Blown power supply caps

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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!
 
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!
 
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
 
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.
 
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
 
Thanks guys. I've traced the problem to a couple of tracks in my rectifier veroboard that i hadn't removed. Now that extra copper's gone i'm getting a healthy 36v of dc.

Getting quite a bit of hum through the speakers but i guess i just need to recheck all my connections to ground.
 
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