Bulk capacitor rating

[ej64]

Hi,

with AMP6 there is a 18000µF/25V cap included in the kit. For the transformer a 15 VAC type is recommended. That looks quite dangerous to me.
A small transformer often has a 15-20% higher idle voltage, what means 18VAC*1.41-1V (rectifier diode loss) = 24.38 VDC. Is this safe with a 25V rated cap?
On page http://www.41hz.com/main.aspx?pageID=106 a 35V cap is referencend for the unstabilized side.

Thanks for any suggestions
Eberhard

[Rune Bivrin]

I don't really see the problem. The cap is specified to work at 25 volts. Why should 24.38 volts be dangerous? While there is nothing wrong with using a higher rated cap, I don't really think it's needed, unless you also have to factor in 10% line voltage variations.

Rune

[ej64]

Well, that's why I ask:
Once I upgraded the PSUs of my old (1980s) MOSFET mono amps with high grade F&T caps. DC is 58V (normally!?), the caps were 10000µF/63V. After working properly for some months one eventually blew -- it gave a flash, some burning and smoke until I reached the power switch. The PCBs (PSU and Amp) were only slightly damaged but as time went on some corrosion occurred. I removed all F&Ts and only the original caps (4700/63) remained. They still work fine since then (~ 20y).

Maybe this was plain bad luck and has nothing to do with the voltage -- maybe it has ...

Thanks for your input
Eberhard

[ej64]

Just a feedback: my toroidal transformer measures 18VAC (no load), after rectifying 22.6V. More voltage loss than expected ... so a 25V cap seems secure under all circumstances.

(I had some waiting for additional/modified parts but now I'm going to finish my amp6 :-)
You may have some german reading at my blog: www.ej64.de -- if you can't read german there are some pics, too, showing my unprofessional soldering ... ;)

[Rune Bivrin]

Sorry for not getting back to you. So many forums, so little time...

Anyway, my guess is your previous upgrade experience went bad not because of voltage, but because of excessive ripple current. The more current your amp uses, the higher the ripple current will be.

This is definitely an overlooked rating for smoothing caps.

Rune