Integrated Amplifiers

So-called hybrid amplifiers – combining a triode tube line/preamp stage with a bipolar/FET power amp stage – can either combine the 'colour' of tubes with the grunt of a solid-state output, or... not. Copland's CSA150 [HFN Jun '21] and PS Audio's BHK Signature [HFN Jul '16] showed us that the inclusion of a high voltage tube line stage needn't translate into higher distortion at the output, and Musical Fidelity's use of Trivistor tubes in various products typically delivered vanishingly low THD. BAT's VK-3500 is rather different because its technical 'fingerprint' is a true amalgam of low-feedback tube/transistor thinking. Power output is generous at 2x170W/8ohm and 2x275W/4ohm, increasing to 220W, 425W, 390W and 215W into 8, 4, 2 and 1ohm loads under dynamic conditions, but distortion is higher at lower power (0.18%/1W to 0.08%/100W) and into lower impedances [see Graph 1, below]. This same decreasing distortion profile remains consistent right across the audioband [see Graph 2, below] and it's this feature that, subjectively, may prove a boon.

Meanwhile, the 0.22-0.25ohm output impedance is high for a transistor amp and though still lower than might be expected from a tube output stage it will still have some impact on frequency response, depending on your choice of speaker. Into a 'flat' 8ohm load the HF response rolls away above 10kHz, reaching –0.25dB/20kHz and –3.6dB/100kHz while achieving –3dB/4Hz in the sub-bass. Overall gain (balanced in) is fairly high at +43dB and the A-wtd S/N slightly below average at 79dB (re. 0dBW), the latter a product of both LF hum and HF white noise. Stereo separation is held to at least 70dB while the fine volume control maintains an impressive 0.08dB channel balance. PM