Pass Labs INT-25 integrated amplifier Measurements

Sidebar 3: Measurements

Prior to measuring the Pass Labs INT-25 using my Audio Precision SYS2722 system, I preconditioned it for an hour at one-third power into 8 ohms. At the end of that time, the heatsinks on the amplifier's sides were hot, at 122°F (50°C), as was the top panel, at 116.6°F (47.8°C). Owners should make sure this amplifier is well-ventilated. The amplifier's maximum voltage gain into 8 ohms was 25.4dB, and the amplifier preserved absolute polarity (ie, was noninverting).

The Pass Labs' input impedance is specified as 47k ohms; I measured 46k ohms at 20Hz and 1kHz and 37k ohms at 20kHz. The small reduction at the top of the audioband will be inconsequential. The output impedance was very low, at 0.06 ohms at low and middle frequencies, rising very slightly to 0.08 ohms at the top of the audioband. (Both measured impedances include the series resistance of 6' of speaker cable.) Consequently, the variation in frequency response with our standard simulated loudspeaker (fig.1, gray trace) was minimal. The response into impedances of 4 and 8 ohms (fig.1, cyan, magenta, blue, and red traces) was flat up to 20kHz, and the output into 2 ohms (green trace) was down by just 0.25dB at 20kHz. The ultrasonic rolloff varied with the load impedance; the –3dB point was close to 200kHz into higher impedances but dropped to 100kHz into 2 ohms. The traces in fig.1 were taken with the volume control set to its maximum. Commendably, the frequency response didn't change at lower settings of the volume control, and the already good channel matching improved. The Pass Labs' reproduction of a 10kHz squarewave into 8 ohms (fig.2) featured very short risetimes and no ringing, though there was a very slight hint of overshoot.


Fig.1 Pass Labs INT-25, frequency response at 2.83V into: 8 ohms (left channel blue, right channel red), 4 ohms (left cyan, right magenta), 2 ohms (gray) (0.5dB/vertical div.).


Fig.2 Pass Labs INT-25, small-signal, 10kHz squarewave into 8 ohms.

Channel separation was very good, but asymmetrical at >77dB, R–L, and >90dB, L–R, below 1kHz. Both figures dropped to close to 60dB at the top of the audioband, however. The wide-band, unweighted S/N ratio, ref. 2.83V and measured with the volume control set to its maximum and the input shorted to ground, was 75.1dB (average of both channels), which improved to 83.2dB when the measurement bandwidth was restricted to the audioband, and to 86.9dB when A-weighted. Spectral analysis of the INT-25's noise floor (fig.3) revealed a low level of random noise, but spuriae related to the power-line frequency were present at low levels. The 60Hz component, which will be due to magnetic interference from the power transformer, was highest in level, at –76dB.


Fig.3 Pass Labs INT-25, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (linear frequency scale).

The INT-25's output power is specified as 25W into 8 ohms and 50W into 4 ohms, both equivalent to 14dBW, with a "class-A envelope" of 50W peak into 2, 4, or 8 ohm loads. With "clipping" defined as when the THD+noise reaches 1%, I found that the Pass Labs amplifier with both channels driven at 1kHz clipped at 60W into 8 ohms (17.8dBW, fig.4) and 98W into 4 ohms (16.9dBW, fig.5). Fig.6 shows how the INT-25's THD+N percentage varied with frequency at a moderate output voltage of 8.95V, which is equivalent to 10W into 8 ohms, 20W into 4 ohms, and 40W into 2 ohms. While the Pass Labs offered very low distortion at all frequencies into 8 ohms (blue and red traces), with only a small increase at the top of the audioband, the distortion at this output level increased dramatically above the midrange into 4 ohms (cyan and magenta traces) and 2 ohms (gray trace). It is possible that this behavior is associated with the amplifier's output stage moving out of the class-A bias condition.


Fig.4 Pass Labs INT-25, distortion (%) vs 1kHz continuous output power into 8 ohms.


Fig.5 Pass Labs INT-25, distortion (%) vs 1kHz continuous output power into 4 ohms.


Fig.6 Pass Labs INT-25, THD+N (%) vs frequency at 8.95V into: 8 ohms (left channel blue, right channel red), 4 ohms (left cyan, right magenta), and 2 ohms (gray).

