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Speakers and Power Amps: Don’t Blow That Cabinet

Q: I’ve been thinking about getting a new amp head, and I want to make sure that it will work properly with my current cabinets. How do I make sure that the head I’m getting won’t blow a speaker or ruin my shiny new head?

A: While I’m not a super “techy” guy, I have learned about some of this the hard way (blowing up a power amp on a tour).

I would encourage you to Google ohms and power handling if you’re looking for more detailed explanations of what I’ll cover here, as there is a lot of information on the web about this as well.

Basically, you need to know what the power handling is for both your cabinets and your power amp. I’ve never harmed a speaker due to using a head that was too powerful, but I have ruined a poweramp because I’ve run it too hot (by connecting too many cabs to the power amp).

It all comes down to ohms. The math is pretty simple.

1. How many ohms can your power amp run at safely?

The lower the number, the more power will be pulled from the power amp.

2. How many ohms are your cabs, and how many can you chain together?

Here’s how it breaks down:

If your cabs are 8ohm cabs, each cab will draw 8 ohms. If you run two 8 ohm cabs together, they will run your power amp at 4 ohms (twice as much power being pulled from the power amp).

If your head runs at a max of 4 ohms, that means that you can run one 4 ohm cab or two 8 ohm cabs safely.

Some heads will run safely at 2 ohms. This means that you can run two 4ohm cabs or one, two, three or four 8 ohm cabs. You could also run one 4 ohm cab and 2 8 ohm cabs, for example.

A 500 watt power amp will generally put out 250 watts into an 8ohm cab, and will allow a 4 ohm cab to draw the full 500 watts.

Read the manual for the power amp and it should lay out how many watts it’ll put out at “X” ohms.

If I’m chaining cabs together, I generally use 8 ohm cabs, but I also have one 4 ohm 410 cab that I use for my hi-output (loud) gigs, as the head I currently use only goes down to 4 ohms.

Again, a quick search on the web should lead you to a good breakdown of ohms and how they work on a more technical level. I encourage you to learn as much as you can about it to avoid any problems.

Readers, what advice do you have to give? Share your thoughts in the comments.

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    Ernie Leblanc

    I’m currently(pun) running 720 watts @ 2 ohms into (1) 2×10″ 175 watt, 4 ohm bass cab stacked upon (1) 4×10″ 350 watt, 4 ohm bass cab with “IMpressively EXcellent” sound-quality results.

    Just because an automobile has 500 horsepower doesn’t mean it can idle in park or run slowly down Any Lane, U.S.A. at the posted speed limit.

    So, if you have (1) 600 watt power amp rated at the correct ohms for your bass cab, just don’t press the pedal-to-the-metal and things should run just fine.

    -Ernie Leblanc