How Much Power Don't You Really Have ?
By Bob Bornack
Today, lets take a look at amplifier output power vs. how much power actually gets delivered to your speakers. This issue can be a heated one, so in short, for now at any rate let's just scratch the surface a little.
As with electricity, The further you travel down a speaker cable, the more applied power you will forfeit. Let there be no misunderstanding, it's not just a "maybe", it's a fact.
It's no secret that to take full advantage of an amplifiers output, it is advantageous to run your system at the least number of ohms possible. Most high end amplifier manufactures build there amps to run smoothly at 2 ohms so let's first use the 2 ohm scenario as an example.
If you make the mistake of parking your amp's next to Front Of House, you are apt to find yourself with 100 plus feet of speaker cable to get to your speakers. No big deal unless you are prepared to have your output power plummet. An 18 gauge (AWG) speaker cable running 100 feet and feeding a 2 ohm speaker load means that you will loose almost 60% in the run. You heard me correctly, 60%. That means that if your running a 1000 watt amplifier, only 400 or so watts are actually showing up at the speaker.
For the "p.a. on a stick" folks, chances are good that your probably using an 8 ohm load. Good news for you in that in the same cable scenario, you are apt to only loose about 24% meaning that same 1000 watt amp is actually delivering about 750 watts. Better, but certainly not stellar by any stretch.
Here's some math to chew on. As you will see, the lower the speaker cabinet impedance, the more watts you will loose in the run as you start going further in distance. % of loss is approximate.
Speaker Lead in Ohms |
Cable Gauge (AWG) |
Length in Feet |
% of Power Loss |
8 |
18 |
25 |
7.00 |
8 |
16 |
25 |
4.50 |
8 |
14 |
25 |
2.75 |
8 |
12 |
25 |
1.75 |
8 |
18 |
50 |
13.00 |
8 |
16 |
50 |
8.50 |
8 |
14 |
50 |
5.50 |
8 |
12 |
50 |
3.30 |
8 |
18 |
100 |
23.50 |
8 |
16 |
100 |
15.70 |
8 |
14 |
100 |
10.50 |
8 |
12 |
100 |
6.50 |
Speaker Lead in Ohms |
Cable Gauge (AWG) |
Length in Feet |
% of Power Loss |
4 |
18 |
25 |
13.00 |
4 |
16 |
25 |
8.50 |
4 |
14 |
25 |
5.50 |
4 |
12 |
25 |
3.90 |
4 |
18 |
50 |
23.50 |
4 |
16 |
50 |
15.75 |
4 |
14 |
50 |
10.35 |
4 |
12 |
50 |
6.50 |
4 |
18 |
100 |
39.50 |
4 |
16 |
100 |
28.00 |
4 |
14 |
100 |
19.00 |
4 |
12 |
100 |
12.25 |
Speaker Lead in Ohms |
Cable Gauge (AWG) |
Length in Feet |
% of Power Loss |
2 |
18 |
25 |
23.50 |
2 |
16 |
25 |
15.75 |
2 |
14 |
25 |
10.40 |
2 |
12 |
25 |
6.40 |
2 |
18 |
50 |
39.50 |
2 |
16 |
50 |
28.00 |
2 |
14 |
50 |
19.00 |
2 |
12 |
50 |
12.25 |
2 |
18 |
100 |
59.00 |
2 |
16 |
100 |
45.50 |
2 |
14 |
100 |
33.00 |
2 |
12 |
100 |
22.24 |
As you can easily see, the lower the load, the greater the loss.
The closer you keep your amp racks to your speakers means the less cable you need and the heavier gauge speaker cable you use means more delivered power to your audience.
Send comments to:
bob@tantrasound.com
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