Pass Labs XA60.5 monoblock power amplifier Measurements

Sidebar 3: Measurements

Before performing any measurements, I ran one of the Pass Labs XA60.5s (serial no. 26384) for an hour at one-third its specified maximum power of 60W into 8 ohms. This is thermally the worst case for an amplifier with a class-B or -AB output stage. A class-A amplifier, however, runs hottest when it is idling. By the end of the hour, the top panel was warm, at 107°F (41.8°C), and the side-mounted heatsinks were hot, at 120°F (49°C). With the amplifier idling for another hour, the heatsink temperature had risen to 124°F (51.1°C), the top panel to 108°F (42.3°C). (Temperatures were measured with a Mastercool infrared thermometer.)

I performed a full set of measurements using Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (see www.ap.com and the January 2008 "As We See It"). The voltage gain at 1kHz into 8 ohms was very slightly higher than the specified 26dB, at 26.4dB balanced and 26.55dB unbalanced. (For unbalanced drive, pins 1 and 3 of the XLR jack were connected with a jumper.) The amplifier was non-inverting (ie, it preserved absolute polarity) with both input modes. The input impedance was 17k ohms, 20Hz–20kHz, for both balanced and unbalanced signals.

The XA60.5's output impedance, including 6' of speaker cable, was 0.1 ohm at 20Hz and 1kHz, rising to 0.2 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 a mild ±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 at 102kHz. The response rolled off a little earlier into lower impedances, but was still just 0.5dB 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 good square shape (fig.2).

114PA605fig01.jpg

Fig.1 Pass Labs XA60.5, frequency response at 2.83V into: simulated loudspeaker load (gray), 16 ohms (green), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red) (0.5dB/vertical div.).

114PA605fig02.jpg

Fig.2 Pass Labs XA60.5, 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 a good 79dB. This improved to 86.6dB when the measurement bandwidth was restricted to the audioband; and further still, to 88.7dB, when the measurement was A-weighted. 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.

114PA605fig03.jpg

Fig.3 Pass Labs XA60.5, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (linear frequency scale).

The needle of the front-panel meter remained just to the left of 12:00 at 8-ohm powers below 60W. At 60W it moved slightly to the right, reaching 1:00 at 100W into 8 ohms and 1:30 at 120W. Specified as putting out 60W into 8 ohms (17.8dBW), the XA60.5 considerably exceeded that power, delivering, at 1% THD, 130W into 8 ohms (21.1dBW, fig.4), 210W into 4 ohms (20.2dBW, fig.5), and 330W into 2 ohms (19.2dBW, fig.6). The THD starts to rise above the noise floor at high powers, but is very low at powers of a few watts with a 1kHz signal, even into 2 ohms. The percentage of THD then slowly rises with increasing power, suggesting that the XA60.5 has only a small amount of corrective feedback (though I'm not sure how and where this feedback is applied in Nelson Pass's Supersymmetry circuit).

114PA605fig04.jpg

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

114PA605fig05.jpg

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

114PA605fig06.jpg

Fig.6 Pass Labs XA60.5, 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 was extremely low in the midrange (fig.7) and hardly changed with load impedance, something that I conjecture is associated with good sound quality, as the amplifier's fundamental transfer function is not being modulated by the output current. However, the THD did rise linearly as the frequency rose—which, I conjecture, suggests that the circuit has a limited open-loop bandwidth. (With a conventional amplifier circuit, there is less gain margin between open- and closed-loop conditions, hence less corrective feedback available, at higher frequencies than at lower frequencies.)

114PA605fig07.jpg

Fig.7 Pass Labs XA60.5, THD+N (%) vs frequency at 9V into: 8 ohms (blue), 4 ohms (magenta), 2 ohms (red).

Fortunately, the XA60.5's distortion is predominantly the subjectively innocuous third-harmonic in nature at both moderate (fig.8) and high (fig.9) powers, though the second, fifth, and seventh harmonics can also be seen in fig.9, all close to the –100dB level (0.001%). With the top-octave decrease in linearity seen in fig.7, it came as no surprise to see some high-order intermodulation products that were fairly high in level when the XA60.5 was asked to drive an equal mix of 19 and 20kHz tones at a level just below visible clipping on the oscilloscope screen (fig.10). However, the subjectively more objectionable second-order product at 1kHz is almost 50dB lower, at –100dB (0.001%).

