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Anyone Blow a Release Valve on Power Caps?

I went to turn on my Sierra Denali power amp yesterday and noticed it was wet on top plus there were dried residue spots everywhere. I dried it off (a bit oily) and opened up the case. It was even worse on the inside. I dried things up a bit and did a visual inspection. The Denali has four "beer can" sized 63v 82,000 mF power capacitors surrounding a 2.2 kva transformer. All four of the caps have a small gray plug on top of them. One of the plugs was extended above the cap with a clean hole in it. Obviously, the liquid was the electrolytic fluid inside the cap and it had blown out through the hole. I called the folks at Sierra and they will take care of it and even upgrade a few things for me. Nobody there had ever seen that happen before. We had not been playing music at a loud volume. Each of my two Denali amps is on a separate dedicated 20 amp circuit. The amps sounded fantastic the previous night (my wife even commented on it). My wife only noticed it before turning the amp on, so she decided not to turn it on. It may have happened just after the amp turned off the previous night. I can't imagine it happened while we were listening to the music -- wouldn't it have been noticeable?

Has this happened to anyone else? What might have caused this? Can I avoid this from happening in the future? If this is just a case of a bad cap, there's not much I can do, but... Thanks in advance for your help. I did upgrade my XLR cables from the preamp to the amp a week ago, but I can't see how that would damage anything. Help!
ozfly
herman
2,254 posts
 
Electrolytic caps blow because:

a: they had too much voltage across them, i.e bad design
b: they broke down due to some internal defect, i.e. bad cap

The volume you play the amp at has nothing to do with it. Assuming a good design that doesn't put too much voltage across the caps, that leaves a bad cap, and that is just luck of the draw. Be thankful you are dealing with a first class manufacturer that stands behind their products. The chances of it happening again are minimal.

I calculated that a push-pull amp with 63 volt caps operating at 75% of the cap's voltage rating would put out about 140 watts into 8 ohms. As long as your amp is at or below this wattage then it should be OK.
herman
2,254 posts
 
A side note to Sean's post, the large "beer can" electrolytics such as you have in your amp all have a small valve (looks like a small rubber bump on the side where the wires connect) that will release the gases that form if the cap breaks down. Whereas a smaller cap will let go with a loud pop, caps the size you descibe would literally blow up without a vent valve and could be quite dangerous
sean
6,231 posts
 
Herman: Thanks for the kick in the head and the explanation. It made me start thinking normally again : ) Sean
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ozfly OP
887 posts
 
Thanks guys. Herman, since my amp is rated at 600w into 8 ohms, I'm hoping that the formula you mentioned allows for an additive wattage. Four of these caps at about 150 w each would cumulate to 600w. If it's not additive, I'll have to talk to the manufacturer. Thanks again. Thank goodness for the vent valve!
herman
2,254 posts
 
Actually, to get 600 watts into 8 ohms requires 100 volts across the caps, so they must be using a pair in series for each voltage rail. The voltage is additive but the capacitance is halved so you have the equivalent of a pair of 126 volt 41,000 uF caps. That should be OK @ about 80% of the rated voltage. They do this because caps that big don't usually come with voltage ratings that high.
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