Building a Speaker Cabinet for Guitar: Wiring and Theory

[twsknight]

I've been toying with the idea of building a speaker cabinet for a guitar amp. I've got a lot of little hifi speakers lying around to experiment with before I go for the anything serious, but I'm confused by wiring, impedance and how it'll all look.

Basically, here's what I'd like to do eventually. I have a 50watt solid state combo amp at the moment. It's nothing special, but I bought it because it'll just get over the rest of the band in rehearsal. I started pondering if I could hook up an external speaker to the combo amp to make it usable in small venues and I also like the idea of wiring and building something myself; so if your first thought is "just buy a bigger amp of ebay" that's not really the point.

First thing is, would I be able to hook up another speaker to a combo amp in theory? Also, would it make any real difference to the overall max volume output? I'm worried I've been sucked into this Marshall stack with a million cabs Rock star thing.

I've googled and the main thing that people talk about is resistance/impedance. Talking 4ohms and 8ohms. This makes little sense to me, but it clearly is important. Presumably if I bought an external speaker it would need to match the ohms against the amp?

Don't worry about the trials and tribulations of building the cabinet itself for the moment. I just really need advice the basics of wiring etc.

I did do a search on here, but couldn't find anything; so apologies if there are similar topics on this.

Finally, I'm a drummer as well as a home recording enthusiast; so please use small words and simple sentences!

 

[moresound]

If the out put of your amp is rated to go down to 4 ohms and the speaker you have is 8 ohms you can add one more 8 ohm speaker in the chain which will drop it to 4 ohms. I think that's how it goes but if the speaker you have matches the ohmage out put of your amp I'd leave it alone.

 

[twsknight]

So if you have two speakers with the same ohm rating (or is it ohmage?) then the overall ohms would half?

From there your overall ohms need to match the amp?

 

[Minion]

There is Paralell wireing and there is series wireing and combinations of the two, two 8 ohm speakers in Paralell is 4 ohms , two 8 ohms speakers in series is 16 ohms , if you take two 8 ohm speakers and wire them in paralell and thake another two 8 ohm speakers and wire them in Paraell and then wire each set of Paralelled speakers in series you will have an 8 ohm 4 speaker cab .....

You also don"t want to run an amp at a lower impedance than it is rated for and you also don"t want to mix 2 different types of speakers in the same cab unless they have the same impedance and sencitivity specs ....


Cheers

 

[mshilarious]

Resistance and impedance are related concepts; resistance is, as it sounds, the amount a component resists the flow of electricity. Higher resistance (measured in ohms) means, given every else equal, less current will flow. This is Ohm's Law:

I (current, in amps) = V (voltage) / R (resistance)

So if your amplifier is cranked to a given level (voltage), then the current that flows through your speaker is a function of its resistance.

Power increases with the square of voltage:

P (power, in watts) = V * I

Impedance is frequency-dependent resistance. Some components, such as speakers, have varying resistance dependent on the frequency of the alternating current flowing through them (an audio signal is AC). But we pretty much ignore that when stuffing speakers in a guitar cabinet, we use the nominal impedance of the speaker, which is often 8 ohm (there are 16 ohm speakers too, and if you dig hard enough 4 ohm).

Back to batteries and bulbs! Resistors in series increases resistance:

Rtotal = R1 + R2 + R3 . . .

So if you wire two 8 ohm speakers in series, you would have a 16 ohm load. That means you'd reduce current in half, and power in half as well.

Resistors wired parallel decreases resistance:

1/Rtotal = 1/R1 + 1/R2 + 1/R3 . . .

For multiple resistors where they all have the same value, that equation simplifies to:

Rtotal = R1 / n

where n is the number of resistors in parallel.

So if you wire two 8 ohm speakers in parallel, you will have a 4 ohm load, current will double, so will power.

If you have a four-speaker cabinet, you'd get 2 ohms. That's kinda bad, not all guitar amps (especially tube amps) can handle that heavy of a load. You could use 16 ohm speakers and get back to 4 ohm.

Or, you could wire two groups of two speakers in series (16 ohm each), and wire those two groups in parallel. Now you're back to 8 ohm. This is called series-parallel, and it's commonly used.

With transistor amps, generally they don't care about light loads (numerically higher in ohms), but you can't hook up a heavier load than rated (4 ohm in your case). With a transistor amp, you can usually get maximum power at the maximum rated load. Although power amps for mains can often be "bridged", which usually requires an 8 ohm load, but can produce more power than either channel can at 4 ohms.

Tube amps are particular: you need to hook up the rated load. Tube amps frequently have switches or separate jacks for different cabinet ratings. Respect them.

As for "ohmage" is it a term you'll encounter when people are talking about speakers and amps, but in other areas people tend to stick to resistance and impedance. In circuit design, it's often important to keep those two separate since you are dealing with both DC and AC at the same time. For that reason, I tend to use the term impedance when talking about speakers. Both impedance and resistance are stated in ohms, but for abbreviations:

R = Resistance
Z = Impedance

since "I" was already taken for current. Why? I don't know, but I blame the French . . .

. . . and sometimes you'll hear people using "amperage" for current, or more usually a current rating. That is mostly electricians: "You need a breaker with less amperage as you only used 14 gauge Romex on that branch."

 

[twsknight]

Thanks to everyone's responses so far, things are starting to make sense.

I realise that ohmage of the speakers shouldn't be lower than the ohmage amp (so if total ohmageof speakers=4ohms but your amp=8ohms that's bad); but if it's higher(speaker=16ohms and amp=8ohms) is that good or bad. What I guess I'm saying is, is the aim to match the ohmage?

Finally, how does this all equate to the overall volume produced? I imagine this is an almost stupid question, but I'm starting to confuse myself.

 

[commanda]

Also be aware HI-FI speakers are designed to a different paradigm than instrument amplifier speakers. Hi-Fi speakers are designed for linear travel. Instrument amplifier speakers are designed so that the coil moves out of the magnetic field on extended excursions. This stops the speaker from physically self-destructing when faced with the extreme dynamic range produced by typical mad guitarists. This excursion limitation introduces distortion, which is a significant part of "The Sound".

Hi-fi speakers will work, in as much as they will produce sound. Pushed hard, they may self destruct. Almost certainly won't beat a good set of Celestions.

 

[Minion]

If your amp puts out 100w into 8 ohms that means it will put out 50w into 16 ohms .... there is no problem running into a higher load ...

 

[mshilarious]

Thanks to everyone's responses so far, things are starting to make sense.

I realise that ohmage of the speakers shouldn't be lower than the ohmage amp (so if total ohmageof speakers=4ohms but your amp=8ohms that's bad); but if it's higher(speaker=16ohms and amp=8ohms) is that good or bad. What I guess I'm saying is, is the aim to match the ohmage?

Bad in a tube amp, doesn't matter in a transistor amp. However, you won't get maximum possible power out of the transistor amp if the load is lighter than rated.

Finally, how does this all equate to the overall volume produced? I imagine this is an almost stupid question, but I'm starting to confuse myself.

That's a function of power and the efficiency of the speaker. But for simplicity, let's say you have a perfect amp (that is, it can produce 2x power at 4 ohm vs. 8 ohm) and you have two speakers of the same type. By adding the second speaker in parallel, total power will double (power produced by each speaker is the same whether you use one or two), and you will get a 3dB increase in volume.