[felixscerri]

G'day all, I was looking at a certain simple RIAA phono stage that I was thinking of buying and building, and noticed a curious choice of components in the kit, what look like carbon film resistors, a couple of ordinary electrolytic capacitors and ceramic capacitors, but a couple of high quality polystyrene capacitors in the RIAA network. 

From past personal experience polystyrene capacitors are excellent, and also tend to be rather expensive.  Is there any specific advantage to their use in RIAA networks?  Regards, Felix. 

[Steve]

G'day all, I was looking at a certain simple RIAA phono stage that I was thinking of buying and building, and noticed a curious choice of components in the kit, what look like carbon film resistors, a couple of ordinary electrolytic capacitors and ceramic capacitors, but a couple of high quality polystyrene capacitors in the RIAA network. 

From past personal experience polystyrene capacitors are excellent, and also tend to be rather expensive.  Is there any specific advantage to their use in RIAA networks?  Regards, Felix.

Hi Felix,

Well, there are pluses and minus to the typical caps. The plus is that the temperature coefficient is excellent. The minus is three fold.

1) They cannot handle high temperatures, say, from a soldering iron for long.

2) They are only available in foil, which is good, except that the termination lead is almost always connected at one point on the foil.
Some films are such that the lead is attached to the entire edge of each foil turn, thus less internal inductance.

3) They are generally limited to PF (micro micro uf) values. I just saw some values in the tenths of a uf.

Here are articles that might help: "Picking Capacitors" by Walter Jung and Richard Marsh.

www.waltjung.org/PDFs/Picking_Capacitors_1.pdf

www.waltjung.org/PDFs/Picking_Capacitors_2.pdf

and "A Chemist's Point of View"

http://www.audience-av.com/capacitors/a_chemistview.html

Hope this helps Felix.
Steve

[felixscerri]

G'day mate, many thanks.  Regards, Felix.   

[peranders]

I would suggest that you choose polypropylene caps. This is the modern alternative.

[Steve]

I did a mix for my test phono stage. I used basic polypropylenes and bypassed them with very very small polystyrenes
to obtain within approx. 0,5% deviation of the RIAA curve. Capacitor values are much more limited than resistor values,
so I suggest obtaining the capacitor values first, then the resistors, if one is designing from scratch.

However, simply changing capacitor types is much easier, especially if one can actually measure the capacitance for accuracy.
Sometimes one can request, for extra cost, capacitors within 1%, maybe even tighter in tolerance.

When ordering polystyrene caps, some come with what appears to be steel leads. One can tell as the leads are thin,
and very springy. There also may be a mismatch, sonically, when combining with polypropylenes, so one must be careful
when adding some polystyrene caps.

Cheers
Steve