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Pass Labs X3505 Stereo Amplifier

I have been listening to Nelson Pass-built amplifiers since 1994. From the Class A simplicity of the Aleph line to today's X and XA series, I have always found Pass' dedication to improving an amplifier's audio reproduction admirable.

(click thumbnail)Technology HighlightThe X350.5 is the accumulation of the improvements made during the product life of the model X350. This is true of the “.5” series, in general, and at some point the products evolved enough to merit an upgraded designation and a new faceplate.

The original X amplifiers used MOSFETs as the input devices in single differential pairs. The .5 series uses cascoded, complementary JFETs that have lower noise and greater linearity, delivering better performance while allowing lower feedback and greater stability. As before, the circuit still consists of the Pass Super Symmetry design — a simple voltage-gain stage driving a simple current-gain output stage, with carefully matched parts balanced in a patented cross-coupled topology.

The output stage has been increased from 40 to 48 power MOSFETs, used as complementary followers in the symmetrical balanced circuit. The frequency compensation of these devices has been improved so that there is much less rise in distortion at high frequencies, and by enclosing the output stage in the feedback loop of the amplifier we have increased the damping factor from about 30 to about 250.

According to Pass, the decision with each product to use feedback on the output stage or not has always depended on the results of listening tests, and in the earlier designs it could went either way. In the present product .5 the preference was the output stage in the loop.

The other major improvement in the .5 series is the application of a small amount of single-ended, Class A bias to what would otherwise be a highly-biased Class AB design. Current sources draw about one-half amp from the output to the negative supply rail, so that for the first watt or so the amp operates single-ended. Above that the amplifier operates push-pull Class A to about 30 watts. The idle dissipation of both output stages is about 400 watts.

Improvements were made to the power supply by working with manufacturer Plitron to produce a power transformer that was electrically and mechanically quieter, particularly under noisy AC conditions. This also improved the power factor (the spread of energy draw from the wall) to about 80%. The AC primary circuits also have improved filtering to reduce high frequency noise, both coming into and emitted by the power supply, and new massive high-speed/soft-recovery rectifiers have been added on the secondary side of the transformer for the same purpose. The bulk capacitance of the supply has remained the same, but the first of the two stages has improved filtering and decoupling.

Pass says that between these changes and improved circuit layout, all forms of noise have been improved by a factor of about two. I have been listening to Nelson Pass-built amplifiers since 1994. From the Class A simplicity of the Aleph line to today’s X and XA series, I have always found Pass’ dedication to improving an amplifier’s audio reproduction admirable.

Steps up in amplifier design usually mean subtle audible improvements. However, Pass’ latest upgrade to his successful X series has resulted in a not-so-subtle improvement to the sound — especially with high resolution playback. In fact the Pass X350.5 auditioned here, just may have the most inner midrange and treble detail of any amp I have ever heard. High-end mastering and engineers and those studios with high-end reference listening rooms are the intended ears for this class of amp.


Price at a whopping $9,500 (Yeah, that is more than your Pro Tools rig, including the G5), the Pass X350.5 is an example of premium, made-in-USA build and parts quality. This heavy brute, weighs in at more than 150 pounds, but all that pain of moving disappears when you hear it.
Fast FactsApplications: Studio, post production

Key Features: Single-channel; SuperSymmetry technology; Class A; JFET input/MOSFET output design; 350W @ 8 ohms; balanced and unbalanced connections, 12V remote trigger

Price: $9,500

Contact: Pass Labs at 530-367-3690,

Product Points


+ Exquisite, accurate reproduction

+ Improved bass and transient response

+ Perfect for high resolution monitoring


– Ouch, my hernia!

– Maxes out most credit card limits

The Score:

It’s heavy, expensive and runs very warm, but can an amp sound any better?
Amplifiers are always minimalist electronics components, and the Pass is no different. On the front panel is the power button, a really cool looking backlit current meter and separate front panel handles. The back panel sports balanced XLR and unbalanced RCA inputs, a set of 12V triggers for remote off/on switching and a large wing nut-based binding post that works with spade or bare wire termination (To make a product line that meets European and U.S. safety standards, Pass does not offer banana plug-capable posts).

The rear also contains the main power supply on/off switch, and a massive set of carrying handles, but it takes a stout man to lift 150 pounds contained in such a compact chassis. It took two to transport and place this bad boy in my studio. The sides of the amps are occupied by large heat sinks that would chop off a foot if you dropped the amp. The amp runs very warm at low volume and hotter at louder levels. Make sure there is plenty of ventilation.

