Welcome to my DIY audio forum.

Per-Anders Sjöström

Main Menu

Review Pass Labs XA100, Xono, X2.5

Started by peranders, September 10, 2005, 11:25:55 AM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.


The danish magazine "High Fidelity", Swedish edition, issue 6, 2005, page 40-49, ads in page 43 and 47.

This review is translated into English but some parts are not exactly but I have tried to write so the meaning get through. In a few places I don't know what the writer means really although it is in Swedish. A part of this problem is that the original is in Danish.

Per-Anders Sjöström

Thanks to Hans Buhl Houmøller, Denmark who translated page 48 and 49.

Well, here we go.

-Page 40 -

Pass Labs - A real American

Nelson Pass has the ability to design high-end which both visually and sounding meets up even to the highest expectations.

By Jan Nielsen (Danish original text)
and Michael Madsen (measurements)

In the hifi world there are some great personalities we all know about. Without any thought we can mention names like Aik van den Hul, Bob Carver, Mark Levinson, Ivor Tiefenbrun and Jean Hiraga but of course there are many more. One of them is the American Nelson Pass which through the years has come up with many solutions how good sounding audio equipment should be built.

His present company Pass Laboratories was founded in 1991 but it all started in 1974 when Nelson Pass started the company Threshold. They produced real big amps which at that time belonged to the coolest real high-end products.

It was also at Threshold he developed the so called Stasis principle which later also (under licence) was used in a couple of well-reputed amps from the Japanese Nakamishi.

The last years Nelson Pass has come with a more or less in two divided concept for his business. One of the side is normal produced products of highest class and the other side is movement of do-it-yourself which he supports in an almost unbelievable way. How many high-end manufactures invite people to build their products by themselves?

At Pass Labs website ( P-A's remark) there are complete building directions for preamps and also very brutal single ended amps. The last mentioned with a huge power consumption which most is gone away in big heatsinks.

You can also find Nelson Pass in various news groups, such as (which he has an own forum, P-A:s remark) and he answers questions in a good mood. You can wonder where he got all his time.

*The ready made products*
If you check the products from Pass Labs we see also that there are a couple of real cool loudspeakers. The latest efforts is a transconductance amp. Nelson Pass calls it for "First Watt" because it is aimed for highly efficient fullrange speakers. "First watt" uses "current mode". They have an output impedance of 60 Ohms which you of course must consider. The concept is very different and rare but in some cases also more right. You can read more about it at

-Page 41 -
The subject for this test is completely different, something that can drive regular speakers. At the beginning we wanted to take a look at a Pass Labs amp but we got a complete kit consisting of X2.5 preamp, Xono, RIAA amp and two mono blocks called XA100. X2.5 isn't the top of the line from Pass Labs but this doesn't it make it worse. We listened to all apparatus but we did concentrate ourselves to the XA100 power amps.

* Very American *
It's easy to see the high-end products which are coming from the US. Most of the time the appearance should be big, fancy and dangerous. They don't deny themselves here either with a enclosure which is one of most brutal we have seen in a long time. X2.5, the preamp, is built of thick aluminium profiles which contributes to the high weight, -  yes this is "heavy hifi". Xono comes with the same enclosure but here are two of them at 6.5 kg (14.3 lbs) and 8.5 kg (18.5 lbs) (preamp and power supply).

The power amp has a front plate which in some places is 45 mm (1.77") thick. This is quite impressing but at the same time almost vulgar. The are lot's of sharp edges and corners which requires great respect. They get also hot- very hot!

* Unusual operating *
X2.5 has a bit unusual operating and requires some adaption. At the front there are only one big dial which can be used for lot's of things depending of which menu you are in. This dial has got company of four push buttons which are for various choices which are volume, inputs and balance. Pratically it's a bit tidious that you first must push "input" then choose which input. The same thing goes for the remote control so also here you need some practicing.

