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What matters about SINAD measurements

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#1
First may I commend this site and its founder\admin for a fine service to several audio communities, ranging from newbies to audiophiles to professionals. I myself am in the latter category – a professional audio engineer since 1958 (my first paying gig after graduating RCA Institutes, and since with a BSEE, long-time member of AES and SMPTE, researcher\presenter globally at ASA\CAA and Deutche Verein, author of two books – you get the picture).

However, I have a different interpretation re some of your measurements that I think will be useful for members…

Even though “we’ve always used 1w” is often the admonition, your arbitrary 5w tests for speaker amplifiers is no less valid, maybe more! But up to 0.05% SINAD such as for the Behringer A500 is nothing to sneeze at, because in most circumstances it will be inaudible, which is ultimately what matters. [SINAD is signal to total noise & distortion, sometimes THD+N that is a measure of audio fidelity.] Here’s a “gedanken test” (thought experiment) with a warning that it contains arithmetic (nothing more).

Beginning with listening and working backwards, say you follow the SMPTE cinema\studio mastering standard 85dB SPL reference, 20dB below full scale (FS), which allow for undistorted individual channel peaks of 105SPL, or about 108 with five (5) surround channels driven. Home listeners typically use 10dB lower (75SPL), perceived as “concert level” for acoustic classical, jazz, or movie sounds (there is no reference for pop\rock, as even hearing damage is not the limit). The 20dB between “reference level” and “FS” is intended as “headroom” for preserving the dynamics that occur in the natural, lifelike sounds of music, speech, and ambience.

Now insert a quality (monitor-grade) loudspeaker with a relatively high sensitivity of 91SPL for 1w listening in an acoustically controlled space at 1m – that’s ~2w at 1.4m “near-field” listening, ~5w at 2.5m (8ft) “mid-field.” For these three levels, each power amplifier channel would need to deliver (all channels driven) 20dB more power, or 100w, 200w, or 500w to pass the 105SPL peaks without clipping. A more common 88dB sensitivity speaker for home use (75SPL reference) would need 1/10 the power but ½ the sensitivity, or power levels of 20, 40, or 100w listening at 1m, 1.4m, or 2.5m – less power in a typically live, acoustically uncontrolled room.

The ultimate test is whether SINAD is audible. That will be limited by the environmental noise of the listening space. My control room and studio are both below 30SPL unweighted, or ~25 A-weighted, but ISO:226 (Fletcher-Munsen) applies mostly below 200Hz, so we’ll use unweighted in this example. 30SPL noise is 55 dB below the 85SPL reference, or 45dB below the nominal home listening level of 75SPL, and assumes an unusually quiet house. We can discern signals 15~20dB below noise, so to be inaudible, amplifier artifacts for typical home listening need to be -65dB below 1w, 2w, or 5w by their respective listening distances. -65dB is less than 0.05%. So if SINAD is <0.05%, the amplifier is not perceived to be adding bogus colorations to the sound. And especially not when you factor in that the individual SINAD components would need to be heard, and that another link in the audio chain distort and mask other links’ artifacts much more (speakers being the main culprits, along with acoustic issues).

So for the Behringer A500 power amplifier channel you measures THD+N about 0.047%. In a typical home environment it would be as good as needed. And while the measured spikes for its power supply could be better, individual even and odd order harmonic spikes are mostly down >80dB – very likely inaudible even in a quiet and acoustically-controlled studio. SINAD for most amplifiers appear best at highest power – why they are often the only figures a manufacture's marketing dept publishes – but as long as they cover peaks without clipping, it’s the 1\2\5w percentages that matter more, because most of the time that’s what we are hearing.

Keep up the good work! – Robin Miller (author of The Better Sound of the Phonograph)
 

Blumlein 88

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#2
If you get SINAD just short of clipping, it is probably mostly distortion. The A500 may be a little odd in that it might be as much power supply leakage as distortion. Generally though for most gear as you drop the level distortion goes down, both in absolute levels and in terms of percentage of the signal. I'd say nearly all 50 watt and larger amps have effectively no distortion at 5 watts and less. What you really have in THD+N is the remaining noise. So rationally, you might think if the amp can make the max power you need at less than -60 db SINAD, and if you hear no noise at lower levels, you are good.

