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Old 30th November 2003, 05:00 AM   #1
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Default Cable distortion and "micro diodes"

In the cable directionality thread in post 144, John Curl made the following claim:

Hate to be the one to tell you, BUT there are diodes in your metal wires. More than you will ever bother to measure. I have measured them.

Indeed, John has measured the distortion of something. But is it distortion caused by "micro diodes" in the wire? Is it distortion caused by some other component of the cable? Or is it distortion produced by his test gear?

Here are three distortion spectrum plots of John's measurements of three different interconnect cables; a Radio Shack Gold series interconnect, a model from JPS Labs, and a model from van den Hul respectively:

Click the image to open in full size.

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When I first saw these plots and the claims made by John that they were distortion products caused by "micro diodes" in the wire itself, I was a bit skeptical.

While I could see such gross nonlinearities in copper wire being overlooked by the audio industry, particularly the high-end audio industry, I didn't think it was very likely to have escaped the notice of the materials science crowd.

Another thing which caught my attention was the 15.75kHz interference spike which in this set of plots seems to diminish in level along with the distortion components.

I thought perhaps what John was measuring was the distortion being produced by his test gear being variously attenuated by the loads of the different cables. There was a correlation to this and cable capacitance, with the Radio Shack cable having the highest capacitance of the three and the van den Hul having the lowest.

To make a long story short, John claimed that the interference spike changed its magnitude depending on how many TVs people in his apartment complex had turned on and that the distortion products were definitely not being produced by his test equipment, which for these plots was a 25 year old Sound Technologies 1700B which John says he's replaced some of the opamps in, and his Macintosh's internal sound card using Mac the Scope software.

So I sought to have John's measurements duplicated.

If the distortion products were being produced by his test equipment, it wouldn't serve to try and duplicate his measurements using the same analyzer so I wanted to try and duplicate his results on a more modern system, preferably an Audio Precision rig.

I don't own one or have direct access to one so I contacted a few individuals who did. While they were interested in doing the measurements, neither of them had the free time to do so at the moment.

Then about a week ago or so I came across a post on one of the newsgroups by Bruno Putzeys, an engineer at Philips' Digital Systems Lab.

Bruno had recently done a suite of tests on some audio interconnect cables using his Audio Precision System Two Cascade and reported that he was unable to turn up any distortion.

His post read in part:

Recently I've done a collection of measurements on interconnect cables to see what I could find that would explain the sonic differences that many people, including myself, have grown accustomed to hearing. The test equipment was an Audio Precision System 2 Cascade. Test objects were a handful of cables of varying construction and claims to audiophile performance.

Distortion: Not only sine wave, but also extremely complex full-spectrum multitone testing (including signal sequences derived from actual music). There was no difference between the cables tested.

Phase noise. While this would have shown up anyway in the above tests, it was separately checked at frequencies well above the audio band. Nothing showed up.

"Micro phase shifts". The AP2's resolution is so good you can read the length of a 1m cable by measuring the phase difference between input and output. Apart from this, nothing turned up.

In-Out difference. Actually, two different cables of equal length were fed the above distortion test signals in opposite phase. The two outputs were summed through a trimmable network to null the output. Well, the output nulled completely (better than 120dB across the audio band).

In short, apart from a constant time delay of a few nanoseconds (depending on length), an interconnect will have the same voltage at its output as at its input.


I replied, mentioning John's measurements and John's claim that the distortion he's measuring only shows up at low signal levels (in the case of the plots above, 30mV) and asked Bruno if he'd done any of his tests at such low levels.

Bruno replied that he had, down to several microvolts, but nothing turned up there either.

I EMailed Bruno asking him if he had any plots of his low level measurements. He responded by doing a set of measurements duplicating John's test conditions of the plots above using a 1kHz sinusoidal stimulus at 30mV.

Here are the plots with Bruno's accompanying text in bold:

Page 1. THD spectra taken from four different cables. One was a cheap A/V cable: a triple coax with serve shield, all bare copper. One was a Japanese manufactured cable with audiophile pretenses by a brand named Hisago. It was the only cable in the test field I've actually listened to, and I remember feeling the sound was exceptionally apalling (without "e" and two "l"). Conductors are bare OFC, insulator is foamed PE. The third cable was a 50ohm RF coax with solid PE insulator and bare copper conductor. Shield is tinned copper. The fourth is a coax with Teflon jacket (sheath), teflon insulator and silvered conductors/shield. Source setting was 1kHz, 30mV, 20 Ohms impedance. The plots show 256 times power averaged FFTs of the residual. This means the 30mV fundamental was notched out such that the distortion/noise performance of the ADC does not affect the result. All dBs are relative to the fundamental. The generator third harmonic at around -130dB just peeks out. No other distortion products are visible.

