Amplifier distortions - what, and how much are audible

[PENG]

It seems that almost every other week, this forum will get questions on something like the following:

- Will adding a power amp to a receive improve sound quality?
- Do amps have their own sound signature even if their THD+N are "low", with flat FR 20-20,000 Hz, DF>20, SNR/DR>100 dB A weighted etc., that is, will they sound the same if the specs are the same and equally good?

I have read tons of articles on such related topics and I believe if one reads most of the one I linked below, one will find the answer, at least the answer they want to hear/read.
Otherwise, all I can say is, it depends.

For example, an amp with THD+N <= 1% at rated output may sound better than one specified <=0.05% but if both are rated <=0.05%, 20 to 20,000 Hz at output level between 0.1 W and 200 W/300 W into 8 and 4 Ohms will most likely sound the same, all else (such as FR, DF, IMD, XT, SNR/DR) being equal, and both driven to say 30 W maximum level, into an 8 ohm nominal moving coil loudspeaker. Other than that, I guess you'll have to read the good stuff, ignore the bad ones, and you'll be the judge.

Articles (I read at least parts of all of them, found no apparent misconceptions in terms of the EE theories and math. that I am familiar with):

Edit: I highly recommend the ones highlighted bold, if any of the linked articles appear under more than one section, it is because they fit under both, so for convenience I put them under the sections they fit in.

Note: The THD listening test at audiocheck.net that someone linked on ASR is fun to do, I hope some will try it and give feedback. The klippel test is fun too, I didn't do well on that so I guess SINAD of 75 dB is more than good enough for me, but again, we don't know how accurate and scientific it is, and your score would depend on your room noise, using headphone vs speakers etc.

For such test using music, try the Klippel one, that is however, more about loudspeaker distortions.

Definitions related:

Total harmonic distortion - Wikipedia
Confused About Amplifier Distortion Specs? | Analog Devices
Dyamic_Performance_Testing_of_Digital_Audio_D/A_Converters
Understanding, Calculating, and Measuring Total Harmonic Distortion (THD) - Technical Articles (allaboutcircuits.com)
Understanding instrumentation for measuring total harmonic distortion (testandmeasurementtips.com)
Total Harmonic Distortion | Universal Audio (uaudio.com)
Total Harmonic Distortion (THD): Is It a Good Indicator of Sound Quality? | Audioholics
10 Things about Audio Amplifiers You've Always Wanted to Know | Audioholics
THD and IMD Distortion—Sidebar | Audioholics Note: (I would be careful reading the first par by Steve Feistein, not saying he's wrong about anything but the part about people's sensitivity to low frequency distortions could be misleading imo).
Distortion In Power Amplifiers (douglas-self.com)
Intermodulation Distortion (sound-au.com)
Microsoft Word - AES06Gedlee_ll.doc
Microsoft PowerPoint - The Perception of Distortion.ppt [Compatibility Mode] (gedlee.azurewebsites.net)
Audio Distortion: What is It and Why Should You Care? | AV NIRVANA
Audibility of Phase Distortion (silcom.com)
Application notes - Audio Distortion Measurements (bo0385) (bksv.com)
Interpreting THD Measurements - Think dB not Percent! - Benchmark Media Systems
How Much Distortion Can We Hear With Music? - Blog | Axiom Audio | Axiom Audio (Be careful reading this one so you don't get mixed up noise with distortion, a couple times the author said distortion but he clearly meant noise).
The sound of Distortion (pmillett.com)

THD Measurements related:

Confused About Amplifier Distortion Specs? | Analog Devices
Dyamic_Performance_Testing_of_Digital_Audio_D/A_Converters
THD and THD+N – Similar, but not the same - Audio Precision (ap.com)
More about THD+N and THD - Audio Precision (ap.com)
AMH-1_2nd_Ed-4.vp (ap.com)
Mathew-AudioPrecisionSpecifications.pdf (sengpielaudio.com)
Total Harmonic Distortion | Universal Audio (uaudio.com)

Listening tests related articles/video:

Amplifier Distortion (Bob Carver, 1973) - Interviews/news - TheCarversite!
Audio Myths Workshop - YouTube
Listening Test - How it works (klippel.de)

Blind listening tests you can try (no info on what kind of harmonics introduced to the test samples):

Listening Test (klippel.de)
THD - The Extended Set (125Hz) (audiocheck.net)
Online Intermodulation Distortion (IMD) Sound Test (audiocheck.net)

Some DBT results in recent tests:

Archimago's Musings: BLIND TEST RESULTS Part II: "Is high Harmonic Distortion in music audible?" Respondent Results

Edit 1: Added one written by Doug Self, on "The distortion mechanisms of a generic power amplifier".
Since I am interested in DIY amps, about to start the 2nd one, I may buy his book:
The Audio Power Amplifier Design Handbook (douglas-self.com)

Edit 2: I think the following AP article is "must read" one for those interested in measuring distortions:
THD and THD+N – Similar, but not the same - Audio Precision (ap.com)

 

[ryanosaur]

Admittedly, my head is spinning just reading the list of articles, but this is very cool, Peng!

