W to 0.065%/10W and 0.3%/20W (all 1kHz) and at low/high frequency to 1.15%/20Hz and 0.75%/20kHz (re. 10W) [see Graph 2]. PM