Click the image to open in full size.

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Page 2: Same objects, same test condition. The FFT analyzer was now set to synchronous averaging. Instead of smoothing out the noise floor, it artificially lowers all non-synchronous components (ie. noise), allowing much lower distortion levels to be resolved. Third, second and fifth harmonic are visible, at the relative levels known from the generator. The noise floor also causes some statistical deviation of the measured levels of the harmonics that are close to it. (most notably the 5th).

Click the image to open in full size.

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Page 3: The test of page 2 was repeated with the generator impedance and the input impedance set to 600 ohms. This should reinforce any nonlinearity present either in the dielectric or in the conductor. Note that the actual signal level on the cable is now 15mV, which accounts for the relative increase in noise level.

Click the image to open in full size.

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For comparative purposes, here are the levels at which John claims his tests are measuring:

Steve, the notch filter in the 1700B notches out to somewhere between -94 and -100dB This can be shown by the RESIDUAL of the THD at all levels is a little less than .002% or -94dB. Of course at very low levels, NOISE makes the residual appear to increase on the meter. This residual includes the unnotched fundamental ( the highest peak on the left side at 1KHz), the averaged distortion residual, and NOISE. At low levels, noise is all important, and this is why I must use signal averaging or very narrow bandwidth filtering to remove enough noise to look at the distortion residual. If you start with my graphs with the IKHz (notch) then you can see that the noise boundary is about -125dB. The distortion occurs in this case about -115 to -120dB, or even as much as -110dB if the notch is not complete.

Bruno's measurements using the System Two Cascade are measuring a good 20dB lower than John's measurements yet even with a cheap A/V cable of the kind typically supplied with mass-market consumer gear, there are no signs of the picket fence high order harmonics that appear in John's measurements.

Whatever John's measuring, it ain't diodes in the wires and it ain't being produced by the cable itself.

So if there are diodes in our wires and they're causing diodic type nonlinearities, their effects are flying below the radar of the System Two Cascade which is significantly below where John's measuring and in spite of John's claims, remain thus far unmeasured and unproved.

se
 
Old 30th November 2003, 05:11 AM   #2
tiroth is offline tiroth  United States
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Very thorough. As much as I have respect for Mr. Curl, there is clearly something odd going on with those graphs. Cables simply don't produce that kind of distortion, and you don't need an AP to know that. This measurement doesn't have anywhere near the fidelity, but doesn't turn up such distortion even though it passes though about 12' of Radio Shack cable. The signal level is about 8-16mV.


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Old 30th November 2003, 05:40 AM   #3
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Quote:
Originally posted by tiroth
Very thorough. As much as I have respect for Mr. Curl, there is clearly something odd going on with those graphs. Cables simply don't produce that kind of distortion, and you don't need an AP to know that. This measurement doesn't have anywhere near the fidelity, but doesn't turn up such distortion even though it passes though about 12' of Radio Shack cable. The signal level is about 8-16mV.[/IMG]
Well the first thing John would tell you is that you need to notch out that fundamental in order to achieve sufficient dynamic range to measure at the levels he's measuring at. And he'd be correct on that account.

Do you have any means of notching out the fundamental in your measurements?

Oh, and by the way, you do have the same picket fence of high order harmonics that John's got. Be aware that the horizontal axis is plotted on a linear scale in John's and Bruno's plots. Yours are logarithmic. Switch to linear and you'll see the same evenly spaced distortion components that you see in John's plots.

se
 
Old 30th November 2003, 05:59 AM   #4
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I'm getting uncomfortable in this lotus position so I have to ask: was there any need to have brought up the directionality thing with Bruno Putzeys? Perhaps moot in light of the above... Happened upon that thread today and now understand what was going on yesterday with the Bybee thread. Very interesting to have the hard data presented here. How long have you known about these results? You're not just trying to psyche out the can-hear-a-difference crowd are you?

