Integrated amplifiers — Pass Labs INT-250

The INT-250 is a stereo integrated amplifier from Pass Labs, the Californian company associated with Nelson Pass. It combines a preamplifier and a power amplifier in one chassis and builds on the smaller INT-60, adding a large increase in output. Its power supply and output stages come from the Point 8 series of Pass power amplifiers, and Pass describes the result as a one-box solution able to drive larger and less efficient loudspeakers. In silver the US price is about USD 13,860, with black finish available to special order.

Key specs that matter

Rated output is 250 W RMS per channel into 8 ohm and 500 W RMS into 4 ohm, an exact doubling into the lower load. Distortion is specified at 1 percent at the full 250 W into 8 ohm at 1 kHz, and response is 6 dB down at 80 kHz. Gain can be set to 30 or 36 dB, input impedance is 45 kohm, damping factor is 150 and residual noise stays below 250 uV unweighted from 20 Hz to 20 kHz. There are four inputs - two offering either XLR or RCA and two on RCA only - plus preamp outputs. The chassis is 48.3 cm wide, 54 cm deep and 23.1 cm high, and it weighs 47.6 kg. Consumption is 450 W in use and under 1 W in standby.

Engineering and use context

The INT-250 runs in class AB with a heavy class A bias. Higher-power MOSFETs form the output stage. The toroidal transformer has both an electrostatic shield and a Mu metal shield, and is vacuum impregnated and filled with epoxy to keep it quiet electrically and mechanically, while the supply circuitry adds extra filtering. A simplified preamp section removes some parts from the signal path, and the volume control, designed by Wayne Colburn, runs a micro-controller that reads an optical encoder and gives sixty-three 1 dB steps. Its control logic takes power from a supply isolated from the analogue side, and no audio signal passes through that circuitry. The XLR and RCA sockets on inputs 1 and 2 are linked, so only one of the pair can be used at a time. Both speaker terminals are live, and a separate ground post is provided for connecting active subwoofers. The amplifier is designed to be left on permanently, and a remote control is supplied. North American units carry a 3-year warranty.

Pairing

With its output doubling into lower loads, the INT-250 suits low-sensitivity floorstanders and loads that drop well below nominal impedance. The preamp outputs, muted together with the main outputs, can feed a subwoofer, a second system or another amplifier for bi-amping. Balanced sources are preferred by Pass for lower noise and distortion.

Alternatives

The Pass Labs INT-60 is the lower-powered integrated amplifier in the same range and the design the INT-250 was developed from.