This is worth making it Sticky in the Amp Sub Forum! ( @shadyJ or any Mods out there? )

 

[PENG]

Admittedly, my head is spinning just reading the list of articles, but this is very cool, Peng!

This is worth making it Sticky in the Amp Sub Forum! ( @shadyJ or any Mods out there? )

I didn't expect you to read them all in an hour, its okay to take a whole day on Sunday.

Talk about Shady, his article is fantastic, and some good links in Matthew's too. Audiohlics.com's articles are hard to beat.

Did you try the Klippel blind tests? I gave up after passing up to -30 dB, I am sure I could do better, but the room was beginning to get too noisy, someone turned on the oven..and I could see it could go for a few hours.

Would love to know your score, only if you don't mind.

 

[ryanosaur]

Did you try the Klippel blind tests?

Doing other stuff... gonna try later when I have time.

 

[AcuDefTechGuy]

Most of us have been saying for years that we just don't have those damn golden ears.

But then again, we know who we are.

THD, Noise, THD+N have been talked about and measured for the last 40+ years?

Didn't seem like many non-golden-ear people cared that much about those numbers the last 40 years because it was inaudible and many people measured those numbers, so it seem commonplace.

But now that most publications have stopped doing these inaudible measurements, it seems like the last 40 years have been wiped out of existence. And now many people all of the sudden cared about these inaudible numbers again.

I think it's a good idea PENG is putting these measurements in proper perspective.

If we can't hear THD+N of -75dB, why do we care if these electronics measure -100dB or better?

But the opposing view is that audibility isn't even the point at all. It's simply about getting that last drop of prestine measurement number for the pure art of science, damn practically, sense and sensibility.
They just don't want to buy anything with THD+N of less than -100dB.

 

[shadyJ]

Admittedly, my head is spinning just reading the list of articles, but this is very cool, Peng!

This is worth making it Sticky in the Amp Sub Forum! ( @shadyJ or any Mods out there? )

I just do little things like delete spam. I am open to making this thread a sticky, if the other mods are OK with that.

 

[lovinthehd]

Then again you'll have those that won't bother with actual technical info/research and just want someone to tell them this sounds good like all those things they've read on the interwebs where that seems to be a "thing".

 

[Steve81]

Would love to know your score, only if you don't mind.

FWIW

 

[Pogre]

You put some work into this one peng! I'll try the test in a little while when I have some time. I'm about to start work right now tho.

 

[highfigh]

It seems that almost every other week, this forum will get questions on something like the following:

- Will adding a power amp to a receive improve sound quality?
- Do amps have their own sound signature even if their THD+N are "low", with flat FR 20-20,000 Hz, DF>20, SNR/DR>100 dB A weighted etc., that is, will they sound the same if the specs are the same and equally good?

I have read tons of articles on such related topics and I believe if one reads most of the one I linked below, one will find the answer, at least the answer they want to hear/read.
Otherwise, all I can say is, it depends.

For example, an amp with THD+N <= 1% at rated output may sound better than one specified <=0.05% but if both are rated <=0.05%, 20 to 20,000 Hz at output level between 0.1 W and 200 W/300 W into 8 and 4 Ohms will most likely sound the same, all else (such as FR, DF, IMD, XT, SNR/DR) being equal, and both driven to say 30 W maximum level, into an 8 ohm nominal moving coil loudspeaker. Other than that, I guess you'll have to read the good stuff, ignore the bad ones, and be the judge.

Articles (I read most parts of them, found no apparent misconceptions in terms of the EE theories and math. that I am familiar with):

Note: The THD listening test at audiocheck.net that someone linked on ASR is fun to do, I hope some will try it and give feedback. The klippel test is fun too, I didn't do well on that so I guess SINAD of 75 dB is more than good enough for me, but again, we don't know how accurate and scientific it is, and your score would depend on your room noise, using headphone vs speakers etc.

