Integrated Amplifiers

With so many tube, hybrid and variations on the Class D theme populating the ranks of hi-fi amplifiers these days, AVID's Sigsum is about as close to the 'textbook' Class A/B amplifier as you'll find. However, with no fewer than four pairs of Sanken A1294/C3263 power devices per side, plus that huge ~1kVA mains transformer in tow, the rated 110W/8ohm power output looks fairly modest. In practice the Sigsum will deliver a continuous 2x128W/8ohm and 2x225W/4ohm with sufficient headroom for 142W, 265W, 459W and 535W into 8, 4, 2 and 1ohm loads [see Graph 1, below] – powerful, certainly, but there's possibly some untapped reserve under the bonnet should AVID's engineers choose to open the throttle. As it stands, the Sigsum offers a sensibly moderate +38.6dB overall gain (balanced in) and an above-average – low noise – 89.3dB A-wtd S/N ratio (re. 0dBW). The response is flat and extended with a –3dB/1Hz DC filter and a HF reach of –0.14dB/20kHz to –0.67dB/100kHz, although there's some progressive loss of extreme top-end into lower impedances, reaching –0.83dB/20kHz and –4.0dB/100kHz into the toughest 1ohm loads.

The parallel/complementary design of the Sigsum's output stage confers an impressively consistent trend of distortion versus output, holding to within 0.004%/1W to 0.006%/10W and 0.0085%/100W (all 1kHz/8ohm). Versus frequency, however, distortion does not simply increase at HF in the fashion of most solid-state designs but gently trends upward from the lowest bass through midrange and treble – specifically 0.0009%/20Hz to 0.006%/1kHz and 0.028%/20kHz (all re. 10W/8ohm). Both these features are clearly illustrated in Graph 2, below. PM