When picking out a car radio amplifier you must familiarize yourself with Ohm’s Law and the type of car stereo speakers you have in the vehicle. The speaker’s power capabilities and impedance will greatly affect the type of car radio amplifier you can use.
Ohm’s Law is about the relationship between voltage (E), Current (I) and Resistance (R). Consider your home as the power source, or battery. Attaching a hose to the faucet is like running a wire. The size (diameter) of the hose determines its resistance. A thin hose will have a high resistance and a fat hose will have a lower one. The water running through the hose is like the current running through a wire. Your home faucet has a large potential to allow a great deal of water to pass through it. This potential is like voltage. If there is no potential, then nothing will flow through the hose. Power (P) is the result of a potential causing a quantity of current to flow through a resistance. Power is expressed in Watts, and is equal to the current multiplied by the voltage (P=IE).
Car stereo speakers have resistance or impedance values expressed in ohms. A car radio amplifier is a device that takes a very small electrical signal (voltage) and increases or amplifies it. All car radio amplifiers are powered by the vehicle’s electrical system, and operate at 12 volts. Car radio electronics are designed to take the small voltage signal produced by radios and to pass it to the car radio amplifier where it is amplified and passed along to the speakers where it produces sound. The larger the speaker, the more power the amplifier must supply. Additionally, the use of many speakers requires the amplifier to supply a larger amount of power.
The type and size of the car radio amplifier will change depending on the size and number of car stereo speakers installed in your vehicle. Under no circumstance should you use a car radio amplifier with a larger power rating than that of the car stereo speaker system. If you do, then the first time you blast the system you will burn out the speakers. If you have the stock speakers that were installed by the manufacturer, a full range amplifier with moderate wattage will suite your needs fine. If you have aftermarket car stereo speakers, then the choice for an amplifier will be different. You may need separate amplifiers to adequately power all of the speakers. In either case, look for an amplifier that has its own crossover networks (filters that direct specific sound frequencies to specific speaker types), and a choice of output impedance. The output impedance is important because it must match that of the car stereo speakers. If it does not match, you will not get the power amplification you paid for, and the sound quality will be affected.
If you pay attention to the power and impedance requirements of your car stereo speaker system, half the work of selecting a car radio amplifier will be over.