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Old 10th August 2012, 06:26 PM   #4781
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The rectification effect between to dissimilar substances in contact is as a first approximation the difference in work function between the two substances.

For typical metals this is listed as -4.5 and for pure Carbon this is -5.0

From this the rectification effect of a typical wiper on a typical Carbon track is in fact very small and will only suffer any relevant increase in the case of the metal being oxidised, and just twisting the pot back and forth a few times is in practice sufficient to remove any oxidisation.

In a semiconductor such as Cadmium disulphide is connected via "Ohmic" connections to out put terminals. Exactly how Ohmic these are can vary widely, and in something like an LDR might well be far more of a Schottky barrier than any pot wiper, measurements I have seen seem to indicate this.
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Last edited by rcw666; 10th August 2012 at 06:35 PM. Reason: misplaced decimal point
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Old 10th August 2012, 07:17 PM   #4782
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So if I understand you correctly the physics which might create a tiny problem with a pot-wiper connection could easily create a far worse problem for the end contacts of an LDR?
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Old 10th August 2012, 08:11 PM   #4783
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Yes, an Ohmic contact is one in which the electrons can cross a barrier with equal facility no matter what direction they are traveling in.

In quantum mechanics this means that the junction must be such that the electrons can tunnel through the barrier with equal ease in either direction, rather than having to cross a potential barrier that statistically favors one direction.

The mass production of semiconductor devices was enabled by developing methods of doing this, and how Ohmic the contacts actually are, is a major factor in performance.
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Old 10th August 2012, 08:27 PM   #4784
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OK. I did several solid state physics courses nearly 40 years ago but never really enjoyed them so I didn't retain much of the detail. I was more interested in elementary particles.
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Old 11th August 2012, 01:20 AM   #4785
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Curiously enough, there's no specific talk about this "diode problem" with the 'trimpots' on the Texas Component site, altho they do take a lot of trouble with the sophisticated contact system - they do emphasize the trouble taken with the connection between the resistance foil and the leads and altho this may have more to do with the physical durability of the junction of the 2 dissimilar metals, perhaps this does contribute to the better performance of these components
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Old 12th August 2012, 08:49 AM   #4786
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Just a quick question about DC coupling capacitors...

Most chip amps use a 2.2F DC coupling capacitor just after the volume pot, and I think I'm right in saying that is is supposed to protect the pot from DC. Does the Lightspeed attenuator need this cap, as I haven't used one with mine? If so, does it matter whether it is placed before or after the attenuator?
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Old 12th August 2012, 09:26 AM   #4787
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That would go (calculated) with the chipamps input impedance to create a LF roll off point to protect it from any ultra low frequencies or dc from the preceeding stage.

If the chipamps input impedance is say 10Kohm this with the 2.2uf would have a -3db down point at 7hz.

Cheers George
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Old 12th August 2012, 09:33 AM   #4788
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Thanks. I'll put one after the attenuator.

Also, is an ultrasonic filter at the input necessary, I see some people use them, other don't
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Old 12th August 2012, 10:50 AM   #4789
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No just after will be fine.

Cheers George
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Old 12th August 2012, 11:03 AM   #4790
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Thanks George. I have to say that the Lightspeed attenuator is great, very clear and smooth sounding.
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