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Old 18th March 2012, 12:55 PM   #1
kumori is offline kumori  Europe
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Angry DC on Lightspeed-attenuator possible

Hallo,
I have two questions about DC on volume pots.
1) I read a few times that DC potential on a line level volume pot is a bad thing and should be avoided. Why is it a bad thing and what happens when you apply DC on your line level pot?
2) Does this limitation also apply to the lightspeed attenuator? I thought that it might be different here since there is no whiper on a lightspeed. But this is just a guess (hope)

I'm asking, becuase I have a Naim-style preamp, which is basically a buffer followed by a volume pot, followed by the gain stage. Since the buffer already has an output cap, I would like to omit the input cap of the gain stage. My problem is that my power supply is not perfectly symmetrical, resulting in an DC offset of about 0,1 Volts at the gain stage input. Without the input cap this potential would be applied to the pot. I guess that this is not acceptable, using a regular pot. Would the lightspeed attenuator be an alternative? Or are there other possibilities?

Thanks for any help
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Old 18th March 2012, 02:20 PM   #2
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A LDR is a four terminal device, the two signal connections that your post refers to are separated by light a medium that does not connect DC. Rather the other two terminals called the anode and cathode connect DC. The Signal and DC do NOT physically touch in any manner.... other than by light see:.
http://www.silonex.com/datasheets/sp...pdf/104057.pdf

I refer you to my thread Light Dependant Resistor Current Control
Light Dependant Resistor Current Control

that discusses a complex subject ...the use of current to control the DC side, In other designs you may have seen when a voltage is applied to a potentiometer it immediately alters current as that potentiometer is turned. LDR's require current control, something a fixed voltage and potentiometer that immediately alters resistance and therefore changes current more quickly than voltage cannot do.

Cheers / Chris

Last edited by Chris Daly; 18th March 2012 at 02:45 PM.
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Old 18th March 2012, 02:45 PM   #3
DF96 is offline DF96  England
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I think you have misunderstood the OP's question. Will DC affect an LDR-based volume pot? My guess is that it won't do any long term damage, as it would to a normal pot track. It might cause excess noise, like DC in many resistor types, but I don't know if LDRs suffer from excess noise.

Personally, I would not let fear of capacitors force me to move to an LDR system. You may simply be swapping one problem for another.
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Old 18th March 2012, 02:58 PM   #4
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Quote:
Originally Posted by DF96 View Post
I think you have misunderstood the OP's question. Will DC affect an LDR-based volume pot? My guess is that it won't do any long term damage, as it would to a normal pot track. It might cause excess noise, like DC in many resistor types, but I don't know if LDRs suffer from excess noise.

Personally, I would not let fear of capacitors force me to move to an LDR system. You may simply be swapping one problem for another.
Yes DC placed on a LDR based volume pot will have similar consequence to placing DC on any other pot. My advice is to not change your Naim preamp to then emit DC.. that is just asking for trouble.

Have no fear with LDR based attenuators they easily surpass any other.
Attached Files
File Type: pdf LDR Stereo Attenuator.pdf (15.2 KB, 65 views)
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Old 19th March 2012, 08:29 AM   #5
kumori is offline kumori  Europe
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Quote:
Yes DC placed on a LDR based volume pot will have similar consequence to placing DC on any other pot. My advice is to not change your Naim preamp to then emit DC.. that is just asking for trouble.
Ok, thank you. This answers my question.
I already have a lightspeed kit half finished, it will soon be installed (with all signal caps)

Can someone please provide a short explanation what is the bad thing about DC on a volume pot?
Im not an electrical engineer, and mainly started diy-audio to learn things.
So, a short hint is welcome...

thank you, flo
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Old 19th March 2012, 09:29 AM   #6
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Quote:
Originally Posted by kumori View Post
Ok, thank you. This answers my question.
I already have a lightspeed kit half finished, it will soon be installed (with all signal caps)

Can someone please provide a short explanation what is the bad thing about DC on a volume pot?
Im not an electrical engineer, and mainly started diy-audio to learn things.
So, a short hint is welcome...

thank you, flo
DC is direct current, pots themselves as a component do not mind DC at low voltage levels, however when DC is allowed to go further into your audio system, it can cause loud noises to occur and can damage speakers and amplifiers.

There are techniques of providing a current of opposite polarity to cancel DC of a known polarity. this is just to prove that thought has gone into this subject before, not as an experiment to try. For instance The TDA1541 a elderly but very popular DAC chip has as part of its design excess current on one channel, the technique to cancel was a current of opposite polarity in this case admitting a slightly more negative voltage derived from a positive and negative connected poles, adjusted by wiper of a potentiometer placed before the i/V stage. that then neatly canceled this current prior to going into a op amp. Ben Duncan discussed this in his article Supertuning CD that was published in Hi Fi News and Record Review.

But simply put avoid Dc in signal paths, Julian Vereker
Julian Vereker - Wikipedia, the free encyclopedia designed your naim pre amp well.

Cheers / Chris

Last edited by Chris Daly; 19th March 2012 at 09:39 AM.
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Old 19th March 2012, 10:05 AM   #7
DF96 is offline DF96  England
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Quote:
pots themselves as a component do not mind DC at low voltage levels
DC on pots tends to make them noisy in the long term, especially if the wiper-track contact has to pass DC.
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Old 19th March 2012, 10:28 AM   #8
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Quote:
Originally Posted by DF96 View Post
DC on pots tends to make them noisy in the long term, especially if the wiper-track contact has to pass DC.
Here is a nice article on pots and their design. The article i note has chosen the cheaper and sadly ubiquitous,variety. Potentiometer

Cheers / Chris
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Old 19th March 2012, 10:37 AM   #9
kumori is offline kumori  Europe
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Quote:
DC on pots tends to make them noisy in the long term, especially if the wiper-track contact has to pass DC.
But this would speak for the light speed attenuator, because it has no whiper.

Chris, thanks for the link. Ill have a read this evening.

Regards
Flo
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Old 19th March 2012, 11:04 AM   #10
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Quote:
Originally Posted by kumori View Post
But this would speak for the light speed attenuator, because it has no whiper.

Chris, thanks for the link. Ill have a read this evening.

Regards
Flo
Correct on its signal side, however the cadmium sulphide etching that makes up the resistor element will still pass DC as George and myself explained. what DF96 refers to is the tendency of pots to get noisy over time when the wiper passes DC.

Just to clarify the anode cathode connection in a LDR circuit is usually controlled with a pot processing DC. There are alternatives such as using digital pots like AD5228 or DS1869, however these also have lifespan issues -however very long lifetimes. I believe it is sensible to limit voltage and current, if one has to use a pot, and to use a quality part in all cases. ..... and it is fun replacing them one day.. er much later

Cheers / Chris
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