Understanding Capacitor specs

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Zero Cool

Zero Cool

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Joined 2004
#1
My big Krell KSA-250 arrived the other day and while it is in perfect running shape, it is time to replace all the electrolytic caps and give her an overhaul! I mean she is 20+ years old!

SO I am looking for some help to better understand El cap specs, which has always confused me to some degree!

So lets just take one of the caps and look at Digikey for an example.

200uf 63v radial cap

Capacitance, voltage, tolerance and lifetime hrs all seem self explanatory.

Now this amp runs HOT! even with those massive heat sinks this baby gets real real toasty! so I know i want 105c rated parts or better. I see there are some rated 125c which sound interesting.

Where I start getting confused is when they start listing ripple current, ESR and impedance. I never know if i want high or low numbers.

I plan to keep this amp a good long time so I want to use the best, most reliable parts I can. I only want to do this ONCE! and i want to sleep well at night knowing i used the best parts I could.


SO, lets talk about cap specs!

Zc
 
#2
Normally one would prefer high maximum ripple current spec and low ESR and loss factor. That is, if the electrolytics have a high ripple current handling and small losses, they won't heat up themselves too much, and they will be closer to ideal capacitors than a high-loss electrolytic. Of course I haven't a clue what the minimum acceptable ripple current value would be for your amplifier, as I don't know how much ripple actually flows through those capacitors.
 
#3
One possibility is to identify the original capacitors and look them up. If no longer listed or traceable to a current manufacturer, someone might have an outdated catalog offline.

Or the original mfr was obscured for 'engineering privacy'...

Murray
 
#4
I've been buying the higher esr e-caps and getting away with it. I don't compromise on life spec, as I've had to recap some equipment I own (dynakit) 3 times in 40 years for low power or leaks. I will buy nothing under 2000 hours life and 3000 hours or more if I can get it. Note some manufacturers have a worse ripple spec at end of life than other manufacturers- saying the former are making up their life numbers to look good. You have to download the datasheet to see that, it is not obvious from the number quoted in the table. And if I can get 105 deg spec caps with adequate life, I buy that. The 2011 recapped ST70 amp equals the power specified, the 2010 recapped ST120 exceeds the power spec except for the heat loss limit, and the 2010-11 recapped Hammond H182 equals the push pull tube datasheet spec power. It does have rediculously low ESR motor caps as the first ones because I wanted film instead of electrolytic to never do it again. Motor caps were too hard to package though, had to install a separate chassis for the caps. The one unit where I went with the lower ESR cap was the PV-1.3k, where the high ESR caps were too tall to fit. All power ratings done with an appropriate resistance dale or ohmite power resistor, and a 200000 ohm/volt Simpson 266 VOM that reads 120 -125 VAC on the wall plug. (my low buck calibration check). Finding out what old caps were is good way to waste a lot of time. I have a 1970 Sprague master catalog weighing about 5 lb, and I can't figure out what the life spec was on the Sprague e-caps caps in my 1968 organ. It sure got louder and better frequency response when I replaced them, though.
 
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#11
5000 hr 125 C rated cap life is much better than 10,000 hr 105 C rated cap life, if both are ran at 50-60 C you might expect even in a "hot" chassis

Google electrolytic temperature life derating

given better processing, materials today you should also be able to up the V rating in the same case size - also contributes to longer life, but not as strongly as temperature derating

if any Electros can be identified as being "in the signal path" then looking for non-polar construction with high V rating could be an improvement according to Bateman's meaurements
 
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