Capacitor suggestion

[EBM_dude]

At first, I have to apologize to you guys. I always tend to ask rather dumb questions lately. Anyways, I have some 30 Panasonic FC caps I want to use for my amp's PSU. They are rated for 50 Volts. The xformer puts out 22.5 VAC, which would be some 31 Volts rectified. Now given that without a load the xformer will likely put out a higher voltage and mains voltage may be 10% off, the air gets rather thin. Should I use 63V or even higher voltage caps?

[benb]

I think 50 volt caps should be fine, and perhaps even more appropriate that 63V. I've read in some manufacturer's literature that electrolytics should be run at at least 50 percent working voltage to maintain long life. I'd have no worry that the caps might suddenly explode if exposed to 51 volts. While they shouldn't be operated over their rated voltage, electrolytics can take a temporary overvoltage of up to 10 percent over their rated working voltage, where temporary means it doesn't happen regularly.

[Conrad Hoffman]

I agree with benb- take advantage of the surge rating (I don't know if they publish it, but you can be sure it's there) and use the 50V caps. I've found zero evidence that modern caps suffer in any way if used at a small fraction of their rated voltage, mostly anecdotal reports concerning old or HV parts, but there's no reason to do it intentionally.

[amc184]

50v caps will be fine for that application.

I had a Musical Fidelity A1, that had a 18VAC transformer, 24VDC rails and 25V rated capacitors. Combined with the fact the the caps were rated at 85oC and the amp was running at around 75-85oC, this was a cap eating machine. Needless to say they were replaced with 35V 105oC rated equivalents when I recapped it!

[Conrad Hoffman]

I could be wrong, but I think the 85 or 105C numbers refer to the internal temperature. If ripple current is flowing in the cap, causing dissipation, the internal temperature can be quite a bit higher than ambient. It's not that hard to exceed the ratings if the ambient is high. Most of the time there's plenty of safety factor in audio equipment because the continuous current draw tends to be low, and things don't run that hot, but if there's doubt you should check it. I'm a big "fan" of ventilation for anything that tends to run warm. amc184, I think you found the perfect storm!

[amc184]

I think you're right there.

Ventilation was the answer for this amp, probably needed active cooling, but I was wanting to sell it and I didn't want to diminish its value. I no longer have it now, it's someone else's problem (at least the guy who bought it was familiar with Musical Fidelity, and knew not to expect long term reliability from it).

[EBM_dude]

Thanks a lot guys! I'm happy I can use those fine caps and don't have to buy new ones.