114PA605fig08.jpg

Fig.8 Pass Labs XA60.5, 1kHz waveform at 20W into 4 ohms, 0.02% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

114PA605fig09.jpg

Fig.9 Pass Labs XA60.5, spectrum of 50Hz sinewave, DC–1kHz, at 120W into 4 ohms (linear frequency scale).

114PA605fig10.jpg

Fig.10 Pass Labs XA60.5, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 120W peak into 4 ohms (linear frequency scale).

The Pass Labs XA60.5's measured performance is very similar to that of the stereo XA30.5 (see ), which was favorably reviewed for Stereophile by Brian Damkroger in May 2009 and by Erick Lichte in December 2009. Both are well engineered, and each delivered more power than its modest specification would suggest.—John Atkinson

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Pass Laboratories Inc.
PO Box 219
Foresthill, CA 95631
(530) 367-3690
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COMMENTS
waynel's picture

 70°C below a rated maximum is 126°F below rated max, not 158°F

25°C above ambient is 45°F above ambient, not 77°F

 

"Nelson Pass says that the actual chip temperatures are about 70°C (158°F) below their rated maximum. But with that much bias current, a class-A amplifier will run hot, "about 25°C [77°F] above ambient,"

John Atkinson's picture

waynel wrote:
70°C below a rated maximum is 126°F below rated max, not 158°F

25°C above ambient is 45°F above ambient, not 77°F

Thank you for the corrections. Looks like we erroneously added the extra 32° in both cases, confusing Celsius degrees with degrees Celsius.

John Atkinson

Editor, Stereophile

jlesnick's picture

Hey John,

Now that you've reviewed both the CP-800, & XA-60.5s, do you think that the two devices would have a good synergy together?

John Atkinson's picture

jlesnick wrote:
Now that you've reviewed both the CP-800, & XA-60.5s, do you think that the two devices would have a good synergy together?

Yes, I believe so.

John Atkinson

Editor, Stereophile

karaflohighendas's picture

Hey John, hope you are fine!

Do you mean the best for this price or the best overal? Comparing XA60.5 with your past reference 33H what is your thoughts?

John Atkinson's picture

karaflohighendas wrote:
Do you mean the best for this price or the best overall?

Best overall. Although other amplifiers exceed the Pass Labs in one or two areas, the XA60.5 offers the best balance of qualities.

karaflohighendas wrote:
Comparing XA60.5 with your past reference 33H what is your thoughts?

Sadly, one of my No.33Hes has been broken for the past 5 years and I haven't been able to afford to get it fixed yet. I couldn't perform direct comparisons, therefore.

John Atkinson

Editor, Stereophile

 

karaflohighendas's picture

Is it possible an amp so old like 33H to compare with something so new with all these new materials and design? I think that 60,5 probably outperform 33H in every way with no direct comparisons needed at all. Dont you?

Regards

O karaflos highendas

John Atkinson's picture

karaflohighendas wrote:
I think that 60,5 probably outperform 33H in every way with no direct comparisons needed at all. Dont you?

Without being able to make the comparisons, I think it foolhardy to conjecture what the differences between the two amplifiers might be, let alone rank them.

John Atkinson

Editor, Stereophile

 

 

pepe's picture

Hi John,

Thanks so much for the great review.  I have a quick question, Do you recall if the front meter was contantly moving when driving the Alexias at normal listening levels?  (e.g. around 70-80db).   Did you feel like the XA 60.5 had enough power for the Alexias or did you feel like a higher XA, such as the XA100.5, would be a much better match?

Thanks for your insight, really appreciate your feedback.

John Atkinson's picture

pepe wrote:
Do you recall if the front meter was contantly moving when driving the Alexias at normal listening levels?  (e.g. around 70-80db).

No, the meters remained steady at the central, bias position.

pepe wrote:
Did you feel like the XA 60.5 had enough power for the Alexias?

In my 27' x 16' room, yes.

John Atkinson

Editor, Stereophile

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