[Elecraft] Re: tube amplifiers and polypropylene capacitors

[Jim Brown]

On Thu, 25 May 2006 16:35:57 -0600, BMW wrote:

>I don't really know of any reason to use them in audio circuits over
>ceramics or bipolar electrolytics (depending on capacitance value
>needed).

I work in pro audio, and am a Fellow of the Audio Engineering Society, 
but I am NOT a High Futility Tweak. Several years ago, our local AES 
chapter heard an excellent presentation by John Hardy, the designer of a 
well respected mic preamp. He spent 20 minutes (at least) talking about 
capacitors! There are significant differences in the amplitude linearity 
of capacitors of different constructions. That's one reason why some 
types are preferred over another. 

It takes many specifications to define the characteristics of a product, 
and many are not listed on data sheets -- you must learn them the hard 
way. Things like temperature coefficient, aging, reliability, 
environmental, stray reactances, and, as noted, non-linear distortion. 
One of the things I taught my kids is that there is usually far more to 
any job than meets the eye, or that is obvious to even the trained 
observer. 

Re: Loudspeaker wire -- BEEF (that is, big copper) is the only thing that 
counts for good audio performance, but TWISTING is critical for good RF 
rejection. RF present on the loudspeaker terminals will often be coupled 
by the feedback loop around the output stage and detected in a driver 
stage. Lots of RFI problems have been solved by replacing glorified zip 
cord with twisted pairs of POC (plain, ordinary, copper). 

Jim Brown K9YC