Pre/Power Amplifiers

CVGT (Continuously Variable Gain Transimpedance) is a method of offering control over volume without simply attenuating the full gain of the preamp. But with variable gain comes variable compensation (feedback) and so distortion and other parameters change with volume setting even if the output level and S/N ratio is maintained. The ‘breakpoint’ with the KX-8 is around –12dB (with the rotary at about 2 o’clock) where the response flattens out to –0.1dB/20kHz and –0.6dB/100kHz, distortion settles to a very uniform 0.002-0.003% (re. 20Hz-20kHz) and, importantly, the channel balance returns to ~0dB. At maximum volume – gain is only +2.5dB – the KX-8 has a moderate 0.5dB channel imbalance, distortion falls to a minimum of 0.0009-0.001% and response rolls away to –0.15dB/20kHz and –1.9dB/100kHz. The A-wtd S/N ratio, meanwhile, is a consistent 89dB, matching the partnering VX-8 power amplifier [see PM's VX-8 Lab Report].

Our sample was equipped with the ES9038Q2M-based digital option which offers a 5V balanced output with a 0dBFs input (the preamp has a maximum 11.5V output, analogue in/out) and distortion that falls to a minimum of 0.0003-0.0009% (20Hz-20kHz) over the top 30dB of its dynamic range [Graph 1]. Response is set by the slow roll-off minimum phase digital filter that trades much reduced time domain distortion (ringing) for very limited stopband rejection (just 6dB for a 22kHz signal at 48kHz sampling). It reaches out to –2.5dB/20kHz, –6.3dB/45kHz and –10.5dB/90kHz with 48kHz, 96kHz and 192kHz inputs, respectively. Digital jitter reduces with increasing sample rate from 150psec at 48kHz to 55psec with 192kHz data, in all cases the sidebands repeating at multiples of ±119.5Hz [Graph 2]. PM