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Pass Laboratories XA160.5 Monoblock Amplifiers Review

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Pass Laboratories XA160.5 Class A monoblock amplifiers

All Business

There is nothing funny, however, about the functionality of the XA160.5. The face is bereft of buttons save for the proportionately large POWER button directly below the Bias meter. The manual explains the meter, which sits fairly motionless during operation, “The meters on our amplifiers are different. They reflect the current consumption of the amplifier, and when the amplifier is operating, they don’t go down to zero like the meters on other amplifiers. This is because the electrical current consumption of our circuits has a fairly high value at all times, a property called the bias. The bias current runs through the amplifiers at a minimum value, determining the class of operation – Class B, Class AB, or Class A.”

This is a wonderful tool for reinforcing obsessive compulsive behavior in the Tweaker audiophile! There is nothing more fun while listening than staring at a Bias meter’s needle to make sure it never leaves perfect Class A output . What is an avid listener going to do if it does wiggle –- put in a new electrical service or spend several thousand on power conditioning gear? I suggest you ignore the meter to the best of your ability. Unless it has obviously strayed from its normal position, around 12 O’ Clock, you will be hard pressed to discern audible differences in the music.

You can also ignore the fact that the amp uses a 15A IEC versus 20A. While the X1000 uses a 20A power cord, Pass Labs does not use such on its other models. Nelson relates, “…we deliberately insert resistance in series with the transformer primaries, so the amplifiers do not depend on the presumption of a low impedance AC line.”

Do not, however, make the mistake of ignoring aftermarket cables. Does the fact that the amp is so well designed to be immune to vagaries in the power supply negate the need for superior power cabling? Not at all! I had a discussion in an online forum in which an electrical engineer stated that an amplifier such as a Pass is unaffected by power cable swapping –- in fact, he used this very amp as his example! As the XA160.5 was under review I did not want to discuss it publicly, so I played down the conversation and drifted away. I would have loved to say, “ARE YOU DEAF?” I suspect this individual has never used anything but a stock power cable, or has profound hearing loss –- or both! I think it an absurd conclusion that if an amp is more robust and better in overall quality that it should be less sensitive to such things as cables, as though they won’t make one whit of difference. If that is what a better amp does, I want no part of it! That’s the equivalent of saying, “This sports car has been designed for such superior performance that it doesn’t need good tires!”

Maybe Nelson is not laughing now. No matter, as he is a gregarious and generous guy, able to accept that the audiophile tent is large enough for individuals with disparate viewpoints. I do not worship at the feet of any designer; I use their gear and report what I hear. I hear distinct differences in presentation when swapping power cords with the XA 160.5. The real test for me is listening, and once again my suspicions were confirmed. The Pass XA 160.5 reacts to power cable changes just as every other amp with an IEC. This should not be surprising. It also was not surprising that the Wireworld Silver Electra power cord was cooler and more tipped-up in sound with the XA160.5 than the Clarity Cable Vortex, which is a higher total-gauge, all copper design. This outcome is in line with every previous use of these cables. The amp is very clean and not particularly weighted toward the bass. I strongly suggest prospective owners in their search for power cables that they consider especially the total gauge and conductor material. If you are not preferring a more lightweight, ultra-airy sound quality, you may want to avoid silver conductors with this amp.

My experience was that this amp cannot be made to carry disproportionately large amounts of low-end dynamic wallop merely by swapping power cords. If nothing else, the XA160.5 is balanced top to bottom in terms of dynamics. It is not a heavy-handed amp, and it will not give the impression as though it has disproportionate low-end power as compared to the midrange and treble. Therefore, if the audiophile’s goal is to have a disproportionately powerful low-end with massive bass then source, preamp and cabling must be carefully selected to extract the maximum in terms of this effect when using the XA160.5.

Nelson’s design parameters

In discussion of the XA160.5, Nelson gave me some insights as to the particular pedigree of his Class A designs. His guiding principle in design is, “…simplicity and linearity, backed up by heavy hardware.” A case in point, most of his designs have been refined continuingly since his Threshold days; at that company his Class A designs were 5-staged whereas Pass amps are 3-staged. Threshold Stasis amps used local feedback only, while Pass X Series amps use local and global feedback. Pass Labs’ patented, “Supersymmetric” topology is employed as well as torroid transformers, says Nelson, “We spec transformers with lots of saturation margin and deliberately insert resistance in series with the primaries, designing our amplifiers circuits around this.” The result is more independence from the AC line character: “The amplifier’s capacitors do not depend on the presumption of a low impedance AC line.” The power supplies are big, classic, computer grade cans, while caps which see the signal may be silk, polyester or polypropylene electrolytics, all of which are vetted through listening in a process nearly as daunting as the selection of a Supreme Court judge.

A warning. Owners are cautioned to refrain from using active line conditioners, “The amplifiers draw their energy on peaks, and that means that the peak is a multiple of the average current. If a line regulator does not offer this extra reserve, then there will be problems.” Over the years’ use of many power conditioning or filtering systems has convinced me that it is not a guarantee of superior sound. Certainly, the XA series is robust enough that use of power conditioner is not an absolute necessity. It is one thing to use power protection circuitry, if so desired; it is quite another to build a system for ultimate performance. To capitalize on the elegance the XA160.5 affords, I did not use power conditioning or filtration.

