Mic Pre PSU - capacitors burns after 2-3 months of usage

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elskardio

Well-known member
Joined
Sep 29, 2010
Messages
560
Location
Montreal - Canada
Hi Guys,

I'm using the following PSU for my small utility Mic preamp. It's powered from a 18VAC wall wart.

It's working great... but after 2-3 months of usage C11 and C17 burned out. This is the second time it's happening so something must be wrong with my circuit. Both capacitors are 35V (50V).

Any hint?
Thanks

Mic-Pre-PSU.png
 
The capacitors are in front of a 48V regulator, so the voltage must be even higher across them. Of course they will break. You will need at least 63V rating or even higher depending on your rectified raw voltage.
 
Hey

This is a P48 rail right ?

So if you have 48V after the reg you probably have little more before...capacitor at 35v rating is not enough...

In other hand it seem you have a voltage doubler from 18V... ending at 36v, hight quality 35v caps should last longer than two month but still not future proof... but then the you don't have 48v at P48 rail

I'm confuse ?!? what is actual voltage at you phantom rail ?

Best
Zam
 
zamproject said:
that's almost a triple cross timed redundancy reply  ;D

Cheers  8)
3 posts hit within a little over one minute span...

There were no answers when I started writing my post.

More answers are better than none, since they all seem to pretty much agree.

JR
 
Hi JR

JohnRoberts said:
3 posts hit within a little over one minute span...

There were no answers when I started writing my post.

More answers are better than none, since they all seem to pretty much agree.

JR

Was same for me as Volker I suppose, no answers when I write
I's absolutely not a criticisms from me !!!

I just find it "funny", also because as you say, answers go the same way

OP should have a strong direction to look at now  8)

Best
Zam
 
Thanks guys for the replies.

I removed the faulty capacitors and they are rated for 50V. When I built the unit 3 months ago, I was using a 16VAC transformer and it was giving me 44V once rectified. I only had 50V capacitors on hand so I installed them. Two weeks later I changed the transformer for a 18VAC (to get the full 48V phantom)... but didn't change the capacitors. This transformer gives 50.7V once rectified. I just installed new 100V rated capacitors and everything should be good now.

Note to myself, always use higher voltage capacitor in the phantom power PSU path  ;)

Cheers and have a great weekend
 
I knew a amplifier design engineer who tried to save some money by using marginal voltage rated capacitors in a high power amp. He ASSumed that the cap makers specified their caps conservatively, and the cap makers may have ASSumed that users applied them conservatively.

Unregulated rails need to allow for maybe 20% high line voltage overload, while these days such high lines are not as common.

JR
 
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