Definitions related:

Total harmonic distortion - Wikipedia
Dyamic_Performance_Testing_of_Digital_Audio_D/A_Converters
Understanding, Calculating, and Measuring Total Harmonic Distortion (THD) - Technical Articles (allaboutcircuits.com)
Understanding instrumentation for measuring total harmonic distortion (testandmeasurementtips.com)
Total Harmonic Distortion | Universal Audio (uaudio.com)


Total Harmonic Distortion (THD): Is It a Good Indicator of Sound Quality? | Audioholics
10 Things about Audio Amplifiers You've Always Wanted to Know | Audioholics
Audio Distortion: What is It and Why Should You Care? | AV NIRVANA
Audibility of Phase Distortion (silcom.com)
Application notes - Audio Distortion Measurements (bo0385) (bksv.com)
Interpreting THD Measurements - Think dB not Percent! - Benchmark Media Systems
How Much Distortion Can We Hear With Music? - Blog | Axiom Audio | Axiom Audio (Be careful reading this one so you don't get mixed up noise with distortion, a couple times the author said distortion but he clearly meant noise).
The sound of Distortion (pmillett.com)

THD Measurements related:

Dyamic_Performance_Testing_of_Digital_Audio_D/A_Converters
THD and THD+N – Similar, but not the same - Audio Precision (ap.com)
More about THD+N and THD - Audio Precision (ap.com)
AMH-1_2nd_Ed-4.vp (ap.com)
Mathew-AudioPrecisionSpecifications.pdf (sengpielaudio.com)
Total Harmonic Distortion | Universal Audio (uaudio.com)

Listening tests related articles/video:

Amplifier Distortion (Bob Carver, 1973) - Interviews/news - TheCarversite!
Audio Myths Workshop - YouTube
Listening Test - How it works (klippel.de)

Blind listening tests you can try (no info on what kind of harmonics introduced to the test samples):

Listening Test (klippel.de)
THD - The Extended Set (125Hz) (audiocheck.net)

Some DBT results in recent tests:

Archimago's Musings: BLIND TEST RESULTS Part II: "Is high Harmonic Distortion in music audible?" Respondent Results

Why don't I see anything about IM distortion? THAT'S what we need to be concerned with since music is made up of complex signals. THD only tests with pure tones and we almost never hear those.

 

[cpp]

I get to the point, I pick the amp or AVR I like, not based on measurements or reviews but how my ears react to what I hear and I hear alone via my speakers, my source and in my room.. Be it tubes, SS or a Hybrid. I really don't care. Its what I like.

 

[PENG]

Why don't I see anything about IM distortion? THAT'S what we need to be concerned with since music is made up of complex signals. THD only tests with pure tones and we almost never hear those.

Thank you, I meant to include IMD but forgot. Good thing it is still editable so it's done.
One thing about IMD is that it is related to HD, and if THD is low, IMD tends to be low too.

I also added the link to an IMD listening test, again, mainly just for fun.

There are other factors too obviously such as FR, SNR, DR, DF but those are easier to understand and hear, much less controversial for sure. Another important one is IMD but from what I gather, like DF, TIM, or TIMD is no longer consider much of an issue:

Intermodulation Distortion (sound-au.com)

"..........TIM (transient intermodulation distortion) (aka TID - transient induced distortion) has been the topic of considerable research, and although it's now considered to be largely irrelevant,......................."

My guess is that slew rate used to be more of a challenge, but modern amps generally have more than high enough slew rate.

 

[PENG]

I get to the point, I pick the amp or AVR I like, not based on measurements or reviews but how my ears react to what I hear and I hear alone via my speakers, my source and in my room.. Be it tubes, SS or a Hybrid. I really don't care. Its what I like.

Yep, that's what many people do, but many don't. We also know there are the so called subjectivists and objectivists after all.

This thread is more about knowledge relevant to the topic though, and I think some of those who don't care about specs and/or measurements may still be interested in knowing what distortions are about, how are they quantified, specified and measured.

 

[cpp]

Yep, that's what many people do, but many don't. We also know there are the so called subjectivists and objectivists after all.

This thread is more about knowledge relevant to the topic though, and I think some of those who don't care about specs and/or measurements may still be interested in knowing what distortions are about, how are they quantified, specified and measured.

Oh agree, but too many times, not saying on this site, but some others, if you are not using measurements then "you basically don't have a clue". Shame. In my past life working, measurements is all we used along with good research and engineering and in depth testing. But in this case, my ears is all I got to go by.

 

[highfigh]

Thank you, I meant to include IMD but forgot. Good thing it is still editable so it's done.
One thing about IMD is that it is related to HD, and if THD is low, IMD tends to be low too.

