Pass Labs X2.5 Preamplifier

Pass Labs X2.5 Preamplifier


Joe Lamano

November 2003


Gain: Selectable 4 dB or 14 dB balanced output, 
selectable 4 dB or 14 dB single-ended output 
Frequency Response: -3 dB @ 2 Hz, -3dB @ 100kHz
Distortion: < .1 % THD typically .003% @ 2 volts @ 1kHz
Max Output: 10 volts RMS balanced out, 7 volts RMS Unbalanced out
Noise: Noise floor < -125dBV, 20-20kHz
Amplifier Turn On: 12 Volts DC mA continuous.
Dimensions: 17″W X 3.5″H X 11.5″ D 
Power Consumption: 35 watts
Weight: 35lbs.

Price: $3,900.00

Pass Laboratories
PO Box 210
Foresthill, CA 95631

Telephone: (530) 367-3690

Today, with all the technology supporting multi-room audio, home theatre, digital signal processing and more, the stand-alone pre-amp is a piece of equipment that is seems to be known by only a small group of individuals. These individuals are the varying degrees of audiophiles that still appreciate good old-fashioned two channel stereo music and equipment without all the bells and whistles found in today’s receivers and processors. Most non-audiophiles think a pre-amp is nothing more than a costly input selector and volume knob. True, a preamp will lack all the new and flashy functionality to make your living room sound like a cathedral or stadium and cost multiple times more than most mass-produced receivers. But to an audiophile, someone who can appreciate music and its reproduction quality, the preamp is the critical bridge between the front-end and back-end components responsible for much more than gain and separation of the selected source.

As I built my first system, I decided to use a Parasound AVC2500 processor as my preamp while I focused on selecting other components. I purchased the volume card upgrade from Parasound and connected my Electrocompaniet ECD-1 DAC to the 5.1 direct analog left and right inputs and the processor performed well as a preamp. While I was satisfied with this configuration for some time, I knew that to bring my system to the next level I needed to break my home theater processor away from my 2-channel components and start evaluating some preamps.

I am not a person that believes in solely building a system using “matched” components from a single manufacturer. However, certain components such as amplifiers and preamps seem to have a very strong synergy between the two and therefore I think it is beneficial to evaluate models from the same manufacturer and product line. So looking back on my decision to hold-off on a preamp until I selected other components may not have been wrong, but next time around I’m sure I will find myself evaluating preamps and amps at the same time.

For me, the most logical starting point with preamps was the Pass Labs X2.5. I have been using the Pass Labs X150 amplifier and by speaking with others at Stereo Times and the people at Pass, I learned that this would be a very complimentary component. The X2.5 is built with similar technology, style and attention to detail as the X series amplifiers. While unpacking the unit, I noticed that its weight was impressive for its size. Obviously, the people at Pass did not skimp on build materials. The chassis and faceplate are made from machined finished aluminum. The front plate is silver and styled similar to the other X series products and consists of a blue display panel, 4 buttons, and a volume knob. The X2.5 is designed to always be on so it is not equipped with a power or standby switch. The rear panel consists of an IEC power connector, tape loop, 1 pair of single-ended and balanced outputs, 4 source inputs (all inputs support single-ended RCA connections, inputs 1 & 2 provide balanced XLR connectivity), and an amplifier trigger binding post. The remote control is extremely simple, consisting of four unmarked buttons that match those of the front panel used to scroll through the various functions, but it is constructed of the same machined-aluminum and is probably the heaviest remote that I have.