It seems that tiroth's graphs are more like John's. Maybe John's notching wasn't so good.


JF
 
Old 30th November 2003, 06:16 AM   #5
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Quote:
Originally posted by johnferrier
I'm getting uncomfortable in this lotus position so I have to ask: was there any need to have brought up the directionality thing with Bruno Putzeys?
Huh? I didn't bring up any directionality thing with Bruno.

Quote:
Perhaps moot in light of the above... Happened upon that thread today and now understand what was going on yesterday with the Bybee thread. Very interesting to have the hard data presented here. How long have you known about the results?
I received the plots from Bruno on November 24th.

By the way, the issue of John's measurements and his claim of the distortion he's measuring being due to diodes in the wire goes back several years, most of the discussion about it done over on Audio Asylum's Cable Asylum.

It's erupted at different times since then but here's the genesis of it:

http://www.audioasylum.com/audio/cab...ges/42469.html

se
 
Old 30th November 2003, 06:26 AM   #6
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Quote:
Originally posted by johnferrier
It seems that tiroth's graphs are more like John's. Maybe John's notching wasn't so good.
Just love that edit button.

Tiroth's not notching out anything that I can tell. John says his notch is about -94dB or thereabouts. Also, you can't directly compare the decibel scale on John's plots to Bruno's. There was some gain applied after the notch and before the Mac the Scope FFT. So you'll have to go by what John says his plots represent relative to the 30mV fundamental. The dB scale on Bruno's plots is relative to 30mV.

Gets confusing.

se
 
Old 30th November 2003, 08:49 AM   #7
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Folks, I am not trying to get a Nobel prize for my efforts. I have and do measure differences in cables at levels of approximately -110dB below the test signal level, BUT I have to use lower level signals than are typically used with a THD analyzer. The graphs that I put out, that have now been put on this website, were out-takes from a series of tests that I did several years ago. They were an example of my test procedure, but they were not my best measurements. These measurements were put up in response to someone deciding that cable break-in was impossible, because he measured the cable with a Fluke multimeter and found no difference in DC measurement before and after a 'break-in' period.
I have never heard the end of it, but I still can and do make similar measurements.
Am I actually measuring cable distortion? I can't be absolutely sure, but I can say this: My measurements are repeatable, and each cable type has its own harmonic signature. Also, if I do not keep my connectors clean, I get a build up of harmonics even on my reference connectors, which will go away if I clean my connections with isopropyl alcohol. This implies some form of diodic distortion. I also know that the order of distortion changes at higher working voltage levels, like 1V or so.
Steve Eddy thinks he is the 'Amazing Randi" of audio. If he can personally discredit me or my associates, he will do so, as he has attempted to do for years.
 
Old 30th November 2003, 11:22 AM   #8
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This is all very curious. It's great that you take the time to try help with something difficult to understand.

Quote:
Originally posted by john curl
I have and do measure differences in cables at levels of approximately -110dB below the test signal level, BUT I have to use lower level signals than are typically used with a THD analyzer.
...
My measurements are repeatable, and each cable type has its own harmonic signature.
However, it seems that the Philips engineer (Bruno Putzeys) was not able to measure a difference in cables. Is there an explaination why?

Quote:
Originally posted by john curl Also, if I do not keep my connectors clean, I get a build up of harmonics even on my reference connectors, which will go away if I clean my connections with isopropyl alcohol.
Do you find connections that are left alone (never made and broken) get harmonic build up too?


JF
 
Old 30th November 2003, 12:44 PM   #9
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Quote:
Originally posted by john curl
I have and do measure differences in cables at levels of approximately -110dB below the test signal level, BUT I have to use lower level signals than are typically used with a THD analyzer.
And -110db below test signal (30mV) is getting down there. Some people consider that inaudible.


Quote:
Originally posted by john curl Steve Eddy thinks he is the 'Amazing Randi" of audio.
Probably not the intention, but it's humorous. Frankly, I wouldn't have known anything about Jack Bybee and the Amazing Randi, if it wasn't for you and Steve. So in a way, you two are advertising them.


JF
 
Old 30th November 2003, 12:58 PM   #10
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I ran into a lighter side of Steve.

The miniature clothespin:
http://db.audioasylum.com/cgi/m.pl?f...dy&r=&session=


JF
 

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