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Pass ACA Class A Power Amplifier Review
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Rudolf
Member
What if there is more to Nelson Pass designs than meets the meter?
I swapped ML 23 for an Pass Aleph 4 today. I had not used the Aleph with Schiit Freya S preamp before, and I was quite impressed (with some other preamps Aleph’s bass seemed too shy; but maybe the woofer is finally broken in after 2 1/2 years?).
I know that my CD and Schiit both measure exemplary, and so does ML 23. Aleph 4 measures worse, not as bad as ACA, but not nearly as good as a ML 23. So Pass Aleph 4 must be wrong, ‘cause it does not measure right; but probably it is wrong in the pleasant way?
I guess this makes me a subjectivist? To quote:
“Years ago my mother used to say to me, she'd say, "In this world, Elwood, you must be," — she always called me Elwood — "In this world, you must be oh so smart, or oh so pleasant." Well, for years I was smart. I recommend pleasant. You may quote me.”
I swapped ML 23 for an Pass Aleph 4 today. I had not used the Aleph with Schiit Freya S preamp before, and I was quite impressed (with some other preamps Aleph’s bass seemed too shy; but maybe the woofer is finally broken in after 2 1/2 years?).
I know that my CD and Schiit both measure exemplary, and so does ML 23. Aleph 4 measures worse, not as bad as ACA, but not nearly as good as a ML 23. So Pass Aleph 4 must be wrong, ‘cause it does not measure right; but probably it is wrong in the pleasant way?
I guess this makes me a subjectivist? To quote:
“Years ago my mother used to say to me, she'd say, "In this world, Elwood, you must be," — she always called me Elwood — "In this world, you must be oh so smart, or oh so pleasant." Well, for years I was smart. I recommend pleasant. You may quote me.”
If you enjoy it that much that your willing to give up ever hearing the recording close to as the artist and engineer intended, fine. It's like buying a preamp with the tone controls locked to something other than flat and not being to ever hear them set to a "transparent to the source" flat position, that's fine.
Not my choice to have someone else season all my food to their taste either.
Killingbeans
Major Contributor
Not really a useful question. It's like sitting in a pub on a dark and stormy night when some guy barges in and exclaim that he was just in the woods and heard a monster screaming at him. Then it's fair to assume that it was a pair of branches rubbing on each other in the wind, or some other non-monster-related thing happening. Even if the guy says: "What if it WAS a monster?"
If the thing that made the amp impressive to you wasn't just a trick of the mind, it would be highly unlikely that the cause of it couldn't be seen in measurements. It might be something as simple as a difference in gain.
Rudolf
Member
Gain is different, Aleph 4 has less gain than most. And they can distort more than most, but at my listening levels distortion should be below audible level (Stereophile measured a couple of 2 gain stage Alephs, 30W and 200W; they all are very similar, just more or less output MOSFETs, Aleph 4 is a 100Wpc stereo amp), and below loudspeakers’ distortion level (Gradient 1.5 Helsinki, they have passively equalised 12” drivers in OB). Alephs have 0.1 Ohm output resistance, many amps have lower; still, purely resistive 0.1 Ohm is low enough to not be audible, isn’t it?
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If that's the case, then it would be a surprise to Nelson Pass, whose own discourse about his amp designs is grounded in electrical principles and a detailed knowledge of amplifier topologies. Reasonable people can debate whether or not Pass' design choices make the right choices and tradeoffs for optimal performance and efficiency of operation - but his designs are not generally based on some kind of mystical voicing disconnected from "the meter."
Rudolf
Member
Mind you, I was not listening to jazz, it was the Clash London Callig (twice, as the amp was warming up; there may or may not be a difference), and then some Rolling Stones greatest hits. Noticeable 2nd harmonic euphonic distortion should have been objectionable.
