Resistor noise question.

[pinkphiloyd]

Are you sick of my asinine questions yet? A lot of the time, at work, I have absolutely nothing to do but sit, read, and think.

So, I know higher power resistors are quieter than lower power resistors, and metal films are quieter than carbon films, so...

What's quieter, say, a 1/2 W metal film, or a 1 W carbon film?

[roberto]

I absolutely don't want to hijack this thread neither to bother anyone with questions that may seem dumb, but do you think that will weight more one kilo of lead, or 1000 grams of plumage?

[xtian]

lead, or 1000 grams of plumage?

I guess we have to take wind resistance into consideration, because your resistors are not falling in a vacuum.

Pink, there are some thorny equations for noise but real world results will vary widely by resistor manufacturer, tolerances, Earth's rotational speed deviations, voodoo, etc.

[pinkphiloyd]

Ah, I see. Thanks.

I'm sure the real world difference is negligible, but like I said, I was just sitting here at work, bored, and wondered, scientifically...

[Leo_Gnardo]

The idea is that heat is part of the noise generating process, the more heat the more noise. So a resistor with a higher wattage rating will (presumably) not heat up as much in use, therefore be quieter.

......................but..................

what if that resistor is heated not only by its own ohmic heating P=IV, but also by its ambient environment, 'specially if that involves some roasty hot vacuum tubes, now all that benefit of spec'ing a higher wattage part is out the window. Phooey, no free lunch.

In practice some build amps with say 1 watt carbon film or metal film parts, sometimes even 2 watt parts. Lower noise, maybe by a little. More likely, better reliability. Maybe a better discount on the bulk parts order too.

Half watt MF will have lower noise factor than 1W CC.

[xtian]

Half watt MF will have lower noise factor than 1W CC.

Now, there is a short, and based on the source, likely reliable, answer to your impossible question!

[Structo]

Pinkphiloyd,

You have received one demerit for asking silly questions.

J/K

You could put on your mad scientist hat and experiment with different types and sizes of resistors in a simple circuit and record the results.

I think a good reason that carbon caps got a bad rap is because they drifted and usually became noisy at one point.
Sizzling, pops, etc.

The early ones absorbed moisture too easily and became problematic.

So they developed film caps which can be sealed.
Film caps have much better tolerance ratings as well.

So you got the carbon comp, carbon film, metal film, metal oxide, tant, etc for different applications.
And within those types there are sub categories with different construction methods concerning the body, end caps and leads.

Look at Parts Connexion to see different types and quality in resistors.

http://www.partsconnexion.com/resis_main.html

[V2]

why not something like this?

[V2]

or perhaps this:

[Littlewyan]

I don't think from reading Merlin's book that there is a massive difference between metal film and carbon film. I do wonder though if a 1W Carbon Film would be quieter than a 1/2W Carbon Film. Someone once mentioned the maximum voltage rating for 1/2W Carbon Films is often exceeded and thus they can be noisy. Not sure about this.

[Firestorm]

Aiken says 2W resistors are 3x quieter in terms of contact noise (which is the only kind of resistor noise you can really do anything about). I'd say go for 1W definitely.

[Littlewyan]

Yeah but he was talking about carbon comp resistors. I just read his article. I may try ordering some 1W Carbon Film resistors just as an experiment to see if it lowers the hiss in my TW Express.

[Firestorm]

Carbon film is inherently quieter than carbon comp, but has contact noise too. Only wirewounds are immune.

[Leo_Gnardo]

Theory: There's also the matter of noise being proportional to the voltage across the resistor. If it's a low or no volt situation you can't expect a reduction in noise. Unless your carbon comps are really super funky.

Practice: FWIW I have occasionally found preamp cathode resistors noisy, typically 1500 ohms with only a volt or two on them at most. Must have been some of those super funky resistors. So if you're trying to beat noise, it is worth a try, go for carbon film, metal film, or metal oxide film. I've seen carbon film used quite a bit in boutique amps.

One drawback of modern miniature-sized resistors, 2-watters the size of an old quarter-watt, they don't seem to hold up well in pulse current situations. We've been all over that one discussing output tube cathode current sensing resistors.

[Littlewyan]

If you mean the Output Valve Bias Resistors I've used 1W for my output valves as the wattage dissipated across them is fairly negligible. However I didn't think about pulse current as I have to admit I don't know anything about it.