Pass Labs INT-25 Integrated Amplifier

Pass Labs builds FET amplifiers with excessive output stages, huge heat sinks, and massive power supplies. They use the highest quality components in simple linear topologies, assembled and tested with care and subtlety. They measure well, sound excellent, and are reliable. Twenty plus years of ongoing innovation have kept the X and XA amplifiers at the forefront of high-end audio.

The INT-25 embodies all of these features and even more. A smaller stereo Class A amplifier at a lower price, it takes advantage of the lower power requirements to have simpler single-ended circuitry and fewer parts in the signal path. This also allows for the elimination of degeneration, “that other negative feedback,” in all stages.

Simpler and fewer gain stages improves speed and stability. Lower voltages mean the ability to run gain devices at higher bias currents, more deeply into Class A.

Newer power FET technology replaces banks of parallel small transistors with a single pair of industrial devices, each rated at 700 watts and 40 amps. With no current-hogging issues and a new constant-current bias circuit to compensate for temperature drift, they connect directly to the loudspeaker without ballast resistors for the lowest possible distortion and highest damping factor.

No degeneration also increases the efficiency and Class A operating current, allowing for greater Class A operation into low impedance and reactive loads.

The simple front-end circuit uses two pairs of our favourite NOS complementary FETs in common-source mode in a current-feedback (CFA) configuration. 

The preamp section is a simplified single-ended version of the one used in the INT-60 and INT-250 with three line-level inputs and no line-level output.

Everything done in the signal chain from the microphone to the loudspeaker forever leaves an indelible mark on the music. The XA25 amplifier section is direct-coupled, requiring no DC servos or frequency compensation, neither adding to nor subtracting from the experience.

As with all Pass amplifiers, the technology and construction are strictly in the service of the listening experience. The INT-25 measures superbly, but its subjective qualities are most special. In an industry where products with exceptional specs often turn out dry and boring, this amplifier breathes new life into the music.