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Old 11th May 2007, 02:41 PM   #1
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Default Blown power supply caps

Hi there,
I'm getting a problem with blown power supply capacitors on the LM3875 inverting gainclone i've just built following the schematic on Nuuk's website.
I'm using one transformer with dual 25v secondaries for both channels but with a separate rectifier board (and caps) for each channel. The caps i currently have are 120uf - 50v electrolytics and both caps on both channels blew after 5 minutes or so.

Could it be that the transformer secondaries are too high a voltage or is there something more sinister going on?
Any ideas?

The amp works btw at least music comes out of the speakers, i just find the sound of caps blowing slightly distracting!
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Old 11th May 2007, 02:49 PM   #2
Hartono is offline Hartono  Indonesia
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wrong polarity ?
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Old 11th May 2007, 02:57 PM   #3
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120 uF caps won't be made for very high ripple currents, so if you listen to high volume the might get very warm.
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Old 11th May 2007, 03:34 PM   #4
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The polarity of the caps looks right to me, i've followed Nuuk's diagrams exactly. As for the ripple how can i tell?

I have some 1000uf - 50v caps, maybe i sould try those?
Is there anything else i should check first - i think the 1000uf would make a bigger bang!
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Old 11th May 2007, 03:40 PM   #5
giulio is offline giulio  United Kingdom
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If you have a multimeter, check DC polarity without the caps. If not buy a multimeter.

Good luck!
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Old 11th May 2007, 03:43 PM   #6
Netlist is offline Netlist  Belgium
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It takes a lot to blow a cap within 5 minutes unless the polarity is wrong or the max voltage is exceeded. Are you sure it's rated 50V?
Take out the caps, which you probably already did, disconnect the PSU from the amp and look what your DMM tells you about the voltages after the rectifier. (AC and DC)

/Hugo
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Old 11th May 2007, 04:56 PM   #7
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Electrolytic caps usually have the "-" terminal marked, not the "+" terminal. When caps are wired backwards it is usually because people expect the "+" terminal to be marked.

I_F
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Old 11th May 2007, 04:58 PM   #8
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it seems i've got about 25v ac going into the rectifier and 70v dc coming out! Guess this would explain why the caps have blown.
Why is the voltage so high though?
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Old 11th May 2007, 05:18 PM   #9
Hartono is offline Hartono  Indonesia
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well, could be the transformer problem , but if you're getting 25 v ac out of it then it's not the transformer.


maybe mistake in the rectifier wiring, and forming a voltage doubler instead of bridge rectifier.

or generally mistake in transformer to rectifier wiring, check how you connect the ground for both power supply.

disconnect the amplifer and supply capacitor and test the bridge output.
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Old 11th May 2007, 06:00 PM   #10
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It almost sounds as though you have your secondaries wired in series.

25 VAC + 25 VAC = 50 VAC

50 VAC * 1.4 = 70 VDC!!!!! (isn't that what you measured?)

quite possibly somewhere in your bridge configuration.

i dont have the schematic so i will assume you are using 2 separate bridge configurations for each pair of secondaries. or are you running one bridge and center tapping the transformer? if you are using the center tap approach make sure that the tap is grounded, that would explain the reasoning for the 70VDC if there is no ground connected.


good luck
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