At the backside you can fins five inputs and three of the are doubled with balanced inputs. The XLR connectors are equipted with shorting jumpers which must be used when you are using the unbalanced inputs. The output has also both unbalanced and balanced connections. At last there are also a DC output which can switch on the Pass Labs power amps.

The backside of Xono is very simple. A sturdy cable is connected between the amp and the external power supply. Besides this connector there are also inputs for MC cartridge and MM cartridge but you can only the one type of cartridge at the time. The output is balanced or unbalanced.

* Lot's of metal *
The power amps are impressing designs. The big round meter at the front and the blue light radiates mystique and is very appealing. Many people almost fall into some hifi trance condition - and can almost touch the feeling of having money flying from your bank account - you only must have this amp.

The meter only shows the bias current so here when it's about class A the needle will be more than half deflection of the full scale. Some claims that the needle will move if the amp is push extremely hard but we haven't seen anything of this. Under this big blue meter there is a push button for standby which normally is used. The mains switch is located at the backside. What the standby switch actually does we don't know but we suspect that the input stage and also the power supply is connected because we don't experience any "lights blinking" when the amp is powered on via the standby button.

-Page 42 -
At the backside there are both a balanced and an unbalanced input. The doubled output terminals have comfortable big wings but they can sometimes be in the way for big angled fork connectors, like WBT. With NLE (Danish brand, P-A's remark) there were no troubles at all.

When the amp belongs to the new "XA" series, it's all a simple circuit solution but in the same time optimised. It works in balanced single end, class A and is in reality two amps in a push-pull connection, a sort of internal bridge connection. It's a combination of the big Pass Labs amps from the X series and the well-reputed Pass Labs Aleph series which now is discontinued. The XA series is described as a combination between the output stage of the Aleh and the patented "super symmetry" X input stage. The goal with the XA series was to give priority to sound quality rather than power and also get a combination of the best from the both amps.

XA100 is specified to deliver 100 watts into 8 ohms but with the working principle it means also that you will get only marginally more power at lower speaker impedance. If you insist to use "difficult" speakers it's possible that the amps from the X series is a better choice. Normally the XA amp should be the optimum choice.

XA100 is 483 mm (19") wide, 240 mm (9.45") high and 560 mm deep, including handles on front and back side. The weight is 40 kg per amp.

* The small - without any lids *
The enclosures for the preamp, RIAA amp and the power supply is made of 6 mm (1/4") thick aluminium plates and brought together with 25(1") x 25(1") mm corner profiles. The cases are nearly as solid as a safe.

We started with the RIAA which you also are forced to get open if you want to change the cartridge load and sensitivity. Everything is well explained in the good written manual. There are two DIP switches for the adjustment of the cartridge load and jumpers to change the gain. The whole amp is built on a big pcb and there are there sections, one for the left channel, one for the right and the power supply.

Here we have the power supply consisting of (which already is fed from a well filtered external power supply) with a impressive regulator plus the amp consisting of JFET's, all discrete design (no IC's!). The quality of the parts is very high and the same can be said about the quality of the whole apparatus. It is manufactured by people who has real control over how high-end should be built.

The external power supply could be in a much smaller enclosure but it must also fit in with the RIAA amp. Internally there is a big toroid transformer, separate rectifier bridges and 40000 uF in smoothing per channel which forms a +/- power supply. In addition to this there is also a smaller transformer with rectifier bridge and 2 x 3000 uF. The power supply is connected to Xono with a 2 m long cable with something which looks like PC connectors (D-subs, P-A's remark).

-Page 44 - (Page 43 ads)
The preamp X2.5 has a big pcb with amongst other things power supply, and 12 input and output relays. At this main pcb there are also two pcb's which contains active circuits for the volume, built by lots of SMD parts such as FET's and belonging control circuitry. The analog amps have also two pcb's with SMD parts and also here it is a very neat and tidy design. At last we have a pcb for the remote control which is located at the back of the front panel. X2.5 give the impression of being a very competent design where the well accomplished building efforts fills up the whole enclosure with parts.

