New Pass Labs X.5 series versus previous X series

Pass Labs is in the process of re-dedesigning their entire X series into the new X.5 series. As of this writing X150.5, X250.5 and possibly X350.5 are already shipping. The X600.5 are expected to start shipping early in the new year. This thread is to discuss experiences with the new amps, their sound signature, perceivable improvements over the old series, and of course dissenting opinions. Everyone's perspective is welcome--X and X.5 owners, X.5 window shoppers, Dealers, and all other audiogoners with a passion for affect by the bug of audiophilia.
Do we know what is changed in the circuitry?

I can GUESS that X.5 series should have a sound somewhere between the X series and the Aleph series, actually probably between the X series and the XA series. Somewhat smoother and more liquid than the X series, but not as much so as the XA's. I haven't heard them yet though.
The info on the X.5 series internal changes are still scant. The best info I have found so far seems to suggest changes in the power supply have been included. Here is an extract from an ad for the x250.5 on audiogon: ". . . addition of single-ended Class A bias, high speed fast recovery rectifiers in the power supply, and some changes to the front end circuitry have been made." I have not however seen a complete discussion of these changes anywhere yet.
I have found the following snippet in a current Audiogon listing for the X350.5: "The new X350.5 features improved output transistors and a redesigned single ended Class A bias circuit . . . ."
I have owned X150, X250, X350 and now the X250.5. The X250.5 is by far the best sounding amp in the X series or maybe the X.5 too. I like the X250 better than the X350 and I assume the X250.5 should outperform the X350.5. The X250.5 is less aggressive, more sweet, relax, extended than the X250.
Thank you Jazzmeup. I have also seen you are putting your X250.5 on sale. What are you going to replace it with? What aspects of X250.5 are you finding dissatisfying, if any?

What you have already said is about all the info that I was told by Peter Perkins at Pass Labs. He did tell me that the X350.0 is there best sounding amp in the X series. The majority of people I have questioned said the X250 was a better sounding amp than the X350 before the .5 version. However, with the X250.5 and X350.5 the majority of people I have talked with said the X350.5 is better sounding. Peter Perkins said the X350.0 has improved the most from the .5 versions. He even said it would be very difficult to have the X600.5 sound better. Please take this all with a grain of salt. I would advise you to call Pass directly and hear it from them. They are shipping the X350.5 without manuals until they get them printed with the new info.
Thanks Irish65. I am planning to call Peter at Pass next week, as -- per my info -- by now they should have a working X600.5 prototype. It would be also interesting to hear from audiogoners who have already heard x350.5 in person, or who are planning to acquire one.
Some more tidbits I just gathered from Pass: while specs on new X.5 series will be virtually identical to older X series, the implementation of the class-A portion of the amps has been changed from push-pull class-A, to a single-ended class-A design. This has apparently significantly reduced time-smearing of low level detail and is generating a much more layered and finely detailed image.
The ultimate goal seems to be to maintain the high current/power of the powerplants of the X series while re-capturing some/most of sound characteristics/magic of the original Aleph series through the single-ended class-A implementation.
X350.5 is complete and is being delivered to dealers. X600.5 still in engineering. Stay tuned.
What was Peter's feedback concerning the X350.5?
Guidocorona, that's a very interesting tidbit! Thanks for passing it on.
Today I spoke to Kent, rather than Peter. Peter was home with the flue. Kent said he spent some quality time a few weeks ago with X350.5.
He indicated that X350.5 has the very best sound he has heard this far, and not by a small margin.
According to his particular taste, the previous series lineup, in descending order of preference for him went: X600, X250, X350, X150.
He seemed to indicate that in the X.5 series things are very different. As nelson Pass proceeded in re-designing from X150 upwards, he put more and more of his own sound philosophy into the new models. So while X150.5 was definitely a positive step in the right direction, X250.5 was significantly better than X250, and the difference between old X350 and X350.5 is apparently even more remarkable.
Kent's new descending order of preference is X350.5, X250.5, X150.5.
The X600.5 will not experience 'first sound' until after mid December, and I intend to call back Pass Labs at that time. Will the delta between X600.5 andd X600 exceed the delta between the immediately lower model? Kent of course would not commit. Stay tuned. I suspect the entire new lineup may be ready for CES 2005--my inference only, not a tidbit from Pass.
Actually I was thinking of getting the X350.5. But I am 100% convinced. Being the fact that X250 outperforms the X350. I really have to audition the X350.5 and go from there. I hope I will make the right move this time around. Anybody auditioned the X350.5 and wants to shed some light?
wonder how X250.5 compare to XA-200 ? Any idea ? Thanks!
I'll be out of touch for 36 hours. Keep the discussion going in the meantime. Some dealers are already stocking X350.5. Someone should give these guys a buzz.
Hi folks: I own a X250 and has been one of the best speakers I have had, the list is quite large to bore you here. I don't feel the need to move to the .5 series, and more into finding theright preamp for my X250. The best I have OWNED is a First Sound Passive preamp. X2 was awfull for me.

