Audio interface keeps blowing capacitors

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mitaux8030

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I have a PRO2022 scanner that I have modded with a line level output, using a LM358. The output of this uses a 10uF coupling capacitor to a 1/4 inch jack on the back panel. From there, I use a 3k>3k isolation transformer, and that feeds the line level input on a PC soundcard to record.
My problem is the 10uF capacitor keeps 'blowing up' - not literally, but it definitely develops a problem in passing audio. I've replaced it three times so far, first was a regular electrolytic, then a tantalum, then a SMD tantalum. When I replace the cap, everything comes good for a few months, then the problem comes back again.

Anyone have any ideas on what to do to stop this problem? I thought the 3k isolation transformer would have kept everything isolated enough to not cause any problems... or might it be passing enough 60Hz AC differential in ground potentials to cause the cap to die? (PC using a switch mode power supply, PRO2022 uses an external switch mode supply)
 

krokus

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What voltage rating is the cap?

Why are you bothering with the cap? The 358 should be buffering the audio, and you are feeding a transformer.
 
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kb0nly

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Either increase the voltage rating of the cap or just leave it out and see how it fairs...
 

mitaux8030

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The cap is there to provide some DC isolation. Remember that the chain goes like:
Audio tap > LM358 > 10uF > back panel jack > cable with isolator transformer built in > PC line input.

At sometime in future, there could be something else plugged in to that back panel jack eg a tape recorder with straight patch cord. So removing the cap isn't a good idea I think.

Besides, if the cap is the sacraficial lamb for whatever is causing the problem, I don't want the LM358 to become the device that dies. Anyway cap rating is 25 volts now, will see what happens. I might get out the multimeter and see if there's any nasty AC voltage differentials floating around. Might have to build a passive, high voltage rated 200 Hz high pass filter if I do find some nasty voltages there?
 

mitaux8030

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Found the cause of the blowing capacitors. Given the two devices are each powered by seperate switch mode power supplies, each supplies its DC voltage 'floating' - and think thats why I'm seeing about 40 volts AC difference between the two chassis of PC and PRO2022, with similar voltages between the audio hot side of the 2022 and chassis of PC. The transformer is DC isolating things, but isn't able to discriminate between audio freqs and 60 Hz AC. Might have to figure out a way to power the 2022 from the PC... a common supply should sort things out.
 

mitaux8030

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Theres a transformer between the two for DC isolation - good for preventing hum caused by ground loops. Might give it a try without, and genuinely tie the two to the same potential. Not sure if that'll be dangerous to the circuitry or not?
 
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