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scumbag

Pass Labs XA25 Power amp

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I am writing this review on the "new" XA25 Pass Labs power amplifier. This design represents what I think is a big turning point for PL. To me it represents a fusion of the established way of doing things at PL with a new more contemporary design. And in design I mean sound character and the actual design. 

 

I am inevitably comparing this this amp’ to the XA30.8 that I owned previously. It's a logical point of comparison as both amp’s are at the bottom of the PL range (bargains LOL). Used XA30.8’s are floating around the same price as a new XA25 at the moment. There was a period of about a week between selling the XA30.8 and getting the XA25 but I think my auditory memory was sufficiently locked away to evaluate the differences.

 

Straight out of the box (couldn’t resist), there was an obvious difference between the character of the 2 amps. I think that anyone who thinks all amps sound the same is operating on incomplete data. There is more to an amp than frequency responses. In the case of the XA25 versus the XA30.8, we have measurably lower distortion (not necessarily a better thing of course as was eventually realised after decades of SS high feedback designs). There is more 3rd order distortion versus 2nd order especially as frequency drops, a much higher damping factor (or a lower output impedance for semanticists). Also, a faster slew rate. We also has lower gain rate as (I was told by a PL employee) the design of the gain stage dictates it. That will affect things as we’ll see!

 

Peak current is less in the XA25 and is limited to 10A which in real terms is still quite high. This is the only PL amp that I know of that has some form of protection other than the prerequisite fuse)! Which is one of the ways that this design is more contemporary. It gives you a little bit more piece of mind that is something catastrophic happens, the amp' will simply shutdown and not go up in smoke.The XA25 is rated down to 1ohm though as per the Stereophile review, you will get a rise in distortion at higher frequencies but these may still be an order lower than some other amp’s out there anyway. But it will thrive on a 4ohm set of speakers and power output will rise as well and according to the Stereophile review it works well speakers with difficult loads.

Similarities are, as far as I can tell, a similar class-A power envelope (50 watts peak into 2,4,8 ohms) but with a smaller power envelope (about 1.5-2dB is real terms) so about 90 watts RMS into 8 ohms for the XA25 versus 120 watts RMS for the XA30.8. Like just about all of pass labs amplifiers, the unit will transition into class b operation at higher output. In reality this extra power is the xa30.8 versus the xa25 is not a consideration for me and I found the XA25 handles the upper ranges of it's power output with more composure than the XA30.8. And the difference in watts only translates to a small difference in spl. 

 

I did read all the reviews on the XA25 and of course read the reviews on the XA30.8. They are pretty much without fail highly positive and there are a lot of comparisons to tubes. Personally, I don’t really buy in to the solid state versus tube thing. I think there are characteristics that are more common in one technology over the other but there is overlap especially as you move up in price. More on this later.

 

Depletion Mode:

From a topology / circuit design POV, the big ticket item is the use of depletion mode MOSFETS. Nelson Pass started playing around with this type of circuit a long time ago I’m sure and he has several First Watt devices that use it but this is the first time he’s introduced it in the Pass Labs range which you might call courageous as he risks upsetting his existing customers. 

So what’s the big deal? Well a enhancement mode mosfet (as per other previous PL designs such as the XA30.8) uses resistors (ballast resistors) to control the Mosfet. From NP’s Burning Amp article –

“There's a reason to try depletion mode Mosfets instead of usual enhancement mode devices. Mosfets conduct current based on the relative voltage between the Gate and Source pins. N channel Mosfets (like this one) conduct more current as the Gate becomes more positive with respect to the Source. In enhancement mode N channel Mosfets, the transistor is turned off when the Gate is at the same voltage as the Source. With depletion mode N channel Mosfets, current flows when the Gate and Source are at the same voltage, and it takes a negative Gate voltage to turn off the device.”

In simple terms (that I can understand), you don’t have to use a resistor to stop the Mosfet from self-destructing. But I think that NP moved to this design as it has subjective benefits (otherwise I think he would have kept doing things the way he had). I’m not going to try to equate how this relates to my subjective observations and I am putting it out there as some reasoning behind why the 2 amps may sound different. Nelson's opinion is that the lack of ballast resistor gives a more dynamic sound over his other designs that use enhancement mode and I am not one to argue. 

Practically we also get an amp’ that weighs half as much, uses half the electricity and has a similar power output. It runs about 5 degrees cooler at the heat sinks from my measurements but it also kicks out less radiated heat. The XA30.8 weighed about 45kgs, the XA25 is 21kgs (a lightweight!) – that was a back breaker so I can see people being drawn to the XA25 simply for reasons of size, weight and power consumption.