The waveform of the residual distortion and noise with a 1kHz signal at this voltage into 8 ohms (fig.7) is dominated by the subjectively benign second harmonic. This was confirmed by spectral analysis (fig.8), though it is appropriate to note that the second harmonic lies at a very low –86dB (0.005%). Higher-order harmonics are virtually absent, though the low-level power supply–related spuriae can be seen in this graph. Both high-order intermodulation products and the second-order difference product with the INT-25 driving an equal mix of 19kHz and 20kHz tones at a peak level of 10W into 8 ohms were very low in level (fig.9). However, as expected from fig.6, repeating this test at the same output voltage into 4 ohms produced high levels of intermodulation distortion. I had to reduce the peak output level to 6.32V into 4 ohms, equivalent to 10W into this load, to get a similar spectrum to that shown in fig.9.


Fig.7 Pass Labs INT-25, 1kHz waveform at 10W into 8 ohms, 0.0092% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).


Fig.8 Pass Labs INT-25, spectrum of 50Hz sinewave, DC–1kHz, at 10W into 8 ohms (linear frequency scale).


Fig.9 Pass Labs INT-25, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 10W peak into 8 ohms (linear frequency scale).

There is much to admire in the Pass Labs INT-25's measured performance. However, its intolerance in the treble for loads of 4 ohms and below means it will work best with loudspeakers whose impedance doesn't drop below 8 ohms at high frequencies.—John Atkinson

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

davip's picture

"... I would never purchase an integrated amplifier with a DAC or phono stage residing in the same box. To me, that's like buying an active loudspeaker. Why would I pollute a fine audio component with a non-upgradeable and possibly third-rate ancillary".

I've often wondered why magazines ostensibly dedicated to audiophilia justify reviewing pre-/integrated amplifiers without phono stages (let's forget about the IC that is a DAC), and this lamest of justifications above provides the context for asking. Why, firstly, would anyone have any faith in the sound-quality chops of such an amplifier when the highest-quality audio source (i.e., a turntable) was not used in its design? Does this reviewer kid himself (or, more importantly, his readers) that such designers use live, all-analogue FM broadcasts to design, test, and voice their amplifiers, or is he content for such amplifiers to be designed with digital audio signals as their source? Even if a turntable source was used (presumably using a non-'third-rate' phono stage), how could any end-user know how such an amplifier would sound with vinyl unless the same phono-stage used by the manufacturer in design or the reviewer in appraisal were also purchased alongside? It behooves amplifier designers -- people like Pass in particular -- to use the best quality source available in designing their kit and if there's no phono input then it wasn't used. If a turntable was used, then there is no justification whatsoever for not including a phono input.

The reason a phono stage belongs in an amplifier is because it equalises and amplifies the output of a phono cartridge, both of which are the only jobs of an amplifier (and, similarly, why a DAC does not belong in an amplifier, any more than a tweeter does). If Nelson Pass or indeed anyone else cannot design an integrated amp ('integrated' being the operative word here) worthy of being reviewed by Stereophile without putting a 'third-rate' phono stage in it, then he is worthy only of our opprobrium and certainly not of our patronage. Of course, he is perfectly capable of doing so, and it's silly reviewer statements like the one starting this review heading this comment that let him and many like him get away with not doing so.

Come-on Stereophile -- you aren't 'What Hi-Fi'. There's plenty of 'magazines' that pander to the computer-as-music-source crowd. Stick to the audiophilia that is implicit in your name...

JackWlwi's picture

The convenient fiction would be the assumption that a turntable is the most superior source which frankly, is just not true. The most pleasantly coloured perhaps, but the most superior? Sorry, nope.

Ortofan's picture

... (or DAC) phobia, Accuphase makes a pair of FET output, class A biased integrated amps, with card slots for plug-in phono stage and DAC option boards.

Have no fear of a built-in phono preamp or DAC?
Then get the option boards.

Rather have an external phono preamp or DAC?
That's covered, too.

I'd want to audition the Accuphase amps before declaring something else to be reference grade.

Incidentally, an "arcane art form" such as opera should have been composed by Verdi and sung by Bergonzi.

Bogolu Haranath's picture

May be HR could review the new NAD M33 with Purifi 'Ultra-Quiet Amplification Technology' 200 WPC integrated amp, $5,000 ......... M33 has built-in phono-stage, DAC, Dirac-Live, tone controls, Wi-Fi access and headphone output :-) .......

Ortofan's picture

... give HR palpitations.

Bogolu Haranath's picture

We don't want that to happen to HR ....... We need HR for a long long time and keep reviewing audio equipment :-) ........

Bogolu Haranath's picture

Well ...... HR could use the M33 as a pre-amp and connect the pre-amp outputs to his favorite tube power amp :-) .........

dc_bruce's picture

I'll only go part way with Daveip's comment, to the extent that he implicitly flags the silliness of the review's expressed prejudice against integrated amps with phono stages (and, presumably, preamps with phono stages). Although I have a great fondness for vinyl (having grown up listening to it) and I own a separate all-tube phonostage (BAT) with a step up transformer, I would not claim vinyl as "state of the art." The best vinyl is certainly very good -- but so is the best digital. The limiting factors seem to be the quality of the recording and mastering, not so much the medium.