Operational features and a frustration

Returning to the amp’s physical description, classically large and visually forbidding heat fins adorn the left and right sides of the amp top to bottom — another reason why one might want to get help moving the amp. As I moved it about in shorts between amps and speakers, a gentle grazing of the heat dissipation fins by my leg could’ve drawn blood. Use caution when walking near them. This should not disparage the potential owner, as once the amps are in place most audiophiles will not be climbing over them to change cables or swap out a DAC. The XA160.5 is hardly the only amp with sharp edges, but one needs to beware before an accident occurs.

The rear of the unit is clean, showing the somewhat thin handles, although a set of handles on the front would be most welcome ergonomically but the eye candy appeal would suffer, dual sets of output binding posts, RCA and XLR Inputs, grounding post and 12V trigger terminals. I was surprised by the 15A IEC and the relatively restricted travel of the binding posts.

The binding posts constituted a frustration, not by their operation but by their design. There are specific situations in audio when wonderfully designed components have idiosyncratic incompatibilities due to their build, and this was one of those instances. It pertained to the incompatibility of the amp, the cables and the speakers. I’m glad it happened to me and not you…

The Manual notes that banana plugs are banned by regulatory agencies in many countries, so Pass abandoned using them. Using a baseball analogy, that’s strike one, due to the regulatory agencies overseas. It precluded my using a set of banana and spade terminations for bi-wiring from one set of the amp’s terminals. The XA160.5 provides two sets of output terminals, which for typical users will be plenty. However, one of my reference speakers is the Legacy Audio DSW, a tri-wirable/ tri-ampable design which can accommodate three sets of cables in either mode. Strike two, the requirements of a speaker which is tri-wirable.

Normally, this would be easily handled by the use of a set of “shotgun” speaker cables which are single at the amp end and doubled at the speaker end, or by using two sets of spade terminated cables on one post. However, the design principles for the Clarity Cable Organic speaker cable precludes a standard bi-wire cable, and the solid copper spades employed are thicker than normal. In addition, the nuts of the binding posts of the amp have limited outward travel meaning only one set of spades will fit even if the spades are thinner! Even thinner Wireworld spades doubled up did not fit onto the speaker posts. Strike three, the necessity of three sets of spade terminations incapable of fitting into space for two.

Who is to blame for all this incompatibility? Neither company, it’s simply the occurrence of an attempted match of two radically differently designed products. It happens. I am a patient man but such things drive me nearly to distraction! Of all the possible amp/speaker/cable combinations in the world! Incompatibility between components of a system is not that uncommon. It’s not unusual at shows for a manufacturer setting up a demo rig to be scrambling in the background to address an unforeseen issue pertaining to incompatibility between components. There are so many unique facets to a product’s design, and so many unique permutations of systems, that somewhere along the line a setup problem will occur. Audiophiles who have worked with many different models and brands will likely have run into this frustrating, but often not intractable, problem. It is in your best interests to explore a component’s operation fully in the system it is being used prior to purchase.

As a stopgap measure, I employed the Wireworld Silver Eclipse shotgun bi-wire set on two sets of the Whisper DSW’s terminals and the single run of the Clarity Organic on the other. This was an intolerable long term solution as the Wireworld and Clarity cables are very different sounding. No matter which arrangement I used there were glaring inconsistencies in the sound. I was not willing to settle for less than ideal, so I contacted Clarity Cable with a special request – would they send me a custom “parallel” bi-wire set of cables for reviewing? Yes, they would; it consists of two regular speaker cables co-joined, jackets and all, with their terminations mated at the amp end with individual sets at the speaker end. While this is unusual, it accomplishes the same purpose as a shotgun cable, only better as an entire cable’s gauge is dedicated to a driver set. Thus, I have come to rely heavily upon Clarity Cable in reviewing. I wouldn’t want to be without the Whisper DSW or the Organic speaker cables in assessing the XA160.5.

When the parallel bi-wire set arrived I knew instantly upon hearing them that I had made the right decision. The system snapped into tonal harmony and no aspect of performance lagged. The improvement in coherency between the driver sets of the Whisper DSW was quite noticeable. It could easily be heard now that the amp has a sultry, smooth character with fantastic transparency. In my review of the Simaudio Moon Evolution 750D DAC/Player, I made the following comments regarding the nature of the amp, and I repeat them here as they are worthy of consideration:

The Pass XA160.5… carries its weight lightly, something most solid state amps do not accomplish. The sound of the unit is disarmingly sweet in the way that a low power amp with a high efficiency speaker sounds sweet. Do not misunderstand; by “sweet” I do not mean prissy, but rather succulent. My in-laws have a grain farm in Manitoba and every time we visit we dine on local beef – wheat fed beef which is very sweet tasting in comparison to corn fed beef. The descriptor succulent perfectly describes the far richer taste of the meat, and succulent is an apt descriptor of the richness of the XA160.5’s sound.

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