* Heat generator *
In the big mono block we have a bit different building design, when the big heatsinks almost carries the whole design. The fins of the heatsink is turned wrong so therefore there are almost no convection at all and because of that the whole enclosure, handles and the front plates become very warm, even hot. High temperatures are never any positive property for achieving a long lifetime of the amp.

My first impression is, "no, so simple it is". Sure, it's simple but the extra dimensioning makes it's quite big.

-Page 45 -
The transformers are 600 VA and despite that the 120000 uF smoothing battery only is rated to 50 volts it's enough due to the internal bridge connection topology. The DC voltage is fed via assumed rather small inductors up to the pcb's located at the big heatsinks. Furthermore those pcb's have some additional 10000 uF caps along with non-polarized caps.
This is in other words a CLC design which normally only seen in tube amps.
On each pcb there are 10 HEXFET output transistors. One of the pcb's has small SMD FET's, located at the input. Somewhat surprising is the AC coupled via 100 uF/35V caps to the balanced input stage. The caps are surely of high quality but it's still a bit peculiar to found electrolythic caps in the signal path in a so pronounced high-end product.

In total everything seems to be very good built and very well thought but it's also a bit confusing with all those loose wires crossing the pcb's.

* Starting problems *
X2.5, the preamp some problems with audible buzz in the speaker (my theory is grounding problems, American unit in Europe, P-A's remark) in combination with our Auccuphase A60 power amp. We tested everything: Ground, not ground, turning mains cables, balanced, not balanced and also a insulation transformer. Nothing helped. The only thing that help was a Nordost Thor mains filter, which we will test in a coming test. When this filter was connected we could hear a very small buzz which not could be heard from the listening position. We must also admit that we had very sensitive speakers (approx. 100 dB at 1 W and 1 m) but we have not had any troubles with any other preamps.
The connection of the Pass XA100 was done with expectation that the buzz problem was gone,  but not! On the contrary, the buzz problem was even bigger. With 88 dB speakers there wasn't any real problems.
It's worth mentioning that the XA100 had no trouble with other preamps we had at disposal.


* X2.5 - calm and excesses *
Solidness and obviousness are two properties of the sound coming from the X2.5 preamp. The sound quality has it strength in the original fundamental tone area, not the subwoofer octaves and the highest treble, which is not immediately making any fuzz. There is an obvious power and presence over the reproduction.

Voices are experienced as located a bit in front of the sound picture, with lots of body and details, when the instruments are primary delivering music. X2.5 has no tendencies to exaggerate or make the message look nicer, plays soberly and clear sighted. The upper top and also to some extent the bottom is somewhat placed in the background. Not that we miss anything but we still experience at bit cautious character and also very "dry" impression. X2.5 manage all the difficult with great selfconfidence, even then most complex passage as the easiest task in the world. It refuses to sound unpleasant and almost boasts with it's calmness and excesses.

-Page 46 -
In other words music is presented with lots of output in frequency extreme or complex music compositions, just like a unity, one part, when everything is there without any unnecessary focusing on any particular areas.
Reference CD No. 70, track 3, Bliss with Evening sun, have we heard many times but via X2.5 we'll get a directness and a overview which makes the music even more well tempered and direct in it's approach. The voice is well defined but also a bit naked without any help from pronounciations or selfproduced sounds. All is delivered in sober and correct way. You could almost say that X2.5 demands some help from other components in order to add a bit roundness and warmth.

We believe definitely that you can achieve good results with X2.5 together with other equipment but with there is a special synergy together with the power amps from Pass. This may not be so peculiar because at Pass Labs they have listened very much to this particular constellation.

