Pass Labs X350.5 Stereo Amplifier


Equipment report
Solid-state power amplifiers
Pass Laboratories X350.5
Pass Labs X350.5 Stereo Amplifier

Another similarity to tube amplifiers is the X350.5’s ability to track musical phrases in ways that many solid-state amps just seem to short change. Rather than cycle through a pattern of the “picking up” and the “letting go” of musical notes, the X350.5 seems to hold onto them better, longer, and more completely than most other solid-state amps in its price range. The typical “just the facts” school of listeners might not appreciate my train of thought here, but the sort of phrasing aspect I speak of occurs in live music. Indeed, I consider this area of music reproduction an important set of “the facts.” Some stereo gear just seems to allow this ineffable aspect of music to come through more readily than others. Though I understand that one man’s “continuous phrasing” could be another’s “liquid embellishment,” I hear the X350.5 as possessing the admirable quality of conveying the emotive aspects of music without sounding euphonic or cloying.

The soundstage conjured up by the X350.5 is fairly wide and deep, but not as wide or as deep as other amps such as the Gamut M250i monoblocks (Issue 229) can muster. To be fair, mono amps usually have the advantage in this regard—all other things being equal. Also, with the X350.5, the front of the soundstage tends to hover in a plane between the front baffles and the rear panels of the speakers, depending on the recording. It is not at all unpleasantly forward, but the effect is still noteworthy because it made it harder for me to separate the soundstage from the speakers. I know some listeners prefer this sort of presentation and consider a soundstage that begins behind the speakers (as I am used to) to be an artificial effect involving a measure of unwanted tonal-shifting.

Despite the soundstage beginning a bit more forward than usual, apparent listener perspective is a bit to the rear of mid-hall, based on my impressions of overall tonal balance, spatial cues, resolution of fine details, leading edges, and string vs. body sounds. Individual images have admirable heft and physicality—rather than a wispy, gossamer-like presence—but the depth of those images is slightly foreshortened rather than being fully rounded and more 3-D-like. Hall sounds and the space around musicians are well represented but, again, bettered by the more revealing $23,000 Gamut M250i mono amps. Admittedly, I am unfairly comparing a reasonably priced stereo amp to twice-the-price monos. I don’t mean to take anything away from the X350.5 by sharing these sonic impressions; I bring them up as a way of providing some perspective on what is possible with these amplifiers.

The underlying technology unique to Pass Labs amplifiers is beyond my understanding. From what I can gather, though, much more is going on in the X350.5 to account for its wonderfully non-fatiguing sound than its first 40 watts operating in Class A. Pass Labs calls its principal noise-reduction technology “supersymmetry.” The basic premise, as far as I can tell, has to do with making the two halves (positive and negative) of a balanced circuit behave more like each other in their respective distortion and noise characteristics, thus enabling the inherent noise reduction of a well-executed balanced circuit to be more effective. Supersymmetry apparently comes closer to ensuring that what is canceled is indeed noise and what is left over is a cleaner signal. Pass also achieves high power output and low noise by using fewer gain stages with more closely matched complimentary output devices (MOSFETS, in this case).

Physically, the amplifier is a large, heavy, nicely sculpted metal beast with the characteristic “cupped meter” faceplate found in XA.5 and X.5 models. It has large banks of heat sinks on its sides to help dissipate the considerable heat. (Class A operation generates lots of heat.) I left the X350.5 on, often with signal running through it, for a few weeks before I did any serious listening, so I can’t speculate about possible “burn-in” changes over those weeks. It warms up adequately for a listening session in about two hours. It has both single-ended RCA and balanced XLR inputs, and the well-written, comprehensive manual gives instructions on how to insert the supplied shorting jumpers across two of the XLR pins if you use the single-ended inputs. Elsewhere the manual reassures readers that the X350.5 is a robust product that can handle difficult speaker loads, but also encourages the user to “relax” if something goes wrong in setup or operation. The manual goes on, “We go to a lot of trouble to make products reliable, and the failure rate of our amplifiers is very low.” The sheer size (19" x 11" x 22") and mass (132 pounds), power consumption (550 watts at idle), not to mention heat generation (the heat sinks run about 127°F, too hot to touch for more than a few seconds), may deter some users. But most serious audiophiles opt for good sonic performance over convenience, and this amp definitely qualifies for the job.

Considerably more than merely good sonic performance awaits X350.5 owners. It provides truly compelling listening sessions with your music collection—fatigue-free, imagination-inspiring, emotion-filled listening sessions. Its prodigious power, natural tonal balance, and liquidity impart a solid foundation to even the most demanding music while caressing delicate passages with a silky fine touch. I can’t think of a more powerful and non-fatiguing amp close to its asking price. Highly recommended.


Power output: 350W (8 ohms), 700W (4 ohms)
Inputs: One pair balanced/XLR, one pair single-ended/RCA
Outputs: Two pairs of binding posts (spades or bare wire)
Power consumption: 550W
Dimensions: 19" x 11.2" x 22.5"
Weight: 132 lbs.
Price: $11,550

Pass Laboratories
24449 Foresthill Rd.
Foresthill, CA 95631 USA
(530) 367-3690


Analog Source: Basis Debut V turntable with Vector 4 tonearm, Benz-Micro LP-S cartridge
Digital Sources: Ayre C-5xeMP universal disc player, Sony VAIO VGN -FZ-490 running J River MC 17, Hegel HD2 andHD20 DACs
Phonostage preamp: Ayre P-5xe
Line stage preamp: Ayre K-1xe
Integrated amplifier: Hegel H200
Power amplifiers: Gamut M250i
Speakers: Dynaudio Confidence C1 Signature, YG Acoustics Kipod II SignaturePassive
Cables: Shunyata Anaconda ZiTron signal cables, Audioquest Coffee USB and Hawk Eye S/PDIF, Shunyata Anaconda and Cobra ZiTron power cables
A/C Power: Two 20-amp A/C lines, Shunyata SR-Z1 outlets and Triton/Typhon power conditioners
Accessories: Stillpoints Ulra SS and Mini footers, Shunyata Research DFE V2 cable elevators
Room Treatments: PrimeAcoustic Z-foam panels and DIY panels