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Pass Labs X350.5 Stereo Amplifier Design Overview


Pass Labs electronics are designed and built in Foresthill, CA USA. Their boards are made in the US and they stuff the chassis themselves at their factory. Pass Labs uses local machine and sheet metal shops and source as much as possible locally.  This is rarity worth mentioning and although it’s a more costly endeavor to produce products in the USA, it puts fellow Americans to work, which is something our country needs right now. I commend Pass Labs for this effort.  This should make forum trolls think twice about bashing the price of this amplifier.

The X350.5 is touted as a minimalist design, which in terms of circuit complexity may be true, but the quality of craftsmanship and the layout of the components is an engineering marvel.  They use very high quality output devices and tight tolerance parts to help minimize the excessive reliance on negative feedback which they claim improves transient response, bandwidth linearity and stability by not having to employ additional gain stages.  The X350.5 is heavily biased into Class A operation up to 40 watts/ch peak or 20 watts/ch continuously.  Pass Labs’ recommends a warm-up period of about an hour before conducting critical listening tests.  At higher power levels, the X350.5 operates in Class AB for increased efficiency.


Pass Labs X350.5 Top View

Removing the top plate, a peek inside the X350.5 amplifier showed it to be a work of beautiful art. The capacitors are connected via sheets of metal which serve as low electrical resistance connection points and heatsinks to keep them cool even during high amplifier output conditions.   The six (6) very large 25,000uF / 75V power supply capacitors are connected in parallel, yielding a total capacitance 150,000uF.  Pass Labs’ literature states a max voltage output of 76V, but that’s not the actual rail voltage.  The rails are actually +-55V which yields almost a 20V margin for the capacitors. This ensures longevity in the capacitors by not running them steady state at their max allowable voltage limits.

The power transformer is a hefty1.5kVA toroid, which is not visible from this picture as it’s mounted at the bottom of the chassis underneath the heatsink flanking the power supply capacitors.  Each channel has 36 output devices (18 on top as pictured and 18 on the bottom).  The output stage is fully differential, which if properly implemented can be a huge advantage for noise immunity and distortion reduction.  Only the very best amplifiers are fully differential from input to output and the Pass Labs is in good company here. 

The X350.5 amplifier is CE certified, meaning it meets or exceeds all safety standards for domestic and international markets.  The X350.5 is not user selectable for 120V/220V operation like some amplifiers we’ve tested. 


Pass Labs X350.5 Back View

The Pass Labs X350.5 has quite a striking rear end.  It’s equipped with two large handles for “easier” transportation of this anvil-sized amplifier.  The X350.5 has both balanced and unbalanced input connections, a detachable power cord, and ground and trigger connections.  The balanced/unbalanced input connections aren’t switchable so be careful about accidentally using both input types simultaneously.  The balanced connections come with a shorting plug installed for this very purpose and to also minimize noise pick up of the unused input connections.

There is only one set of speaker terminals on the X350.5 and they only accept spades (no bare wire and no bananas).  This means you can’t bi-wire or run two pairs of speakers simultaneously unless you construct a breakout cable to do so.  I am not a fan of spade terminations as they can be dislodged with a minor jostle even when twisted down.  It was difficult to get my Kimber 8TC cables tightened down while twisting the lugs as they rubbed against the terminations.  I would have preferred conventional 5-way binding posts, but Pass Labs claims they use this connection scheme for safety purposes.  I don’t get this since they could have still used binding posts affixed to plastic standoffs to prevent arching like high power amplifiers produced by other manufacturers often do. 


Pass Labs X350.5 Front View

Perhaps the most striking feature to the Pass Labs X350.5 is the large round blue backlit meter that actually tracks current consumption instead of power.  The sculpted aluminum faceplate is simply stunning and is instantly noticed by visitors when they enter the room.  Both sides of the amp are completely flanked with extruded heatsinks that serve their purpose to draw off heat from the output devices but also give it that badass look.  I have to admit I found myself quite infatuated with the blue hue of the front panel glass on this amplifier during my extensive listening sessions.