* ... and with Xono *

-Page 48 -
There are inputs both for MM and MC cartridges but you can't use them at the same time. This way you have the advantage of avoiding switches in the signal path. During the use of this unit we didn't experience any noise – buzz or hiss. You find yourself unwillingly playing a little louder than normal, because everything seems so effortless and fluent. The sound is almost tube like with a wonderful effortless and spontaneity. Yes, those are the exact words, because everything sounds as if the music flows through the RIAA without being squeezed or forced. Everything seems easy and with plenty of energy reserves. On the classic "Sultans of Swing" from Dire Straits, the guitar pickings has new life and dynamics. At the same time there is a spontaneity that would prove characteristic for everything we played through the Xono. On Jennifer Warnes "The Well" it is the size of the soundstage and the treble range that draws your attention. Once you hear how a good pickup through this MC/RIAA manages to reproduce the highest octaves with weightlessness and plasticity, you're sold. We can very well understand why many regards the Xono as being amongst the best available on the market. Though, the Xono can seem a little more cool and sober than most of really good tube RIAAs that typically uses a MC transformer at the input. Compared to those, the Xono, as well as the X2.5, can be said to lack a little bit of warmth and roundness, but on the other hand you get almost all other qualities that you can think of. The Xono will be missed on our shelf.

* Pass Power *
After about one hour of heating up, the two XA100 are ready. Not just ready, but impressing from the first notes, with a large, very open soundstage, where especially the important midrange is wonderfully present and three-dimensional. Musicians seem physically present in a way we only have heard from the best single ended triode amps. Already now we have found these amps strong side. They simply have an almost organic way of reproducing the whole large fundamental keynote range – the same qualities we found in the X2.5 preamp, but still in a slight different way.

Here we have a little bit of the roundness and warmth, that very distinguished completes the preamp.

Take for example the new CD with Allan Olsen/Gæst (Danish artist). It is very well recorded, and you very much hear recording room, and wholeness in a very captivating and "poisoning" way. By poisoning we mean in relation to everything else we have heard. This is very much the well known openness and spontaneity you hear from the very best tube amps, in a way that make these amps a dangerous competitor to almost all other amps in the very highest class.

-Page 49 -
Moving up in the tonal spectrum, it's safe to say that the XA100 is a little bit forgiving. It never sounds direct or harsh. It's laid back and compared to other alternatives in the price range also a bit on the nice side. You can also say that it holds back a small part of the details. We are talking about wholeness instead of focusing on single instruments or parts of the music. We have heard our ribbon tweeters deliver more air and detail, but again, this was with amps in the same price range as the XA100. Another way of putting it is to say that reverberation of the recording room fades out more rapidly. It's like there's missing a little bit of air in the highest octaves.

A good example of dynamics and space is High Fidelity Reference CD No. 78 (the magazine's own CDs that used to come with every issue, not anymore...) track 5 and 7 with "Tango Orkesteret" and "Freedom". Both tracks offers lots of details and dynamics – micro dynamics as well as when the whole soundstage is crashing in to your living room. Here the XA100 has a good grip on everything. XA100 focuses on the instruments and their messages, and also more on the space around the music, instead of the tiny details. Dynamically there is also a little bit less "jump-effect" than we experience from our Accuphase A60, which has its focus on the dynamics and liveliness in the music.

On the other hand the XA100 has the ability of placing a guitar more realistic in the soundstage, with lots of space and air to the imaginary rear wall and the other instruments. The XA100 excels in two areas – the liveliness and the organic in all instruments and the reproducing of space. Especially on the organ recording, track 7, where the large room opens and expands a bit into the track. The speakers simply vanish in the soundstage - that is if they have the necessary quality. Here you can say that XA100 is demanding, because they definitely deserve a good pair of speakers.

Now that we are at it, lets take the bass. With the available speakers there certainly weren't any problems, but again it's an organic and very musical reproduction rather than a reproduction that seeks to impress you. Electronic music as for example Yello is handled nicely, but its not here that the quality is revealed. Instead take a large orchestra where everything suddenly explodes. It's this ability to expand and at the same time preserve the overview in the music that the XA100 masters. Speaking of classic music, this has to be the amp of dreams for opera fans. Listening to Cecilia Bartoli brought tears to our eyes. With music of this character you easily forget to listen to what the amp does and does not. Here you simply enjoy the music, which, in the end, has to be the meaning with it all.