Setting up and using the X2.5 is very simple. Once you make the connections and provide power, the preamp is ready to go. And remember, you don’t even need to press a power button. Four arrows are used to scroll between and set the preamp’s options; the X2.5 does not have any direct assigned function buttons. This design keeps the preamp simple, although it requires several clicks of the arrows at times. Settings are easy to read on the display panel because it uses large readable words versus small LED indicators. All functions available on the front panel are available via the remote control. The volume is configurable from 0 – 100 in steps of +1db and provides good control of the listening level. The default gain is set at +4db, which is the optimal setting for Pass amplifiers. I tried the high gain (+14db) setting with the Pass X150 amplifier and did not find it very pleasing; the system seemed very loud and lost some of its normal dimensionality. A nice feature of the X2.5 is the unity gain function assigned to input four. This feature allows for the connection of a component that has its own volume control, such as a home theatre processor, to be used in conjunction with the preamp in a pass-through manner. Other features of this preamplifier include tape loop control, muting, balance, display brightness, and amplifier turn-on/off.

Internally, the X2.5 is built using a fully balanced class-A always-on design which utilizes Pass’ patented SuperSymmetry technology. This technology is used to reduce distortion and noise by attempting to precisely match the two balanced halves of the class-A circuit and eliminate anything non-musical from reaching the balanced outputs. The same balanced design using SuperSymmetry is found in the entire Pass X series of amplifiers. I have been told in the past that balanced connections are only needed for long runs and it doesn’t make a sonic difference for short connections. However, I found with the Pass X2.5 preamp and X150 amplifier, that they do benefit from using the balanced inputs and outputs. The circuitry design of these component’s balanced and single-ended (RCA) connections are converted to balanced so that the SuperSymmetry processing can occur. To me it makes sense that using balanced inputs and outputs sounds better. The most significant sonic benefit I heard was the increased dynamic capabilities and slightly better definition within the soundstage. The X2.5 already exhibited a very quiet backdrop, and I did not notice a greater silence when switching from single-ended to balanced. However, I used an interconnect which I find to be extremely quiet too: the Harmonic Technology Pro-Silway MKIII, with XLR termination. I would think different makes or models may sound different. 

The low noise floor of the X2.5 allows for subtle ambient details like the slow fade of instruments and vocals to sound more real. I really enjoyed listening to slow jams, whether it be jazz, blues, or anything else, because the X2.5 not only can reproduce the music it can set a mood. It has a wonderful ability to define space within the soundstage and render it with the subtlest details at any volume level, without imposing restriction on detail or dynamic power. The Cowboy Junkies’ Trinity Sessions [RCA 8568-2-R] is one of those discs with a mellow sound that really takes advantage of the X2.5’s capabilities and is really a pleasure to listen to. The natural acoustics and ambient details of the cathedral where the recordings occurred are well presented. The room’s size and emptiness can be felt as the instruments and vocals slowly fade into the air and space of the expansive soundstage. The details of the entire band are extremely well defined throughout the disc. The electric bass is mellow and never overpowering and the guitar riffs jump out into the room. The soft drum work is resolved with precision easily placing the drummer back in the soundstage without losing and detail. The first track, “Mining for Gold”, demonstrates the strong vocal replicating capabilities of the X2.5 against a quiet backdrop. From that track forward the soft vocals are natural sounding and place the singer inside the listening room. 

The renditions of Charlie Parker songs played by Hargrove, McBride, and Scott on Parker’s Mood [VERVE 314 527 907-2] is a good test for any system and the X2.5 performed very well. This disc will easily test the resolution and dynamic capabilities of any component or system. The midrange of horns and piano is revealing and dynamic but not overpowering. The control of transients and dynamics that the X2.5 brought to my system is amazing. The curt trumpet blows are sharp and very natural sounding but not bright or piercing. The horns and piano explode from the speakers into the listening room with excellent imaging and separation, it is very easy to visualize the location of each musician in this three-man band. The acoustic bass is reproduced with a precision and speed with assistance of the Talon Raven-C’s throughout the disc. The entire disc demonstrates how well this preamp can cleanly drive low and mid-bass without booming or becoming muddled.

In summary, I found that the Pass Labs X2.5 preamplifier brought a higher level of refinement to my system by precisely bridging the front-end components with the amplifier and speakers providing seemingly limitless dynamics and detail.

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