Design-wise, the X350.5 has the same basic SuperSymmetry design that Nelson Pass and crew developed in the late 1990s, which utilized carefully matched output devices in a low feedback, fewer-stage configuration. The new tweaks are employed in the .5 series, which includes the X150.5, the X250.5, 350.5 and 600.5 monobloc. Among the changes: JFET input devices have replaced MOSFETS (more linearity, Pass says), high-bias Class A on the input stage with limited feedback loop, (a feature seen previously on the Pass’ flagship XA Series), better filtering on the power supply and more power supply capacitance. The refinements are claimed to net better bass performance and transient response at lower distortion than the last version.

Relevant specs for the Pass include 350 watts- per-channel at 1 percent distortion or less; dampening factor is listed at 250 ref 8 ohm nominal load (hence, the audibly tighter, faster bass presentation). Frequency response is listed from DC to 100 kHz at minus 3 dB. Gain is 30 dB. The X350.5 idles at 600 watts and consumes 1,800 watts at maximum output. All this power is still possible through the 15 amp 120V power supply.

In Use

I connected the Pass X350.5 to my reference listening system, which includes, a Legacy/Coda High Current preamp, Esoteric DV-50 Universal DVD-A/SACD player with upsampling PCM converter, modified Vacuum State Electronics Sony SCD-777ES SACD player, Benchmark ADC-1 A/D converter and TASCAM DV-RA1000 high resolution recorder.

Primary speaker cables included Alpha Core solid silver, spade-terminated interconnects and Westlake Low PE Distortion Balanced cables. Amps on hand for comparison included the first generation Pass X250 and the Bryston 14BSST — one of my favorite studio amplifiers.

I played a number of my own high resolution 24-bit/192 kHz guitar recordings and auditioned a number of commercial and DMP label SACDs to get a feel for the X350.5’s sonic signature.

On the DMP SACD release, Steve Davis The Quality of Silence, I immediately noticed the difference comparing the original X250. The transient response with recorded drum cymbals, and the upper register piano was more clearly delineated than the older version. You did not have to A/B the amps to hear the difference in presentation, and the more realistic reproduction of those instruments via the .5 version.

On my Martin D-35 acoustic recordings, the strums and delicate pick attack was much more out front in the stereo image with the X350.5. The details were much easier to pick out in the mix. I always thought that X250 always had a good width and depth to the image, but the extra detail on the X350.5 makes it starkly more present. The extra information contained in high resolution recordings made the amp’s improvements much more audible. The 24/192 kHz playback’s improvement over 44.1 kHz was obvious using the old Pass amp, but the new amp brought it to an even higher degree of realism.

The other big difference was the tightness of the bass. With the big improvement in dampening factor, the X.5 series has lost almost all the warm bloom of the older version, making it just about as tight and fast as the bipolar-output Bryston 14BSST. Speaking of the Bryston, the Pass now also nearly equals or perhaps surpasses it in transient response. I always thought that bipolar amps would always be better at the upper-frequency reproduction with more clarity and speed (and sometimes a slight harshness), but the X350.5 shows that the JFET design is up to the competition.

I also auditioned the Lipinski L-505 closefield monitors with the Pass X350.5. Lipinski recommends Pass Labs amps to power its speakers. Those same qualities I heard through the Legacys were also delivered in closefield range with the Lipinskis. The L-505s do not deliver the bottom end of the Legacys, which have three woofers to extend bass to under 20 Hz, but the transient response and the mid bass were quite impressive up close.

My nitpicks with the X350.5 are few. It weighs twice as much as the Bryston, even though the Bryston produce about 600 wpc (8 ohms), and it does not accept banana plugs. I have several sets of high-quality cables that terminate with banana plugs, but I could not use them with the Pass.

I know there will be complaints about a $9,500 price tag, but a Dodge Viper does not cost $20,000. Low-volume, made-in USA high-end components are not cheap. Workers who assemble Pass amps don’t make 50 cents an hour, and you don’t see a power supply like the 350.5s in a $400 amp.


If your speakers are passive, and you want to hear the most accurate two-channel playback possible, I strongly recommend the Pass X350.5. Its new design has resulted in a clearly audible improvement in sound over the previous generation of X Series. The sonic enhancements are more clearly magnified when listening to high resolution sources.