Actually, wirewound
resistors are the quietest of
all, having only thermal
noise. Metal film resistors
are the next quietest,
followed by carbon film,
and lastly, carbon
composition. 

Noise is also proportional
to the resistance value and
inversely proportional to
the power rating (a 2W
resistor is less noisy than a
1/2W resistor). 

Following is an excerpt
from an old post of mine to
the alt.guitar.amps
newsgroup (before it went
down the tubes): 

Resistor noise is made up
of three main types:
thermal, contact, and shot
noise. Thermal noise is
dependent only on
temperature, bandwidth,
and resistance (ignoring
Boltzmann's constant),
while shot noise is 
dependent on bandwidth
and average DC current, 
and contact noise is
dependent upon average 
DC current, bandwidth,
material geometry and 
type. 

The thermal noise of a 1K
carbon resistor is the same
as a 1K metal film; it is
independent 
of material. The only way
to reduce this noise is to
reduce the resistance
value. This is why you
don't want those 10MEG
resistors on your input
stage. 

Shot noise is dependent
upon current, so the more
average DC current
through a resistor, the more
noise you get. In order to
reduce this type of noise,
you must keep the DC
current to a minimum. This
is best done in the first
amplifier stage or in
low-level stages such as
reverb-recovery amps,
where it is the most
critical. Unfortunately,
higher DC currents usually
sound better in tubes, so it
is a tradeoff. Best practice
is to use a wirewound or
metal film in these
applications 
(unless you are making a
high-frequency amp where
the inductance of the
wirewound resistor comes
into play, this is not
generally a factor in guitar
amps). 

Contact noise is dependent
on both average DC
current and resistor
material/size. The most
significant contributor to
noise in guitar 
amplifiers is the use of
low-wattage carbon
composition resistors.
Since the noise is
proportional to resistor
size, the use of 2W carbon
comp resistors will
improve the performance
over that of 1/2W
resistors. 
Studies have shown a
factor of 3 difference
between a 1/2W and a 2W
carbon comp resistor
operating at the same
conditions. 

Wirewound resistors are
the quietest, having only
thermal noise, followed by
metal film, carbon film,
and lastly, carbon
composition. 

There were a couple of
other posts I made on the
subject as well. If you are
interested, you can do a
dejanews search on my
name to find them. 

Randall Aiken 
reaiken@innova.net