PS Audio Stellar M700 monoblock power amplifier Measurements

Sidebar 3: Measurements

I measured the PS Audio Stellar M700 with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). Before performing any tests of an amplifier, I usually precondition it by running it at one-third the clipping power into 8 ohms for an hour. As the Stellar M700 has a class-D output stage, this was not necessary. But I did take the temperature of its case: the top panel was slightly warm, at 80°F (30°C). The voltage gain into 8 ohms was 30.5dB for both the balanced and unbalanced inputs, and the M700 preserved absolute polarity (ie, was non-inverting) for both inputs, meaning that the XLR jack is wired with pin 2 hot.

The unbalanced input impedance was close to the specified 50k ohms, measuring 47k ohms at low and middle frequencies, and dropping inconsequentially to 35k ohms at the top of the audioband. The balanced input impedance was twice the unbalanced impedance, as expected. The output impedance (including 6' of speaker cable) was low, at 0.1 ohm at 20Hz and 1kHz. However, the output impedance was actually negative at the top of the audioband, perhaps due to the behavior of the necessary low-pass filter. The action of this filter can be seen in fig.1, which rolls off the output very sharply above 50kHz, with the actual frequency of the response "knee" dependent on the load impedance. The variation in frequency response into our standard simulated loudspeaker was small, at less than ±0.1dB (fig.1, gray trace), but there is an implication of an ultrasonic resonance developing with loads of higher impedance. This gave rise to a small degree of overshoot and one cycle of damped ringing with a 10kHz squarewave (fig.2).


Fig.1 PS Audio Stellar M700, 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 PS Audio Stellar M700, small-signal, 10kHz squarewave into 8 ohms with AP AUX-0025 low-pass filter.

As the M700 is a class-D design, all measurements were taken with Audio Precision's auxiliary AUX-0025 passive low-pass filter, which eliminates noise above 200kHz that might otherwise overload the SYS2722's input circuitry. Without the filter, 877mV of ultrasonic noise was present at the speaker terminals, with a center frequency of 465kHz. (This is very similar to the Peachtree Audio nova300, which Art Dudley reviewed in June 2017 and which also uses an ICEpower module.) This noise can be seen riding on the waveform of a 1kHz sinewave when I didn't use the filter (fig.3). With the Audio Precision filter, the Stellar M700 produced relatively low levels of noise in the audioband, with no trace of AC-supply harmonics (fig.4). The unweighted, wideband signal/noise ratio was 66dB, improving to a respectable 81.7dB when the measurement bandwidth was restricted to the audioband, and to 84dB when A-weighted.


Fig.3 PS Audio Stellar M700, small-signal 1kHz waveform into 8 ohms without AP AUX-0025 low-pass filter.


Fig.4 PS Audio Stellar M700, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (linear frequency scale).

We define clipping as when the percentage of THD+noise in the amplifier's output reaches 1%, and fig.5 indicates that the M700 slightly exceeds its specified power of 350W into 8 ohms (25.44dBW). Into 4 ohms (fig.6), the M700 clips at precisely the specified 700W (25.44dBW). Fig.7 plots the THD+N percentage against frequency at a level at which I could be sure I was looking at distortion rather than noise: 20V, which is equivalent to 50W into 8 ohms, 100W into 4 ohms, and 200W into 2 ohms. The THD is very low at low and middle frequencies into 8 and 4 ohms, but does rise into 2 ohms and at high frequencies. The distortion signature primarily comprises the relatively benign third harmonic (fig.8) and lies at just –80dB (0.01%), even at 200W into 4 ohms (fig.9). Intermodulation distortion at the same high power was also very low (fig.10).


Fig.5 PS Audio Stellar M700, distortion (%) vs 1kHz continuous output power into 8 ohms.


Fig.6 PS Audio Stellar M700, distortion (%) vs 1kHz continuous output power into 4 ohms.


Fig.7 PS Audio Stellar M700, THD+N (%) vs frequency at 20V into: 8 ohms (blue), 4 ohms (magenta), 2 ohms (red).


Fig.8 PS Audio Stellar M700, 1kHz waveform at 100W into 8 ohms, 0.0063% THD+N (blue); distortion and noise waveform with fundamental notched out (red, not to scale).


Fig.9 PS Audio Stellar M700, spectrum of 50Hz sinewave, DC–1kHz, at 200W into 4 ohms (linear frequency scale).


Fig.10 PS Audio Stellar M700, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 200W peak into 4 ohms (linear frequency scale).

