Pass Laboratories XA60.8 monoblock power amplifier Measurements

Sidebar 3: Measurements

Before performing any measurements, I ran one Pass Labs XA60.8 (serial no. 28984) for an hour at one-third its specified maximum power of 60W into 8 ohms—thermally, the worst case for an amplifier with a class-B or -AB output stage. By the end of the hour, the top panel was warm, at 96.4°F (35.8°C), and the side-mounted heatsinks were a little hotter, at 109.1°F (42.8°C). (Temperatures were measured with a Mastercool infrared thermometer.) The XA60.8 ran a little cooler than the XA60.5 I reviewed in January 2014, probably due to its larger heatsinks.

I performed a full set of measurements using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). The voltage gain at 1kHz into 8 ohms was 25.3dB for both the balanced and unbalanced inputs (for unbalanced drive, pins 2 and 3 of the XLR jack were connected with a jumper), meaning that the XA60.8 will output 1W into 8 ohms with a drive signal of 155mV. The amplifier was non-inverting (ie, it preserved absolute polarity) with both inputs. Its input impedance was usefully higher than that of the XA60.5, at 49k ohms unbalanced and 100k ohms balanced at 20Hz and 1kHz, dropping to 38k ohms at 20kHz for unbalanced signals. (The balanced input impedance at 20kHz was 92k ohms.)

The XA60.8's output impedance, including 6' of speaker cable, was 0.07 ohm at 20Hz and 1kHz, rising to 0.1 ohm at 20kHz. The modulation of the amplifier's frequency response, due to the Ohm's law interaction between this source impedance and the impedance of our standard simulated loudspeaker, was just ±0.1dB (fig.1, gray trace). Into an 8 ohm resistive load (blue trace), the XA605's response was flat up to 20kHz, then rolled off to reach –3dB around 120kHz. The response rolled off a little earlier into lower impedances, but was still just 0.3dB down at 20kHz into 2 ohms (red trace). With this wide a small-signal bandwidth, the amplifier's reproduction of a 10kHz squarewave into 8 ohms featured short risetimes and a well-squared shape (fig.2).


Fig.1 Pass Labs XA60.8, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red) (0.5dB/vertical div.).


Fig.2 Pass Labs XA60.8, small-signal 10kHz squarewave into 8 ohms.

The unweighted, wideband signal/noise ratio, ref. 1W into 8 ohms and taken with the input shorted to ground, was an excellent 80.9dB. This improved to 81.8dB when the measurement bandwidth was restricted to the audioband—and further still, to 93.6dB, when the measurement was A-weighted. This is a quiet amplifier. Fig.3 indicates that both the odd and even harmonics of the 60Hz power-supply frequency were present, though these all lay at or below –100dB ref. 1W into 8 ohms.


Fig.3 Pass Labs XA60.8, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (linear frequency scale).

Specified as putting out 60W into 8 ohms and 120W into 4 ohms (both 17.8dBW), the XA60.8 considerably exceeded that power, delivering, at 1% total harmonic distortion (THD), 150W into 8 ohms (21.8dBW, fig.4), 240W into 4 ohms (20.8dBW, fig.5), and 380W into 2 ohms (19.8dBW, fig.6). The THD begins to rise above the noise floor at high powers, but remains at or below 0.1% below the specified output power. The percentage of THD then slowly rises with increasing power, suggesting that the XA60.8 has only a small amount of corrective feedback.


Fig.4 Pass Labs XA60.8, distortion (%) vs 1kHz continuous output power into 8 ohms.


Fig.5 Pass Labs XA60.8, distortion (%) vs 1kHz continuous output power into 4 ohms.


Fig.6 Pass Labs XA60.8, distortion (%) vs 1kHz continuous output power into 2 ohms.

I examined how the percentage of THD+noise changed with frequency at 9V, a level where I could be sure, from the earlier measurements, that I was looking at actual distortion rather than noise. The THD+N was extremely low in the midrange into 8 and 4 ohms (fig.7, blue and magenta traces), and only just above 0.1% into 2 ohms (red). It rose linearly as the frequency increased, the three traces converging in the high treble—which suggests that the circuit has limited open-loop bandwidth.


Fig.7 Pass Labs XA60.8, THD+N (%) vs frequency at 9V into: 8 ohms (blue), 4 ohms (magenta), 2 ohms (red).

Fortunately, the XA60.8's distortion is predominantly the subjectively innocuous low-order variety. Fig.8 shows the waveform of the THD spuriae at two-thirds the specified power; it suggests that the third harmonic is predominant, though at low frequencies at this power (fig.9) the second harmonic is highest in level, at –72dB (0.02%). With a 1kHz signal, the second harmonic remains at this level, but the third has risen to –63dB (0.07%, fig.10). Some higher-order harmonics can also be seen in fig.10, but all are much lower in level than the second and third harmonics.


Fig.8 Pass Labs XA60.8, 1kHz waveform at 20W into 8 ohms, 0.072% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).