The remaining issue with power amps however is reactive loads. The A500 might barely meet your needed specs. Does it maintain those into a reactive loudspeaker load? More than likely it cannot. If you had SINAD measurements connected to your loudspeaker, or an exact model of your loudspeaker's impedance then the measures would be much more valuable. Of course picking .1% distortion (-60 db) as limits of audibility is from testing with sine waves. With music it probably is more like 1%. So maybe even 60 db SINAD gives a bit of headroom for doing worse with a loudspeaker load. I do believe this issue is why I'd like a little more headroom than that.
 

amirm

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#3
Welcome to the forum @RCAguy. Great to have more people here with long industry experience.

I must confess I don't understand the argument. :) SINAD or THD+N is not perceptually correlated measure. So it is hard to make an argument for audibility of any value regardless of conditions selected. You can have two amps with identical SINAD and one may have audible distortion and the other not.

The reason to measure SINAD is to detect engineering excellence as a general measure. Here, I am not a fan of using 1 watt because I personally have no value for any amp that only produces that much power. So I went up to 5 watts. I could have gone higher but not all amplifiers I test have much power.

The data one seeks is available in my measurements in the THD+N (SINAD) versus power. So if it is the SINAD before peak power is desired, that is there as well. I just don't compile them in a table since that point is different for every amp.

Finally, I don't believe in any sensitivity computed loudness figures. Those are marketing values (for speakers) and don't represent real life situations. Such math may convince you that you need just a few watts but in my experience, it is easy to need and want hundreds of watts.
 
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#4
Welcome to the forum @RCAguy. Great to have more people here with long industry experience.

I must confess I don't understand the argument. :) SINAD or THD+N is not perceptually correlated measure. So it is hard to make an argument for audibility of any value regardless of conditions selected. You can have two amps with identical SINAD and one may have audible distortion and the other not.

The reason to measure SINAD is to detect engineering excellence as a general measure. Here, I am not a fan of using 1 watt because I personally have no value for any amp that only produces that much power. So I went up to 5 watts. I could have gone higher but not all amplifiers I test have much power.

The data one seeks is available in my measurements in the THD+N (SINAD) versus power. So if it is the SINAD before peak power is desired, that is there as well. I just don't compile them in a table since that point is different for every amp.

Finally, I don't believe in any sensitivity computed loudness figures. Those are marketing values (for speakers) and don't represent real life situations. Such math may convince you that you need just a few watts but in my experience, it is easy to need and want hundreds of watts.
Thanks Amirm for the welcome. However if as you say "I must confess I don't understand the argument," then I respectfully suggest, as in your reply you are quoting me inaccurately, that you read my post again carefully.

In essence it is that what matters is whether the artifacts such as noise & distortion can be heard. SINAD or THD+N (and especially non-harmonically-related IM) is precisely a "perceptually correlated measure." Otherwise why measure it? Measurements are not intended for their own sake, but for the sake of modeling our perception of sound, usually intended to point to what to fix it if it's not good. Common ground for audio subjectivists and objectivists, not only "engineering excellence" but good sound is confirmed by good measurement &v.v. In my measurements over the decades, two amplifiers that measure in every way the same also sound the same - any too tiny differences are too low to register in our perception. Furthermore, measurement data varies with loudspeaker sensitivity - not a "marketing value" but a critical engineering one* - which along with acoustics controls the power we demand of the amplifier for the same sound level.

We agree - and I said - that for the 20dB of headroom in a movie or dynamic music recording, 100 times the power of nominal levels on the order of 1 or 2.5 or 5w in my example. So we certainly must also measure that clipping has not reached its onset for instantaneous peaks at 100x the more typical 1 or 2.5 or 5w levels. However all amplifiers of any maximum rating reproduce typical audio content at 1 or 2.5 or 5w a lot more of the time than they produce maximum power of 100 or 250 or 500w, and so your measuring SOP at 5w is quite relevant. This is why constant low-level crossover distortion artifacts of a mediocre AB amplifier are more clearly audible than they are amidst loud sounds, where fixed distortion is now a much lower percentage, drowned-out, or masked.

