[R-390] filters

[Charles P. Steinmetz]

John wrote:

>Would any of the knowledgeable folk on the list care to comment about the
>donut material and whether the old material was chosen as an acoustic
>isolator to prevent microphonics?

I think the ultimate would be Teflon foam.  See, for example:

<http://www2.dupont.com/Teflon_Industrial/en_US/products/product_by_name/Teflon_FFR/index.html>

<http://www.blinexindia.com/ptfe-soft-flexible-foam-gasket-material.html>

However, AFAIK it is not commonly available in sheets, rounds, or 
other structural engineering forms.  One might be able to find Teflon 
foam-dielectric coax with a suitable size dielectric (ID/OD), but 
buying just a few inches could be a problem.  If one wanted to 
explore the possibility of Teflon foam, this might be a good place to start:

<http://www2.dupont.com/Teflon_Industrial/en_US/contact/index.html>

If I were rebuilding a mechanical filter, I think I'd consider slices 
from an appropriately-sized (ID and OD) piece of the softest 
available (lowest durometer) silicone rubber tubing.  Here is one 
source:  <http://www.acmerubber.com/siltube.htm>  Part 
No.:  CATB-.375-.125-65957  (ID = 0.375", OD = 0.635"); 
CATB-.250-.125-65909  (ID = 0.250", OD = 0.500")

Failing that, I might try donuts punched from a sheet of silicone 
rubber of the appropriate thickness using circular leather or gasket punches.

But I've never rebuilt a mechanical filter, so I cannot say from 
experience whether silicone rubber would be the best choice.

Best regards,

Charles