Carbon Comp Resistor Question

[Joe Hart]

I keep hearing about Carbon Composition Resistors.  The ones that I get at Radio Shack that look like a little peanut, I believe, are Carbon Film.  Or at least that's what SOME of the packaging says.  Some packs don't say either way.  Then I have some older resistors that are dark brown and look almost like electrolytic caps (perfectly cylindrical).

Now, I know that the word "composition" just means what it is made of -- so Carbon Film, I would guess, is "composed" of carbon.  But are these Radio Shack resistors the "Carbon Comp" resistors that people talk about?  And are the older ones that I have any better?  My understanding is that "Tropical Fish Caps" are really no better than some modern caps you can get (except for the mythical "mojo" that they may or may not have), so what about the resistors?

And as always, thanks.

-Joe Hart

[brett]

There's an excellent discussion of carbon "mojo" in RG Keen's intro to tube amps over at geofex.  My understanding of it is that unless you're dealing with high voltages and situations where voltage-dependent resistant changes will make for some good-sounding distortion, there's no "good mojo" to be had from old, new or borrowed carbon that you can't get from modern metal film resistors.  In fact, you'll probably just wind up with a noisier, wider tolerance circuit.

That's my two pennies.

[Peter Snowberg]

I totally agree with brett.

If you break open a carbon comp resistor you'll see the resistor body is a tube filled with the carbon composition material. The resistors that RS sells are based on a little solid ceramic cylinder with a coating of carbon film on the outside. They look like well formed cylinders rather than peanuts.

In pedals, I'm convinced no mojo rears it's head.... only noise and low tolerance. I don't even like CCs in most places in tube amps. Carbon comp resistors also drift over time (at least in amps where they get heated).

You're now up to 4 cents worth of replies.

-Peter

[will]

Hi Joe,

I totally agree with Peter & Brett. There is no advantage to carbon composition resistors. Especially if there is any voltage drop across the resistor, noisy as hell.

Years ago the tube audiophiles used small value carbon film resistors in series right next to the grid of a tube this reduced any harsh highs. They would never put them where there would be any voltage drop. This effectively rolls of some of the ultra high (radio) frequencies and prevents unwanted ringing (oscillations).

Now if you wanted the effect of an old cheap stereo sound you could use them to generate noise.

Regards,
Will

Add 2 more cents to the bank!

[sfr]

I have tons of Xicon 5% carbon film resistors - would I be much better of with metal resistors?  I know the metal is tighter spec and lower noise and all, but how much of a noticable difference would there be in pedals?  Are there certain types of pedals I should really be using metal resistors for?

[Doug H]

I have tons of Xicon 5% carbon film resistors - would I be much better of with metal resistors?  I know the metal is tighter spec and lower noise and all, but how much of a noticable difference would there be in pedals?  Are there certain types of pedals I should really be using metal resistors for?

I wouldn't particularly worry about it unless you are building really high-gain circuits. Even then I believe you can get the majority of the low-noise advantages of metal film by just using them in the early stages of the circuit, to lower the noise floor before the signal gets amplified. In other words, hang on to your CF resistors- you can always use them.

I buy my stuff surplus so I usually end up with CF or CC resistors- whatever I can get my hands on. I really should order some metal films from mouser for the high-gain stuff. But the noise hasn't bothered me so much up to this point. I don't think with pedals and effects (esp distortion) that resistor composition is as critical as with amps. Just keep in mind you can use metal films to help you out if you run into noise problems.

Doug

[bwanasonic]

I have tons of Xicon 5% carbon film resistors - would I be much better of with metal resistors?

Those are not the same as carbon comp, but I can't say I know technically how they differ. The *mojo* carbon comp resistors are usually these big honkin' brown jobs. If you check Analog Mike's page about the Sunface ( http://www.analogman.com/fuzzface.htm ) you can see them on the inside view. I don't subscribe to the *mojo* theory, but I would be curious to try an experiment with socketed resistors in a Fuzz Face.  

Kerry M

[R.G.]

I would be curious to try an experiment with socketed resistors in a Fuzz Face.

Be aware that *sockets* can themselves cause oddities, especially when you use round resistor leads in sockets designed for IC pins.

[Johan]

from what I understand, elektricly, carbonfilm behaves in the same way as carboncomposite in every aspect, including the second order distortion-mojo that R.G accounts for in his exelent article (read it!!!)...some people will probably insist there is a great difference but i really beleive it is a "look how good it sounds"-kind of thing...


Johan

[Ansil]

i have built distortions with them and it sounded harsh and acrid..
i built lpb1s with them and loved every note..