And I will say again Amirm - keep up the good work!

*Often revealing an intentional trade-off between efficiency and quality, such as between PA speakers and audiophile ones.
 
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QAMatt

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#5
SINAD for most amplifiers appear best at highest power
I don't think that is true. SINAD is simply the reciprocal of THD+N (eg -105 THD+N = 105 dB SINAD). And as you know, the THD+N versus power curve has 3 distinct regions: Noise limited, optimum and harmonic limited. The best THD+N and SINAD will always come from the optimum region which is far below the highest highest power.
 
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#6
If you get SINAD just short of clipping, it is probably mostly distortion. The A500 may be a little odd in that it might be as much power supply leakage as distortion. Generally though for most gear as you drop the level distortion goes down, both in absolute levels and in terms of percentage of the signal. I'd say nearly all 50 watt and larger amps have effectively no distortion at 5 watts and less. What you really have in THD+N is the remaining noise. So rationally, you might think if the amp can make the max power you need at less than -60 db SINAD, and if you hear no noise at lower levels, you are good.

The remaining issue with power amps however is reactive loads. The A500 might barely meet your needed specs. Does it maintain those into a reactive loudspeaker load? More than likely it cannot. If you had SINAD measurements connected to your loudspeaker, or an exact model of your loudspeaker's impedance then the measures would be much more valuable. Of course picking .1% distortion (-60 db) as limits of audibility is from testing with sine waves. With music it probably is more like 1%. So maybe even 60 db SINAD gives a bit of headroom for doing worse with a loudspeaker load. I do believe this issue is why I'd like a little more headroom than that.
Hi "Blumlein." "Just short of clipping" (what is termed "onset") is usually still mostly distortion-free, as nearly all distortion curves show. AB biased amplifiers can have crossover distortion that is both measurable and audible at 1 or 2.5 or 5w in my example. And music signals, being Fourier summations of sine waves, retain the same order of % distortion as their individual components, not multiplied by 10. Distortion might about double in % because now IM sum & difference artifacts are created by the multiple frequencies. Finally, your tolerance for SINAD is also dependent upon your listening level and room noise, as my post attempts to explain.
 

daftcombo

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#7
If an amp has THD of -60dB @1kHz @5W, and if I play the songs for the 1kHz peaks to be at 80dB (which is not so loud), then I also hear artifacts at 20dB, right?
The question is: do the 1kHz peaks mask those artifacts?
 
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#8
I don't think that is true. SINAD is simply the reciprocal of THD+N (eg -105 THD+N = 105 dB SINAD). And as you know, the THD+N versus power curve has 3 distinct regions: Noise limited, optimum and harmonic limited. The best THD+N and SINAD will always come from the optimum region which is far below the highest highest power.
1556033023524.png


Hi QAMatt. I was generalizing, based on typical SINAD v. power curves such as the one shown for a Denon AVR-X3300W. The "knee" is the "onset" of clipping, above which distortion rises rapidly (we don't want to listen there), and at which is calculated the lowest percentage, here about 0.015%. So marketing today usually uses this "best" figure, although it is valid only for instantaneous peaks. Properly sized for speaker sensitivity and room acoustics, most of the time, this amplifier output would be less than 1w, where SINAD is closer to 0.1% - but you won't see that advertised.
 

daftcombo

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#9
View attachment 25226

Hi QAMatt. I was generalizing, based on typical SINAD v. power curves such as the one shown for a Denon AVR-X3300W. The "knee" is the "onset" of clipping, above which distortion rises rapidly (we don't want to listen there), and at which is calculated the lowest percentage, here about 0.015%. So marketing today usually uses this "best" figure, although it is valid only for instantaneous peaks. Properly sized for speaker sensitivity and room acoustics, most of the time, this amplifier output would be less than 1w, where SINAD is closer to 0.1% - but you won't see that advertised.
Yes but at 1W it will be mainly noise, not distortion, right?

Isn't the "knee" the point where distortion becomes audible, whereas it was masked by noise before?

You took an example where there is no "optimal" region.
See this one:


Here you can see noise domination up to 7W, then distortion domination from 7W to 100W, then clipping.
 
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andreasmaaan

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#10
If an amp has THD of -60dB @1kHz @5W, and if I play the songs for the 1kHz peaks to be at 80dB (which is not so loud), then I also hear artifacts at 20dB, right?
The question is: do the 1kHz peaks mask those artifacts?
It depends on the frequency of the fundamental and the artefacts, both in absolute terms (due to the nonlinearity of human hearing acuity) and relative to the frequency/ies of the signal content.

1556033423198.png


The graph on the left shows audibility threshold vs frequency in silence (bottom trace). It indicates that, without any masker tone, frequencies lower than about 120Hz will not be heard at 20dB, even in silence.

The graph on the right (from Zwicker and Fastl) shows the masking threshold of a maskee tone (which you can think of as an artefact in terms of your question above) by a masker tone (which you can think of as the signal content or fundamental).

Looking at an 80dB masker tone at 1KHz, we see that for a 2KHz tone to become audible, it must exceed 45dB or so. For a 4KHz tone to become audible, it must exceed 30dB. However, for an 8KHz to become audible, for example, it need exceed only about 10dB.

Of course, this graph shows masking by a single masker tone (or "fundamental frequency" if you want to think about it in distortion terms) only. But masking thresholds for narrow band noise are more generous, and for multiple tones or wideband noise they become more generous again. So it's seems quite safe to conclude that the graph above gives a worst-case scenario in terms of masking thresholds.
 
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#11
If an amp has THD of -60dB @1kHz @5W, and if I play the songs for the 1kHz peaks to be at 80dB (which is not so loud), then I also hear artifacts at 20dB, right?
The question is: do the 1kHz peaks mask those artifacts?

Hi "daftcombo" - I think your implication is correct that the artifacts would be audible if not masked. It is complicated by whether the "80SPL peaks" contain harmonics that overlay in the same critical frequency band* with the harmonic distortion artifacts, which they might indeed mask.

*Usually a fraction of an octave, critical band masking is the basis for data reduction ("compression") of the perceptual coding of Dolby, DTS, etc.
 
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#12
Yes but at 1W it will be mainly noise, not distorsion, right?

Isn't the "knee" the point where distorsion becomes audible, whereas it was masked by noise before?

You took an example where there is no "optimal" region.
See this one:


Here you can see noise domination up to 7W, then distorsion domination from 7W to 100W, then clipping.
 

QAMatt

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#13
Hi QAMatt. I was generalizing, based on typical SINAD v. power curves such as the one shown for a Denon AVR-X3300W. The "knee" is the "onset" of clipping, above which distortion rises rapidly (we don't want to listen there), and at which is calculated the lowest percentage, here about 0.015%. So marketing today usually uses this "best" figure, although it is valid only for instantaneous peaks. Properly sized for speaker sensitivity and room acoustics, most of the time, this amplifier output would be less than 1w, where SINAD is closer to 0.1% - but you won't see that advertised.
Yes, OK, agree in that case. But most marketing folks are specifying max power well up the right side of the curve. The plot you included is claimed to be a 105W amp into 8 ohms which looks like 10% distortion assuming this plot is into 8 ohms. That's about where all the silicon vendors are spec'ing their max power too
 
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#14
Yes but at 1W it will be mainly noise, not distortion, right?

Isn't the "knee" the point where distortion becomes audible, whereas it was masked by noise before?

You took an example where there is no "optimal" region.
See this one:


Here you can see noise domination up to 7W, then distortion domination from 7W to 100W, then clipping.

By definition, the "knee" is "onset of clipping," which is extreme distortion and not a operating region (except for electric guitars). So your point "Isn't the "knee" the point where distortion becomes audible, whereas it was masked by noise before?" is not precisely true. It might be true if, in dB terms, the distortion at onset of clipping just happens to be comparable to noise, electronic plus ambient. If you digest my OP, you will see the interplay of noise, listening level, speaker efficiency (sensitivity), and amplifier power. Your headphone amplifier example would need those parameters in order to be analyzable, but the curve itself is not typical of a well-behaved power amplifier (the red dashed line is). The "optimal" region would be properly scaling the amplifier performance (measured) within specific listening parameters, such that the entire dynamic range of the recording is free of audible distortion & noise. (This is entirely possible, as I've achieved as a consultant installing film studio control rooms and home theaters.)
 
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#16
Yes, OK, agree in that case. But most marketing folks are specifying max power well up the right side of the curve. The plot you included is claimed to be a 105W amp into 8 ohms which looks like 10% distortion assuming this plot is into 8 ohms. That's about where all the silicon vendors are spec'ing their max power too
Your point underscores the importance of independent measurements, such as by amirm and presumably others on this forum (I'm new so don't know you all yet.) Denon's claim as you report it is false, but I would hope that it is not true that "all the silicon vendors are spec'ing their max power." I traffic mostly not in consumer but professional audio, where such deceitful marketing (it's not engineering) would not be tolerated for long.
 

amirm

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#17
In essence it is that what matters is whether the artifacts such as noise & distortion can be heard. SINAD or THD+N (and especially non-harmonically-related IM) is precisely a "perceptually correlated measure." Otherwise why measure it? Measurements are not intended for their own sake, but for the sake of modeling our perception of sound, usually intended to point to what to fix it if it's not good.
A few points. Whether someone can detect non-linear distortion is highly depends on perception qualities of the listener. In testing lossy compression for example, we know that trained listeners do a ton better than non-trained. So this is not a static number.

On why measure, in every case I show SINAD it is accompanied with FFT. It is the FFT that is instructive of whether the distortions can be audible. I realize people mostly focus on the SINAD since it is easier to digest but I do make an attempt to show the picture at the same time.

SINAD by itself then is best thought of engineering excellence when not focused on too accurately. I show the SINAD figures in four buckets for this reason. No question in my mind that a device in forth bucket is much worse design than those in first or even second bucket. In this manner, we use SINAD as a rough figure of merit without falling into the pitfall of using it too accurately when it doesn't have such predictive power.

The other reason to show SINAD is that the industry uses THD+N which is the same thing as SINAD. By measuring this we can tell how truthful the manufacturer is in their specifications.
 

Shadrach

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#18
It's about engineering excellence/competence; it's not about audibility.
If it was about audibility we wouldn't have much to discuss.
There are lots of sometimes interesting discussion on the merits of this design, or that. Most competently designed equipment reaches a performance standard that most people will find acceptable and indistinguishable from another such product.
By the way, It hasn't been my experience that professional audio is above all the bullshit.;)
 

DonH56

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#19
I don't really have anything to add but when has that stopped me...

I tend to agree with Amir; to me, SINAD is one of those necessary but not sufficient sort of things. If the SINAD is very high then noise and distortion products are likely very low and I don't have to worry. If it is poor then it takes more work to see if it is a noise problem, a distortion problem, or combination. Taking the case of a power amplifier, low distortion and a higher noise floor may work well with less sensitive speakers. High noise and highly-sensitive speakers is probably going to mean hiss. High distortion means potential for being audible, particularly if it is say in the -40 to -60 dB range (1% to 0.1% in voltage). Then there are the cases where something like power supply spurs contribute significantly to SINAD, leading to hum that may be objectionable. And of course it is always better to see a curve than just a single number; if there are significant shifts over frequency that is good to know, as is a steeply rising noise floor at low power levels. SINAD is good for a pass/fail sort of test but lumps everything together so is just a starting point for looking at measurements. If an audio amp has SINAD of 60 dB I probably won't look further and if it is >120 dB chances are it is good enough for anything I'd throw at it. If it is in between, say 80 to 100 dB, I would want to look deeper, especially with so many excellent-measuring amplifiers available these days.

Multitone testing for distortion and step testing for stability were a couple of my "to go further" metrics back when I did more measuring of audio stuff. For that matter, still good metrics for most of the HF stuff I worked on.

FWIWFM - Don
 

garbulky

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#20
First may I commend this site and its founder\admin for a fine service to several audio communities, ranging from newbies to audiophiles to professionals. I myself am in the latter category – a professional audio engineer since 1958 (my first paying gig after graduating RCA Institutes, and since with a BSEE, long-time member of AES and SMPTE, researcher\presenter globally at ASA\CAA and Deutche Verein, author of two books – you get the picture).

However, I have a different interpretation re some of your measurements that I think will be useful for members…

Even though “we’ve always used 1w” is often the admonition, your arbitrary 5w tests for speaker amplifiers is no less valid, maybe more! But up to 0.05% SINAD such as for the Behringer A500 is nothing to sneeze at, because in most circumstances it will be inaudible, which is ultimately what matters. [SINAD is signal to total noise & distortion, sometimes THD+N that is a measure of audio fidelity.] Here’s a “gedanken test” (thought experiment) with a warning that it contains arithmetic (nothing more).

Beginning with listening and working backwards, say you follow the SMPTE cinema\studio mastering standard 85dB SPL reference, 20dB below full scale (FS), which allow for undistorted individual channel peaks of 105SPL, or about 108 with five (5) surround channels driven. Home listeners typically use 10dB lower (75SPL), perceived as “concert level” for acoustic classical, jazz, or movie sounds (there is no reference for pop\rock, as even hearing damage is not the limit). The 20dB between “reference level” and “FS” is intended as “headroom” for preserving the dynamics that occur in the natural, lifelike sounds of music, speech, and ambience.

Now insert a quality (monitor-grade) loudspeaker with a relatively high sensitivity of 91SPL for 1w listening in an acoustically controlled space at 1m – that’s ~2w at 1.4m “near-field” listening, ~5w at 2.5m (8ft) “mid-field.” For these three levels, each power amplifier channel would need to deliver (all channels driven) 20dB more power, or 100w, 200w, or 500w to pass the 105SPL peaks without clipping. A more common 88dB sensitivity speaker for home use (75SPL reference) would need 1/10 the power but ½ the sensitivity, or power levels of 20, 40, or 100w listening at 1m, 1.4m, or 2.5m – less power in a typically live, acoustically uncontrolled room.

The ultimate test is whether SINAD is audible. That will be limited by the environmental noise of the listening space. My control room and studio are both below 30SPL unweighted, or ~25 A-weighted, but ISO:226 (Fletcher-Munsen) applies mostly below 200Hz, so we’ll use unweighted in this example. 30SPL noise is 55 dB below the 85SPL reference, or 45dB below the nominal home listening level of 75SPL, and assumes an unusually quiet house. We can discern signals 15~20dB below noise, so to be inaudible, amplifier artifacts for typical home listening need to be -65dB below 1w, 2w, or 5w by their respective listening distances. -65dB is less than 0.05%. So if SINAD is <0.05%, the amplifier is not perceived to be adding bogus colorations to the sound. And especially not when you factor in that the individual SINAD components would need to be heard, and that another link in the audio chain distort and mask other links’ artifacts much more (speakers being the main culprits, along with acoustic issues).

So for the Behringer A500 power amplifier channel you measures THD+N about 0.047%. In a typical home environment it would be as good as needed. And while the measured spikes for its power supply could be better, individual even and odd order harmonic spikes are mostly down >80dB – very likely inaudible even in a quiet and acoustically-controlled studio. SINAD for most amplifiers appear best at highest power – why they are often the only figures a manufacture's marketing dept publishes – but as long as they cover peaks without clipping, it’s the 1\2\5w percentages that matter more, because most of the time that’s what we are hearing.

Keep up the good work! – Robin Miller (author of The Better Sound of the Phonograph)
RCA guy thanks for talking common sense. If you can’t hear it, what’s the use for engineering excellence? Isn’t engineering supposed to accomplish a practical task ? So if one doesn’t hear a difference in real life listening, I would say the task is accomplished. I say this because I have listened to gear referred to as “poor engineering” which is completely inaudible to me. I wish people realized the very low impact a difference between 70/80 and 120 dB makes.
 
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