 

Running in:

Nelson Pass doesn’t seem to believe in run-in. Well I’m not going to pick an argument with him and other EE’s but I can only relate my experience. If you listen to a Pass Labs amp’ that doesn’t have 1-2 weeks on it then IHMO you are doing yourself and the amp’ a disservice. Of course I have no data to back that up but I do have comments from people who have popped in over the first month or so to listen to the amp’ and they’ve agreed. Plus a trust my ears.

When I got my XA30.8 it sounded like crap for the first week, sounded better on the 2nd and only started to properly come on song in the third week with some improvement over the next week after that. So 4 weeks all up. The XA25 was bit different. Rather than sounding gritty and anemic during run-in, the bass sounding flabby and weak. Everything else sounded OK but there was no magic. Glad I stuck it out.

 

My system:
I have an Audiophonics I2S streamer feeding into a Direct Stream Junior (DSJ) via I2S (HMDI). I also have a BHK Signature preamp - it is a hybrid design that uses mosfets paralleled to tubes - either 6922 to 12AU7 variants. For some of this listening I ran the DSJ without the BHK preamp because I wanted to hear it without any enhancement from the tube preamp. I have recently used a pretty sonically transparent Clay Geiseler preamp though as the DSJ isn't really happy driving a power amp' and there was more gain needed in the setup). So most of the comments are sans BHK preamp.

 

Fit & finish:

This a Pass Labs. So whilst it will never have the bling factor of some high end stuff, you expect there to be a high standard all round. The XA25 is a simpler, less over the top construction compared to the XA30.8 (good news for your back). It doesn't feel compromised though - there is a nice degree of workmanship on the both the visible parts and the internal parts. It looks the part and feels like it will last forever. Good quality components are in use. Those Mosfets are rated at 750 Watt RMS and 150 centigrade so there is the usual over-designed feel there too.

 

Some ownership notes:

This amp' weighs in at about 22kg which is light-weight in the Pass Labs scheme of things but is still worth noting. It is a standard 430mm or wide and but quite deep (about 42cm including the grab handles) and quite high at about 22cm. The big factor with all Class-A amp's is getting rid of heat so there are the obvious fins at the side that need to be kept clear of things. You should also try to leave as much room above the amp's as possible too. It gets up to about 50C in my office which is about 25C above ambient - that's right on the figure that Pass Labs specifies. If you place something near the unit then the heat will creep up and place anything over the amp' and you might start to reduce the life span of the components in the amp. 

 

Subjective observations:

I listen to wide range of musical styles but I gravitate towards electronic music so that will show in my choices of evaluation music. Electronic music is useful to show the character of an amplifier and its limitations too. You need acoustic music to reveal other things of course. So straight into it:

 

Imaging Width–

I was listening to Moderat (German IDM) and I realised that the cymbals were all around me. Of course this is a contrivance of the producer but the fact is that this never happened with the XA30.8. The sound stage did spread out quite wide on some tracks but on the XA25 it was enveloping.

 

Imaging Depth–

With the right source you can get great depth from the XA25. I think that's all about low level detail retrieval. The XA30.8 did depth quite well but I don't remember there being as much ambient detail. With the right tubes in my BHK Preamp (12AU7 Haltron Tunsgram Holland) you can get a soundstage that seems to go off into the distances. Fun stuff.

 

Imaging Accuracy–

Performers and sounds are precisely placed in space (height, depth, width). There is no etching around the images. 

 

Treble-

The XA25 sounded just right in the treble. It really projects treble detail without steering into any sort of sterility or exaggeration. This level of clarity makes low level listening quite satisfying. And it keeps its grip on the balance of the treble even when pushed – the XA30.8 started to get a little ragged when you gave it some wellie.

 

Midrange-

The midrange is probably where the most change happened over the run-in period. That is the magic that I hinted at earlier. No, it's not a tube amp' but it reveals a nice sweetness without any mushiness. Or as that guy over at 6Moons said "I like Pass Labs because they get so much of what tubes can do without the disadvantages of tubes". Don't take that the wrong way. I love tubes. Again, with the right source you can get a nice delicacy that draws you into the music.

 

 

Bass-

Bass started out a bit flabby and lacking in depth when it was fresh out of the box. It became more and more extended, tight and tuneful over time.

Bass definition – New Order – Blue Monday – the initial kick drum which punches away is well controlled, the woofers seems to stop and start much faster than they do with the XA30.8. Better still a little bit into the music, when the bass synth joins in, the elements are clearly differentiated. The XA30.8 did not do this in my system and once I’d heard this, I started to notice it in other tracks as well. Bass kicks (real drum or synthesised) are tight and very detailed and loud bass lines retain control even when the amp’ is driven hard. Hans Zimmer’s intro’ track to Black Runner 2019 has some nasty bass which borders on percussive – I played a game of dare to see how loud I could go before the XA25 lost control and I managed to get to 100 on my preamp. Higher damping factor I assume. By contrast, the XA30.8 would loss grip with an injection of what seemed like more 2nd order distortion - a deliberate move by PL of course and part of the character of the XA30.8.

