Originally Posted by nikongod
I disagree that high capacitance or low voltage rating (provided the parts are rated for more voltage than the application) could hurt downstream gear in this app. The poor leakage spec might, but it can be dealt with.
The issue with supercaps is that they are designed to replace small batteries as I posted previously and basically no thought is given to how well they pass AC. Since this application is purely based on how well the cap passes AC they are a poor choice.
To put it bluntly nothing about these caps except their capacity is super. Since you don't need the capacity, the only thing super about them is their name.
A super cap for this application would have the following characteristics:
Nominal DC leakage
Low AC distortion
Reasonable physical size
Low ESR (ESR is not very important in this application, but maybe someone will find this and be saved from the poor choice of using a supercap for the power rails of their amp after considering ESR)
Hi Nikongod - Completely in agreement with your thoughts about super caps and what would be truly super in this application.
The reason I mentioned damage to downstream gear, is that on the DIY iMod thread, the voltage rating of these caps were posted. I can't remember the spec off the top of my head, but it was very very low, like a volt or two. Please anybody correct me if I am remembering this wrong, I'm not going to look it up on the thread. While the iPod isn't running any high voltages on the output, I would be more comfortable having it a little higher than a volt or two. I admit that I do not know the actual voltages in play on that circuit, but it would stand to reason that if the proper spec cap (4.7uf film cap for example) has a voltage rating of 50v why not use that? Then you know you are covered. Mostly any cap, film or electrolytic that is sensible to use in the application will have much a much much higher voltage rating than the "super" caps. And you mentioned there are ways to deal with the poor leakage spec...Why bother? My point of this is why not use the proper cap? It has been proven by many others performing iPod mods that using the proper part works.
The point of using a cap in the signal path is to block voltages from going downstream. (yes, which can damage gear) If you are using a cap that does not provide this basic function, just don't use a cap at all. If you don't care, and want to experiment with your own stuff, fine, but please don't show up to a meet with that gear. And please don't tell someone wanting to try the mod themselves that this is a a part they should use. It's obvious that the super cap is not a good choice in this application. Why search for a workaround just to have the word "super" attached to a part?
I am not meaning to disagree, I just don't want someone who is researching doing the mod themselves to get confused thinking this super cap thing is a viable option. There is a formula of what value to use, there is no reason to go willy-nilly selecting anything outside those parameters. Say your car needs new tires. Can you put square tires on? Sure. But why use square tires? Round is what you need.