# Powering cabs

Discussion in 'Amps and Cabs [BG]' started by Bluerskiesburnt, May 20, 2004.

1. ### Bluerskiesburnt

Mar 25, 2004
San Jose, Ca
would it be better to power a 400-watt cab at 8 ohms with a head at 8 ohms but only 150 watts of power, or the same cab but at 4 ohms with 300 watts?

2. ### Petebass

Dec 22, 2002
QLD Australia
It's actually not that different. I'll explain.

Lets start with option 1. Cab = 400w at 8ohms, and Head = 150W at 8 ohms. This will work fine. However 150 watts isn't very much, so you'll be tempted to run that amp so hard that it starts to distort (also called clipping). At this point the amp actually puts out more than it's rated 150 watts, but it's a dirty signal that can harm speakers. In this case it's not likely because even at 100% full clipping, the amp still won't put out 400w.

OK Option 2. Same cab, but an amp rated at 300w at 4 ohms.
This means the amp can haldle a 4 ohm minimum load, and at that level of resistance, it puts out 300w. You'll be running an 8 ohm load which will work fine, but at that level of resistance, that amp can't push the whole 300w. A 30% reduction is a quick rule of thumb when you double the impedance. So with an 8 ohm cab connected, tha amp puts out about 200W.

So what's the difference between 150w and 200w. Not much actually. To double your volume, you need to multiply your amp watts by 10. Doubling your watts gives an increase in volume of 3dB.

However Option 2 leaves you with a little more headroom. It also allows you to extend your rig later on by adding a second cabinet. Provided the second cab is also 8 ohms, the 2 cabs will combine to give a 4 ohm load, so now you're getting the full 300w from the amp. Not only that, you also get an increase in dB because the 2 speakers will couple and help each other out.

3. ### Joris

Petebass, I second that fully. Excellent advice!!