Fig.9 Pass Labs XA60.8, spectrum of 50Hz sinewave, DC–1kHz, at 40W into 8 ohms (linear frequency scale).


Fig.10 Pass Labs XA60.8, spectrum of 1kHz sinewave, DC–10kHz, at 40W into 8 ohms (linear frequency scale).

As with the XA60.5, the XA60.8's top-octave decrease in linearity seen in fig.7 was associated with some higher-order intermodulation products that were fairly high in level when the XA60.8 was asked to drive an equal mix of 19 and 20kHz tones at a level just below visible clipping on the oscilloscope screen (fig.11). However, the subjectively more objectionable second-order product at 1kHz is almost 20dB lower, at –78dB (0.012%). At lower powers, the intermodulation products were all much lower in level (fig.12).


Fig.11 Pass Labs XA60.8, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 40W peak into 8 ohms (linear frequency scale).


Fig.12 Pass Labs XA60.8, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 2W peak into 4 ohms (linear frequency scale).

The measured performance of the Pass Labs XA60.8 is very similar to that of the XA60.5. Both are well-engineered amplifiers that deliver more usable power than their modest specifications suggest.—John Atkinson

Pass Laboratories Inc.
13395 New Airport Road, Suite G,
Auburn, CA 95602
(530) 878-5350

dalethorn's picture

The differences you describe confirm what I've experienced so many times, although I'm not privileged at this point to hear these monoblock beasties. Our ears, when normal, hear a dynamic range of nearly a trillion to one, a frequency range of 500-1000 to one, and numerous other effects like timing and phase differences. Despite not being a mathematician, I'd guess that the combination of these hearing properties are near-astronomical, which means that we have plenty of room for improvement in sound reproduction.

smargo's picture

"A youngish lady approached our table in G-string and pasties and did a tableside dance. My friend's jaw scraped the floor; I, noting her lack of enthusiasm, was unmoved. The stripper noted my impassivity and stated, with irony that at the time I somehow missed," "You're a hard man."

Isnt this too much information - really

mrkaic's picture

It isn’t.

es347's picture

..a hard man is good to find...apparently

amgradmd's picture

Just off the top of my head, the Decware model SE84UFO at 17 lbs and 4.6 total watts RMS (2.3 per channel) gives a weight/power ratio of 3.7, slightly higher than the ratio of the XA60.8.

tonykaz's picture

First the Surprise: PS Audio in the same breath as Pass! Those years ( 1980's ) that I was selling PS Audio stuff were years where Audio Research seemed to be the Top Gear Brand, PS Audio was "Entry" level.

Now-a-days, PS Audio's DACs, Pre-Amps, Amps & Power all stand with the Greats. No more looking down at PS, no more up-grading from PS. PS Audio has Arrived!!!

Yet again,

has there been any designer, who's work found it's way into more reviewer's personal music systems than N. Pass ? ( other than T.Edison )

I ( kind-of/sort-of ) tried to carry the Threshold Line ( back in the 1980s ) but already had the superb Electrocompaniet Line. I even had the Threshold National Sales Manager staying at my house for a short time but he couldn't/wouldn't let us have his stuff ( not even a try-out sample ). Sooooooo, I've never had my hands on any of these many Pass designs.

Now, today, it's too late for me. I have a rather pronounced leaning towards Active Loudspeakers with built-in Amps. Of course, Pass amps could be configured for an Active Loudspeaker System but it'd take some hefty convincing for me to consider it.

Tony in Michigan

ps. the little 30w. Pass Aleph SET Amp. still seems exciting

mrkaic's picture

The distortion at higher frequencies is unacceptably high. Also, Since when is 0.05% THD+N “extremely low” (see Fig. 7)? That is -66dBFS — average performance that you can get from many mass produced amplifiers.

johnnythunder's picture

over your comment that the THD is for this amp is "unacceptably high." JA didn't flag any measurements as being objectionable, flawed and able to heard (JA feel free to chime in here.) There is ZERO correlation between the sound quality of a mass market amplifier and this PASS amplifier even if the mass market amp has lower THD. Was your idol the late Julian Hirsch?

mrkaic's picture

So, is JA your god? He needs to flag something before you agree? Don’t you have your own views?

The Pass amp is mediocre and overpriced. And you could not tell it from a $300 TEAC class D in a blind test.

Finally, how much science and engineering have you studied? ANSWER WITHOUT EVASION!

ChrisS's picture



Shame on you, mrkaic!

mrkaic's picture

So unable to counter my devastatating scientific arguments. So angry as a consequence.

johnnythunder's picture

are what they do to your reputation here.

mrkaic's picture

Being disliked by subjectivists and/or anti science types is a badge of honor. I would hate to be liked by anti vaxxers, for example.

johnnythunder's picture

and you can continue to make your inane remarks in a forum where you are happily a rare, isolated minority opinion. The audio obectivists have long been on the verge of extinction anyway. Find a medium and have a nice laugh over cheap wine with Julian Hirsch.

mrkaic's picture

As far as I can see, subjectivists are very anti science. If they were not, they would admit several things.