I do have an issue with reviewers --almost all of whom are unqualified to opine on the subject -- repeating with praise the manufacturer's description of the advantages of his unique circuit design and so on. And if we're going to talk about fetishes, why is this amplifier not balanced (which seems to be a fetish praised by many).

The proof, of course, is in the listening and in the measuring to some degree. Thankfully JA's measurements suggest that this is not the amp to use with 4 ohm speakers unless one like an additional flavoring in one's music reproduction.

JackWlwi's picture

Nailed it.

The power supply noise on the output is pretty substantial too.

tonykaz's picture

Exciting and thought provoking. What more could we hope for from Stereophile's Poet Laureate.

A reviewer or even a dedicated neurotic/psychotic Audiophile should own gear that encourages separates evaluation. ( I prefer Active Loudspeakers and/or Manufacturer-Designed complete systems as in : Meridian, Linn, PS Audio, Schiit ( in-spite of their horrible name and rear mounted power switches ) and even Diaveliet from France .

No phono, No problem.

The World's population today doesn't own Vinyl, doesn't plan on owning vinyl or doesn't even know what vinyl records are. Designing new Audio Gear for a 21st Century Marketplace won't include 33.3 ( why should it ? ) There are plenty of previously loved ARC Vinyl Preamps for any of us old geezers that are still holding large collections and want something vintage and interesting.

There are some very nice vinyls coming out but I can't play any of them on my mobile rig, so why bother? When I'm home, my family don't want the house filled with Russian Pianos, maybe if I'm a single guy but no way around wifey.

I will probably never own PASS gear but I do like reading HR's opinions. I'm waiting for Mr.HR's review of the 1,800 Watt Class D from PASS ( the size of an iPhone ), when it finally comes out next year or so. ( doubles as a remote control )

Tony in Venice

ps. I miss Enid Lundley and wire comment.

davip's picture

"...The World's population today doesn't own Vinyl, doesn't plan on owning vinyl or doesn't even know what vinyl records are. Designing new Audio Gear for a 21st Century Marketplace won't include 33.3 ( why should it ? )".

Straw-man argument. Do you really think that Nelson Pass' few-10s-of-watts multi-$1000 amplifiers are designed for "...the World's population" (when they can get a Bluetooth-enabled one with 10x the power for 1/50th of the price)?

Pass and his peers are not designing for 'the World' but a tiny subset of that world in the audiophile. Were the analogue element to disappear from Stereophile the magazine would go bust in fairly short order -- not everyone (at least not every adult) wants their musical enjoyment to be endlessly portable or regards their music collection as being on an SD-card as a good thing.

tonykaz's picture

what argument am I refuting? i.e. "straw-man"

Vinyl isn't an argument, is it?

Mr. Pass's concept of "First Watt" is certainly pertinent to the music reproduction by the vast citizen population. Who needs more than one Watt?

Of course his pricing and high manufacturing quality isn't aimed at "everyman", neither is vinyl now-a-days.

I do agree with you on your important point : Pass builds for a tiny segment of Audio gear Marketplace.

but disagree on Stereophile's not covering vinyl

Stereophile would do just fine because Stereophile exists and thrives on the journalistic integrity of JA1&2, the superb writing of some of it's staff and it's maintaining a strong connection to a thriving Industry. ( Automotive Audio itself exceeds $20 Billion )

Vinyl is a minute niche of hoarder/collector hobby that cannot itself support a proper print magazine. ( I myself own a rather large collection of 33.3 & Moving Coil phono Cartridges - kinda frozen in time ) Today's Vinyl greats are folks like Cassem, Chesky and a small few Europeans creating beautiful pressings, all of them combined would probably fit in a high-school gym. god bless em.

Vinyl's aggressive proponents tend to be Gas-lighters and have been since the mid 1980s . Some are acid spitters. I'm a vinyl guy that loves Redbook - a disruptive person from some Vinyl believer's point of view.

Tony in N.H.

mrkaic's picture

Look at the distortion at low powers — really bad stuff. Not cheap either. I will pass (pun intended).

jimtavegia's picture

Not what one would expect from Nelson. I suspect something is amiss. The HF at 8 ohms is also very odd. Should we not expect some component failure in this amp?

