Originally Posted by rocksure
Resistors: Metal film and Carbon
Does it really matter?
Here's a few things you should understand when using specific resistor types in mic preamps.
1) All resistors have slight second order distortion, especially carbon resistors, which is why carbon comp resistors sound warmer than metal film resistors. This distortion figure can be obtained from the resistor manufacturer and is called the Voltage Coefficient (VC) which is expressed in PPM. A typical VC for Metal Film is 1 PPM for each resistor in the signal path which is equivalent to approx .0001% distortion when your preamp is running at a +15dBu output level.
A typical Carbon Comp resistor has a VC of 320 PPM which equates to .034% for each resistor distortion at +15dBu output. So, for every three carbon resistors you place in the circuit, you will introduce approximately .1% even harmonic distortion which is audible. FWIW, higher resistor values have greater VCs but the relationship is not linear.
2) Carbon resistors are substantially noisier than metal film resistors. All resistors have Johnson noise but what I'm talking about here is what is referred to as Excess Resistor Noise (i.e., Current Noise) beyond normal Johnson noise, and this noise is directly proportional to the voltage across the resistor as well as proportional to the resistance value. Without getting into a long drawn out discussion, suffice it to say that current noise of carbon resistors is approximately 100X that of metal film resistors. In other words, don't use carbon resistors unless you want your preamp to hiss.
To give you an idea how much noise can come from a carbon resistor: a 1/4 watt 100K carbon resistor with 10v across it will perhaps have 2uv of noise generated within it which is added to your signal. This is approximately 20 PPM which is -95 dBu which is about .002% hiss. Clearly the hiss from a single carbon resistor is audible. Multiply that by 5 similar resistors and you get approximately .01% total hiss which represents -60 dBu which is ungodly noisy.
3) Metal film resistors are about 10X more stable than carbon comp resistors in terms of temperature stability. More importantly, they are vastly more stable over the lifetime of the resistor, meaning they won't drift much with age.
4) There can be a difference in max voltage between carbon and metal film resistors, but that depends on their physical construction so I can't say much further on that other then to suggest that you check the voltage ratings of whatever resistors you use to make sure they work.
In sum........The obvious choice for your preamp is metal film. I've had good luck with cheap metal films from Xicon as well as more expensive MFs from Rodersteins among others. Both sound good.