Integrated Amplifiers

This is a bold attempt by BAT to offer a valid triode-based alternative to the more traditional 6550/KT90 tetrode-powered output stage favoured by many (valve-focused) amplifier brands. The 6C33C-B power triodes used in the VK-80i can match pairs of KT90s used in push-pull configuration for raw watts, particularly into low impedances, but there's a trade-off with distortion which increases steadily with output. The latter increases from an already high 0.65%/1W to 1.5%/10W and fractionally under 3% at 50W (all 1kHz/8ohm). Unlike the typical KT90-based design, distortion does not increase at high frequencies [see Graph 2, below] although there's no escaping the marked increased in bass distortion through, presumably, output transformer core saturation. Noise, too, is slightly higher than average, leading to an A-wtd S/N ratio of 78.5dB (re. 0dBW).

All this feeds into the maximum power output available from the VK-80i which amounts to 21W at 2% THD, 34W/2.5% and 53W at the rated 3% THD into 8ohm (all via the 'High' 6-8ohm tap). Into 4ohm loads via the 'Med' 4-6ohm tap this translates into 20W/2%, 37W/2.5% and 51W/3% THD, and into 2ohm via the 'Low' 3-4ohm tap at 16W/2%, 33W/2.5% and 46W/3%. There is significantly more juice available under dynamic conditions where the VK-80i achieves 97W, 102W and 105W into 8, 4 and 2ohm loads via the aforementioned taps at up to 3% THD [see Graph 1, below]. Into very low 1ohm loads this is limited to just 15W where the source impedance of the 'Low' tap is already twice this value at 2ohm. I discuss the impact of this high impedance on frequency response in our sidebar although the 'native' response is flat to within ±1dB from 13Hz-60kHz. PM