Surprised this could be considered a "reference" amp. It is true that I do not trust my ears anymore. These is so much at play to make an audio system likable, that we miss some things that aren't quite right. I know I do everyday.

jmsent's picture

..about the high frequencies at 8 ohms?

jimtavegia's picture

For an amp at this price range to not be extremely low at either 4 or 8 ohm and be consistent across the entire band at something under .0 something % I don't get. I am also not saying that low distortion is all of it, but it does raise the question brought by another of what % of distortion can we hear, does it matter in the overall presentation, and in the context of microphones and mic pre-amps does it matter. Obviously it was still like by HR.

I guess for some a straight wire with gain is out the window, but I think that as your speakers get more and more revealing and many that are power hungry, these things do matter.

tonykaz's picture

Can we actually hear low distortion?

The people that manufacture the Distortion measuring gear contend that humans can't quite detect distortion below 20%, ( strange as that might seem ). They prove their claim at every Audio Show they display at. It's quite surprising.

Tony in Venice

Archimago's picture

Great question Tony. Starting this weekend on the blog, we can test this out in the comfort of one's home :-).

tonykaz's picture

Maybe you can test it out and report.

I'd ask JA to make some sort of contribution as well as those Precision Gear Manufacturers.

It was the Japanese in the 1970s that got us all those zeros infront of a distortion number, wasn't it. Zero distortion doesn't sound good from our Absolute Sound perspective.

Personally, I'm fond of what ever distortion tubes yield.

So, In my opinion book, distortion is a good thing, which might explain my Alcholic life ( recovered in 1999 ) .

I love JA and his measurements, I just don't judge my gear based on the numbers. ( maybe I should )

Tony out in the dam cold winds of the Planes.

Archimago's picture

"Is high Harmonic Distortion in music audible?" Internet Blind Test is up now on the blog.

This is step one. To see if THD can be discerned and at what kind of level among audiophiles / music lovers who want to partake in the test.

In time, we can perhaps looks at things like high order vs. low order distortion. And of course odd vs. even order harmonics.


tonykaz's picture

I wonder if revealed facts will change our decision making decisions.

I'm related to a still suffering alcoholic that loves Beer so much that he has it with his breakfast cereal. ( he's divorced and living in his 85 year old mother's basement )

Your explorations will make you a legend. ( at least, in my mind )

Tony in Venice

Bogolu Haranath's picture

Compare the measurements of this Pass INT-25 integrated ($7,250) with twice as expensive BorderPatrol amp ($14,500) :-) .........

Les's picture

At the volumes Herb was listening, the distortion levels are quite reasonable. (And of course at lower volumes the distortion would be even less perceptible.) Also, the rising THD+n per frequency wouldn't come into play for opera or really any reasonable music. In what circumstances would we listen to such high frequency content at such levels?

This is not a defense of the amp but rather to point out that its "weaknesses" don't really come into play in real life situations. YMMV.

volvic's picture

But, for me who has quite a few components; tuners, cd players, computer audio, SACD and several turntables, only three inputs is a bit of a deal-breaker. It is not a knock on Mr. Pass and his gorgeously sounding equipment but on some of these manufacturers who make fewer and fewer inputs. Guess I'm old school.

Gerd Bethge's picture

I cannot understand why the difference in distortion at 100mW is so different between the two amps.
What went wrong with measurement.....


AaronGarrett's picture


Have you heard 75 Dollar Bill's album "I Was Real"? I think you will enjoy it!

Herb Reichert's picture

"I Was Real" right now. I am listening with the RAAL AR1a headphones and the new Schiit Jotunheim R dedicated amplifier.

You are right! I dig 75 Dollar Bill a lot.

Thanks for pointing it out.


AaronGarrett's picture

Glad you like it!

Bogolu Haranath's picture

As long as you don't print it, that should be ok ........ Just kidding :-) .......

helomech's picture

Herb has a new reference - most, most, most predictable.

unitygain's picture

Price-equivalent comparisons to the Pass INT-25 are just waiting to be made, namely:

- The venerable Sugden Masterclass IA-4 solid state class A integrated putting out 33wpc (particularly given the history of Devore / Sugden show combinations)
- The just-released Modwright KWH225i hybrid integrated biased 25 wpc Class A then A/B to 225 (with the added twist of 6922s in the preamp section and enough juice to push those lovely Studio Electric F2s)

Always a fan of the chimeric admixture of a very reasonable form factor (integrated amp) allied to a wasteful, Dionysian impulse (Class-A-or-be-damned!).

aRui's picture

I has Pass Labs XA25 and it is an excellent power amp, great measurement. I wonder Why the measurement of this INT-25 is much worse than the XA25 on the power side? Isn't the INT-25 is built based on the XA25?