carbon resistors noisy? why?

I'm sitting at work with 3 drawers full of carbon 1 and 5% resistors. a few metal films too. my 1176 boards are here with me also... bosses not here today...hmmm. I've read may places that carbon resistors are a bit noisier than metal films.. I don't recall why though. can someone explain before i stuff these boards with them? How much noise are we talking about?

the metal films are close to some of the values i need but not all of them are dead on. what percentage should i be within? 1%, 5 or 10%?

Carbon composition resistors are noisy (and not available in 1%). Carbon film resistors should be ok, but they are more noisy than metal films.

Best regards,

Mikkel C. Simonsen

much appreciated MCS! can you explain why though?

Variations of resistor chemical compositions will cause the electron flow to generate differing thermal noise signatures. Try this search for some info:

http://www.google.com/search?hl=en&ie=UTF-8&q=carbon+metal+resistors+thermal+noise

(Do excuse my layman's physics, I'm not an academic you know :wink: You knew that right? :green: )

Justin

Hi,

Here's a quick explanation of the principles behind resistor noise- it's related to guitar amps, but it's a good primer:

Resistor Noise

Don't forget that lots of equipment we use from the 50's/60's/70's were built using carbon composition resistors- this was a time when metal film and 1% and even 2% resistors were expensive! Today, carbon film and metal film resistors are as cheap as the carbon types, and can be used for everyday DIY use.

Mark

I normally use metal films for audio but can I also assume that carbon films/comps can be used for non audio paths with no ill effects or will they cause noise due to PS and /or switching noise in your experiences?

Metalfilm resistors last longer. Carbons alter their resistance when they age..

[quote author="sismofyt"]Carbons alter their resistance when they age..[/quote]
I think that only applies with carbon comp. resistors. Carbon film resistors should be stable.

Best regards,

Mikkel C. Simonsen

I've read conflicting reports about the Riken resistors. One site says they are carbon film, another says they are carbon comp. Anyone know for sure?

IIRC some carbon films can have less noise than some metal films. There is a good page about this in "The Art of Electronics"


I believe some of the noise is due to the crimp at the lead connections and the deposting of the film.

Carbon comps can absorb and lose moisture, this is probably why they change value and can get noisy.
When you use comps for tube plate resistors, use 1 watt or better. All those 1/2 watt 100K's in Fenders for instance, are over worked, which can also lead to noise.
Useless trivia: Drop your plate voltage for less microphony.
cj :guinness: