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Old 7th April 2007, 11:36 PM   #1
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Default Rambling on about cables and perception

Long ramble on a couple topics here, and the point is actually in the very last paragraphs- got a cold and sore throat so thought it would be a good day to do something quiet on the electronics bench. In another thread, the idea came up that different conductor materials, silver for instance, might affect sound. I happened to have some pure silver tubing, so I made up a 30" parallel run of 3/8" tubing, spaced about 30mm apart, with an RCA jack on each end. I made up a similar test piece using brass tubing, figuring that had as good a chance as anything of sounding really bad. The numbers to note would be the very low capacitance of each assembly, about 10 pF, and the very low dissipation factor, since air is the insulator. The silver also has a resistance down in the milliohms, and is equivalent to something like AWG 9 wire. Since it's tubing, skin effect isn't an issue, and the thing is probably good to many GHz if one figured out the characteristic impedance for a parallel line. Note that these "cables" are pretty awkward, since they are completely non-flexible.

Since I have a cold, I didn't do much with actual listening, other than to listen to some white noise with each piece inserted in-line with some headphones. Eventually I'll do some real listening in my system. I can convince myself that the silver sounded a bit smoother, but as you'll see in a bit, I don't trust that conclusion as far as I can throw it.

As a first check, I paralleled a 20k resistor with my scope input, so I had that plus it's 15 pF. That's probably not far from a pre or power input. I then terminated my sig gen with 50 ohms, plus a 150 ohm series resistor. Again, similar to what you might find on a preamp output. Connected the scope and sig gen together and started looking at sine and square waves. Pretty much flawless perfect edges and corners, as one would expect. Then I inserted the silver and brass tube assemblies between the instruments. Still looked ok at audio frequencies- I would have been shocked if it didn't, but I noticed a bit of overshoot on the square wave edges, and a little glitch on the peak and valley of the sine wave. Silver or brass gave the same result. Now, to digress, you'll find various signal generators on my bench, but I tend to use a Wavetec 185 sweep gen for most things. It's ok, but not super low distortion, and has a tiny point on the top and bottom of the sine wave, from the internal conversion method. IMO, this is a feature, not a flaw! I also still keep some "boatanchor" scopes around, because the long CRTs can be focused to razor sharpness, and I don't usually need more than 40 Mhz for the things I do. They let you see fuzz that's otherwise invisible. Though I have other scopes, and can borrow decent digital scopes from work, IMO, their 8 bit vertical and LCD displays are useless for audio work.

Ok, so crank the frequency up to about one Mhz, and here's all this ringing around the square wave edges and the peak and valley of the sine wave. Not subtle stuff, but a good 3-10 cycles of significant amplitude, at 30 Mhz. Who gives a rat's behind, you ask- that's way above audio. Agreed. OTOH, unless you've tested your amp with some extra 30 Mhz injected into the input, who knows. I haven't, but I still expect signal integrity, with damping, not high Q ringing, at pretty much any frequency. That amp test is a test for another day.

So now I started replacing the super conductive, low resistance, high Q tubes, with standard audio cables. Here's where it gets interesting. My old favorite Belden 8421 low capacitance cable rang almost as much as the tubing. Higher capacitance cables showed no ringing at all. The value of the 150 ohms output resistor has a large influence on the ringing, as expected, and not using it can make almost any cable a disaster. Also, and parallel capacitance at the input of the cable (output of the sig gen) increases the ringing, whereas any extra capacitance on the output end (the scope or power amp) will kill the ringing. Maybe ultralow capacitance cable isn't the ideal? Before you bring it up, I'm not addressing dielectrics, DF, and the like, right this second.

IMO, that's a fairly valuable lesson when designing preamp outputs and amplifier inputs, and RF filtering should never be overlooked.

None of this is any surprise really, since it's all just ordinary cable and termination theory. Here's the good part. I started holding a small silver mica cap across the conductors, while listening to white noise on the headphones. It was really obvious that when I touched the capacitor across the conductors, the noise became a tiny bit smoother, because the highs were knocked down a bit. Makes perfect sense. Then I got confused and discovered that I could easily convince myself that the capacitor increased the high frequency response. In fact, the capacitor was having no effect at all, but the act of expecting a certain change, and the motion of my hand to apply the capacitor, made it happen. The scary thing is that I could do it either way entirely at will. Bottom line, as I've always believed, is that audio comparisons are entirely BS, unless the effect is large and obvious, or the test is done double blind.
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Old 8th April 2007, 12:28 AM   #2
v-bro is offline v-bro  Netherlands
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I second this totally, human ears are really clunkers... Nice of you to share this test

Any pictures of the scope images would have been nice though...

I've tested lots of socalled 'audiophile' cables upto extremely high prices (siltech, ixos, rhapsody, vandenHul etc. etc.) during the four years I worked in a hifi shop and back than allready came to the conclusion that either I didn't have adequate ears or all of it was BS...

Nevertheless I like certain cables for certain purposes, and there are differences (like for my MP3 player with FM radio I prefer to use badly or unshielded cable as the cable is also the antenna....).

I also like flexible cable for minijack plugs, I like Neutrik RCA plugs, I like thick (rugged) RCA cable, I do have very thick speaker cable, but not very expensive and just plain copper...(talking about the ones I use, because I own some silver vandenhul speakers cable and RCA cables with silver plated copper wire, but no audible difference IMO...don't use them becasue they're too short and too thick to handle... )

I also solder a lot with silver (5%), but mainly because it is tougher material and seems to fix things like board connectors better to whitstand mechanical force...

Just my
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Old 8th April 2007, 01:11 AM   #3
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Default Re: Rambling on about cables and perception

Quote:
Originally posted by Conrad Hoffman
Bottom line, as I've always believed, is that audio comparisons are entirely BS, unless the effect is large and obvious, or the test is done double blind.
Absolutely!!
Golden ears or golden ego???
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