[Necroist]

What's the difference between them? I noticed in tangent's cmoy guide, he specificly stated to get the blue metal film resistors instead of the normal beige resistors.

Why is that so?

What's the main difference between then.

 

[jboehle]

http://www.aikenamps.com/ResistorNoise.htm

 

[averydonovan]

Carbon films can change value over time, while metal films are far more resistant to that, iirc. Not to mention it is far easier to match metal film resistors since they usually have a 1% tolerance and carbon films have a 5% tolerance. Main downside to metal films is that they are more expensive, but not hugely so. For example, a 50-pack of metal films costs $4.99 at RadioShack, while a 100-pack of carbon films cost $5.99 there so 10 cents each for the metal films and 6 cents each for the carbon films.

 

[Gariver]

Some people like to use carbon resistors because they add warmth. Thus, tube amp lovers do like carbon resistors for that added warmth. The downside is what Donovan said, they change in value over time and are not accurate.

 

[00940]

"change value over time"

Does someone has an idea of the time needed before this shift occurs ? I mean, if it's like 20 years, who cares ? The caps will have gone bad before the resistors. If it's a year or two, it's a different matter.

Actually, I checked the value (using the color code) of a few resistors i took out of a 20 years old onkyo and they were still in a 1% range.

 

[tangent]

Quote:

Does someone has an idea of the time needed before this shift occurs?

There are two different kinds of changes going on here.

One is the temperature coefficient: the amount the value can change with respect to temperature and still be within spec. For common metal film resistors, it's in the 50-200ppm/deg.C range. That is, they'll change in value by 0.0005% to 0.02% per degree C of change. Carbon films are somewhat higher, 350ppm or so as I recall. But carbon comps are around 1500ppm! That's 0.15% change per deg. C of change. If the resistor is passing significant current, most of the heat difference it sees is probably its own power dissipation, so the resistor's value will be modulated by the current. That's right, the resistor value changes continuously, and if it's a carbon comp it can easily be changing by a good fraction of a percent, continuously.

The other way carbon comps change in particular is by humidity absorbtion and such, since they have a kind of spongy nature. Your resistors can be a different value from day to day, just depending on the weather.

 

[Gariver]

FWIW...

http://www.sasaudiolabs.com/theory10.htm