Icon Audio ST40 MkIV 6L6 Integrated Amplifier Lab Report

Lab Report

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

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Dynamic power output versus distortion into 8ohm (black trace), 4ohm (red), 2ohm (blue) and 1ohm (green) speaker loads. Max. current is 4.7A

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Distortion vs. frequency (5Hz-40kHz at 1W/8ohm, black; 20Hz-20kHz at 10W/8ohm, red)

Power output (<1% THD, 8/4ohm) 28W / 28W
Dynamic power (<2% THD, 8/4/2/1ohm) 32W / 32W / 30W / 22W
Output imp. (20Hz–20kHz/100kHz) 1.75–1.85ohm / 2.8ohm
Freq. resp. (20Hz–20kHz/100kHz) –0.15dB to –0.2dB/–3.6dB
Input sensitivity (for 0dBW/28W) 71mV / 380mV
A-wtd S/N ratio (re. 0dBW/28W) 78.5dB / 93.0dB
Distortion (20Hz-20kHz, 10W/8ohm) 0.052-1.14%
Power consumption (Idle/Rated o/p) 153W / 209W
Dimensions (WHD) / Weight 390x210x410mm / 20kg
Price £2200-£2400

COMPANY INFO
Icon Audio
Leicestershire
Supplied by: Icon Audio
0116 2440593
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