PS Audio's Stellar M700 packs a lot of power into a small, cool-running case, and offers respectable measured performance.—John Atkinson

PS Audio
4826 Sterling Drive
Boulder, CO 80301
(720) 406-8946

tonykaz's picture

Of course, I'm only asking your opinion.

Is your review a useful comparison to your Theta Mono Amps ?, in your opinion.

Why be vague ? , $3,000 for a "hell-of-a" pair of powerful mono Amps, Made In USA ! ( for gods sake ) is something to "Crow About", isn't it ???

Did the lads in Colorado do a hell of a job, again ?, is that what your Say'n ?

But we're still waiting on those French Tube Amps ( and JVS ) that look like gold plated Heathkits, I'm wondering if they'll be Class A recommended ? ( presuming they don't explode during the review period )

I was hoping to learn if the Stellars did "density" very well, "density" being the latest Reviewer's descriptive terminology.

Tony in Michigan

ps. I'm becoming a fan of PS Audio again, after being one of their Dealers 35 Years ago. Seeing YouTube & Paul and the inside of the Company is a "Power-Close" for a Factory man, like me, the Place looks as clean and organized as Sennheiser.

ps.2). I wonder if I could get a Signed Pair ?

Glotz's picture

I was wondering that as well, after reading the measurements. I believe High Class B... ?

What do I know... I haven't heard these amps, but after the 2nd review, they seem like real values.

I wonder about that new preamp as well. It looks like a great sweet spot, but digitizing my analog sources... NO thank you. I could use clarification on that though.

Once again, Tony you are a pleasure to read and ask all the right questions!

Frans's picture

I am confused by the reviewer improved results from placing these amps side-by-side to get better results. Then came the expensive isolator pedestals, and there's Paul Gowan claiming a 2-week minimum break-in period... Is that the break-in period for your ears of the equipment?

Are these amplifiers not designed to be placed on top of each other, and are they so sensitive to minute vibrations as to requiring pedestals?

Is there any science behind this, and if so, why sell these amps without these features from the factory, or are these actual manufacturer's recommendations?

Glotz's picture

No shame, every component on the planet has issues with vibrational feedback. Cost is the culprit as it is expensive to isolate a component from external and internal vibrations. In the case of turntables and disc players, it is critical.

As well, every component extant needs break-in. Not one's ears. If a mfg. decides to break-in the components themselves before shipment, that is a rarity and will not affect the review process.

If a component has excellent damping characteristics, ie: the company has added damping materials or chosen substantial materials for construction, then the ability to stack components can be met.

I guess I don't understand why the review decided to stack mono-blocks in the first place, perhaps out of rack space issues or just simple curiosity (he was reviewing the footers in other issues).

Lastly, I really needed more comparisons, as Tony as elucidated.

AJ's picture

No, although some mumbo-jumbo about "microphonics/vibrations, etc." is likely.
Scientific method listening tests, ears only, delusion and self deception controlled for, would yield zero change in the sound.
Of course to anti-science types, the conclusion wouldn't be that the uncontrolled viewing "listening" daydreams were the source , but rather the scientific listening tests were faulty. Duh!! ;-)

GeneZ's picture

Just learned about this the other day.Toroidal transformers are usually great for not generating noise on their horizontal plane. That is good for being on a chassis with the electronics.. But, not as good on the horizontal plane. So? Stacking them might not be as good for that reason.

tonykaz's picture

All things have a resonant frequency and will microphonically resonate. Increasing Mass & Dampening will lower the resonant frequency but not eliminate it.

Now, the big question : Will the Amp try to play the ultra low frequency content in Bob Katz's Bombay Dub Orchestra's masterings annnnnnd will the Amp become a resonant "tuning fork" ?

I suspect that the Vinyl Era reviewers won't notice an Amps low frequency capability with Digital's performance envelope in mind, vinyl gear has a natural low frequency limitation.

Clearly, the PS Stellar is a Digital Era Amp needing a contextually relevant evaluation of it's musical performance capabilities.

I wonder if PS Audio Engineering even own a functioning Turntable? , Ps has been a committed Digital outfit since the 1980s.

Tony in Michigan

ps. here is a chance for PS to (YouTube) demonstrate the Stellar's playing into a Full Range Loudspeaker while Mr. Paul pounds 'em with a hammer ( shoe ) of some sort. By the way, Paul is starting to look like a Veggie, Tea totaling & non-smoking Johny Carson sitting on a White Sofa. Next comes The "Guest" Appearance with a Side Man doing the drum rolled Introduction: heeeeerrrrrrrs Paul McGowan !

supamark's picture

you may have done in your audio dealer days (if you sold open reel tape decks) - with inputs disconnected start recording on an open reel tape deck and yell into the record head. It will record the sound you made onto the tape... talk about microphonics lol.

