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How To Hook Up Cabinets, Heads, Speakers Without Causing Fire

I recently picked up a couple of JBL K140 15″ speakers for dirt cheap. At the time I had only my trusty Fender Deluxe 90 for amplification, and I was yearning for more “beef” in my sound, so I thought I might be able to somehow use one of these in a cabinet or just replace the 12″ inch in the amp to beefify things a bit. Having just had the amp repaired from a mysterious meltdown mid-gig recently, I was understandebly a bit concerned about plugging things in where they normally don’t go.

My amp’s output is 4 ohms, and these speakers are 8 ohms. Without going into exactly what that means, suffice it say that amps are designed to supply juice to a speaker with a certain amount of resistance. When you change that resistance, you necessarily change the amount of juice the amp sends out. Change it the wrong way, and your amp will try to send it out too much juice and it might blow up.

So how to avoid this? I did some research and found a really clear explanation from Andrew Pouska at www.tunemybass.com, check his site for it and loads of other useful info, especially if you are a bass player.  For a simplified explanation, keep reading.

Basically, the impedance rating of your speaker/cabinet must be equal to or higher than the output of your amp.  Using my situation as an example, my K140’s are rated at 8 ohms.  One speaker at this rating can be safely connected to my amplifier with a rating of 4 ohms.  Because the speaker has more resistance than the amplifier’s rated output, the volume will be diminished, but no fire will occur.  If those numbers were reversed, you have a formula for fire.  An amplifier with an 8 ohm output connected to a 4 ohm speaker will overheat and possibly burn up.  Less resistance means the amp will try to send more power than it should, which is BAD.

Remember, amplifier must have a lower impedance output than your speaker/cabinet to be safe.  Ideally they will be the same.

The concept applies to multiple cabinets connected to the same head/amplifier, but there is now math involved to calculate the resistance to make sure you aren’t endangering your equipment.  If this describes your situation, take a moment, visit www.tunemybass.com and read Andrew’s great article on this subject.  Following his math might just save your gear and hard-earned cash.