Speaker Wire?

Question

Would anyone please specify why, If i am running 8 guage power wire to my amp under the seat it needs to be this large when right after the amp there is 10 feet of smaller speaker wire? Whould anyone tell me whether or not the impedence on speaker wire is ignored or if it is not a major factor? Thanks.

Answer 1

Large wire is required for high current. The power wire has to supply enough power for all the output channels of the amp, and it has to do it using the relatively low voltage of the vehicle's alternator or battery. Power is directly related to both voltage and current; if the voltage is low, then the current must be high to supply the power demanded by the amplifier. Furthermore, amplifiers aren't 100% efficient; much of the power it draws from the vehicle is wasted as heat. The current flow through the power wire has to supply that, as well.

On the other hand, the output power through the speaker wires isn't limited to the 12-14 volts of the vehicle's electrical system. In a high powered amplifier the voltage at the output terminals is increased to deliver power to the speakers, and a higher voltage means a lower current flow for a given amount of power. Therefore speaker wires can be much thinner than power wires.

Ohm's law and Watts law equations can provide us with numbers: if you have an amplifier that's producing 300 watts of output power, and it's 60% efficient, then it draws 500 watts of power from the car's electrical system. The relevant equation is I=P/E, where I = current, P = power and E = voltage. If we say the battery is providing 12 volts then the current in the power wire is 500/12, or about 42 amps of current.

On the other end, the amplifier's providing 300 watts of power into a 4-ohm speaker. The current in the speaker wire is equal to the square root of P/R, which comes up to about 8.67 amps of current. (R=resistance; technically it's impedance but the equation is the same). This is possible because the amplifier boosts the voltage on the speaker outputs to around 35 volts to produce 300 watts of output.

Answer 2

Basically your converting a DC current to an AC audio current by amplifiation. In this process the supply must be adequate enough to provide the current needed to amplify the low level audio signal coming in many times over. In addition to this, a good percentage of the supply is lost to heat.

The bigger supply wire is needed to carry all of this current which only part of gets used.

Wire doesn't have impedance, it has resistance. Impedance is resistance for AC circuits (the amps outputs), ohms is resistance for DC circuits. The impedance and ohms both measure the same with no outside force upon them, but impedance will change with frequency added to it.