Bill Leebens's picture

I'll try to respond to several points here. First off, thanks for the kind comments from everyone.

Glotz: I believe Tony's "is it Class A?" comment was not referring to the class of operation of the M700s, but to the class rankings in Stereophile's Recommended Components list---as PS is honored to have four components ranked Class A or Class A+ on the list.

As Bob Deutsch indicates in the opening paragraph, the Stellar amps are Class D, and the company does indeed have decades of experience with Class D amplification. After extensive testing and auditioning we chose an ICEpower module that is also used by a number of other companies; we believe that the sound quality of our implementation of the module is superior, naturally---and thankfully, Bob seems to agree.

Frans: Regarding break-in---PS' experience over the past 45 years is that the sound quality of nearly all components will improve with extensive warm-up and break-in, and most PS products undergo a burn-in period before they leave the factory.

While this is a contentious issue for some, it seems pretty straightforward to me: capacitors take time to form, values of resistors stabilize over time, and a number of physical changes occur in most circuit elements over time.

Regarding vibration: the Stellar casework is quite robust, we often use the Stellar components stacked, and they were designed to be used that way. Having said that, they may well benefit from the use of the IsoAcoustics Orea isolation devices. I personally use IsoAcoustics Gaia II feet on my speakers, with excellent results---but as you note, the Orea are expensive, prohibitively expensive for inclusion with our amps.

Finally---yes, we do own a functioning turntable. While PS has been a leader in digital audio, our first product in 1973 was a stand-alone phono preamp, and we still manufacture the NuWave Phono Converter. Downstream, you can expect to see phono preamps in our Stellar and BHK Signature lines.

Bill Leebens
Director of Marketing
PS Audio

AJ's picture

While this is a contentious issue for some, it seems pretty straightforward to me: capacitors take time to form, values of resistors stabilize over time, and a number of physical changes occur in most circuit elements over time.

It's only contentious for those who are blissfully unaware of, or reject perceptual science/psychology outright.
No contention among those who do not.

Bill Leebens's picture

Having spent a good chunk of my career having to respond to folks who consider almost everything in audio snake oil or worse, I am perhaps hypersensitive to such concerns.

I was also trying to respond fully to the previous poster, for whom such issues seem to be uncharted territory.

Cheers, Bill

Frans's picture

Bill - Please point me to actual verified measurements that show what changes after the first 5 minute of the device being on, and after 45 days. What is the % improvement in these KPIs over this time? I'm interested specifically in amplifiers, and not a generalized rule of thumb.

I'm struggling with understanding the science and statistics behind extended break-in periods, it's usually the listener's auditory adjustments that change over time versus the equipment.

Bill Leebens's picture

The audio biz is a mature industry---which means little is spent on fundamental research. So it goes.

We primarily rely upon what we hear. Given the collective several hundred years of experience at our company, we're pretty able to differentiate between effects, artifacts and psychoacoustics.

If that's not good enough, sorry. But cars improve after break-in; speaker surrounds generally loosen up after break-in; and while you might not think of electronic components as having physical components, they do. And they change.

Life would be easier if that were not the case.

tonykaz's picture

Certainly, any device will deteriorate at a measured pace. There is no engineering group that does not take component spec. changing into careful consideration. Everything on earth is "unstable" in it's composition and performance envelope.

We have to take into consideration that shooting a bolt of Electricital energy thru a device will effect and affect its existence and it's functionality.

Audiophile Industry is tiny, which is why no-one bothers to spend time educating it's nitchey hair shirt hobbyists.

One look at any piece of gear that sits on the Repair Bench will prove that these tiny components have all shifted in their ability to perform, many are near the end of the device's life-span.

Electrolytic Capacitors in power supplies commonly fail after a brief life-cycle.

Valid and verifiable Proof of electrical component stability is accessible throughout the electrical component Industry. Call a capacitor manufacturer to discover these things.

We have to assume that all Hi-Fi Components are in constant change. We're lucky that we don't notice it until smoke starts floating thru the air.

Audiophiles live in happy oblivion.

Tony in Michigan

ps. have a look at the power cord of electrical devices to see active deterioration. Cars Electrical systems are designed to NOT fail for 15 years ( or so ). Older Audi Cars were known for bad wiring ( I've been told )

ps. 2 ) Electrolytic power supply capacitors are though to have 8 Year life-spans which suggest that we should turn off our Gear instead of leaving it on "4 Eva", like some silly folks recommend. go figure!

