For logistical reasons, I measured a different sample of the Dan D'Agostino Master Audio Systems Progression M550 monoblock (serial number PM006P) with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). I repeated some of the measurements with the magazine's Audio Precision APx500 analyzer. Usefully, the box included a set of tests performed with the APx500.
Before I performed the testing, I preconditioned the amplifier by running it at one-eighth its specified power into 8 ohms for 30 minutes, as recommended by the Consumer Electronics Association. After the preconditioning, the M550's top panel was surprisingly cool, at 96.8°F (36°C). The temperature of the massive heatsinks on the amplifier's sides was 101.8°F (38.8°C) and didn't increase significantly with the amplifier idling.
The voltage gain into 8 ohms for the balanced input was 29.7dB, and the input inverted absolute polarity, the opposite of what I had found with earlier D'Agostino amplifiers (footnote 1). The input impedance at low and middle frequencies was very slightly lower than the specified 100k ohms, at 95k ohms, dropping inconsequentially to 50k ohms at 20kHz. The output impedance was higher than the specified 0.1 ohm, at 0.37 ohm at 20Hz and 1kHz, rising slightly to 0.4 ohm at 20kHz. The response with our standard simulated loudspeaker therefore varied by ±0.26dB (fig.1, gray trace). The amplifier offered a wide small-signal bandwidth, the output into 8 ohms (blue trace) not reaching –3dB until 140kHz. The Progression M550's reproduction of a 10kHz squarewave (fig.2) accordingly featured very short risetimes with, commendably, no apparent overshoot or ringing. (This is something that has been a consistent feature with amplifiers designed by Dan D'Agostino.)
Measured with both phases of the balanced input shorted to ground, the M550's wideband, unweighted signal/noise ratio was 55.3dB, ref. 1W into 8 ohms. This ratio improved to 80.5dB when the measurement bandwidth was restricted to 22Hz–22kHz, and to 89dB when the reading was A-weighted. The blue trace in fig.3, taken at 1W into 8 ohms, reveals that the primary source of noise is magnetic interference at 60Hz and its odd-order harmonics, which I assume stem from the massive toroidal transformer. However, at high powers (red trace), the spuriae at 120Hz and its harmonics increase in level.
When I examined how the percentage of THD+N varied with output power, with clipping defined as when the THD+noise in the amplifier's output reached 1%, the M550 clipped at 555W into 8 ohms (27.44dB, fig.4), which is marginally higher than the specified 550W (27.4dBW). The clipping power into 4 ohms (fig.5) was 840W (26.23dBW); though this is 1.17dB lower than the specified 1.1kW, I don't hold the wall voltage constant during these tests, feeling that this is more representative of an amplifier's behavior in a typical system. The wall voltage was 118.7V with the amplifier idling but had dropped to 115.1V with the amplifier clipping into 4 ohms. Note from these two graphs that the distortion percentage at low powers is higher than it is at high powers. This behavior was consistent with both Audio Precision analyzers and is almost identical to what I found with the Dan D'Agostino Progression Mono amplifier that JVS reviewed in October 2017.
As with the earlier amplifier, I examined how the THD+N percentage varied with frequency at a high level, 28.3V, which is equivalent to 100W into 8 ohms, 200W into 4 ohms, and 400W into 2 ohms. (The front-panel meter's needle lay just above "500" at this level.) The results are shown in fig.6: The distortion and noise remain low over most of the audioband into the higher impedances with only a minuscule rise in the top octave. (This graph confirms the test results sent by the manufacturer.) However, when I examined the waveform of the THD+N at 100W into 8 ohms after notching out the fundamental (fig.7), I saw spikes in the residual waveform (bottom trace) that coincided with the zero-crossing points in the signal's waveform (top trace). This behavior indicates the presence of crossover distortion.
Fig.8 shows the spectrum of the amplifier's output while it drove 50Hz at 100W into 8 ohms. The subjectively innocuous second harmonic lies at –74dB (0.015%), but a regular series of high-order harmonics can be seen, which correlates with the waveform of the spuriae shown in fig.7. The harmonic spectrum was very similar with the amplifier driving 50Hz into 4 ohms and 1kHz into 8 ohms at 28.3V (not shown). At 1W into 8 ohms, the second harmonic was still the highest in level but lay at –60dB (0.1%, not shown) and was still accompanied by higher-order harmonics. Fig.9 shows the spectrum of the M550's output as it drove an equal mix of 19 and 20kHz tones into 8 ohms at a peak level of 100W. While the difference product at 1kHz lay at an okay –73dB (0.016%), there are also many higher-order intermodulation products visible in this graph.
The Dan D'Agostino Master Audio Systems Progression M550 is a powerful amplifier indeed and will not be fazed by being required to drive low-impedance loudspeakers. However, the presence of what appeared to be crossover distortion, which suggests insufficient output-stage bias, is puzzling.—John Atkinson
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COMMENTS
Let me get this straight. Serinus reviewed a $45,000 pair of amps and didn't notice that one of them wasn't working properly THE ENTIRE TIME? This guy has turned into an ongoing source of embarrassment and if it wasn't for Mike, Herb and Ken I would have canceled my sub strictly because of his incompetence, not to mention his elitism. This review should have been shelved and rewritten, preferably by someone else, until both amps were working properly.
Let me get this straight. Serinus reviewed a $45,000 pair of amps and didn't notice that one of them wasn't working properly THE ENTIRE TIME?
