Polystyrene and polypropylene film capacitors

Hi there,
While this two types or cap are widely in audio application, when do we want to use one instead of the other ?
Also, correct me if I am wrong, but metal film resistor are used in DIY project while some says that it should ne avoided at all cost for audio appl.
Thanks
Alex

Both are electrically good dielectrics. Polystyrene may be a hair better.

In general polystyrene caps are physically bigger. Largest value I've ever seen in polystyrene was .1uF and they were as fat as my thumb. Polypropylene caps are more robust, so stand up better to production stresses. Polypropylene caps are only slightly larger than polyester (Mylar).

I have never noticed a huge difference between resistor types but have encountered resistors with enough voltage coefficient to make measurable distortion at power amp signal swings.

I suspect this mechanism would be more of an issue at vacuum tube operating voltages.

JR

JohnRoberts said:

In general polystyrene caps are physically bigger. Largest value I've ever seen in polystyrene was .1uF and they were as fat as my thumb. JR

Click to expand...

I have some 5uF polysytrenes I bought years ago from Southern.  They are gigantic.  I think they were quite pleased to get rid of them  ;D

An important aspect of polystyrene capacitors is their very high isolation resistance - which remains very constant even in harsh environments (humidity, temperature).

This is why we use them at front-ends of condenser microphones.

Back in the '80s I had a business partner kill a bunch of polystyrene caps, drying off the PC boards with a high pressure air hose after washing off  the organic flux with water. (that flux bit us in the ass later, but that's a whole 'nother story).  Knowing my partner I suspect he was using way too much air pressure... but my point is that polystyrene while a great (really excellent) dielectric, is not physically robust.

That said I used them almost exclusively in my kit business back in the '70-80's with only a handful of problems from amateur hand soldering.

JR