Frans's picture

I have no problems with deterioration, we can measure this, observe this, repeat this. PREDICT this.

However, electrical components improving with time due to a break-in period is what I was asking about. With car engines, the break-in period (although no longer a common practice because it's done at the factory) is more important because of the thousands of moving components and oil distribution through the engine. My diesel didn't require any break-in, these are mostly myths these days.

Electrical solid-state circuits are not the same as internal combustion engines, they work, fail fast, or fail slow. If there is a build-defect it will happen regardless of a break-in period.

There are plenty of mass production industries that are not niche like high-end audio that have more applicable use cases around break in I'd imagine. For example:

Do we break in light-bulbs, refrigerators, smoke detectors, video cameras, Smart phones, PCs? Not niche and esoteric enough?

After 45 days of using this smart phone, will your calls connect better, will WiFi range increase, will it SOUND any better?

tonykaz's picture

Arn't we simply thinking about a drifting performance capability?

I'm not "drifting" into being contentious, rather I'm suggesting that in active electrical circuits, the values of any component will shift, sometimes in predictable ways.

As a Consumer, I'd rather not notice it.

As a Reviewer, I'd be nervously worried about it.

As a Manufacturer, Importer, Retailer I'm buying from reputable "Name Brand" suppliers so that gear achieves consistently high performance levels with a minimum of failures. Still, I'll accept the deteriorating of Tubes as the price of lovely. I'll even accept Car Tires not lasting long enough.

Reviewers speaking about "Break-in" times is probably a nervous personal obsession, not a recommendation. We reading consumers will agree ( or not ) with the "Slowing down" of the process that Breaking-in requires, it's almost a "More haste, Less Speed" philosophy embodiment .

I have to say that my old NewBalance Shoes broke-in and "improved".

I have a number of bicycles, only the few that I ride have been broken in to improved performance.


Tony in Michigan

tonykaz's picture

I've owned NEW loudspeakers that improved from use, was it the elastic surround that lost it's stiffness ?

Still, I'm not taking any sort of stand on this Breaking-in strategy.

However, my Audiologists tell me that my brain and it's associated nervous system will re-calibrate themselves as I correct my various gear's performance EQ curves.

Career wise, I've worked with a wide range of sensitive transducer systems, enough to realize that system hysteresis is a common ingredient to erroneous summaries.

The one thing that remains a puzzle ( that I'd like Paul McGowan to establish ) is the arrows on various, pricy Audio Cables. My question is : How the hell do they figure out which direction the cables should be going? , which way is the electricity going? , for gods sake !!!

Tony in Michigan

ps. lets stump the experts

Strat's picture

There is no improvement. Some people finally
got a wake-up call from their intelligence and
realized that this new "break-in period" was long
enough to take them beyond the equipment return date.
First it was speakers, but why not use the same for
electronics. This break in time was/is all over the
board, but it was always beyond the standard 30 day
return policy. Now you're stuck with it...unless you
want to pay a ridiculous "re-stocking fee." But hey look,
now it's two weeks. How did that happen?

I see this effort as a direct response to Legacy Audio's
Powerbloc2. They use the same 700ASC ICEpower module.
No coincidence there.

rom661's picture

"Whenever I connect a new amp to my system and turn it on the first time, it's always with a certain trepidation. Will the system be quiet, or will I hear noise?"
Really? That's what you listen for? I listen for music. I quit reading here. No offense to Paul. I want to hear the amps. I don't want to read more of your musings.

Bill Leebens's picture

A reviewer has to take note of all a product's characteristics, and noise level of electronic components is a major concern for many customers/readers.

Certainly, it's a concern we hear all the time, and exceptionally low noise was a primary requirement when we designed the Stellar line.

tonykaz's picture

But I do own a considerable CD Collection and boxes of Vinyl from my previous life as a Turntable Specialist Dealer.

Anyway, I've just been going back in time to read "Vintage" PS Audio reviews. Curious thing is that I'd mostly lost track-of and interest-in all things Audiophile. One 2011 day I got stranded in Colorado and ran into Tyll and Steve G. at RMAF , I bought some headphone gear as the result.

All this time I've been missing my Electrocompaniet Ampliwires/Theil CS3s & my Conrad-Johnson MV-45a/Audible Illusion & Magnapaan MG-2s and ProAc Tabletts. ( and my MiT-750s Cables ) Oh-dear, absence makes the heart grow fond.

