Guest Video - Karl Adams - Audio Distortion Measurement

[EEVblog]

In this guest video Karl Adams shows us how to measure low levels of distortion in audio circuits. The example used is an LM1875 in a compound configuration.

https://www.youtube.com/user/Keicar67

[CrashO]

I think a link to his channel in the description would help him a lot. (Joel's video had links?)
Not everyone reads the forum, (or watches the last 5 seconds of the video for the links). 

_{*I linked his channel in the comments, but it got removed for some reason}

[SAI_Peregrinus]

Mike Renardson's site has some good info on measuring distortion in audio amplifiers. http://renardson-audio.com/dtest.html

[witnessmenow]

I think a link to his channel in the description would help him a lot. (Joel's video had links?)
Not everyone reads the forum, (or watches the last 5 seconds of the video for the links). 

_{*I linked his channel in the comments, but it got removed for some reason}

I did too, it's anti spam measures though. It's currently sitting in Dave's held for review box. All comments that contain other YouTube Urls go there.

If Karl posted a comment asking people to click through to his channel we could all like it and it could go to the top?

EDIT: Just checked, it's in the description now 

[InterestedTom]

Question: Is it really necessary to have a very high spectral purity input to measure vanishingly low distortions?

Surely since all distortion measurements are actually just the change in distortion from the input to the output does it matter whether you use a normal 14 bit signal generator or a special ultra-low distortion oscillator?

I would have thought that the change in the difference in distortion is pretty small, even with "only" 14 bits.

[Brumby]

It seems to me that to answer your question, you should try defining a setup where you do what you suggest ... then look at all the points where it might be problematic.

I'm no T&M guru, but my first thoughts were of a number of issues where, instead of having to supply one high quality source, you needed to provide a lot more high quality stuff.


IMO, it's not impossible - just hugely impractical.

[amirm]

Question: Is it really necessary to have a very high spectral purity input to measure vanishingly low distortions?

Surely since all distortion measurements are actually just the change in distortion from the input to the output does it matter whether you use a normal 14 bit signal generator or a special ultra-low distortion oscillator?

I would have thought that the change in the difference in distortion is pretty small, even with "only" 14 bits.

It depends on what you are measuring.  I recently did a bunch of measurements showing the differences in USB cables when driving an external DAC (yes, you read that right ).  Here is the article and measurement from it: https://audiosciencereview.com/forum/index.php?threads/do-usb-audio-cables-make-a-difference.1887/



As you see, all the spurs are at -128 dbFS and lower.   This is done with 24 bit ADCs with some 32 db of FFT gain.  Otherwise the noise floor of the measurement system will be well above the distortion spikes we want to see.

Now the source here was digital.  With respect to analog in/out systems, ideally we would want the harmonic distortion to be below our noise floor as to not be confused with the harmonics generated by the device under test.  I suppose if we knew the signature of the source we could try to subtract it visually but ideally you want to be free of that.  Just as I am in the above measurement with respect to distortions of my Audio Precision Analyzer.

[Berni]

Yes you want very low distortion in your test setup mainly because its difficult to tell apart what what distortion is your test setup and what is your device under test.

If your test setup has higher distortion that the DUT then the final distortion will only change by a tiny bit, also it doesn't just sum together since the phase of the harmonics matters too, so its possible for the harmonics of your test setup and your DUT to destructively interfere and result ina harmonic peak thats actually slightly lower.

Oh and on that USB testing, did you ever try any of those ferite lumps on your cables? It seams to me whats causing it could be common mode interference over USB getting modulated into the signal.

[amirm]

Oh and on that USB testing, did you ever try any of those ferite lumps on your cables? It seams to me whats causing it could be common mode interference over USB getting modulated into the signal.

Common mode chokes for USB have responses well over hundreds of Kilohertz/Megahertz.  What I am showing is in the audio band so I don't expect them to do any good.

That said, there are some audiophiles who believe in wrapping their cables with aluminum foil!  Don't laugh but I next plan to measure the effects of that.  I can also try the choke there and see what happens.

I should say that I am using a DAC that has high susceptibility to such noises (Schiit Modi 2).  Other DACs (even cheap ones) don't show such differences or remotely as much.

[bitseeker]

Nice tests, Amir. I like the reveal at the end of the article.

[Berni]

Well GSM phones operate at 800 to 1900 MHz but audio equipment loves to pick it up anyway, tho the protocol used there makes it especially good at this because the transmit power is constantly pulsed at low frequency rates. This is mostly due to the RF signal exploiting a nonlinearity inside a component to AM demodulate itself on it, but there are other complex way it can modulate itself into other signals.