Pass FrontUnboxing and moving around the X350.5 was arduous.  It was grueling lugging this 132 lbs behemoth up the flight of steps for testing in the Audioholics Showcase Theater room.  I connected the X350.5 to my Marantz PM-11S2 integrated amplifier via Bluejeans Cable interconnects and Kimber 8PR speaker cables with compression WBT banana plugs to the new Status Acoustics 8T speaker system I currently have under review.  The source devices included the Marantz TT-15S1 turntable and Oppo BDP-95 Blu-ray player via the analog balanced outputs.  The listening space is a 6,000 ft3 room that is moderately acoustically treated, courtesy of Auralex Acoustics.


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Recent Forum Posts:

Rich Davis posts on October 12, 2013 16:33
There a LOT of VERY expensive amps on the market. In terms of how loud they go? Any good amp will probably drive most speakers past what is considered normal listening levels. I don't know if you've done any dB checks, but most of the time I only have my system at around 85 to MAYBE 95 dB on a continual basis. Anything louder than that is not typically good for your ears to have a system operating much louder than that. Most systems playing at that volume level are not using maximum output. Some speakers can handle large amounts or need large amounts of power to drive. Electrostatic, MBL, and other speakers might require or like large amounts of power, but in most cases, you'll probably not use much more than 20 or 50 watts, especially if you do things like bi-amping.

Don't judge a system by how loud it goes, since most of these places won't even turn up the system to maximum levels, because you can't sit within 60 feet of the speakers when they drive them to maximum spl. I have a pair of Meridian DSP5500s for mains, a DSP5000 center channel, with some other DevTech rears and a Talon sub and I can't sit in the same room after I turn up the volume knob past a certain point. Listening to a system in the 100+dB range is just not something good for your ears. When you go to a concert, break out your smartphone and a dB meter app and see what the dB is at your listening position.

Go to these sites and check out some of the most insane systems available. HigherFi - Worlds Best HiFi High-End Speakers and Audio or HIGH-END PALACE - The place for the world's best music systems!

And $50K for a pair of monoblocks isn't the high end, it's not more mid high end in terms of price. one can spend upwards of $500K for the Pivetta Opera or $200K for the Goldmund Telos 5000's, etc. so $50K for a pair of Pass is pretty cheap. :-)
Rich Davis posts on October 12, 2013 16:23
Pass makes nice stuff

Personally, when it comes to power amps, there are so many good amps out there and it's all a matter of matching up with the pre amp, cables, and speakers to achieve the desired end result.
Irvrobinson posts on October 26, 2012 23:21
surveyor, post: 917475
In the fact that it's more powerful than either the Mark Levinson or the ATI amps says it all- it truly a beast!

Actually, both the Levinson and the X350.5 are both merely in the middle of the respective manufacturers' product lines. If you want to get really silly with amplifiers both companies will fully accommodate you. Pass makes an X1000.5, and Levinson makes the completely ridiculous No53 monoblocks for $50K per pair. The Pass Labs monster is a comparative bargain at only about $35K per pair.
surveyor posts on October 26, 2012 22:00
gene, post: 917055
Yep an amp spends most of its time operating at low power so it's good to see how quite and clean it is there.

No doubt the ATI amp is a good one but it's not as powerful as the Pass Labs.

Using square waves to test an amp has little value these days when you can power sweep from 10-20kHz and beyond with modern test equipment.

The Pass Lab's amp displayed extreme linearity at all power levels for the entire audio band.

In the fact that it's more powerfull than either the Mark Levinson or the ATI amps says it all- it truly a beast!
craig7 posts on October 26, 2012 21:37
wow you guys reviewed a product ive actually seen in person before
and yeah… it is a pretty beast amp. ive never heard any system louder than it… it was incredible
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