Integrated Amplifiers

Although equipped with the same complement of KT88s, the performance of Icon Audio's Stereo 40 MkIV is superior in every key area of performance to its predecessor, the Stereo 40 MkIIIm [HFN Mar '13]. Power output has improved from 40W into 8/4ohm loads (via 8/4ohm taps, respectively) to 2x55W into 8/4ohm loads at <1% THD while its dynamic output has lifted from 40W, 40W, 51W and 54W to 52W, 52W, 69W and 73W into 8, 4, 2 and 1ohm loads [see Graph 1, below]. Power output in Triode mode is typically ~60% of that in Ultralinear, the Stereo 40 MkIV achieving 2x35W into 8/4ohm loads. In Ultralinear guise, and with 'Low gain/moderate feedback' selected, the output impedance has reduced from 0.75-1.05ohm to 0.32-0.27ohm (20Hz-20kHz), improving low frequency damping, while the response is both flatter and more extended, reaching out to –0.22dB/20kHz and with –3dB points of 2Hz-90kHz/8ohm. There is no appreciable difference in frequency reponse between Ultralinear and Triode modes but there is a boost in subsonic bass (+0.5dB/5Hz vs. –3dB/5Hz) between 'Low' and 'High' sensitivity settings.

Again, compared with the MkIII, distortion is reduced across the audio bandwidth [see Graph 2] to 0.042-0.36% (10W/8ohm) and versus power output from 0.007%/1W, 0.05%/10W to 0.5%/50W in Ultralinear mode, and 0.005%/1W, 0.021%/10W to 0.31%/30W in Triode mode. Noise has also been reduced in the MkIV, the amplifier achieving an A-wtd S/N of 91.6dB in Ultralinear and 92.2dB in Triode mode (re. 0dBW) and with any residual hum/flicker noise less than –75dBV. By any measure, the MkIV represents a significant upgrade on the MkIIIm. PM