Bass on the XA25 is anything but dry or mono-tonal. It has great pitch definition and it can give you a new appreciation for the skills of your favourite bassist. Any album with well recorded drums will show up the XA25's ability to pick apart bass lines, drum lines and the acoustics of the venue (or the artificial acoustic of a studio). Definite Pitch by Greg Bendian is a delight through the XA25 - you can hear the minute changes in pitch in the percussion in this album - play it through a lesser amplifier and it is a merely "interesting" album.

On a subjective level, the XA30.8 makes a fat, rich bass sound (if you wanted to resort to the stereotype, maybe a tube like bass). I found it captivating and it adds a physicality to music. This is a point where the XA25 is more contemporary in its design. Surely the relative popularity of high damping factor SS designs did not go unnoticed by Nelson Pass. So the bass is tighter and has more definition in the XA25 but doesn't have that rock solid foundation. Both are nice but I'd have to give it to the XA25 as I like to listen to fast electronic music where pitch definition is a necessity. Still I can still see the appeal of the XA30.8 and understand why someone would want that - try any classical music or Jazz or even live music on the XA30.8 and you'll hear what I mean.

 

 

Dynamics / speed (PRAT?)-

Even at low volume the first  song on the Rapor EP by Active Child actually made me jump as the vocals kicked in. That never happened on the XA30.8 or any other amplifier that I’ve had. I had a bit of fun with a stereonetter who came into my office – I put on the first track entitles Discombobulation off the Sherlock Homes OST. Turned up the gain about 2/3rd of the way on the preamp and watched him flinch as the sudden clap of percussion kicked in about 10 seconds into the track. That was a mean trick I know but it proved a point. The XA30.8 just didn’t do this – it swelled up and presented the peak of sound but it took a microsecond to get there. The XA25 seems to just get there. Also the blat of the horns on the track is crisp and realistic (without being in any way harsh or metallic) on the XA25. The XA30.8 plays it safe here – yes, you can hear the horns are bright but it rounds off the edges a bit.

This amp is quite fast. Not super fast like a Class-D but sufficiently fast to keep the propulsion of bass heavy music driving long.

Your feet tap along to the music so I guess that covers the "R". Timing - well it just sounds right - no lagging or hardened edges - just right.

 

Detail –

Yes, I admit I love detail. That was a characteristic that drew me to the amplifier and being very low distortion and having a low noise floor, there is lots of detail on tap but it does not get in the way of enjoying the music. This won't get to Devialet levels of detail (at least not with the source and cables that I am using) but you want the detail to serve the music i.e. you want it to inform about what the performer intended. 

 

Factor X-

Whatcha talkin bout Willis? Well Herb from Stereophile put it nicely (I paraphrase). He said, one of the guys who were gathered at one point listening to the XA25 said that it makes an album sound like an event. I think he was right on the money. Listening to studio recordings sucks you in and live recording helps you feel the atmosphere of the event. Then there is that top-tapping, leg jiggling, getting lost for ages in a song or in an album. Priceless.

 

Cables:

The XA30.8 really allowed me to hear differences in interconnects, digital cables and speaker cables and the XA25 is no different in this regard. Which of course is impossible as all cables sound the same ( :sarc:). Given the very low distortion and noise floor you would expect that this would be greater in the XA25. The XA25 runs on unbalanced interconnects which has less "audiophile" cred but the reason NP employed balanced was to lower distortion through the use of his Supersymmetry circuit. There's no need for that circuit here as the design has lower distortion and noise than any Pass Labs amp to date. Adding balanced inputs would perhaps unnecessarily add to the cost.Distortion in the XA25 is predominantly 3rd order with a bit of 2nd. The XA30.8 injects mainly 2nd, at least until it gets nearer it's power limits. The other thing is that in comparison, the level of distortion at low and moderate levels is a number of orders lower in the XA25.

 

With the BHK Preamp:
Everything just cranks up a bit. Bass is more powerful and seems more defined. Midrange is sweeter. Depending on the choice of tubes, treble can be varied to taste and mode. With the right tubes, the speed is amazing- Kraftwerk "Numbers" from "Minimum Maximum" just rattles off like a machines gun but each nuance of pitch can be followed. Backing vocals and ambiance sound great as you'd expect with tubes. 

This is a great preamp which I guess it should be for the price. It seems to operate like a studio-remastering process - cleaning things up - tweaking things, making the mix sound nicer. And it works really well with my XA25.

