Review: Pass Labs X-350 Amplifier

Category: Amplifiers

Talk about taking an expensive chance! I bought this amplifier solely on the basis of the advice of a couple of audiophile friends of mine, and on the recommendation of Stewart Marcantoni of A Sanctuary for Sound. a Port Orchard, WA dealer whose advice has proven to be consistently reliable and accurate. Two weeks after placing my order, the amp arrived on the doorstep of my North Idaho ranch. The first thing you'll notice about this amplifier is that it's a brute: 150 lbs w/carton. I managed to heft this package up a long flight of stairs, but it’s helpful to have a friend handy to assist you with unpacking and placement. A sturdy hand truck is also a Good Thing to have, especially if you have to negotiate stairs as I did.

A Shocking Development
Since I had ordered the amp with the IEC 20 amp power cord option, I hadn't reckoned that my conventional AC outlets were the wrong configuration for this type of AC cord - NEMA 5-20 outlets are required instead. Doh! So, I had to do a retrofit to my dedicated AC outlets. This inadvertent upgrade was easy to do and resulted in better quality AC outlets. Upgrading to to these outlets seems to have reduced the overall noise level in the system. Cheap tweak, come to think of it: $20 for four pair of these NEMA 5-20 outlets at your friendly Lowe’s hardware store.

Looks don't hurt
The X350 is a gorgeous piece of industrial design. Massive - and I do mean massive - face plate and fascia, with one Cyclopean bias meter dead center on the front panel. Lit up, the meter face assumes a beautiful brilliant blue hue, with the same Cherenkov-like hue spilling from the amp's ventilation slots. The meter’ serve to indicate when the amp transitions from class A operation to class AB; this event occurs as power demands exceed 90 watts, according to the manual. Sturdy rack mount handles on the back of the X350 are another Good Thing, considering its sheer mass. Five soft rubber feet support the chassis; the somewhat soft rubber makes it difficult to slide the X350 on a smooth floor (and just forget about carpet!). However, the composition of the feet suggest that there may well be some damping and isolation benefit there.

The back panel features XLR and RCA inputs. I have used it so far only with single ended RCA cables, so I can’t comment on the differences between the two input options. Speaker connections are made via two pair of five-way binding posts; each are enhanced with sturdy plastic wing nuts that allow a firm connection.

Initial impressions
Right out of the box, the X350 beat my Krell KSA 250S hands-down ,and my Bryston 4B ST wasn’t even in the running. My initial impression was that the X350 delivered a far more organic feel to the music in that there was more of a sense of seamlessness and continuity. Each passage that I heard seemed to have more ‘weight’ somehow. The amp delivered scads of inner detail – I could actually hear the impact of fingers on keyboard on good piano recordings, for example. Voices are to die for - whole, deep and real. Even without the lengthy break-in period recommended by other reviewers, the mids on this amp are pure and breathtaking.

Listening at length
There’s a crystalline clarity up top that I believe is more a reflection of the X350s ability to provide additional information in the form of detail. But this detail is delivered without a tilt towards the top, and without a hint of grain. The five foot ribbon tweeters of the Magnepan MG20Rs are mercilessly revealing, but at the same time they are quite possibly the best high frequency drivers on the planet. At no time did they sound hard or harsh - unless that was present in the recording. There was no sense of solid state hardness or the infamous 'MOSFET fog' heard in other MOSFET designs.

Some reviewers have reported a initial mid-bass emphasis that disappears after 200+ hours of run-in - I didn’t hear it, and neither did my wife, Candace. When it comes to deep bass performance, I would agree with other reviewers in that this amp isn’t the last word in Krell-like low end slam, but I have a feeling that it’s more a matter of the fact that the X350’s doing a much better job of sorting out low-end detail and definition. The X350 appears to be presenting more information in the low end than then did the Krell. I can hear the transition from one low-pitched note to another, something that simply wasn't anywhere nearly as obvious with the Krell. Low-pitched timbres emerged with distinct identity where they were not evident before.

The X350 delivers a better presentation of the entire dynamic envelope, and this is key to what makes this amp better than many of its higher-priced competition. Smaller, lower-level bits of musical information aren’t masked by larger events, so you’re actually hearing more of what’s there. And all of that is kept in perfect proportion up and down the dynamic range of the performance. This is what I believe lends the impression of wholeness and weight to each musical passage.

