[R-390] Resistors and capacitors in audio

[Tisha Hayes]

You should not have any problems with metal-film resistors in the audio
section of the radio. At the most you are going to see 3 KHz through those
components and whatever inductive differences there may be between a carbon
comp and a metal film will be insignificant.

In non-inductive applications a metal film resistor very well may be the
superior choice. They are usually more temperature stable and have less
inherent noise than a carbon composition.

Even on the issue of induction I think there are some variations on
metal-films; In dissecting components there seem to be two or three
different manufacturing styles for metal film resistors. You can see these
differences too; take a metal film and use an razor blade to scrape off the
enamel coating. You will find that many resistors are made of a grooved
glass rod and only the groove around the glass contains the metal film
layer. By varying the number, depth and fill of the grooves the
manufacturer can control the value of the resistor. I have seen others
where there is not a groove but the entire body of the glass rod is coated
uniformly from end to end. I think the latter design of not using the
groove makes the resistor non-inductive by its very design. It is probably
more of a manufacturing challenge for consistency to maintain a specific
thickness of metal film but so much of that technology has evolved with the
making of semiconductors and vapor deposition.

This unusual tendency by me to take things apart down to their fundamental
level goes back to the questions I asked as a child.. "What makes a
resistor resistive?" My daddy would break open a resistor and hand me an
ohm meter to let me figure it out. Sometimes that resulted in dining room
table calamities like making an arc-lamp out of graphite pencils with two,
six volt lantern batteries (stunk the house up) or exploding chemical
concoctions when I discovered electrolysis and the oxygen and hydrogen that
makes up water (btw, do not try the same experiment with salt water, that
was a completely different disaster, you do not end up with sodium metal
but one of the gases will clear out the house).

*Ms. Tisha Hayes, AA4HA*