Pass Labs XA25

At Pass Labs we build Fet amplifiers with excessive output stages, huge heat sinks and massive power supplies. We use the highest quality components in simple linear topologies, assembled and tested with care and subtlety in Auburn, California.

They measure well, sound great, and are reliable. Twenty years of ongoing innovation have kept the X and XA amplifi ers at the forefront of high-end audio. The XA25 embodies all this and less. 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 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 uses two pairs of NOS complementary Fets in common-source mode in a current feedback (CFA) confi guration. The amplifier is direct coupled and has no DC servos or frequency compensation. As with all Pass amplifiers, the technology and construction are strictly in the service of the listening experience. The XA25 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.


Specifications

Output power:

2x 25 Watts Class A @ 8 ohms
2x 50 Watts Class A @ 4 ohms
2x 100 Watts Class A/AB @ 2 ohms

Distortion: 0.1% @ 25 Watts, 8 ohm, 1 KHz, 0.1% @ 50 Watts, 4 ohms, 1 KHz

Freq. Response: DC to -3 dB @ 200 KHz

Gain: 20 dB gain

Noise: Output 100 uV unweighted 20 to 20 KHz

Damping Factor: 200

Slew: 100 V/uS

Input Impedance: 47 Kohm

Peak Current: 10 Amp output (200 Watts into 2 ohms)

Independent of loudspeaker load phase

Idle Power draw: 200 Watts total.

Dimensions: 17”W x 17.375”D x 6”H

Weight: approx. 55 lbs

Reed Muse 1C turntable & 5T laser guided tonearm

.

CH Precision T1 Clock

Audio Research VT 80

$8000

Power output - 75 watt

Driver tubes - 6H30
Output tubes - KT120

Analog Domain DAC1

All input data, regardless of its format, is converted to 24 bits and asynchronously resampled at a very high rate. DSD is unpacked, converted to PCM and resampled. The resampled data enters a Digital Filter. The Digital Filter is configured in optimal mode for the output format of the Sample Rate Converter, therefore there are no user-selectable filter modes. Data then enters the DAC stage where it is converted into an analog signal.

Jitter essentially loses its meaning in this configuration. Input data jitter is practically irrelevant. The Asynchronous Sample Rate Converter takes care of proper word alignment and timing during conversion. The three digital blocks are synchronized to a single, high precision clock to ensure perfect timing. Any CD transport can be used with the DAC1, not requiring external re-clocking or clocking from the DAC1.

The actual DAC stage is a differential output (balanced) design. Its residual distortion is approximately 0.00035% at full scale output, consistent for all input formats. At typical output levels of -10dBFS the distortion falls off to 0.0001% (-120dB) and becomes essentially zero at levels below -20dBFS, “lost in the grass” as we say, at -130dB.
This matters! It’s during the quiet passages that DAC distortion becomes most noticeable. One bit is lost with every 6dB reduction in level, therefore a 16-bit DAC will be working effectively at 14-bit resolution or less most of the time. This is the reason why early designs created a bad reputation for digital audio. We hope to rectify this misunderstanding.

So, is 130dB enough? It is more than enough, and here’s why: a signal which is -130dB lower relative to the full scale output voltage of 2Vrms has an amplitude of 3 million times less, or 0.7 microvolts. That is less than many amplifiers’ input-referred noise voltage. We can confidently say that the DAC1 will have an insignificant noise contribution, if any, to the signal chain.