The only way to test for electricity or electrical characteristics like OHMS, VOLTS, or AMPS, is by using proper testing equipment. One of the most common and useful testing tools is a Digital Multi-Meter (DMM). MCM has #72-7770, but again, they’re all over the place. Hardware stores, construction supply stores, electronic stores and auto parts stores all will have them. Please read the Owner’s Manual and practice on battery-operated stuff. You can figure out how to use a DMM from the manual, but it’s best to be taught by someone who already knows how… And this training needs to include you actually testing stuff – a simple explanation would be insufficient. You do need to know some basic electrical principals to use a DMM safely. Please don’t check your wall outlet with a DMM until you’ve been shown how. Damage to your self and/or your new meter can be the result of unsafe experimentation.
You want to find the symbol that looks like a horseshoe ( Ω ) on the DMM. That’s the Greek letter Omega, and is used to denote “Ohms” or resistance. You can find the word OHM on the back of your speaker and on the back of your amp. You will also find it in the Owner’s Manual. Please read your Owner’s Manual and when you see “8 Ohms” or “4 Ohms”, know that what they’re talking about is the electrical load of your speakers. This is a measure of the load, or “weight”, that your amp is “lifting”. How strong your amp is, is the same as how many WATTS it has. A 50-Watt amp is stronger than a 15-Watt amp. You want to be sure (it’s really critical) that your amp has the right load. Having too light a load on the amp is like stepping into an unexpected hole for you. Whoops! Except for the amp, it’s happening over and over again, and things will blow up eventually if not right away. Having too heavy a load cuts the amp’s power, just like putting extra rocks in your backpack would for you.
While a DMM reads DC resistance, and the “OHMS” on your amp and speaker refer to AC Impedance, and they are different, right now we don’t care! We don’t care because when you measure the DC resistance of a healthy speaker, it will be close to, but less than, the AC impedance. The DC resistance of an 8-ohm speaker will be around 6.8 ohms. It may fluctuate a bit, but what you’re looking for is whether or not it varies drastically from what is printed out the outside. You’re going to do the actual testing using the speaker cord itself, which will add a little resistance, so it will be quite close. You should read about what’s printed on the speaker or speaker cabinet. If you have no reading (open), or close to zero (shorted), then you’ve got a sick speaker.
Here’s how to do it. First, make sure everything is turned off!!!
If you have a combo amp, look to see if you can unplug the speaker from the back of the amp. If not, look to see if the speaker is available in the back. The point is to isolate the speaker and put the two DMM leads across the speaker. If you can do this, fine, if you can’t without taking stuff apart, then don’t. It’s cool. Check a stereo speaker for drill. Make sure the DMM is in the lowest Ohms setting. If you have a head plus cabinet set-up, plug the speaker cord into the speaker and taking the other speaker cord plug, put one probe of the DMM on the tip, and the other on the shield of the plug. The tip and the shield are separated by a spacer, and are connected to the red or white (+) and the black (-) of the speaker. Whatever you see on the meter should be close to what’s printed on the outside. A 4 ohm speaker will read 3.2 or so; an 8 ohm around 6.8; a 16 ohm speaker will read 12 or 13, and a 32 ohm speaker will give you about 24-26. This test doesn’t guarantee a good speaker, but it gets us most of the way there. I test speakers 3 different ways in my shop, but this would be a good test for general acceptance of a speaker or speaker cabinet, provided nothing else was hinky
This speaker test is important because you need to know what you’re plugging into your amp. The danger is “not enough load”. Using a properly matched speaker load of sufficient wattage, your amp will sing all day long without a problem. What you’re looking for is that your amp and your speaker load be the same wattage (strength) for a given load (weight), and that the speaker can shed all the heat (watts) left over when the sound is made. If your Owners Manual, or on the back of the amp, it says it has an output of 100 watts with an 8 ohm load, the speaker load should be 100 watts or more, and the back of the cabinet where you plug in should say “8 ohms”. It’s OK to plug a 100W amp into a 200W speaker cab, but if you plug a 200W amp into a 100W speaker load, the speaker, and then probably the amp, will go “Poof!” Happens every time!
Here’s the tricky part. We’re smart, but amps are not. On the back of your amp, or in the Manual, it will say something like “100W / 8 Ohm”. This means that the amp can put out 100 watts of “lift” into an 8-ohm load or “weight”, and will do it safely all day long. However, if you half the load, you about double the power, or watts, the amp has to put out. These “WATTS” basically equal “heat generated”. Since your body is smart enough to not use as much power in your arm to lift 4 pounds as you do to lift 8 pounds, your arm won’t hurt itself. Your amp is not that smart. It still uses the same “strength” to lift 4 ohms as it does for 8 ohms, and it will hurt itself, sooner or later. This is because the amp is working too hard and generating too much heat, and will soon destroy itself, the speaker, or both. For a 100W / 8ohm amp to be given a 4 ohm load means that the watts generated are doubled, and that means lots of extra heat that the amp can’t get rid of.
Be careful how you hook up your speakers. The life and survival of your gear depend on it.
Armand Blake, Owner: AMPWERX Repair
1935 E. 7th. St.Long Beach,CA90813
Tel: 562-591-1423 Fax: 562-591-1423