Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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Re: All very well and fine, except for one little detail.

Posted by Steve Eddy on November 26, 2003 at 13:12:40

In Reply to: Re: All very well and fine, except for one little detail. posted by john curl on November 26, 2003 at 10:49:24:

I must say that I am impressed by the smoothness of the distortion residual, especially not seeing any spurious input from airborne RFI. The guy must work in a RFI free zone, or a shielded enclosure. It is either that, or something is very different in our test procedure.

Well yeah. The Audio Precision doesn't do its measurements across two completely separate equipment chassis using a second length of cable.

For the life of me I can't understand why you would claim that using two completely separate pieces of equipment to do your measurements INCREASED the dynamic range of your measurements. You wouldn't split a phono stage into two completely separate chassis and expect to improve its dynamic range would you? Why would a measurement system be any different?

I stand behind my measurements, until a reasonable explanation is given why I have a problem.

And I'll stand behind Bruno's measurements until a reasonable explanation is given why his didn't turn up the levels of distortion that yours do. It's clear that the System Two Cascade is measuring significantly lower than you are measuring. And it's clearly showing the distortion of the generator which is also significantly lower than yours. Yet even with a cheap A/V cable, we don't see the picket fence of high order harmonics that appear in your measurements.

So it seems to be pretty clear that your system does have a problem. And just because the problem hasn't been pinpointed doesn't mean that there is no problem.

I still keep thinking back to the first time I saw the three plots from your Mac The Scope and that 15.75kHz spike that stuck out like a sore thumb and how the level of that spike changed with the level of high order distortion you were measuring.

In the Radio Shack plot, which had the highest distortion, the spike was highest. In the JPS Labs plot, which had the second highest distortion, the spike was a bit lower. And in the van den Hul plots, which had the lowest distortion of the three, the spike was significantly lower still.

You have claimed that this spike appears in all of your measurements and that its level changes depending on the number of people in your apartment complex who have their TVs turned on. But I find it very curious that over the course of those three measurements that the number of TVs turned on in your apartment complex just happened to change in step with the level of distortion you happened to be measuring at the time.

How have you verified that the level of that 15.75kHz spike changes depending on the number of people in your apartment complex have their TVs turned on? Did you check with all your neighbors each time you made a measurement and tally the number of TVs turned on when the measurement was made? Or is your claim that its level is due to the number of TVs in your apartment complex being turned on merely speculation on your part?

Unless you can confirm this, how can you know whether the change in the level of the 15.75kHz spike and the change in the levels of distortion aren't both symptoms of the same problem?

No explanation offered so far, makes any sense to me. Crossover distortion? Why would changing a cable bring out crossover distortion in the audio equipment. A critical capacitance value? The equipment is resistively buffered at both the oscillator and input, more than any normal audio equipment. I have matched capacitance in cables, yet found differences.

I don't claim to have the answer. However odd behavior in complex electronic circuits is hardly anything new and the answers are not awlays readily apparent or necessarily make much sense.

I'm simply approaching this from a logical point of view. By all rights if the distortion you've been measuring is indeed being produced by the cable, it should have shown up in the Audio Precision measurements. That it didn't show up (and assuming Bruno is being truthful) leaves two possibilities:

1. That for whatever reason there is a problem in your system which is producing the distortion you're measuring.

2. That the System Two Cascade which is significantly lower than where you're measuring is for whatever reason suppressing all of the harmonics of the 1kHz stimulus.

Since it's known that the System Two Cascade does indeed measure distortion products of a 1kHz stimulus in equipment known to have distortion within the range of the System Two Cascade's range, that pretty much leaves us with possibility number 1.

The first step toward addressing a problem is to first admit that you have one. Denying that you have a problem until such time as someone can tell you what the cause is gets you nowhere.

Why don't you approach this from the assumption that there is a problem and try and figure out what it is? You're always bragging about your education and many years of experience while belittling others for their lack of education and experience. Here's an opportunity for you to show that you really do have bragging rights and aren't simply thumping your chest trying to intimidate others.

My critics are under the impression that it is virtually impossible that low level distortion in an audio cable can exist, so they insist that something is wrong with my test equipment. Well, I would like to know what it is, so that I can address it. However, I don't see a problem with my test equipment at this time.

I'd never insisted that something is wrong with your test equipment. I suspected that there MIGHT BE, but I had never insisted that there was. That's why I wanted to try and have your measurements duplicated. To see if they would confirm your results. And now the same measurement has been done on four different cables and none of them show any signs of the distortion you have been measuring.

Ok, so if there's nothing wrong with your equipment, there must be something wrong with the System Two Cascade. Since you expect others to figure out what's wrong with your equipment, perhaps you'd care to tell us what's wrong with the System Two Cascade.

You'd said previously that you'd looked at its specs and concluded that it wasn't up to the task. Exactly what specs did you look at which led you to that conclusion?

Please remember folks, my critics have NOT made ANY personal effort to buy or rent the test equipment that they insist that I use for my test measurements. They just expect me to pony up $30,000 to satisfy their whims.

Please remember folks, this critic has not insisted or expected any such thing.

Also please remember folks that this critic did make an effort to have John's measurements duplicated using just such a system as John describes above (again through the good graces and generousity of Bruno Putzeys). And because those measurements didn't turn up the same levels of distortion that his measurements do, he's making every effort to detract from that by trying to make a scapegoat out of the messenger.

This way my results can always be berated for not being 'reliable' or conclusive. My problem is that I actually run the tests, and can do so at any time, and I still get the same results, except when I make a mistake, or my contacts get dirty, then even my references show extra distortion.

Who cares if you keep getting the same results? If you've got garbage going in you'll get garbage coming out now matter how many times you keep putting garbage in. If I keep beating my head against the wall, can I expect to eventually stop getting headaches?

se

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