Power to spare
Then there's the sense of sheer, effortless power. The X350 exhibits a sense of utter and effortless control over the MG20Rs, and at any level. The MG20Rs can be driven to ridiculously high levels, and the amp never loses its composure or alters its spectral balance. I could clearly hear the Bryston 4B ST run out of gas on piano passages in Mozart’s Concerto No. 21 in C major (Reference Recording RR 68CD) played at concert levels. The Krell KSA 250S was less prone to this, but I could still hear it give up. Not so the Pass X350.

I must run the X350 really, really hard in order to get the blue-lit front panel bias meter to budge off dead center. That takes the full chorus, orchestra and organ from the last movement of Mahler's 8th (Solti/Chicago Symphony) at realistic levels to make that bias meter move. So it would seem that the X350 is running within that class A regime (up to 90 watts) most of the time. This says a lot about this amp's reserves and current delivery, especially since it's driving the MG20Rs - a notoriously inefficient speaker that benefits from a lot of current and iron-fisted control. In sum, this is absolutely the most musical and utterly transparent solid state amp I've ever heard. There may be tube amps that might offer a shade more in the way of micro dynamics or inner detail, but they do so at the trade-offs of less power, more heat and the sheer dollars spent to get there.

Soundstage
As the X350 warmed up and broke in, the soundstage began to unfold and deepen. Placement side to side and front to back is nothing short of phenomenal. If anything, the X350 has improved after more than 300 hours or so of break-in. It seemed to improve in subtle ways almost day by day, becoming even more detailed and seamless from top to bottom. Low end took the longest to 'settle in', and was the most subtle of the changes, manifesting itself in a degree and clarity of low frequency low level detail that I simply hadn't heard on any of my CDs and LPs until then. It's a funny notion, to think of deep bass as being both strong and delicate at the same time, but that was the overall effect. The X350 made acoustic bass 'action' and fingering come alive in a wholly unprecedented way, imparting a 'right there in the room' sense of reality to well-recorded acoustic instruments in particular. Check out Michael Gettel's San Juan Suites II and Pat Metheny and Charlie Hayden's Beyond the Missouri Sky. Absolutely lovely piano and acoustic bass duos and a real system test. Midrange appeared to, well, relax is the term I'd use. As it broke in, the sound space that this amp delivers has expanded beyond its initially impressive sense.

Still, there’s something that I can’t really put my finger on when trying to describe what this amp does (or maybe doesn’t do). Candace may have got it right when she said that everything seems alive now. For me, it seems as if everything I listened to was somehow more compelling, more worthy of my attention. Perhaps it’s that the X350 allows more of the emotion and what we might call the ‘inner truth’ of each performance to come through. While there's nothing euphonic or theatrical about this amp, it somehow manages to come across as the most musical design I've ever heard. If money were no object, I suspect that I'd move up to the X350s more powerful cousins, the X600 or the X1000 monoblocks.

Other reviewers have consistently said that this amp really takes around 300-400 hours to truly settle in. Our listening experience suggests that this is an accurate perception. The overall presentation became deeper, more articulate and more effortless - it’s as if the system is doing a slow dissolve and leaving just the musical event in its place. And that's what it's all about, folks.

Associated gear
System used for audition
ARC LS22 preamp - retubed with NOS 70's Russian 6H23s
Classe CDP-1 CD player
Bel Canto DAC-1 24/96 upsampling DAC
Synergistic Research Designer’s Reference Digital link
Linn LP12 / Ittok LVII / Dynavector 10X4 turntable/arm/cartridge combo
Magnepan MG20R speakers w/XO20 crossover.

Cables and interconnects
Cardas Golden Cross interconnects
Cardas Golden Reference bi-wire speaker cables
4 pair of .5 meter Audio Magic Spellcaster II jumpers (to go from the XO20 crossover to the MG20Rpanels themselves)
Electra Glide Fatboy power cables.
Bybee Signature Pro power purification.

Similar products
Krell KSA 250S
Bryston 4B ST
Levinson 335