Balanced Line
09-25-2006, 05:57 AM
How many ohms in that wire?
I’d like to take you through the reason wire resistance can be an important consideration in the operation of a low impedance speaker system (the type we normally use as a DJ). In this installment we’ll find out how to determine wire resistance. In the next, we’ll put it to good use to see how it might affect operation in typical and not so typical setup.
The resistance of a conductor is dependent upon its cross-sectional area, its length and the material of which it is made. The material in audio work is almost always copper. Copper is an excellent conductor, better than gold but about 5% less conductive than silver. Why not use silver? Well obviously expense but the cost could never be justified as a larger gauge of copper will more than make up for the slightly poorer conductivity and do so at a much lower cost even if we do have to use more of it.
The formula is: Resistance = resistivity X length / area. Given the formula we can see that if either the resistivity (material) or the length of the conductor increase, the resistance increases. If the cross sectional area increases, the resistance decreases. I’m sure you’ll all agree this is wonderful but how does this help me? Well actually you could use it once you looked up the resistivity for copper and the cross sectional area of the wire you intended to use but there is an easier way. There are tables available such as:
http://www.powerstream.com/Wire_Size.htm
Scroll down the page a bit. The values are calculated for us in this table. I’ve taken the values we are most likely to need and listed them here. Let us look at those values that will be most useful for our work:
AWG_________Ohms per 1000 ft.______Ohms per ft.
10___________.9989________________.0009989
12___________1.588________________.001588
14___________2.525________________.002525
16___________4.016________________.004016
18___________6.385________________.006385
20__________10.15_________________.01015
22__________16.14_________________.01614
24__________25.67_________________.02567
To determine the resistance of your cable look up the AWG (American Wire Gauge) and find the resistance per foot for that gauge. Now multiply that by the number of feet in the cable. The length has to be doubled because the current has to go out on one wire (conductor) and return on the other.
A fifty foot #14 speaker cable @ 50 X 2 = 100 ft of wire. 100 X .002525 = .252 ohms.
A fifty foot #22 speaker cable @ 50 X 2 = 100 ft of wire. 100 X .01614 = 1.6 ohms.
Use the table to determine the resistance for any gauge and length. The next article will put this new found information in an operational context.
I’d like to take you through the reason wire resistance can be an important consideration in the operation of a low impedance speaker system (the type we normally use as a DJ). In this installment we’ll find out how to determine wire resistance. In the next, we’ll put it to good use to see how it might affect operation in typical and not so typical setup.
The resistance of a conductor is dependent upon its cross-sectional area, its length and the material of which it is made. The material in audio work is almost always copper. Copper is an excellent conductor, better than gold but about 5% less conductive than silver. Why not use silver? Well obviously expense but the cost could never be justified as a larger gauge of copper will more than make up for the slightly poorer conductivity and do so at a much lower cost even if we do have to use more of it.
The formula is: Resistance = resistivity X length / area. Given the formula we can see that if either the resistivity (material) or the length of the conductor increase, the resistance increases. If the cross sectional area increases, the resistance decreases. I’m sure you’ll all agree this is wonderful but how does this help me? Well actually you could use it once you looked up the resistivity for copper and the cross sectional area of the wire you intended to use but there is an easier way. There are tables available such as:
http://www.powerstream.com/Wire_Size.htm
Scroll down the page a bit. The values are calculated for us in this table. I’ve taken the values we are most likely to need and listed them here. Let us look at those values that will be most useful for our work:
AWG_________Ohms per 1000 ft.______Ohms per ft.
10___________.9989________________.0009989
12___________1.588________________.001588
14___________2.525________________.002525
16___________4.016________________.004016
18___________6.385________________.006385
20__________10.15_________________.01015
22__________16.14_________________.01614
24__________25.67_________________.02567
To determine the resistance of your cable look up the AWG (American Wire Gauge) and find the resistance per foot for that gauge. Now multiply that by the number of feet in the cable. The length has to be doubled because the current has to go out on one wire (conductor) and return on the other.
A fifty foot #14 speaker cable @ 50 X 2 = 100 ft of wire. 100 X .002525 = .252 ohms.
A fifty foot #22 speaker cable @ 50 X 2 = 100 ft of wire. 100 X .01614 = 1.6 ohms.
Use the table to determine the resistance for any gauge and length. The next article will put this new found information in an operational context.