Coupling capacitors - polystyrene cap shunting/by-passing...

I remember reading somewhere on the web a couple of years ago (but now can't find it!) an articla that suggested adding a very low value polystyrene capacitor to each film coupling capacitor would improve high frequency clarity.

Does anyone else have any knowledge of such or experience?

Steve

[John Caswell]

Hi Steve,
Yes this is similar to adding extra capacitors across decoupling elcos etc.
It is down to things like Equivalent Series Resistance (ESR), self resonant frequency etc.
Some people work on a 10:1 ratio ie. a 100nF bypassed with a 10nF, and some on a 100:1 ratio, ie. 100nF bypassed with a 1nF and try to use different types ie Polystyrene, Polypropylene, PTFE (Teflon) Silver Mica etc.
I have no opinion as to which types are the "best" just try until you get a combination that suits you.

John

Hi John

Thanks for the quick reply! And I thought you might have some info on this!

What you say makes sense. I think I'll give it a try.

It seems surprising there isn't more about this on the web - or maybe I'm phrasing the questions wrongly!

Thanks Steve

[Richard]

Hi both,

try it by all means but it's usually used across electrolytics to improve their hf performance. I wouldn't expect much difference across film caps at audio frequencies.

Steve, do you feel hf is lacking a bit of sparkle? What valves are you using as it's quite likely there are brighter ones you could try. The small valves would be cheaper to try first. If you're using Svet KT88 they have lovely midrange but do not shine at the top in the way JJ and (best I've heard) the new Genelex Gold Lion re-issues do.

[John Caswell]

Hi all,
Richard wrote "I wouldn't expect much difference across film caps at audio frequencies."
You'd be surprised. It's all down to ESR, self inductance, self resonant frequency etc etc.
I agree in some cases no apparent "differences/changes/improvements would be noticed, but not all.
I reiterate try it and see it ceratinly will not harm

John

[Richard]

Well I have tried it across the film caps in the xover of my Pro9TLs 15 years ago and noticed nothing. They were 0.1uF 1KV polyprops across 4.7uF filmcaps so I left them out when I later rebuilt the xovers.

More recently, 0.47uF 630V polyprops across the 100uF psu electrolytics inside Phono3 and Pre3. Didn't notice anything but thought they wouldn't hurt so left them in.

Most recently, 3.3uF polyprops across the 100uF electrolytics in the psu of 300B Glasshouse SE and I thought it was a little better air and transparency so they are still in place.

Hence my own conclusion but ymmv

(edit; they were polyprops, not tried polystyrenes)