Speaker Cabinets and Impedance Issues

4 ohms, 8 ohms, 16 ohms? What the heck are they talking about?

Your speaker is the final step of the amplification stage and affects the sound greatly. Let's look at the main variables:

When you connect a speaker to a guitar amp, or any amp for that matter, you are loading it. Impedence (load) is measured in ohms and it's the resistance that is produced when the speaker is in operation. Most speakers have their impedence rating labeled on the back, which is usually 8 ohms, but you can find other values like 4 ohms -I've even seen 3, 5 and 16 ohm speakers, but this is not so common.

According to Ohms Law, two equal resistances or impedances connected in parallel, i.e. positive from the amp to positive on both speakers and negative to both negatives will create a total impedance of half the value of one speaker. So if I connect my amplifier to two 8 ohm speakers connected in parallel the total impedance will be 4 ohms. If the back of my amplifier head states that it can take a 4 ohm load, there's no risk of blowing the power amp stage. Also, if I wish to use a cabinet with a single 8 ohm speaker there's no problem because the amp can take a minimum of 4 ohms - anything higher than that is no problem for a solid state amplifier. With tube amplifiers, if the impedance is too high you can cause damage to the output stage and power supply. The main point here is to read the manual and work within those parameters. If you don't know what your doing, don't do anything!

Parallel Connection = positive from the amp to both positives on the speakers and negative from the amp to both speaker negatives. If the speakers are both 8 ohm, you get a 4 ohm load.

Series Connection = positive from the amp to one speaker's positive the negative of that same speaker is connected to the other speaker's positive, and the negative of that second speaker is connected to the amp negative. The resulting impedance is 16 ohms with two 8 ohm speakers.

4 ohm, 8 ohm? What's the difference?

Quite simple really, an amp working with a 4 ohm load will deliver twice the power, i.e. if an amplifier working with an 8 ohm load can deliver 50W of power, the same amp with a 4 ohm load can deliver 100W of power -get the picture? What would it be able to deliver if it had a 2 ohm load on it? That's right... 200 Watts! Don't try it! Why? Because you'll be making the power amp stage work twice as hard and you'll be lucky if it lasts you 30 seconds in operation. There are some PA amplifiers that can do this but that's an uncommon exception to the rule. Study your guitar amp manual so you know what you can and cannot do as a mistake of this nature can be costly.

What about speaker size?The basic rule is quite simple, a bigger sized speaker can deliver lower frequencies more easily and a smaller speaker can deliver higher frequencies more easily. Although equalization is very important factor which governs the sound of your rig, 10 inch speakers are very typical for a guitar amp and for a good reason: this size delivers a fair amount of body and definition naturally.

8 inch speakers also work quite well, but how many guitar amps have you seen with a tweeter in the cabinet? I can't remember having seen any unless we're talking about a amplifier for an electric-acoustic guitar. If you've ever put your guitar in distortion mode directly through a full range cabinet, you'll have noticed that a tweeter makes a fuzzy, undesirable sound -but who knows' You might even like the effect for some part of a song!

Bass guitar cabinets come in all shapes and sizes too. these rigs are more complex because the bass player's sound can make or break the sound of the band. Some combinations include 4 8 inch speakers, for definition and a 15 inch speaker for body. Others just have just one 12 or 15 inch speaker -and the variables go on...

When engineers are designing a speaker cabinet they carefully calculate how much loss in body needs to be compensated electronically and in cabinet design when using a smaller speaker to do a bigger speaker's job, for example. So don't be too surprized when you hear good bass sound from an eight inch speaker, thank the engineer.