What is expected to be that different in the .5 series to really make me change my mind?

I will appreciate your feedback.

Hi Folks,
I just received my new 350.5 48 hrs ago. I owed a 350 for two years, and was very happy with it. But the the boys at Pass talked me into an upgrade. Peter promised me great things. And as you might have it they delivered in spades. Without any break in time the 350.5 dropped my jaw. The sound is sweet, detailed, soft and musical, and all of the usual euphamisms. The bass is the real winner, it has the size and rythm of a tube amp and the controll of the best solid state. I picked up a full octave of bass. By peeking into the top, it looks like the power supply is completely different. The new face plate is real cool, a hybrid between the old and the XA. The 350 took a real long time to get as good as it got, so I really expect great things from the 350.5 in the future. I have to say that his amp has been one of the most exciting changes in my system since a significant speaker upgrade.
Sorry, I meant to say 'I am NOT 100% convinced'
I recently purchased an X250.5 and love it. I've seen several amps go through my system over the years including an X250 and an X350. I've never felt content with my amp and always felt the desire to try something different although I really enjoyed the X350 better than any other I had had at the time. But I doubt the X250.5 will be leaving my system for quite some time. Every time I sit down to listen, it seems to sound better. I would strongly recommend a listen to the X250.5 or the X350.5. I can't really imagine the X350.5 sounding a lot better than the X250.5, but I was surprised at how much of an improvement there was in the X250.5.
Sigmund, it will be interesting to hear your impressions of your new X350.5 as you keep breaking it progressively in.
Last night I stayed up way too late listening. I experienced something with the 350.5 that I didn't with the 350. The 350 was a very exciing amp to listen to in my system. The transients were explosive and exciting. The reproduction of insturmental timbers could not have been better. But the 350.5 sucked me into the music....the only thing I can figure is that the inner detail is so much better I kind of felt like I was listening to a 300B amp. The 350 seemed to take sweveral months to really break in, so it will be interesting to see how far the 350.5 will come.
It is interesting to learn on the inner detailed presentation that Sigmund refers to. I have always felt that it is currently one of the main virtues of my X250. Inner detail, dynamics, articulation (particullary with the First Sound Passive preamp). I just can't imagine a step-forward these sonic attributes.. congratulations!


PS: If I am pushed to hard to say what would I like to have more from my X250 would be depth of image. Have you experienced this as well?
Hi guys,

I've been confuse about the new .5 upgrade for a very long time and still is...Therefore, I've talked to Pete at Pass...and certainly, the x300.5 is the best in the x series he have ever heard compare to only the x250, x250.5, and the x300!!...however, I've been wanting the x600 mono block for a very long time, and Pete is recommending me to get the x600 over the x300.5 for my speaker(e.g Sony SS-m9ed - e.g that's sony uses for the demo)....What does this really mean?...could the x600 still sounds better than the new x300.5...but the x600.5 will be the very best of the best?...

I've seen the new faceplate of the new x300.5, and thought that the older look looks more mucho and heavey duty looking. Whereas the newer look is more elegant similiar to the new xa series....

