This means that for low noise operation, the DC and AC currents should be kept low. The contact noise increases as the current is increased. If no current (AC or DC) flows in the resistor, the noise is equal to the thermal noise. This noise is directly proportional to both the current flowing in the resistance and a constant that depends upon the material the resistor is made of. Wirewound resistors do not have this noise, only resistors made of carbon particles or films. The predominant noise in carbon comp, carbon film, metal oxide, and metal film is composed of contact noise, which can be very large at low frequencies because it has a 1/f frequency characteristic. Studies have shown a factor of 3 difference between a 1/2W and a 2W carbon comp resistor operating at the same conditions. 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. The most significant contributor to noise in guitar amplifiers is the use of low-wattage carbon composition resistors. This is why you don't want those 10 Meg resistors on your input stage.Ĭontact noise Contact noise is dependent on both average DC current and resistor material/size. The only way to reduce this noise is to reduce the resistance value.

Therefore, the thermal noise of a 1K carbon resistor is the same as a 1K metal film it is independent of material. If we are dealing with pure resistances, the thermal noise is equal to the thermal noise produced by an equivalent resistance. In general, the thermal noise of any connection of passive elements is equal to the thermal noise that would result from the real part of the equivalent total impedance. Therefore, the total noise remains the same. The above equation shows that the noise varies in direct proportion to the square root of the resistance, so if you take two resistors of half the value and square the square root and add them and take the square root of the sum, you end up with the exact same value as you would if you took the square root of a single resistor of twice the value. If you model the individual resistors as noise generators, the output noise voltage will be equal to the square root of the sum of the squares of the individual noise sources. Since the characteristics of thermal noise have a Gaussian probability density function, and the noise of the two separate sources is uncorrelated white noise, the total noise power is equal to the sum of the individual noise powers. Thermal noise The thermal noise of a resistor is equal to: Wirewound resistors are the quietest, having only thermal noise, followed by metal film, metal oxide, carbon film, and lastly, carbon composition.įollowing is a description of each type of noise, along with methods of reducing its impact on the circuit, along with some general guidelines for low-noise amplifier design. Thermal noise is mainly dependent on temperature, bandwidth, and resistance, 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. Resistor noise is made up of three main types: thermal, contact, and shot noise. From a noise aspect, there are several things to take into consideration. Who is correct? Well, the answer depends on what your design goals are.

Some people will recommend only using carbon composition resistors, others will tell you that metal film resistors are better. General The subject of resistor types comes up quite often in guitar amplifier discussions.

Plate resistor signal path