Film Caps: Polyester vs. Polyprop. and Metallized...?

There will be no difference.

None. Nada. Only when ampere-level currents are being pushed through the capacitor does it make a diddly squat difference whether the plates are metallized (i.e. - a thin layer of aluminum sputtered directly onto the polypropylene) or foil, and even then the advantage translates into slightly less internal heating due to lower bulk resistance. Fillm and foil construction does, however, cost more and take up more space for a given capacitance/voltage combination.

To put it another way, on a rating of 1 to 10, pp film and foil is a 9.9 (maybe teflon, definitely vacuum, is a 10) and pp metallized is a 9.89.

Course, purveyors of snake oil may wish to argue, but unless they've done A/B/X comparison and identified a difference substantially better than 50% of the time, I don't want to hear their opinion

I always look forward to your responses jeffreyj; they're always well written
and edifying....

But what about polyester verses polypropelyne? On a diffrent topic,
which type has the lowest inductance?

Flattery will get you everywhere, Arzela; or most places, at least

Technically speaking, they are very different beasts. My only A/B experience comparing them, though, was for the input capacitor of small amplifier. Honestly, I couldn't tell the difference. Not even could I identify a difference. RMAA couldn't tell a difference, either, btw.

But that doesn't mean I think you can go ahead and substitute polyester (aka - mylar) for polypro anywhere! The dissipation factor is typically an order of magnitude worse for mylar caps, so forget high peak current usage. The temperature stability isn't linear, so critical timing and filter circuits are out of the question. The bulk capacitance is far superior to pp, though, so they typically make superior bypass caps (especially since the higher DF serves to limit the Q of any resonant circuits they might form with stray inductances).

As input coupling caps? Well, let's put it this way: if you have the room, the budget, and a target market that has tossed reason out the window in favor of voodoo and mysticism, then by all means use a PP cap. If you have to choose between a larger value capacitor (mylar) versus a smaller value but higher quality (pp), then I'd go with the higher value any day of the week. Frankly, the phase shift produced by too low a corner frequency, or the noise pickup that results from being forced to raise the shunt resistance of the input network, are far more real errors than whatever sonic signature might be attributed to this dielectric (in this case only. Once again, I am not saying that mylar is a good choice everywhere; just input coupling and power supply bypassing!) Ymmv. Other people may truly have more sensitive ears in this respect and can identify a difference, if not which one is "better," but their arguments will fall on deaf ears if they haven't determined the difference with at least somewhat scientific means.

Parasitic, or "self," inductance is strictly a byproduct of the construction of the capacitor, not the dielectric, per se. Even the fact that mylar has a higher k, and therefore capacitors using it will be smaller than pp ones for a given CV product, doesn't affect the self-inductance value much. Note, however, that this is another area where film/foil types are slightly superior to metallized types because the foils for each side are offset slightly so that they can be welded to the end caps. Metallized versions are usually constructed by welding the wire across the spiral. This results in a small, but noticeable, increase in effective path length; ergo, higher inductance (yeah, I saw Matrix Reloaded... what of it?).

That's enough on this one for now!

You may find this link interesting.

http://members.aol.com/sbench102/caps.html