 

Conclusion:

I hope this hasn’t come off as a slag on the XA30.8. And I hope it doesn’t sound like someone trying to confirm their choice of purchases - I didn’t have to sell the XA30.8 though for practical reasons it made sense to. The price between a 2nd hand XA30.8 and a new XA25 is not a huge gap so I’m not trying to get over buyer’s remorse -  I made a reasoned decision and I am really happy I did it. Is this is the “best” Pass Labs amplifier? It is for me, maybe not for others.  Everyone will have their own answer based on their tastes, their system and their listening room.I could have made an argument on keeping the XA30.8 - it was beguiling and just had such a solid sound. It is also a piece of luxurious industrial design. The XA25 is a bit more middleweight in comparison (size, weight and subjective bass) but still packs a big punch.

Anyway, I have stopped the search for my perfect amp' (and the BHK preamp has ended my search for the perfect preamp too). I am sure there are amp's out there that surpass the XA25 - maybe even for less money but I'm just not interested in the chase any more - I've found an amp' that works for me and suits my tastes.

 

 

Description: 

Stereo power amplifier using JFETs and MOSFETs. Inputs: 1 pair single-ended (RCA). Outputs: stereo pair of Furutech binding posts.

Power output: 25Wpc into 8 ohms (14dBW), 50Wpc into 4 ohms (14dBW). Peak current: 10A (200W peak into 2 ohms). **These official figures are not 1% distortion levels. If we move the goalposts to 1% then power envelope becomes 80watts RMS into 8 ohms).

Bandwidth: DC to –2dB at 100kHz. Input impedance: 47k ohms. Voltage gain: 20dB. Distortion: 0.1% at 25W, 8 ohms, 1kHz.

Noise: <50µV output, unweighted, 20Hz–20kHz.

Damping factor: 500. Slew rate: 100V/µs.

Idle power draw: 2.0 amps at 120VAC (240W).** I tested this at 205 watts into 245V.
Dimensions: 17" (430mm) W by 6" (150mm) H by 17.3" (440mm) D. Weight: 45 lbs (20.4kg).

 

 

https://www.stereophile.com/content/pass-laboratories-xa25-power-amplifier-measurements

 

 

 

Edited by scumbag

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The specs seem similar to the M2 and the DIY M2X that I am building, I hope I am as pleased with the M2X as you are with the XA25.

Great write up, thoroughly enjoyed reading that.

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The xa25 is a funny beast. Whilst it is in some ways a more contemporary pass labs design it also has a lot in common with the first watt stuff too. Very clever on Nelson's part. JFET input stage, no feedback, single ended, simple design, fewer output devices (fewer then a traditional pass labs that is) and most importantly, depletion mode mosfets that eliminate "that other type of feedback". https://www.stereophile.com/content/nelson-pass-circuit-topology-and-end-science

I hope you get a much enjoyment from the m2x as I have gotten from the xa25. 

Edited by scumbag

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@scumbag Great review, very thorough.

 

As a very happy XA30.8 owner I was wondering what the difference was between them. The XA30.8 has some sort of hold over me which I can't explain, I just enjoy the amp. I think it is possibly because I used to have valve amps and their are similar characteristics.

 

I agree with your comment of running in of PL amps before critical listening. This is critical, my XA30.8 changed dramatically after running in. Likewise letting the amp warm up (a minimum of 1 hour IMO) has a significant effect. 

 

 

 

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Just now, Whites said:

@scumbag Great review, very thorough.

 

As a very happy XA30.8 owner I was wondering what the difference was between them. The XA30.8 has some sort of hold over me which I can't explain, I just enjoy the amp. I think it is possibly because I used to have valve amps and their are similar characteristics.

 

I agree with your comment of running in of PL amps before critical listening. This is critical, my XA30.8 changed dramatically after running in. Likewise letting the amp warm up (a minimum of 1 hour IMO) has a significant effect. 

 

 

 

Yes, the first hour to me sounds recessed and weak.

The run in for the XA25 didn't seem as dramatic as the XA30.8 but I think the changes between the "before" and "after" are just as profound. When I got my XA30.8 I thought I'd made a big mistake in the first week. It was grainy, dull and sounded like a cheap 1990's Japanese transistor amp'.

The XA30.8 does suck you into it's cosy little world of music whereas the XA25 draws you in via the vitality of the performance.

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@scumbag or should I say Harry from Sea Cliff. Great review.

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Posted (edited)

Thanks Scumbag,

 

Good review. I had the pleasure of listening to an XA30.8 recently and your words do nothing to dissuade me from trying to listen to the XA25 as well.

 

I may need to make the trip to Sydney Class A here in Melbourne didn't have one on the floor last time I asked.

 

Cheers, Nick

Edited by nandrzej

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