Help guys...x600 or x350.5?.....any advise?..I'm lost.
The amp I had before the 350 was a Sonic Frontiers Power 2, That amp had geat depth, but left me wanting for the last word indetail and resolution. The 350 fixed that yearning. However, I don't recall losing anything in depth in the switch. I currently have Tyler Acoustics Woodmeres sitting six feet from the back wall. The Woodmeres throw a huge detailed sound stage. The image I am currently getting resides well behind the speakers against the back wall with lots of layering. I have also had Wilson speakers, the image was also very deep, but much closer to the floor and smaller, but that is a characteristic of the speaker. So now that I have made a short answer long, no I don't feel like depth has ever been an issue.
The 250, which employs smaller caps, is a more current design from Pass than the 350, which employs those big caps. Most other amp mfrer like Classe etc. also uses small cap in their current amp design. Smaller caps dissipate current/energy much more quicker. Sonically IMHO, it is a lot more faster and better pace and usaually more extended. That could very well be the reason that X250 outperforms the X350. Talked to Pass people a few times and have learned they didn't do anything extra to the 350.5 than they did to the 250.5. I really doubt the 350.5 would outperform 250.5. Maybe in situation that requires the extra power. But, it is interesting to see the feedbacks from other 350.5 and 250.5 owners.
Longho68, the X600.5 will not be available until the first part of the new year. You won't be able to audition them until then. I do not know the efficiency of your Sony speakers, you I can't really comment on what kind of amp you may need.
My situation: Bought a X-250 demo 3 months ago for $4200 and had a PS Audio power cable thrown in on the deal. Now, for $1500 and the X-250 I can have the new X-250.5...Thoughts? Is the list price of $6500 a firm price for the X-250.5?
thanks Guidorona, I think I'm going with the x600 since I like the look of it better..and save the extra 2k in other upgrades.....The new retail for the x600.5 is 18k vs 16k for the x600.......However, the new x600.5 will have RCA inputs as well....

Speaking of having RCA input for the x600.5?..could that mean more circuitry to deal with RCA input makes the x600.5 not as good as the x600?...
Longho68: I am not that familiar with your speakers, just heard them very briefly at Madison 500 some years back when SACD was hot (jejeje).

If I were in your shoes I should go with the new Pass amp, there is a review somewhere in the net and at the PASS site where the reviewer found quite small differences between the x600 and the x250 in terms of musicality, tonal palette, etc.. the x600 is, from my perspective, only neccesary where TONS of power is needed.


Sigmund: I own Avalon Eclipses, which also throw great backstage and imaging scenario. The sound of the combo is terrific for my tastes, but.. I recall a little more depth from the Gryphon amps.. anyway.

I am buying more CDs and less gear since the Pass is in the system...

Thanks Flg2001,

Frankly speaking, I recently sold my x250 in favor of the I'm confuse again..:-).heheheh isn't this a fun and expensive hobby..:-)

Did anybody compare the x350.6 to the x600 yet?...
Longho68: The x600 is a great amp, you will both enjoy it and will surely find a sonic upgrade.

".. expensive hobby.."???, not quite sure..., I thought the same, but if you talk deep with some friends who are into other hobbies such as Photography, wrist watches, pens .. or even more ego-indulging ones based on other sort of pleasure (you know..), we are in heaven....


The Pass Labs web site has just been updated with information on the new X.5 series. See the following url:
In particular, the page on the new x600.5 has information on some of the electric design improvements, as well as some photographs:

"**Expected to ship in the first quarter of '05**
The X600.5 is the natural progression of the X600. Changes include improvements to power transformers, paralleled high speed / soft recovery rectifiers, improved AC EMI filters, more paralleled power supply capacitors, increased supply RC filtering, refinements to the front-end input circuits, and the addition of single-ended Class A bias circuits for balanced single-ended Class A at low power levels.
All of these changes bring the X.5 series closer to the sonic signature of the XA series amplifiers.
The X.5 series feature the distinctive front panel meters indicating of the level of current running through the output stages that have long been the visual signature of X amplifiers.
The X600.5 accepts both single-ended and balanced inputs. . ."