1. The human ear is an imperfect instrument, not suited to analyze electronic components like modern amplifiers, DACs etc. You should admit that auditory illusions, placebo effects, and psychological factors play a major role in "hearing or listening to music". See this video and maybe you'll understand that what you "hear" is actually a product of your brain where the brain produces the "hearing" based on prior information and sensory inputs:

2. You would accept blind testing, no ifs, and or buts.

3. You would stop making unscientific claims about being able to hear the difference between different high resolution components, like modern DAC chips or amplifiers with distortion that is small enough to be below the threshold of hearing.

4. You would stop making unscientific claims about the effect of "audio grade" cables, fuses etc. on sound. There is none, unless the cables or fuses are damaged or really poorly made. Also, stop making unscientific claims about directionality of cables and fuses. It is stupid, flies in the face of science and engineering, as well as the practical experience of never noticing the directionality of wires while building an industrial and information society -- an effort that has required trillions of wires and connectors to be installed.

5. Stop claiming that audio memory is too short for blind testing. If it is too short for blind testing, it is too short for sighted testing as well.

6. There are other indications of anti-science stance by subjectivists, the above five should suffice for now.

ChrisS's picture

This is retail.

No one does blind testing.

No one does "science".

Josh Hill's picture

Last I checked, that was almost -5 dB SPL at 3 kHz:

The audibility or lack thereof of distortion in fact depends on psychoacoustic masking and weighting. The ear is extremely sensitive to some kinds of distortion, e.g., crossover notch distortion (which can be ABX'd, by the way), and very insensitive to e.g. low-order harmonic distortion.

Nelson Pass has studied the audibility of distortion extensively and has chosen to trade off offensive forms of distortion for benign ones. That's a wholly different level of expertise than is require to design an amplifier with low THD, a figure which has little to do with audible quality.

Anyone who isn't a tin ear can hear differences of this kind between amplifiers. But, of course, one must actually listen, something that few "objectivists" seem to have done.

You misunderstand completely the matter of audio memory, by the way. Audio is a complex, changing signal, and so poorly suited to ABX tests. The ear however will come to recognize the sound of a component with long-term listening. So while short-term switching does have a role to play, it is seldom the best way to hear subtle differences between amplifiers. One will eventually identify and start to hear and remember characteristics of the kind mentioned in the review -- is the bass wooly and uncontrolled, how much apparent depth is there, are the highs grainy and harsh or warm and smooth, etc.

Try it -- I can almost guarantee you'll be surprised.

mrkaic's picture

Read this:

ChrisS's picture

...scorned and ridiculed is the badge of a troll.

ChrisS's picture

Your posts have nothing to do with science.

Your baiting and trolling only inspire scorn and ridicule.

You have a devastated ego, mrkaic.

mrkaic's picture wanted to say a DEVASTATING ego. :))

ChrisS's picture

Only someone with a devastated ego baits and trolls.

ChrisS's picture your spelling!

johnnythunder's picture

he writes primarily about how MUSIC sounds to him on the components being reviewed. You never talk about MUSIC. You fixate on measurements and which immediately disqualifies your opinions on equipment to me (and to many many others here.)

My dad was in the record business. He was the district manager of a major music/hi-fi chain. i grew up living with good stereo equipment (Marantz etc.) I play bass guitar. I listen to music and go to classical concerts regularly. i know what an emotional connection to music is and i trust the writers of Stereophile to illuminate those things to me. I make my own decisions.

You troll and make inane remarks about THD. Argue with Paul McCartney about the high harmonic distortion in his bass guitar amp. Tell him that his VOX amp is overpriced and could be bettered by a mass market amp that can be bought for less at Guitar Center.

Allen Fant's picture

Excellent review- JA
I like your sense of humor and play on words- "stripper" and "hard man". She should have been more grateful ;)

Allen Fant's picture

2nd Note;
one could never own too many copies of Getz/Gilberto!

dumbo's picture

I noticed in the recently posted Golden Ear Triton video interview by the real JA that he was using what looked to be the XA60.8's. Perhaps the real JA will also offer his own 2cents on how the new XA60.8 sounds compared to XA60.5 he liked so much before?

Of course if he says the .8 is bad or worse I will disagree either way being a happily biased owner of this very Amp myself :)

Josh Hill's picture

Nelson Pass's remarks about the relative phase of second harmonic distortion and the perception of depth are one of the most interesting things I've read in a long time.

I've long wondered why some components, in particular tube amplifiers, exhibit more depth than others and have never found a satisfying explanations for this. I've long suspected that it was because the tubes were adding something -- the question being what.

Perhaps this, or a similar phenomenon, is the explanation?

gizmo101's picture

The battle btw objectivist and subjectivist will go on and never ends...
Nelson has scientific knowledge but if he pursues solely the scientific path, his amp will never sell. He knows this and therefore makes up something to appease those audio fools and be happy ever after...