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Old 22nd June 2004, 04:21 PM   #1
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Default Large power supply with Leach Amp

I plan on using 2 68000uF 50V caps in series on each side of my leach amp power supply connected to a 80VCT 800VA torodial transformer. I'm going to be driving some loads that dip into the 2.85ohm range and I'm wondering if the 200V 25A Bridge Rectifier (DigiKey 26MB20A-ND) will be able to handle the current or if I'd need a different one. Any other advice about this power supply design would be helpful as well.
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Old 22nd June 2004, 04:51 PM   #2
markp is offline markp  United States
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You are better off using a bunch of 10000 at 80 volt caps and a larger bridge like 35 amp at 400 volt. With series caps that big you are not going to have the same actual capacitance on both the plus and minus sides as the tollerance of caps is 20% or more. You should at least balance the caps with 1% resistors so each gets the same voltage. Using them to save money is going to hurt the project rather than help by getting a big supply out of it, after all, 34000 isnt that big.
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Old 22nd June 2004, 05:39 PM   #3
K-amps is offline K-amps  United States
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I have several 50v 68000uF caps. Let me know if interested. (My cap meter only reads till 20,000uF otherwise I would have matched them for you.)

If you plan on buying 3 pairs, I could match the closest (by series connecting and measuring.... but I guess 3X 68000uF per rail would be too much of a killer.
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Old 22nd June 2004, 09:23 PM   #4
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Interesting, I hadn't even considered the tolerances. What do you think about 3 80V 12,000uF caps wired in parallel on each side? Would it still need the resistor? If so, please explain how I would use a resistor to balance the voltage... Also, I was under the impression that 34,000uF per side was a VERY large amount of capacitance for this amp, is that not true?
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Old 22nd June 2004, 09:30 PM   #5
markp is offline markp  United States
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Yes, 36,000 is big! Use the 3 x 12,000uF in parallel to get 36,000uF per side and you will have a very large power supply. In parallel you dont need to worry about the tolerance of the caps. I use 16 x 6,800 in one of my amps! Now that is big, but there are bigger.
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Old 22nd June 2004, 09:37 PM   #6
K-amps is offline K-amps  United States
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Mark I am curious, why dont you need to worry about tolerances ina parallel situation? I guess with the law of averages, you are better off but you still could have a mis-matches set... right?
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Old 22nd June 2004, 09:41 PM   #7
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Why is it that I don't have to worry about tolerances if the capacitors are wired in parallel, but I do if they are in series? Is there a way to solve the for the size of the transformer (in Amps), the voltage rating and the amperage rating of the bridge rectifier, and the amount of capacitance I will need in order to drive a load of a particular resistance? For instance:

I want to know how large of a transformer, what kind of bridge rectifier, and how much capacitance I will need in the power supply to be able to power a 2 channel Leach Amp, each channel driving a 2ohm load.
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Old 22nd June 2004, 09:46 PM   #8
K-amps is offline K-amps  United States
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I guess what mark means is that by having several caps, their tolerances will average out... still not fool proof nor better than a matched pair but its one way...

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Old 24th June 2004, 02:00 AM   #9
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Hmm, I don't have a capacitance meter so there really isn't a good way to tell if they are "close'. What would happen if one side of the power supply has more capacitance than the other? Is there a way to solve the for the size of the transformer (in Amps), the voltage rating and the amperage rating of the bridge rectifier, and the amount of capacitance I will need in order to drive a load of a particular resistance? For instance:

I want to know how large of a transformer, what kind of bridge rectifier, and how much capacitance I will need in the power supply to be able to power a 2 channel Leach Amp, each channel driving a 2ohm load.
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Old 24th June 2004, 03:37 AM   #10
K-amps is offline K-amps  United States
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Many well respected people use the yardstick of 10,000 uF per rail for a 100 wpc amplifier (2 channels) so for 2 ohm loading, this amp would need 40,000 uF per side... this is more than enough.

The krell KSA-250 uses 80,000uF per rail and that can drive a 1 ohm load to 2000watts!

Some 375 watt carver used 13000 uF per rail. So as you can see there is a lot of variance and they all are proven commercial offerings.

BTW Motion, a 80VCT Trafo will give you rails of +/-55 volts (or loaded 53v) and that will probably fry your 50v caps.....
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