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Old 8th March 2004, 12:27 PM   #1
trancy is offline trancy  Slovenia
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Default PSU choke value

this is my PSU schematic (for tube buffer), and i want to include choke, but I dont know what kind of value it should be....

if I"m understand corectly, choke give caps more capacitance (theoreticly)....Am I right?


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Old 8th March 2004, 12:30 PM   #2
Magura is offline Magura  Denmark
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2mH is the rule of thumb.

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Old 8th March 2004, 04:06 PM   #3
dibbi is offline dibbi  United Kingdom
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Quote:
2mH is the rule of thumb.
NO. THAT IS WRONG!!!!

Somewhere in the region 5H-10H is usual for filter chokes for valve circuits. It depends on the current draw really. Get the PSU designer program from www.duncanamps.com and you can simulate the power supply. Its a must for power supply design.

The circuit seems a little odd to me. I'd have put the larger caps after the choke so that the power transformer is less stressed. Also IMO bypassing the later caps does more good than the earlier ones. I'd swap the positions of all the capacitors.
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Old 8th March 2004, 04:09 PM   #4
Magura is offline Magura  Denmark
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For a FET amp it sure is 2mH, I didnt know its different for tubes.

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Old 8th March 2004, 04:19 PM   #5
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Quote:
if I"m understand corectly, choke give caps more capacitance (theoreticly)....Am I right?
Not really. It does the exact opposite of what a capacitor does. It presents a high "resistance" to high frequencies.

A capacitor has a high resistance to low frequencies and passes the high frequencies easier.

So the choke tries to hold back the high frequencies while the capacitor passes the frequencies.

Because the capacitor is from the b+ to earth it "shunts" high freqs like noise and ripple to earth.

Dibbi is right that generally the choke is 5-10 H

For solid state amps the figure is so much smaller because there is a LOT more current being pulled from the PSU so 2mH usually used since making a choke 5H that can pass 5A would be pretty HUGE!


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Old 8th March 2004, 04:25 PM   #6
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First of all, reduce your first caps to like 20uF and pull one of the two 2nd stage caps. Choke should be between 5 and 50H, doesn't really matter. Anything over 5H will provide more than ample filtering but much more than 50H will have a lot of resistance. I'm assuming you're not drawing the 400mA your capacitor values suggest.

And guys, Magura is perfectly correct, that is, if he were filtering 3kHz.

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Old 8th March 2004, 11:32 PM   #7
Magura is offline Magura  Denmark
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Obviously Nelson Pass thinks its worth filtering 3kHz, cause he uses a 2mH inductor in the power supply of the SOZ.....or it could be the case that 2mH does the trick pretty much??

Ive used 2mH for a ZEN V1 with great succes.

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Old 8th March 2004, 11:41 PM   #8
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Hi,

Quote:
or it could be the case that 2mH does the trick pretty much??
Yes, for the SOZ and most other similar topologies.
Not for a tube based amp though, the voltage/current relations are diametrically opposed to what you do with semis.

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Old 9th March 2004, 03:56 AM   #9
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Yep, 2mH would also be quite sufficient if the caps above were several tens of mF. But even at 160V those would be enormous and last the preamp through the night on a single charge!

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