Amplifiers often have resistors attached to the input jack. This is especially true of amplifiers used with electric guitars and keyboards. Resistors are measured in Ohms for resistance and watts for heat dissipation, as well as in manufacturing tolerance percentage. The only difference in resistors has to do with the resistance values assigned in their manufacture. Ohm's law states that "direct current flowing in a conductor is directly proportional to the potential difference between its ends." This law is normally represented as "V = IR," with "V" for voltage, "I" for current and "R" for the resistance of the conductor.
Input Jacks
Input jacks are where the male plug from the instrument is plugged into the amplifier, sending the electronic signal from the instrument to the amplifier. Once the signal is received, the amplifier lets you adjust the volume and tone characteristics of the signal, producing the desired sound. The sound is obtained as the signal is sent through a variety of circuits, tubes, transistors and other components, often with resistors added at various points to alter the signal.
Resistors
Resistors reduce the voltage sent to a circuit. Different resistor values vary the amount of voltage applied. If a signal going to a circuit is too strong, a resistor of a certain value is placed at that point to help control it. Resistor values are either printed on the resistor itself or represented by colored bands. A resistor with two red bands followed by a yellow band has a resistance value of 220,000 Ohms, written as 220k.
Input Jack Resistor Function
Most amplifiers have multiple input jacks, or channels, for high- and low-gain signals. High gain is often used in rock and roll, and the input jack normally has a high-value resistor wired across the jack and the ground. The low-gain channel has a lower-valued resistor attached, taming the signal further. These resistors are often called grid-load resistors. They control the signal in the first stage of the amplifier and ensure resistance in the signal in case there is no resistance originating from the instrument.
Different Resistors
The different resistors used on input jacks determine the signal strength when it first enters the amplifier. Because the only difference between them is their resistance value, a variety of different resistors are connected to the input jack. These resistors are easily changed if a stronger signal is desired. A common resistor value for high-gain inputs is a one-meg resistor, with a value of 1 million ohms. This allows a louder signal, typical of the high-gain channel. The low-gain input jack usually has a much lower-valued resistor -- 68k, for example. Since the low value resistor lowers the signal strength, the cleaner tone typical of the low-gain input is produced.
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