Quote:
Originally Posted by Radio Wrangler
Electrolytic capacitors are made out of two strips of aluminium foil and two strips of paper separator. The paper is soaked in conductive goo (electrolyte) the layers are alternated on top of each other. the long side of one foil protruding a little at one side, the other at the other then it's swiss roll time. Connections added and a case with an extra squirt of electrolyte.
Note that it's symmetrical. Why is ther a need for a positive or negative marked terminal?
But it's also not much of a capacitor.
Next comes 'forming' stick a good high voltage on it and keep it on for a long while. Electrolytic action strips all the oxide off of one foil and builds it up on the other. The naked foil is our connection to the goo, the built up oxide is our dielectric. Oh, and we textured those foils to boost their surface area.
NOW we have a capacitor!
If we leave it alone for a very long time, that electrochemical forming we did undoes itself. But we could re-form it.
We don't want to have to keep re-forming capacitors in equipment in use, so we need to run them with a minimum amount of DC bias. With modern capacitors, we want a minimal amount of DC across them when the gear is turned on. >10% of rating is a decent choice. S look at your circuit, look at the running DC voltage across a capacitor. Choose one of a higher voltage rating by all means, but best stay below ten times what it will run at.
63 v rated replacing something rated 6v might be pushing it a bit. Not asking for failure soon, but then not giving as long a life as it could.
Similarly, life starts dropping if you run them close to rating. Try not to exceed 80%
Output capacitors for audio amps have to work fairly hard. Think of your power rating, think of what AC current this is into the speaker and this is your ripple current. Good capacitor makers give ripple current rating specs, often at different frequencies in this world of SMPS. A good way to spot better quality capacitors is to look to see if they even have a ripple current spec. Again you can push them a bit, but probable life suffers.
That goo is water based and slowly dries out as water vapour escapes leaking past seals, through rubber etc. As this happens ESR gets bigger, heating gets bigger and the capacitor dies in a runaway process. So you want plenty of goo to start with.
Bigger capacitors have a bigger volume to surface area ratio. Seals are bigger, So leakage may be a bit more, but it's disproportionately less significant because of the bigger volume of goo inside.
So for your output capacitors I'd choose 56v or 63v rating I might put the capacitance up a size to say 2200uF (1500u is a bit parsimonious for 8 Ohm speakers) I might even put the voltage up a bit more to find something in a can close to the original size
105 centigrade rated parts means that their sealing is extra good to keep that precious water in. Therefore generally desirable.
David
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Thanks for the revision course David, although I am au fait with the various construction methods of capacitors it does no harm to be reminded and others will be reading this too
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I feel better that you explained things again. I've got a lot of very low voltage electrolytics that would be ok in less critical areas like decoupling so I might be ok there. If I can sub 10uF for the original 6uF coupling caps as the amp is AC coupled all the way through then again I have stuff that will work. I did seriously consider using 5,6uF Plastic film caps as couplers but I doubt I'd get any measurable or audible advantages.
One things for sure, it looks like Rogers went very close to the limit on all the electrolytics voltage ratings.
It makes sense to me to use high ripple caps in output coupling as I guess they pass a fairly high AC current which is a similar effect to AC ripple on a psu?
Thanks.
Andy