Integrated Amplifiers

In common with Pass Lab's previous-generation INT-30A amplifier [HFN Dec '10], this latest 'entry-level' integrated vastly exceeds its rated output power, the 25W specification bested to the tune of 2x65W/8ohm and 2x120W/4ohm, and with headroom to accommodate 75W, 147W and 281W into 8, 4 and 2ohm loads up to 1% THD under dynamic conditions. The 16A current limit is reflected in its maximum 255W/1ohm delivery [Graph 1, below]. In practice the 25W specification better reflects the envelope of its claimed Class A operation – and with an idle power draw of 180W and ambient heatsink temperature of 45oC this is not unrealistic. Nevertheless, and in common with earlier Pass Labs' amps, the INT-25's bass/mid distortion trend increases gently with power output from 0.001%/1W to 0.004%/10W and up to 0.009% at the rated 25W, subsequently increasing to 0.01%/30W, 0.016%/40W, 0.03%/50W and 0.05%/60W (all re. 1kHz). Versus frequency there's a gentle increase in THD with proportionally higher distortion at higher outputs [Graph 2] – a hallmark of Nelson Pass's complementary, current feedback power amp that employs just one pair of 500V/40A industrial FETs.

The 63-step volume control operates in accurate 1dB steps (there is an error of just ±0.15dB over the last 5dB of its range) while the A-wtd S/N ratio is a generous 90dB (re. 0dBW). There's some slight variation in response with volume position (not level) but at the full, and sensibly low, +25.5dB gain the INT-25 reaches out from a near-DC bass extension to –0.10dB/20kHz and –1.25dB/100kHz into 8ohm. Output impedance is a moderate 0.01ohm but this is flat from 5Hz-10kHz, increasing gently thereafter to 0.015ohm/20kHz and 0.08ohm/100kHz. PM