The X600.5 should become available early in the new year. It's smaller brothers are already shipping. While we wait for X600.5 to be completed and to start shipping, I love to hear from any new X350.5 owners with their live impressions/findings.
I am still in the process of breaking it in. I have about 30 hours on it and each time I listen to it it gets better and better. When I get at least 50 hours on it I will post the results.
Irish65, which model are you breaking in exactly? Thanks, Guido
On your last post on 12-4-04 you encouraged any with the X350.5 to give their impressions/findings. So I was referring to the X350.5.
How many hours do you have on your 350.5? I would like to hear what has transpired with the X350.5 since your last post when you first received it. In other words how has the amp improved or not.
Thank you Irish65 for the clarification.
Just spoke to Peter perkins at pass today and got an update on the upcoming X600.5. The engineering prototype device has now been making music for a while.
According to Peter, the team at Pass is now auditioning what may very well be the final tweak of the design. Everyone seems to be happy about the results, but the final go-ahead to close the engineering/design phase and transition to full scale production, will come only after a final audition/approval by the company CEO.
Peter suspects that full production may be under way by the end of May 2005.
I asked specifically about audible differences with X350.5. Peter answered that while there is a lot of similarity with X350.5, the X600.5 has even greater ease, liquidity, bass control, channel separation, and overall 'sparkle'. He suggests that the difference is not due to any design change, but to the complete separate chassis implementation and the dedicated power supplies in each monoblock.
I have no doubt that the monoblocks will be a refinement of what the single chassis x.5s will offer. I did post a formal reveiw here on the GON, hope you took a look at it, so I won't go into the many virtues I shared in my reveiw of the 350.5 amp. But what I want to share is the following; A new audiofile friend and music lover came to my house to hear my system and said" Come on were are the tube amps you can't get this warmth and timbres out of a solid state amp can you?" He has the 20000.00 Lamms and Karma speakers at home ,so he knows good sound.Yes you can along with dynamics,bass control, etc. with these new Nelson Pass's x.5 babies.
TJ, not only I was impressed by the findings in your review of the X350.5, but I was equally amazed by the consistency of all the positive comments of the other users following your excellent review. Nelson must truly have created something special!
Has anyone compared the X350.5 with the Parasound Halo JC-1's, or owned both of them?
If so, how do they compare sonically?
I had both X250.5 and JC-1 for 3 months driving my Studio. I had such high hope for the JC-1, but shockingly the X250.5 blows it away cold out of the box. Much faster, a lot more extended high end(JC-1' weakest area) , much better defined and controlled bass. And now I moved up to the X350.5 and it is in a class by itself. A lot more authority, dynamic, body, sweetness,....... I really don't know whatelse to expect from a SS amp. I heard some people prefer this amp over the XA160.
Anyone compare the Pass Aleph 2, X 350.5, and XA 160s?

I own latest update Soundlab U-1s with the benign impedence transformer upgrade and the latest PX panels, both of which, BTW, I recommend to any SoundLabs owner - you may contact me for details.

But, back to your opinions of the amps in this situation - driving Soundlab panel speakers or other electrostatics - they don't have to be U-1s.
The.5 designation is deceveing. It could have been a totally new model with the great degree of improvement. G
Hi there, I am considering the X350.5 against the X600.5 and would like to know if you have any views / experience on comparing the virtues of both these great amps?
hocklin, I suspect you may find the old X600 to have greater authority and to be slightly leaner, while the newer X350.5 to have greater refinement, delicacy and control.
Apologies to all. . . I corrected my previous post. I meant to say that the x600 was likely to be found a little leaner, not the 'x600.5'. Clearly a 'ooops!'
Hi Guidocorona, Thanks for your comments. So the X350.5 appears to be "better" vs. the old X600 in the areas of refinement, delicacy, control- which are what I am looking for. What about the X600.5? How big is the gap between the X350.5 and the X600.5? In a previous post there was mentioned that the X600.5 would have more sparkle!?
hocklin, have you considered calling Pass Labs directly? Folks at Pass are extremely helpful. Try:

Pass Laboratories,
PO 219,
Foresthill, CA
95631 -
Voice: 530. 367. 3690 -
Fax: 530. 367. 2193 -
www. passlabs. com

have a chat with Kent and let us know what you hear.
Peter is no longer with Pass Labs.