Differences Between Types of Capacitors and Resistors
R.G. Keen is an electronics guru of monumental proportions. He posts regularly on the Music Electronics Forum and other DIY guitar sites, and is a font of  constant information. Go to his site at geofex.com and you will find an enormous amount of info about guitar effects and tube amps, as well as dozens of projects to build.

This is a thread I pulled from the old Ampage forum by R.G. about the differences between the various resistor and capacitor types, and their effect on tone.

Subject: resistors and caps
I've read that metal resistors have less hiss then carbon ones, but that carbon ones have vintage Mojo.
Also there are many different caps, some which should sound great for audio, some very bad...
What is the real deal here? Or does anyone know a webpage where I can read about this stuff. I read one once but I forgot the url...

From:   R.G.  
Subject: Re: resistors and caps
I guess I really should have done the carbon composition resistor FAQ.
Carbon composition resistors are composed of a cylindrical space filled with carbon granules. They have excess noise above the minimum that a pure resistance of the same value would have. Sometimes much more.
Metal film and wirewound resistors have noise levels very close to the minimum that any resistor can have for a given value, so from strictly noise considerations, they're more optimal that carbon comp, although they may have other characteristics that are a handicap in some situations, principally at high voltage or high power.
Carbon comp resistors have a relatively high voltage coefficient of resistance. That means that the resistance changes with the voltage across them, so the same resistor has a lower resistance at low voltages/currents than it does with high voltages/currents. While all resistors have something like this, carbon comp stands out for having a particularly large one compared to the other commonly available ones.
In this context, "high" means around 100V and up. In tube amps, this can mean that carbon composition resistors have a measurable second harmonic distortion, hence the reputation for sweeter sound.
At low voltages the effect is usually too small to measure. Your 9V (or 12V, or +/- 15V) effect will not show the good distortion, only the noise. There is no vintage mojo, only the high voltage resistance change, or the occasional one that has *bad* point contact and rectification effects.
As to caps, I reproduce an earlier post I did on this. I guess I should just post this stuff every week or two.


The hifi guys are more nuts on this subject, but I'll give you in a capsule the more-or-less accepted rules of thumb. I do hate to see the hifi tweako mystique infiltrating the musical and effects arena... sigh...
For tonal qualities, caps are largely chosen by their dielectric absorption. The semi-accepted order is:
  • Teflon - thought to sound ultimately clear, use for circuit boards too
  • Polystyrene - ditto
  • Polycarbonate - ditto minus a hair
  • Polypropylene - ditto, plus available in uF sizes
  • Polyester (mylar) - Ok if you can't get something better, not a lot worse than the
  • Other plastic films, and much better than those lower in the list
  • NPO ceramic - only available in tiny sizes
  • Aluminum electrolytic - the accepted standard for uF and up sizes simply on $ and size
  • Paper - obsolete, can't get them anymore; thought to be "tweedy" or some such nonsense, by the tubies
  • Other ceramic types - thought to sound "grainy"
  • Tantalum - unreliable and thought to sound grainy
Above teflon is vacuum then air, which are impractical for all normal circuits. There exist other exotica, like glass dielectric capacitors for special purposes.
There are capacitance size and voltage limits, obviously. 100uF polystyrene caps don't exist, while aluminum is barely working up a sweat at 100uF. Likewise, there are no 1000V aluminums, but 1Kv ceramics are easy and cheap. Also the construction of the capacitor, whether stacked, rolled, folded, extended tab, etc., etc., can have real effects (as distinguished from silliness like cryogenic stress relief or water jacketing).
I personally have staged a couple of capacitor shootouts, and haven't yet found someone who could tell simply by listening whether they were hearing music through a polypropylene versus ceramic, although I don't doubt that someone might be able to.
"Best sounding in FX" is not something you can nail down, as you will find that each person may perceive "better" and "best" as meaning something different. I'd be willing to make a significant bet with you that I could set up a test where I changed only the capacitor and you would not be able to do more than 50% +/- 2% correct telling the caps apart by sound alone.
IMHO you would be wasting your money chasing down and buying super premium caps for effects. Use mylar and aluminum, get it working, and when it's going good, substitute out for some super-premium stuff until you satisfy your curiosity. On the other hand, don't just use the high-value ceramic caps for audio coupling (even though many highly sought after vintage effects did exactly this).
There are people who disagree with me strenuously, even violently.