2. amps-preamps
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Ripple current Q for amp capacitors

I have a pair of EAD powermaster 1000's in my theater. The older one is at least 25 years old I think, and developed a slight hum I could hear in the theater.  So I popped it open, did a cap inventory and replaced them all. I checked every one of the main amp caps as I pulled them out, and surprisingly they all seemed really good still, within tolerance.

 

But I didn't replace the two large filter caps. I had searched for a 100,000uf CD cap online, clicked on the link to Mouser, and purchase 4 (2 for each amp.. figured I would do both). However when all the caps arrived, I realized in my haste the link was to 10,000uf caps instead.. DOH! So I went back and tried to find the matching caps.. expensive! And to top that off, when I "could" find some (Ebay mainly), the date code was around the year 2000 as well so they were about the same age. Digikey has a lead time to about December.. and also over $200cdn each.

The original spec was a Cornell Dubilier, 100000uf 50v -10%/+75% cap. They do have a 100v version in stock, half the price, and actually shorter than the 50v version.

But my question is this.. when I search online for whether a low or high ripple current is preferable for an audio amp, I get both answers.. low is better because then it filters better, to high being better because that means the cap can handle more ripple and heat. Which is correct?

The 50v version is:

7mOhm ESR @ 120Hz, 29.3 A @ 120Hz, 38.09 @ 10 kHz

the 100v version is:

10.2mOhm ESR @120Hz, 23.4A @120Hz, 24.57A @ 10kHz

I have no idea if these numbers are incredibly close, or meaningfully different.. and which is better?

tester007

Showing 2 responses by erik_squires

erik_squires
16,892 posts
 

BTW, I should have mentioned this before.  DIYaudio is a great place for this kind of question as many there touch this subject regularly. 

erik_squires
16,892 posts
 

Replacing vintage caps it’s often expensive to try to get a perfect match.  New caps are significantly smaller in physical size and have longer life spans. 

Ripple current for the main power supply caps is high, so you need caps with relatively large ripple handling capabilities.  The currents that come from the bridge rectifier are, by definition, high ripple.  A low ripple current cap here will overheat and die in an hour, even when V values are exceeded.  Trust me on that one.  laugh

The DC current that comes OUT of the power supply (transformer, bridge, caps)  to the amp boards is not high ripple though, so small buffer caps (50-100uF or less) don’t need to worry so much.  

Also, all other things being equal, higher temp caps will last longer, even if neither is really temperature stressed, and also improves ripple current handling since  ripple currents cause heating.

Check the physical size, try to stay close in uF but feel free to go up in V and temperature ratings and make sure that you get caps designed to handle AC ripple. 

Of course, a physically smaller cap is also a good thing, so long as all conditions above are met.   If they are too much smaller you may need to apply some glue to ensure they aren’t rattling around on the board.  If your original caps are glued in and these are smaller, don’t rely on the tin leads alone for mounting. 

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