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Capacitors: Who is Right? What ages them faster?

Power supply electrolytics - the main thing audiophiles worry about getting old and having to replace.

What's the truth here?

Manufacturers can be quoted as saying the life expectancy of caps is anywhere from 10 - 15 years. Is this true? Why is it that Krell amps seem to need capacitor changes in 8 years, while people have receivers from the 70's that never had caps replaced?

Another controversy, with three viewpoints:

(1) I was told that caps age faster in amps that are left on continuously. The rationale here is that the lack of a discharge cycle is not good for the caps since they are in a constantly charged state.

(2) Others claim that leaving a unit unused in a box for years also shortens the life of the caps, as they never see a charge.

(3) Others say the lifespan is the same whether on the shelf, or in the unit.

Who has the technical background or the knowledge to sort this out?
kevziek

Showing 3 responses by aball

aball
2,016 posts
 
One of my collegues does research on capacitor reliability (for aircraft systems) and he agrees that thermal cycling is the main problem. I will ask him again now that he has had time to look more into the issue and let you know. In my experience with using caps in designs is that 10 to 15 years is typical in 55C ambient conditions (for electrolytics).

As for the on/off bit, I have thought about it at length since I am a thermal guy by education and have decided that off once a week and when I leave town is the best balance.

For large value caps, the thermal lag time is longer so perhaps they normally last longer than smaller (physically and value) ones. I will ask my buddy tomorrow about this too if I see him. Arthur
aball
2,016 posts
 
Sean - 130* at the heatsink is WAY too high for any amp to operate. Most transistors can't operate above 120*C without burning up. Including thermal impedance effects, 130* at the heatsink means about 150*C at the junction (in a good design) which is impossible. Your temp measurement needs to be verified.

Capacitors are even worse: many can't function over 85*C. This is a problem I face nearly every day in my applications. the darn caps are typically the thermal limiter in many designs and so they must be kept as far away from the transistors as possible. Caps have a low loss character so they aren't too affected by the load - time is the bigger problem since the heat present around them exacerbates the drying process.

Also, the temperature rise caused by cranking the volume cannot be overlooked as may have here. The initial temp rise at turn-on is only half the battle (less than that even in class B designs) - the other half is when you turn the volume up due to I^2*R conduction losses in transistors. Since current is squared, the volume control results in non-linear heating which can overcome initial turn-on effects rather easily. A much lower time constant driven by high power supply energy in this case is also to blame.

Anyway, I haven't seen my buddy yet but I will start a thread after I talk to him. Cheers! Arthur
aball
2,016 posts
 
Ah, perhaps Sean meant 130*F... I had long forgotten about Fahrenheit so I didn't think of it till now. Otherwise, see my reply above.
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