Since 2011, I've been trying to come to an understanding of Digital Audio Conversion gear. Then: along comes Paul McGowan doing YouTube Vlogs about all things Audiophile and I suddenly feel like it's 1980 again, I'm getting gear tutorials from PS. Phew, just like the good old days and now Paul brought-in a talented group of seasoned designers who seem rather excited to be part of a Winning & Stable Audiophile organization, finally! #METOO
PS Gear has been getting darn good reviews, how did I manage to not notice?

I guess I'll have to get one of those "Audiophile" hats : "In for a Penny, In for a Pound" !

Tony in Michigan
aka B&K Imports & Esoteric Audio

windansea's picture

I just got the baby brother of the M700, the stereo S300. Works beautifully with my mini maggies. No coloration, lots of power and detail, doesn't have the air of a SET but for me it's a great tool at a reasonable price. And it uses a fraction of the energy of my Krell. Hot amps are a pain in the summer here in SoCal! Thinking Class D for summer, Class A for winter.

tonykaz's picture

"the Air of a SET", hmm, interesting concept.

PS Audio seems to have a Company Philosophy relating to the Superior Sound Quality Compromise of Sealed Baffle Loudspeakers which ends up with them building HIGH Powered Amps.
Aren't SET Amplifiers rather low power and therefor used to power high efficiency loudspeakers ( like Devore ).

I wonder if PS Audio could also have a Parallel Line of Amps for high efficiency loudspeakers and if there is any sort of market for 'em. SET Amp outfits seem to charge High Prices for their Gear which they tend to justify by using the highest quality parts available ( and many instances "no-longer" available, made from solid silver capacators )

Hmm, can Krell and PS Audio both serve the same need? You owning both says something significant.

Tony in Michigan

David Harper's picture

IMO there's a lot more going on in our heads than we're aware of, maybe even things going on that we can't be aware of since we can't get outside of our heads.Obviously this would explain why all of these differences in sound quality that audiophiles describe disappear when subjected to double-blind testing. A long time ago Julian Hirsch had a group of audiophiles listen to what they were told was twenty thousand dollars(which was really big bucks in those days)worth of high-end gear. After the listeners all waxed euphoric about the superb sound quality, it was revealed to them that they were actually listening to a three hundred dollar sansui receiver(which had been hidden from their view).They were furious. They all figured out a way, however,to argue that the test had been flawed and meaningless.

windansea's picture

Could anyone elucidate the Gain Cell? The reviewer just quotes PS Audio's cagey description. Because as far as I can tell, the amp is essentially just a stock B&O module with the proprietary Gain Cell input stage, so I'd like to know more about what's going on with that.

jeffreybehr's picture

Everything vibrates. I was e-mailing recently with Steve Herbelien (sp?) of Herbies Audio Labs about coupling or isolating audio equipment including speakers. One thing he wrote, and I paraphrase--'You may think your concrete floor doesn't vibrate, but you're wrong'. I'm now with him. To those of you with poured-concrete-over-earth floors--ever wonder why a 'thud' sounds when you drop to the floor quickly and on your knees? It's the floor vibrating, and its sound is of a very low frequency because the concrete slab is heavy. FWIW--and that's virtually nothing to anyone else--after over 50 years as an audiofool, I now mass-load and isolate electronic equipment, generally with Herbies Tenderfoot products*, and I couple speakers--with sharp spikes--to that concrete floor.

On more thing--William Bruce Cameron wrote "...not everything that can be counted counts, and not everything that counts can be counted." I believe very strongly that we still don't know--and may never know--how to measure the sonic quality of music-reproduction equipment. Those I call 'measurists' disagree with me. :-)

My one M700 amp sounds excellent on my centerchannel speaker.

* I may be able to spend thou$and$ on preamps, amps, speakers, cable, etc., but I can't yet spend hunded$ on isolation feet. Maybe sometime...

Armando's picture

You sound like a bunch of quibbling little boys (and girls if there are any). Ask yourselves why you are so vested in being right and defending your viewpoints so vehemently.

I have a pair of M700. They sound great. I truly don't care how they measure. I care about SQ, not a pretty or correct graph.

I don't have them stacked because I like to minimize heat buildup and they do get warmer stacked. Maybe the extra heat, small or large as it might be, does not impact sound or long-term component longevity, but maybe it does. I don't obsess about it.

I have noticed positive changes in SQ as time goes using these amps. Is it my ears or components breaking in? Who cares? Why can't you just enjoy what you hear? Blessed is anything that gets better over time.

Trying to understand why something happens is fine. I love being curious and inquisitive. That is how progress happens. However, doing it the way you guys do it is pointless, unless you are vested in drama and winning arguments. There is no attempt to understand each other's point of view or observations. You must love drama to argue positions with such religious fervor.

conanb's picture

A recent review: