Are distortion ratings accurate for most amps?

In first watt, Nelson pass pays very close attention to keeping distortion to a minimum, yet in the m2 distortion reaches .1% distortion at 1khz at 10 watts in 8 ohms and 6 watts into 4 ohms. Then by frequency at a modest 1 watt unto 20hz you're already above .5% distortion! Now I wholeheartedly believe these amplifiers have the most attention to detail and are done with good engineering, but then how do we live in a world where everyone seems to throw around a 20hz-20khz .08% distortion or less at full rated power? Usually the wording is in a way to make the rating easy to fib, but that seems pretty straight forward that at any power at any frequency you get less than .08% distortion in most cases according to the rating.

So what's missing from this story? I doubt Mr. Pass labored away to make an amazing low distortion design only to have 10x the standard distortion levels with noticeablely less power.
 
it's simple

just go and read all his papers , at FW article page

especially :

The Sweet Spot

Audio Distortion and Feedback

Talk at CAS

etc.

at least , if you're interested in hiss approach ,meaning of life and everything

(missing article "Machine , which makes Ping!" is in editing stage , written by Mighty Moi )
 

benb

Member
2010-04-24 1:52 am
The "percent distortion" is THD or Total Harmonic Distortion, and that single number is a conglomerate of all the harmonics generated. Some harmonics are much more audible than others, and this number gives no indication of which harmonics dominate the distortion. In short, the commonly used THD figure has little correlation to how an amplifier might sound. So it's easily possible that an amplifier with higher THD can sound better than an amplifier with low THD. People have known this for many decades, but the use of THD persists because too many other people haven't noticed this, and haven't been clued in on it.

I'm looking through Nelson's articles (I recall reading his design philosophy a few years back), and these two look applicable:
http://www.firstwatt.com/pdf/art_cas_2015.pdf
http://www.firstwatt.com/pdf/art_dist_fdbk.pdf
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
Sometimes we just want to listen to a particular approach to design, and each
comes with it's own set of baggage.

In the case of the M2, we have to put up with a transformer which supplies
all the voltage gain for a no-feedback follower.

In the case of the SIT-1 we have a single device surrounded by only passive
components operating common-source without feedback or even degeneration.

And so on.

Easy to make amplifiers that simply have good distortion specs - I've done it for
about 45 years. Now I'm old(er), and I only care that they are fun to listen to.

If they are instructive and entertaining to DIYers, that's all the better.

:cool:
 
The "percent distortion" is THD or Total Harmonic Distortion, and that single number is a conglomerate of all the harmonics generated. Some harmonics are much more audible than others, and this number gives no indication of which harmonics dominate the distortion. In short, the commonly used THD figure has little correlation to how an amplifier might sound.

+1

Single number THD is an essentially meaningless specification. Except when very large (NPA say 3%)

dave
 

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
If your THD = 0.0005% then every and any harmonic present is going to be inaudible etc.

That's my approach. That's not to say it's the only game in town.

Many approaches are valid. It all depends on what you want. If you want the lowest THD possible, there are ways to do that. If you aim for a certain sonic signature, you may want a bit of THD with a specific distribution of the harmonics. Or if you're trying to see the best you can do with a certain architecture, you may not get the lowest numbers on the market, just the lowest for that architecture.

As for the confidence in the numbers, I'd say "it depends". If the designer shows measurements and list the test conditions that were used when taking the measurements, I would trust them, assuming there aren't any glaring oddities to trigger my curiosity or suspicion of "photoshopping" the test results. I certainly trust the test results that come out of my Audio Precision APx525, for example.

Tom
 

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
If THD is very small i start worrying about how they got it… some really mediocre amps with THD that low.

Lots of 1970ies era amps were like that. The poor sound quality was traced down to slew-rate limitations of the internal stages of the amp. We pay attention to those things now... :)

I can bring my MOD86 amp to the Vancouver Island DIY Fest this summer so you can hear what a "no THD" amp can sound like.

Tom
 
Note the THD vs perceived sound quality plot in Figure 8.

What to note in relation to THD figures is that Brockbank and Wass showed decades ago that intermodulation distortion on music is going to outweigh harmonic distortion by two orders of magnitude or more. Hence talk about 'harmonics being inaudible' misses the point.
 

scott wurcer

Disabled Account
2004-01-26 3:03 pm
Belmont MA
Here's the full paper: Belcher (1978). Note the THD vs perceived sound quality plot in Figure 8. Also note the perceived sound quality vs multi-tone test results in Figure 7. Both measurements correlate with the perceived sound quality. The multi-tone test results just correlate more strongly.

Tom

I see the graph on figure 8 going "down" to -70dB, as far as I can see distortion by any measure has been eliminated from the equation for years. Several amplifiers have been presented here that are in the ppm level at full power, THD is a non issue.
 
The paper doesn't say anything about how those THD numbers were arrived at, seems a shortcoming of it. If they're measured into a resistor, I would agree -70dB is nowadays rather a high figure. However if they measured into a speaker load, at the speaker terminals through 5m of typical speaker wire then -70dB wouldn't be atypical but would be more speaker and cable dependent than amplifier dependent.

Some measurements of 'far end' speaker terminal distortion from amps here : http://www.eetimes.com/document.asp?doc_id=1274851
 
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tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
I see the graph on figure 8 going "down" to -70dB, as far as I can see distortion by any measure has been eliminated from the equation for years. Several amplifiers have been presented here that are in the ppm level at full power, THD is a non issue.

I agree that THD is a non-issue for many solid state designs. I have measured solid state amps designed within the last decade, which showed THD in the 0.01-0.1 % range even at relatively low power levels (1-10 W). If you're going for inaudible THD, that's not quite low enough.

Tom
 
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scott wurcer

Disabled Account
2004-01-26 3:03 pm
Belmont MA
I agree that THD is a non-issue for many solid state designs. I have measured solid state amps designed within the last decade, which showed THD in the 0.01-0.1 % range even at relatively low power levels (1-10 W). If you're going for inaudible THD, that's not quite low enough.

Tom

You will have to produce data on .01% distortion being audible. Sighted testing not acceptable.
 
The paper doesn't say anything about how those THD numbers were arrived at, seems a shortcoming of it. If they're measured into a resistor, I would agree -70dB is nowadays rather a high figure. However if they measured into a speaker load, at the speaker terminals through 5m of typical speaker wire then -70dB wouldn't be atypical but would be more speaker and cable dependent than amplifier dependent.

Some measurements of 'far end' speaker terminal distortion from amps here : Loudspeakers: Effects of amplifiers and cables - Part 5 | EE Times

I have done similar THD measurements of amplifiers when driving loudspeakers instead of resistor loads and while the distortion is a little higher, it's not appreciably so with well designed amplifiers. Of course the degree to which the distortion rises is directly proportional to how well designed the loudspeakers are. If the loudspeakers are poorly designed monster loads then you're going to see a greater rise. Also if the amplifier has been designed to drive ridiculously difficult loads that also helps.

What to note in relation to THD figures is that Brockbank and Wass showed decades ago that intermodulation distortion on music is going to outweigh harmonic distortion by two orders of magnitude or more. Hence talk about 'harmonics being inaudible' misses the point.

Yes but IMD is (generally) directly relatable to single tone THD measurements.