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Old 30th July 2011, 10:51 PM   #14501
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Originally Posted by SY View Post
Scott, if I understand what you're saying, the issue is that the surface contamination is not homogeneous. There is a roughness on a microscopic scale as well. Where metal pokes past or is not covered by contamination and can touch the other contact, there is no real resistance. Where the contamination insulates, there's far less (or far far less) conductivity. The ratio of these two areas determines how much the contact fouls the sound (in the form of excess noise, sometimes correlated with the music), modulated by vibration, creep, and other mechanical forces.

There is no metal-thing-metal heterojunction with a 1nV (or whatever) deadband, especially in common metals used for electrical contacts.

Is this a correct summation?

If so, the answer is still the same- clean and tighten contacts periodically and try to minimize them.
Essentially, but I would place the excess noise issue in a separate category. The normal model often sites electrons jumping tiny gaps which is a well intentioned oversimplification. There is a HUGE bibliography on this.
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Old 30th July 2011, 11:06 PM   #14502
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Originally Posted by scott wurcer View Post
Essentially, but I would place the excess noise issue in a separate category. The normal model often sites electrons jumping tiny gaps which is a well intentioned oversimplification. There is a HUGE bibliography on this.
Then the next test if I have time is to increase the current in the test contact. If the barrier allows microscopic contact then there should be an increase in distortion with increased current.
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Old 31st July 2011, 12:12 AM   #14503
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John Curl.....

Not sure if this is the right place for this post but I am a potential buyer of your vendetta scp1; Other than having only one power supply and a different chassis, are the guts basically the same? I can find a lot of info on the scp2 but very little on the scp1. Looking for a new phono stage to mate with my Rega P7.

By the way, I have 4 of the parasound john curl designed amps - love them. Run cool and sound great!

THANKS!
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Old 31st July 2011, 12:23 AM   #14504
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Then the next test if I have time is to increase the current in the test contact. If the barrier allows microscopic contact then there should be an increase in distortion with increased current.
Don't ignore electro-migration or other tendencies for high current density to improve the contact.
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Old 31st July 2011, 01:15 AM   #14505
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Wilco, the SCP-1 is only a gain stage. It has no EQ. It will NOT substitute for a SCP-2.
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Old 31st July 2011, 07:31 AM   #14506
wrinkle is offline wrinkle  United Kingdom
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Thank you for the explanation on the graph Scott.

In theory metal touching metal conducts, but in practice it doesn't conduct very well. I had this experience with some new 3 inch lithium coin cells I was using as a voltage reference. I stacked them to get the correct voltage, but no dice. Even when loaded with just a cap, the voltage was 3 volts off, without significant pressure applied. Used some silver contact enhancer between them to get the correct voltage.

I don't remember if I tried just cleaning the contact area with Deoxit first, but I may have.

Just other day my dad was complaining that the system was sounding bright after I had switched and then replaced the components back. I told him he was crazy (not exactly those words), because nothing had changed in the system. A week later after lots of complaining and arguing, I noticed that when I'd listen to just the left channel the sound was bright. I investigated and found the woofers weren't working because of a loose connection at the woofer binding posts.

I think metal slammed together in a vice conducts well.
Lithium coin cells that are designed for a long service life self passivate, you need to draw some current from them first to wake them up as it were, so that would have confused your assessment of what worked, if you want to repeat the experiment leave the cell for 6 months in its packet again...

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Old 31st July 2011, 08:41 AM   #14507
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Then the next test if I have time is to increase the current in the test contact. If the barrier allows microscopic contact then there should be an increase in distortion with increased current.
Ed,

This is coming late, I know, but this morning with my first cup of coffee I remembered one of your first curves showing a 'flat' portion around the zero crossing.
That looks to me as exactly 1 LSB of your equipment. Is that correct, and if so, what would that mean for the measurement?

jan didden
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Old 31st July 2011, 03:14 PM   #14508
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Ed,

This is coming late, I know, but this morning with my first cup of coffee I remembered one of your first curves showing a 'flat' portion around the zero crossing.
That looks to me as exactly 1 LSB of your equipment. Is that correct, and if so, what would that mean for the measurement?

jan didden
It was two samples long sometimes three, and it occurred where the dv/dt should have been at maximum. I do not see that result with other samples so I assume it is not the scope.

So one issue may be if this is due to contamination in a mechanical contact what are the properties of the contamination? I suspect I will try sulphur on silver wire. Once a film has formed I can cross two pieces and measure the properties of the surface barrier.
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Old 31st July 2011, 03:26 PM   #14509
SY is offline SY  United States
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Ed, do it electrochemically. More homogeneous and controllable.
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Old 31st July 2011, 04:05 PM   #14510
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Ed, do it electrochemically. More homogeneous and controllable.
The issue is; do mechanical contacts have an electrical dead zone? I and many others have the experience of a dead audio circuit temporarily returned to life by a voltage surge, such as tapping on a dead microphone. This is occurring at a level of tens of millivolts.

The switch industry considers any voltage below 12 volts to be a dry contact that will not clear itself by a small arc. The use of self wiping gold plated switches has proven successful for handling lower level signals.

The example of a microphone tap restoring function would seem to be the same arc mechanism except it happens at a much lower voltage.

In my play I showed at the limit of my scopes resolution what may have been such a dead zone inside an electrolytic capacitor. (With your knowledge of chemistry I suspect you understand why I would pick that example.)

The idea then seemed to be to look at the distortion spectra of the signal passing through a contact. If there was no disruption the spectra would be relatively clean. I measured a soldered jumper for this baseline. I tried other mechanical contacts and I found a silver contact switch that exhibits excess noise when passing a low level signal around 1 nv at 1,000 cy. I assume the contaminant is silver sulfide. This is known as a semi-conductor (IEEE Xplore - Sign In).

Now if the resulting spectra had nice even order distortions I would have suspected rectification or at least non-symmetric current flow. The result had much more than that and so much that it appears as a rise in noise level.

The issue has been raised that at a low level of contamination that there would still be metal to metal contacts that would prevent such a dead zone. Also that the surrounding air would not conduct at such low voltage levels.

So although electrochemical methods would produce a uniform and repeatable surface, I think the issue is would natural contamination from the air born coal fired power plants form a uniform film? Or more interestingly can such a film form that would revert to conductive silver at a very low level of excitation.

It may be that the restoration of conduction is not caused by an arc type clearing but rather an electrochemical response!

I think the chemistry of silver sulfite and electrical contact problems is not in dispute.
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