Anyway, it should not have distorted, at least not in audible terms: Aleph 4 is a 100 Wpc amp, identical 30Wpc and 200 Wpc designs as measured by the Stereophile demonstrated sufficiently low distortion at low to medium power levels, the figures are below what is audible. Granted, is not a grand achievment to outperform the Williamson amplifier’s 0.1 % @ 20W, but once it is done, as long as higher power levels are not required (I have measured my volume preferences, never much above 90dBA, usually less), there is limited room for meaningful objective criticism.
I have a Dynacord L1800FD for a sub project, 2 Ohm stable, 850 Wpc in 4 Ohm, fine distortion (0.05% @ 600 W 4 Ohm), fine other specs; as a stereo amp for normal volume listening it is crude, seems to loose details (AD/DA conversion?), and fatigue builds up remarkably quickly (or rather it invokes a sad feeling of hopeless boredom).
Rudolf
Member
Mr Pass has dwelt quite a bit on several things, in particular, that if complex circuits and a simple circuits measure the same, the simple ones sound subjectively better; that going from 3 to 2 stage Aleph measured slightly worse, but subjectively sounded better; also, he has been very particular about his amps’ harmonic distortion, not only limiting the amplitude, but also looking into it‘s phase. And research shows that human hearing in fact is phase sensitive (the old assumption that it is not seems to be disproved). I am not saying that distortion phase is “The key”, other, basic things are more important to get right in the first place, but I still find it a worthy idea that when a circuit starts distorting, it should do so in the least subjectively annoing way.
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solderdude
Grand Contributor
uh huh.
Nelson Pass is no fool.
He found that higher distortion in amps, using music, is not audible and thus can design simple designs that do the job and made a nice story around it.
Where is the research that shows slow phase shifts in amps are audible in blind level matched conditions where only phase is altered ?
Nelson Pass is no fool.
He found that higher distortion in amps, using music, is not audible and thus can design simple designs that do the job and made a nice story around it.
Where is the research that shows slow phase shifts in amps are audible in blind level matched conditions where only phase is altered ?
Rudolf
Member
This is a tad specific request, but more general research arguably implying that can be found (this caught my eye: https://www.researchgate.net/public...of_Human_Hearing_to_Changes_in_Phase_Spectrum, points to some interesting things in references, too).
solderdude
Grand Contributor
One would have to take into consideration that sudden and local phase shifts do not occur in amplifiers. Usually the phase only marginally shifts near the ends of the audible range.
Substantial phase and time shifts occur in transducers and rooms. Add to that test signals are not the same as complex and inpulse like music signals.
Substantial phase and time shifts occur in transducers and rooms. Add to that test signals are not the same as complex and inpulse like music signals.
This is a good link. I myself did some tests with all-pass simulated filters (no change in amplitude response, different phase responses) and also found differences that were distinguishable in foobar abx DBT, undoubtedly. For certain settings of the all-pass only.
The audience here sometimes tend to trivialize audibility of everything.
restorer-john
Grand Contributor
Absolutely it can and does. Ever used bass, midrange, treble, turnover, loudness or filters?
solderdude
Grand Contributor
The moment you use tone controls the changes in FR are far more tone changing than the marginal phase shift that occurs.
Neither sudden nor local.
As @solderdude said, phase shifts in actual physical amplifiers are not the same as contrived all-pass filters.
restorer-john
Grand Contributor
Absolutely sudden and local. Go from bypass to tone active, flat to boost/cut, and filter to no filter and revel in the phase shift.
Ask a typical audiophile what an 'amplifier' is. Or a salesman. Or a hobbyist. Tell me a typical, representative 'amplifier' as sold in the millions or billions since the dawn of high fidelity does not have massive amounts of phase shift depending on the actions of the incorporated controls and filters. To suggest otherwise is facetious.
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Please show an example with phase plots. And while you’re at it, show how it deviates from minimum phase expected from the frequency response variations.
Hint: it doesn’t.
Killingbeans
Major Contributor
I was thinking about something way simpler: That you were comparing the amps with a small but audible difference in SPL. A bit higher volume automatically gives the illusion of greater overall performance.
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