The aluminum foil id expect to have no effect because USB cables are always shielded inside anyway, but id say the only way to find out for sure is to test it.

Oh and i also seen a dangerous practice of cutting the 5V and ground wires in the cable and supplying there own cleaner 5V supply to try and improve a USB DAC. I wouldn't test that one before at least placing a crocodile clip between the case of the DAC and the PC to avoid killing the USB data lines with common mode voltages.

[InterestedTom]

Some interesting responses 

I was thinking of Douglas Self's Power amplifier book. In some of the measurements he makes the distortion of his Audio precision system is plotted as a reference trace, to show that the distortion at some frequencies is almost entirely from the measurement system, rather than the DUT.

As pointed out, directly subtracting the THD figures could cause some errors. I wouldn't like to rely on the idea that all the distortion is only directly additive.

However, if it was done within the measurement system, it might be possible that phase could be accounted for, i.e. by using the complex output of the FFT, rather than by subtracting the magnitudes?

[EEVblog]

Well GSM phones operate at 800 to 1900 MHz but audio equipment loves to pick it up anyway, tho the protocol used there makes it especially good at this because the transmit power is constantly pulsed at low frequency rates. This is mostly due to the RF signal exploiting a nonlinearity inside a component to AM demodulate itself on it, but there are other complex way it can modulate itself into other signals.

[amirm]

Nice tests, Amir. I like the reveal at the end of the article.

Thank you so much.  Just like Dave, I strive for some entertainment value in my audio articles. 

[amirm]

Well GSM phones operate at 800 to 1900 MHz but audio equipment loves to pick it up anyway, tho the protocol used there makes it especially good at this because the transmit power is constantly pulsed at low frequency rates. This is mostly due to the RF signal exploiting a nonlinearity inside a component to AM demodulate itself on it, but there are other complex way it can modulate itself into other signals.

I am familiar with that.  Putting aside the AM demodulation for a second, and using the 217 Hz frequency from Dave's video, majority of power would be in the first few odd harmonics.  As I noted, common mode chokes have no filtering ability in that range.  Here is an example: http://www.fair-rite.com/wpapers_anotes/ferrite-cores-for-low-frequency-emi-cable-suppression-2/



As you know, these are all made to reduce conduction emissions which are not measured remotely close to audio band.

I have by the way tested modern phones close to USB cables and have not found a measurable effect.  It was a small test though.

Recently however I ran into this problem that someone post common mode noise leading into audio ground from DECT "wireless" phones:



I plan to replicate this. 

Note: he corrected himself later that what he says at the beginning with iFi USB filter fixing this is not correct.  There are a ton of devices being sold to "clean up USB" but none of them are effective.  See this one as an example: https://audiosciencereview.com/forum/index.php?threads/uptone-iso-regen-review-and-measurements.1829/.  The company behind has sold some $2M worth of these products!

[amirm]

The aluminum foil id expect to have no effect because USB cables are always shielded inside anyway, but id say the only way to find out for sure is to test it.

Well, I test things not because I believe in them.    I test them because so many others do and without objective data that showing that, the argument just becomes war of words.  Indeed that is why I created AudioScienceReview.  I like to have data behind our engineering opinions, even if it confirms what we already know.  Interestingly enough, the data sometimes shows things we don't even know!  The USB cable is one such example of showing actual analog output of a DAC depending on which USB cable you use.  Every engineer out there assumes it is a "digital interface" and hence can't impact the analog output of the DAC.

This whole "audio tweak" category has become massive.  It is no longer just beefy audio cables.  They sell any and all things.  Take these "ERS" papers from Stillpoint: http://www.stillpoints.us/index.php/product/ers

"The ERS ( EMI- RFI Suppression) material redirects, absorbs, and diffuses EMI/RFI frequencies.  The benefits of ERS for audio/video are best heard with digital circuits due to the RFI creating aspect of the circuitry itself. Benefits of ERS can also be heard with AC power lines, signal cables, loudspeakers and electronics of all types.

EMI/RFI materials have been around for a long time. Most of them are heavy and hard to apply. Many need to be grounded to be effective. None of them are as light weight or easy to deploy as ERS, where grounding is not necessary.

WHAT IS IT MADE OF?
The core of ERS is made of a blend of carbon fibres of various lengths and sizes. Using a proprietary method, these fibres are coated with metals, such as nickel. The exterior is laminated with a polyester textile on both sides to make the material easier to handle and use."

Yeh I know.    Check out this dude putting this paper everywhere:



No I don't know why he is whispering.