John Curl's Blowtorch preamplifier part II - Page 15 - diyAudio
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Old 2nd July 2009, 05:47 PM   #141
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Don't you see the resistor(s) in parallel on one of the schematics?
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Old 2nd July 2009, 06:23 PM   #142
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DOH! Chalk that one up to a momentary brain fart! Somehow I deluded myself into thinking you were talking about something inherent to the choke, not seeing the obvious meaning of "external".

Thanks for the gentle reminder!
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Old 2nd July 2009, 08:03 PM   #143
Magura is offline Magura  Denmark
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Quote:
Originally posted by john curl
I like common mode chokes, hi H, low R, with external R damping.

I usually wind the inductors as common mode where applicable, this far I follow your point, but what is the benefit of using external damping VS. winding the inductors to have the desired DCR?


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Old 2nd July 2009, 08:36 PM   #144
KSTR is offline KSTR  Germany
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Damping the self-resonance of the chokes would be my guess.
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Old 2nd July 2009, 09:50 PM   #145
flg is offline flg  United States
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As long as we're putting forth educated guesses I always like to have some pure R in filters due to these inductors having leakage, parasitics, etc. that will just pass high freq garbage right through them. That is not so easy with an R-C filter...
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Old 2nd July 2009, 09:53 PM   #146
Magura is offline Magura  Denmark
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True, but it's at a very high frequency, in most cases too high to be of interest I think, no?


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Old 2nd July 2009, 11:09 PM   #147
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Fellow hi tech experts, the resistors are added to damp the resonance of the choke and the caps. Cut and try.
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Old 3rd July 2009, 06:01 AM   #148
1audio is offline 1audio  United States
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I use chokes as well BUT they are both more capable of generating interference and receiving it than any other component. When close to a rectified supply it could be a can of worms.

A well designed RF choke can have good inductance to very high frequencies. Adding a core will lower the dcr and raise the inductance, but for a more limited frequency range and current level. They are no less tricky to use than active components, and possibly harder since they are mostly custom made for an application different from yours.

Cut and try seems to be the best way to get results. Resistive damping externally avoids excess DC loss. An RC can be used effectively as well. And adding a resistor in series insures that the impedance will be positive and resistive at a minimum. Look at the self resonance of the inductor like that of a cap.
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Old 3rd July 2009, 06:12 AM   #149
PMA is offline PMA  Europe
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Quote:
Originally posted by 1audio
I use chokes as well BUT they are both more capable of generating interference and receiving it than any other component.
You have to shield them.
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Old 3rd July 2009, 06:21 AM   #150
1audio is offline 1audio  United States
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Quote:
Originally posted by PMA


You have to shield them.
Lots harder to do than it may seem to be. And the shielded chokes are limited in size and value. A Mu Metal shield around a power inductor may work, or it may saturate real fast if the current is high.

Actually this is a real problem for AC line filters. If the load is large (power amp with small conduction angle pulling 30 0r 40 A peak charging currents) the line filter chokes will saturate at the line frequency modulating any RF on the line making it worse that it might have been before the choke was inserted.
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