It's probable that the output-stage bias of one of the amplifiers, slowly drifted while he was using the pair. It was only when he started his auditioning anew with the repositioned Wilson speakers that he noticed the problem with that amplifier.
John Atkinson
Technical Editor, Stereophile
With all due respect, that explanation doesn't seem to scan -- unless bias can drift in a matter of minutes. Serinus clearly says that everything was fine until the speakers were moved, which probably took 30 minutes given their size. And then there's D'Agostino's belief that the bias drifted during shipment. Stereophile has always had some writers I trust more than others but this is pretty ridiculous.
With all due respect, that explanation doesn't seem to scan -- unless bias can drift in a matter of minutes. Serinus clearly says that everything was fine until the speakers were moved, which probably took 30 minutes given their size.
If the bias had slowly drifted during his auditioning, the incremental change in sound quality would have been difficult to perceive. (Think of the "frog in water that is slowly heated" analogy.) It was after the amplifiers were turned on again after the speaker moving that JVS realized that something was not right.
Props to JVS for his honest disclosure.
John Atkinson
Technical Editor, Stereophile
The second sentence of Jeffreylee's first post nails it. However, we should be grateful for JVS's music choices and reviews. They have led me to a lot of wonderful music that I likely would not have heard otherwise.
I want to leave the substance of this comment intact, but I dislike the incivility; I've deleted two other comments for precisely this reason. But I can't think of how to perform surgery on your comment while ensuring that it remains your own, so I'll leave it alone. (No promises, though, about future comments.)
I'm also leaving it because it provides a good opportunity to respond.
I believe that a few critical readers, including you, are missing two points. Or maybe it's three. The first is that we're being radically transparent. For good or bad, instead of covering our collective ass, we've left this out there for all to see. The decision here was to present the whole process, good and bad, so that people can judge for themselves.
I think Jason was courageous for putting this out there--these uncivil responses prove that point. Perhaps I was wrong to allow him to do so, and yet I think it was the right thing to do.
After all, he didn't miss the biasing issue. He heard it eventually. Many writers--specifically writers at other publications--would doubtless have reworked the story to make themselves look better. Jason is to be commended for his honesty and--that word again--transparency.
The second point is this: There is no reason to assume that this was an abundantly audible effect. It may have been quite subtle. You weren't there. Neither was I. Such things can easily be swamped by room, stereo, and (other) psychoacoustic effects, especially when listening in stereo. There's a reason that at Harman and some other places it's standard practice to listen to just one speaker. We audition in stereo because that's how people listen to music, but one speaker can easily mask what's happening with the other. It certainly is possible, if not likely, that I could have missed it.
Either way, we told the truth.
Jim Austin, Editor
Stereophile
The harshest critics didn't read, or at least cherry-picked, as it was all there in the article warts and all from the start. JVS takes more crap than anyone, all the more reason for him to write more.
My older brother was a Horn Player for the DSO.
I recall one performance that seemed wonderful to me, I'd heard him practice that music for a Month.
He got a serious ass chewing for missing a few notes which he fully realised and expected.
The Audience never noticed anything amiss. Me included.
People's synapses adapt and adjust.
That is what makes MR.JA's laboratory work so very important.
Tony in Florida
Are you gonna own these Mono Amps ???
and
Can I buy your old, tired & worn outs ?
I'll send a Wooden Ship builder to pick them up!
Tony in Florid
are you gonna trade in your current amps?
I got first dibs on these tired & warn out old pieces of sadness.
but.,
I'll give you first right of refusal for a couple thou. $2,000.
I can't quite estimate a Final Sale Price because they once belonged to a Famous Reviewer for the World's greatest Publication.
Expect them to sell for about 125% of original Retail... of course they both will be properly Autographed and accompanied with beautiful Framable Photos .
You'll be getting the bargain of the Century.
Tony in Florida
ps. for a small additional few thousand dollars I'll try to entice both Mr.JA1 & JA2 to Sign em.
The strange distortion vs. amplitude plot that you can see here
https://www.stereophile.com/images/1021Dag550fig5.jpg
speaks for itself - it is a medium biased class AB amplifier with very low feedback factor or no global feedback at all. Then you see the rise of distortion above 1W and a minimum near to 200W, because the crossover area and distortion compared to the high output voltage gets relatively lower. Shame on the designer.
This and similar amplifiers are its own category with "special" customers. They do not care about measurements at all and probably would not hear any problem. John Atkinson stays too much on a polite side in my opinion, this should be clearly condemned how poor is the result.
I posted something in response to the glaring deficiencies of this equipment, and it very much appears Stereophile staff deleted it. In no way did I use any foul language, and in terms of spirit, I was no more personal in my attack than @Jeffreylee who rightly called JVS an embarrassment and received a reply from JA instead of a deletion.
... you referred to another forum member as "a cancer."
Jim Austin, Editor
Stereophile
Your assertion that anyone is "stealing money" from anyone is of course ludicrous. People who buy D'Agostino equipment (and equipment from other expensive brands) do so with eyes wide open and all their faculties about them. I'm sure their enthusiastic customers would be amused at your presumptuousness.
Jim Austin, Editor
Stereophile
" a conspiracy of audiophiles " is at least as good as " a murder of crows " .
Jim Austin, Editor
Stereophile
Due to the uncivil nature of some of the posts, this article has been closed to further comment.
Jim Austin, Editor
Stereophile
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