VAC Sigma 170i Integrated Amplifier Lab Report

Lab Report

VAC's co-founder and guiding light, Kevin Hayes, has chosen an output transformer configuration that best suits power transfer into 8, 4 and 2ohm loads at the expense of high source impedance. As a result, the choice of 8-16ohm, 4-8ohm or 2-4ohm taps fundamentally influences power, distortion and response into different loads. Gain is highest at +48.3dB (balanced in) via the 8-16ohm tap as is power output into 8ohm at 2x25W (1% THD), 83W (2%) and 102W (3% THD), with distortion increasing from 0.038%/1kHz to 0.65%/20Hz and 0.35%/20kHz [all 0dBW, black trace Graph 2]. Power is slightly lower into 8ohm via the 4-8ohm tap but so is distortion [grey trace, Graph 2], yielding 2x48W (1% THD) and 95W (2% – clipped), and 0.023%/1kHz to 0.13%/20kHz (re. 0dBW). Into 4ohm, power via the 4-8ohm tap reaches 2x23W (1% THD), 68W (2%) and 105W (3%) and, again, slightly lower power but lower distortion via the 2-4ohm tap at 2x48W (1% THD) and 93W (2% – clipped).

These trends are very clear under dynamic conditions [see Graph 1] with 120W (8-16ohm) and 99W (4-8ohm) available into 8ohm, and 128W (4-8ohm) and 99W (2-4ohm) into 4ohm. Into lower impedances, 130W and 105W is possible into 2 and 1ohm via the 2-4ohm tap. Into a non-reactive 8ohm load, the response shows a –0.6dB dip at 20Hz and 30kHz with a (presumed) transformer peak at ~70kHz. Stereo separation is >60dB through the midband but noise – exacerbated by the excessively high gain – is higher than 'average' and the A-wtd S/N lower at 73.5dB (re. 0dBW, 4-8ohm tap) and 71.3dB (8-16ohm tap). PM


Dyn. output vs. THD into 8ohm (8-16ohm tap; black, 4-8ohm tap, grey), 4ohm (4-8ohm tap, red; 2-4ohm tap, pink), 2ohm (2-4ohm tap, blue) and 1ohm (2-4ohm tap, green). Max. current is 10.3A


THD vs. freq. at 1W/8ohm (8-16ohm tap, black; 4-8ohm tap, grey) and 10W (8-16ohm tap, red)

Power output (<3% THD, 8/4ohm) 95W / 105W
Dynamic power (<3% THD, 8/4/2/1ohm) 99W / 128W / 130W / 105W
Output imp. (20Hz–20kHz/100kHz) 3.45–3.8ohm / 5.8ohm
Freq. response (20Hz–20kHz/100kHz) –0.6dB to +0.3dB/–1.4dB
Input sensitivity (for 0dBW/85W) 10mV / 99mV
A-wtd S/N ratio (re. 0dBW/85W) 73.5dB / 92.3dB
Distortion (20Hz-20kHz, 1W/10W) 0.023–0.13% / 0.20–0.95%
Power consumption (Idle/Rated o/p) 242W / 390W
Dimensions (WHD) / Weight 457x203x440mm / 32kg
Price £10,000

Valve Amplification Company, Inc.
Florida, USA
Supplied by: Signature Audio Systems, UK
07738 007776