No doubt about that it's the XA100 that makes the icing on the cake in this trio. These impressive mono stages don't belong in the analytical and 100% neutral part of the high-end world, but on the other hand you get a present and three-dimensional reproduction that's unheard of. You have the perfect combination together with the Pass Labs own X2.5 preamp. We can only imagine how it will sound with the even better alternatives in the Pass Labs selection of preamps.
Xono offers the best conditions to even the most expensive pickup and turntable. We have a hard time thinking of what more to wish for – in any case it would only concern use and facilities. We have absolutely no complains about the sound.

All in all, these products from Pass Labs have been a great experience, visually as sonically, exactly as it should be with real high-end gear

-Page 44 -
* Measurements *

- Xono Deviation from RIAA standard No. 1 -

Pass promises an exact RIAA correction. That is very true, less deviation than +ä- 0.1 dB - excellent!

- X2.5 Frequency response, volume control No. 2 -

Pass Labs X2.5 has an upper limit  (-1dB) of just above 20 kHz and this is independently of the volume setting or if the gian is "high" or "low".

- X2.5 THD+N 10 Hz - 200 kHz No. 3 -

Indepedently of if the gain of the amp is "high" or "low", the THD+N is well under control at 1 V in /1 V out. At "low" and 600 ohms load the output become only 0.59 volts at 1 V  in.

- X2.5 THD+N 0.1-30 volts, 1 kHz No. 4 -

At any apparatus constellation the distortion and noise ought be be kept at minimum with this pre amp in the equipment.

- X2.5 FFT 1 kHz Gain "high" No. 5 -

At 2 volts out and a load of 100 kohms the THD+N is virtually non-existent. The distortion is only 0.0035% caused by the first harmonics at - 105 dB!

- X2.5 FFT 1 kHz Gain "low" No. 6 -

A bit surprising to see that the unlinearity increases when the Gain changes from "High" to "Low", but still, the distortion is only 0.007%

- XA100 THD+N 1 kHz, 1-200 W No. 7 -

The XA100 mono blocks don't behave like other similar mastodonts. The THD+N increases slowly and evenly without the usual sharp increase at the end when the amp clips.

- XA100 THD+N 1 kHz, 1-200 W, without load No. 8 -

The reason why the THD look like this even without load is because of the voltage amplification.

- XA100 THD+N, 10 Hz-200 kHz, 50 W No.9 -
The class A Hexfet amp has a tendency to increased distortion at the highest frequencies. At lower frequencies is 50 W delivered preferable at 4 or 8 ohms load.

- XA100 FFT 1 kHz, 1 W No. 10 -

At 1 Watts of output power the FFT spectrum is comfortable free from higher harmonics. Only first and second harmonic is visible at -91 and -93 dB, which is the same as 0.007% THD+N

- XA100 FFT 1 kHz, 50 W, 8 ohms No. 11 -

At 50 Watts at 8 ohms load the complete spectrum of harmonics will be there. Notice the repetitive pattern of harmonics. THD+N is now 0.23%.

- XA100 FFT 1 kHz, 50 W, 4 ohms No. 12 -

At 50 Watts at 4 ohms load the FFT spectrum looks almost the same as at 8 ohms but now even order harmonics is more accentuated.

- XA100 FFT 1 kHz, 50 W, 2 ohms No. 13 -

Of this along with graph no. 7 shows that Pass XA100 is not particular happy about load less than 4 ohms. THD+N is 1.4%

- XA100 Impulse No. 14 -

Impulsewise there is room for 40 V peak into 8 ohms, which is 5 A peak. 4/2/0.6 ohms is limited to approx. 9 A.
/Per-Anders Sjöström, owner of this forum

Homepage with my DIY hifi stuff