I also added the link to an IMD listening test, again, mainly just for fun.

There are other factors too obviously such as FR, SNR, DR, DF but those are easier to understand and hear, much less controversial for sure. Another important one is IMD but from what I gather, like DF, TIM, or TIMD is no longer consider much of an issue:

Intermodulation Distortion (sound-au.com)

"..........TIM (transient intermodulation distortion) (aka TID - transient induced distortion) has been the topic of considerable research, and although it's now considered to be largely irrelevant,......................."

My guess is that slew rate used to be more of a challenge, but modern amps generally have more than high enough slew rate.

It's related, kind of- plenty of amplifier designs were good with THD, but not TIM. It's the whole 'Sum & Difference' thing that bites some designs on the butt- if the source material can mask it, it's not too obvious.

 

[Pogre]

Note: The THD listening test at audiocheck.net that someone linked on ASR is fun to do, I hope some will try it and give feedback. The klippel test is fun too, I didn't do well on that so I guess SINAD of 75 dB is more than good enough for me, but again, we don't know how accurate and scientific it is, and your score would depend on your room noise, using headphone vs speakers etc.

Okay, that was kinda fun. My results line up with what they're saying about higher frequencies masking distortion. I could hear it most easily at 31 and 63 hz.

At 31 hz I could hear it from my seat down to 0.5%. At 0.1% I could not hear it from my seat, but when I moved closer I could. I had to have my ear right against my speaker to hear 0.05%. I'm inconclusive at 0.01%. I *think* I can hear it with my ear right up to the speaker, but the neighbor's ac keeps running and I can't tell if that's what I'm hearing. 0.005% is completely inaudible.

It is interesting that as I climbed in frequency it became harder to detect. By the time I got to 1000 hz I was struggling to hear the distortion at 1% from my seat. I can hear 1% from my seat pretty easily below it.

 

[PENG]

You put some work into this one peng! I'll try the test in a little while when I have some time. I'm about to start work right now tho.

It was done over time, when I read something I found informative I bookmarked them. So not really hard work, but thanks.

 

[PENG]

Okay, that was kinda fun. My results line up with what they're saying about higher frequencies masking distortion. I could hear it most easily at 31 and 63 hz.

At 31 hz I could hear it from my seat down to 0.5%. At 0.1% I could not hear it from my seat, but when I moved closer I could. I had to have my ear right against my speaker to hear 0.05%. I'm inconclusive at 0.01%. I *think* I can hear it with my ear right up to the speaker, but the neighbor's ac keeps running and I can't tell if that's what I'm hearing. 0.005% is completely inaudible.

It is interesting that as I climbed in frequency it became harder to detect. By the time I got to 1000 hz I was struggling to hear the distortion at 1% from my seat. I can hear 1% from my seat pretty easily below it.

You just highlighted one example how misconceptions could become hearsay and then "truth", given time on the internet. We often read how how the opposite was "true" that people are not sensitive to low frequency distortions but very sensitive to high frequency distortions so be careful with so called low 0.05% THD at higher frequencies but no concern with even 10% distortions in the bass frequencies. Such talks, as you found out now is very misleading, and that's why I added cautionary notes in parentheses to the Axiom, and Audioholics articles. I had to read those twice to convince myself there were correct but worded in ways I believe people could be mislead and may misrepresent them by quoting them out of context or using different wording.

JA of Stereophile often measure THD with ower frequency sine waves, in addition to the typical 1 kHz, so did Amir of ASR. Harmonics are multiples of the fundamental frequency of the test signal. So by the time you get to 1,000 Hz, the 2nd harmonics would be at 2,000 Hz and the 3rd would be 3,000 Hz, right where humans are very sensitive to. If you climb up to say 6,000 Hz, the 2nd and 3rd harmonics would be at 12,000 and 18,000 Hz, how many of us over the age of even 35 have no hearing loss at frequencies that high?

 

[PENG]

Okay, that was kinda fun. My results line up with what they're saying about higher frequencies masking distortion. I could hear it most easily at 31 and 63 hz.

It takes longer, so need patience, but you may find it more fun with the Klippel test because you can select music instead of tones. Its gear to speaker's distortions but to me the result is applicable to amps too to some extent.

 

[Pogre]

It takes longer, so need patience, but you may find it more fun with the Klippel test because you can select music instead of tones. Its gear to speaker's distortions but to me the result is applicable to amps too to some extent.

I did try the klippel test but started getting impatient and kinda rushed through the last few tests. -21 they said, which is a li'l embarrasing? I dunno. With a little more practice and maybe some headphones I know I could do better.