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Old 21st April 2012, 07:06 AM   #931
Mooly is online now Mooly  United Kingdom
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If you are chasing low AC hum signals on a DVM the old trick of a simple amp to boost these tiny levels to something more measurable is worth trying. I think the output would need to be AC coupled too to be universally suitable for all DVM's

This is from Radford.

(Andrew might need a few of these cascaded for that DVM mentioned)
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Old 21st April 2012, 04:08 PM   #932
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In line filters, do they usually use common mode chokes or differential mode? I thought common-mode chokes had the circle symbol and differential chokes had a figure eight on the schematic...
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Old 21st April 2012, 04:34 PM   #933
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Do you mean power lines ? The rails ?

If so you can use either.

Common mode chokes in the + & - rails effectively clean up the earth - to some extent - and because the fluxes cancel they can be small & quite cheap.

Differential chokes need much bigger cores to deal with the magnetic fluxes that are cumulative and can tent to saturate the core if there is no magnetic gap. So they tend to be big, heavy and expensive - but if used in all legs they clean the power rails very effectively.

Not sure about the symbols. I watch out for the dots at one end of the coil symbol to indicate polarity.
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Old 21st April 2012, 05:33 PM   #934
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In line filter I have used in my Fetzilla amp has differential mode chokes (Qualtek 880-06/003). Symbol for differential choke is like letter C with two arrows at the ends that is placed between two inductor symbols.

George
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Old 21st April 2012, 06:14 PM   #935
mikelm is offline mikelm  England
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whoops - lost in translation - I never did get the hang of the american language
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Old 21st April 2012, 10:01 PM   #936
JKC is offline JKC  Canada
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This is line EMI filter schematic with differential choke.

George
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Old 21st April 2012, 10:11 PM   #937
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Mine doesn't have arrows, just a circle. I thought I had seen a symbol with a figure eight though for the other kind...
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Old 22nd April 2012, 09:28 AM   #938
AndrewT is online now AndrewT  Scotland
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Is that post936 symbol a common mode inductor or a differential mode inductor?
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Old 22nd April 2012, 03:53 PM   #939
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It is symbol for differential mode inductor.

George
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Old 22nd April 2012, 04:13 PM   #940
AndrewT is online now AndrewT  Scotland
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I saw, I asked, I got the answer.

Any other answers?

Why do I ask again?

To me common mode choke requires two separate coils that have 4 pins to connect to the outside world.
I thought that differential mode chokes had one coil that requires 2 pins to connect to the outside world.

What am I misunderstanding about the difference between common and differential modes?
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