Blown power supply caps

big russ · 11th May 2007, 01:41 PM

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!

Hartono · 11th May 2007, 01:49 PM

wrong polarity ?

Christer · 11th May 2007, 01:57 PM

120 uF caps won't be made for very high ripple currents, so if you listen to high volume the might get very warm.

big russ · 11th May 2007, 02:34 PM

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!

giulio · 11th May 2007, 02:40 PM

If you have a multimeter, check DC polarity without the caps. If not buy a multimeter.

Good luck!

Netlist · 11th May 2007, 02:43 PM

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

I_Forgot · 11th May 2007, 03:56 PM

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

big russ · 11th May 2007, 03:58 PM

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?

Hartono · 11th May 2007, 04:18 PM

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.

drummer_Dave · 11th May 2007, 05:00 PM

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

11th May 2007, 01:41 PM	#1
big russ diyAudio Member Join Date: Jan 2004 Location: London	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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11th May 2007, 01:49 PM	#2
Hartono diyAudio Member Join Date: Nov 2002 Location: currently in China	wrong polarity ?

11th May 2007, 01:57 PM	#3
Christer diyAudio Member Join Date: Sep 2002 Location: Sweden	120 uF caps won't be made for very high ripple currents, so if you listen to high volume the might get very warm.

11th May 2007, 02:34 PM	#4
big russ diyAudio Member Join Date: Jan 2004 Location: London	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!

11th May 2007, 02:40 PM	#5
giulio diyAudio Member Join Date: Dec 2004 Location: ???	If you have a multimeter, check DC polarity without the caps. If not buy a multimeter. Good luck!

11th May 2007, 02:43 PM	#6
Netlist diyAudio Moderator Emeritus Join Date: Jan 2003	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

11th May 2007, 03:56 PM	#7
I_Forgot diyAudio Member Join Date: Jan 2005 Location: Phoenix, Az.	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

11th May 2007, 03:58 PM	#8
big russ diyAudio Member Join Date: Jan 2004 Location: London	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?

11th May 2007, 04:18 PM	#9
Hartono diyAudio Member Join Date: Nov 2002 Location: currently in China	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.

11th May 2007, 05:00 PM	#10
drummer_Dave diyAudio Member Join Date: Oct 2006	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