PASS Labs INT25 - Class A Integrated Amplifier
PASS Labs INT25 - Class A Integrated Amplifier
The PASS Labs INT25 is a high voltage pure Class A integrated amplifier that has a simplified circuit topology to allow the immediacy and delicate details of music to shine through your system in a way that is natural and full.
Douglas HiFi PASS Labs INT25 Notes : The PASS INT25 sounds a LOT more powerful than the specifications may suggest (Lab tests do show the power output to ultimately be 65W ). Due to the simplified circuit design and minimised number of internal components the INT25 has a pure, immediate mid-range that projects vocals and instruments in a lifelike manner out into the room coupled to a bass that is subterranean, deftly controlled, fast, detailed and muscular. The INT25 sounds like a great 300B tube amp - but it grips the bass drivers with Nelson Pass Class A Mosfet Solid State authority and a fast yet warm detailed finesse. This amplifier is best matched to speakers that are around 89db efficient or greater.
Pass Labs make some of the finest HiFi audio electronics in the industry. Continually on the "Reccomended Components" lists of Sterophile and The Absolute Sound you really need to hear the beautiful music making magic that PASS Labs products bring to any HiFi system.
PASS Labs INT25 design features
The INT-25 embodies a smaller powered stereo Class A amplifier with circuit design around "less is more". As the circuit outputs 25W it takes advantage of the lower power requirements to have simpler single-ended circuitry and fewer parts in the signal path. This also allows elimination of degeneration, “that other negative feedback” in all stages.
Simpler and fewer gain stages improves speed and stability. Lower voltages means 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 having a 700 watt / 40 amp rating. 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 lowest possible distortion and highest damping factor. No degeneration also increases the efficiency and Class A operating current, allowing greater Class A operation into low impedance and reactive loads.
The simple front end circuit uses two pairs of our favorite 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.
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 too or 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 great specs are often found to be dry and boring, this amplifier breathes new life into the music.
PASS Labs INT25 Test results on power output :
"... 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 "
( https://www.hifinews.com/content/pass-labs-int-25-integrated-amplifier-lab-report )