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What's ohms and IMPEDANCE have to do with amps and speakers?

We need to discuss the “Law of Ohms” or "Ohms Law"…whatever you feel like calling it. It's pretty straightforward stuff So, let's learn by looking at an example of “Ohms Law” at work. First though, let's remind ourselves of Mr. Ohm's often forgotten formulas. There are two of 'em and they enable us to find out the combined impedance of speakers (or cabinets) when joined together in either Series or Parallel (note: if you don't know the difference between series and parallel in electrical terms, go look 'em up!)

Let's imagine that R1 is the impedance of our first speaker (or cabinet); R2 is the impedance of the second one and R3 is the resulting impedance when they are hooked-up together. Got that? Good Here are the two formulas:

The Parallel formula is R3 = (R1 x R2) / (R1 + R2) and the Series formula is R3 = R1 + R2

 So, let's look at the Parallel law in action. This is the one you need to be most familiar with because although speakers inside cabinets are often wired up in series, when it comes to hooking-up cabinets to combos or heads it is ALWAYS done in parallel (well it is for Marshall guitar amplifiers and we'll use them as an example).

The most common question asked in mega amp world is, "Hey dude, what impedance do I set my tube head to when I'm using two cabs?" Well, let's take a look…

Let's suppose that our buddy has a Marshall DSL100 head and 1960A and 1960B cabinets. Here's an idiot proof hook-up list:

1. Make sure both cabinets are switched to Mono operation.

2. Make sure you have two SPEAKER cables to do the hook-up with (NEVER use guitar cables).

3. Plug one end of one lead into the 16 Ohm MONO input of the 1960A

(NOT the 4 Ohm input…remember, there are two because of the stereo option).

4. Plug one end of the other cable into the 16 Ohm MONO input of the 1960B.

5. Now, as we're hooking two 16 Ohm cabinets up to our Marshall DSL100 head in parallel, let's plug in R1 (16 ohms)and R2 (also 16 ohms) into our trusty formula:

R3 = (16 x 16)/(16+16) = 256/32 = 8 Ohms

6. Set the Ohm switch below or use the correct speaker outputs on the back panel of the amplifier, plug the speaker cable ends into the outputs, switch the amp on, plug in yer axe, dial in a tone and rock!!

And there you have it. In fact, a simplified offshoot of the Parallel law we've just learned is this:

Whenever you're hooking-up two cabinets of the same exact impedance (and let's call that impedance "X") in parallel, the resulting impedance is X/2, fout cabinets would be x/4 . 

For PA gear, if you have four 8 ohm PA cabinets hooked up to your PA amplifier, your amplifier will be pushing 2 ohms.  Make sure it can handle this load!  If your PA cabinets are 4 ohms, then your PA amp will be pushing less than 2 ohms and no PA amplifier is happy at this load.  It will be one hot, unhappy and soon-to-not-work unit!

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