There are a lot of threads running in the forums about using extra speakers cabinets to increase your sound or improve your tone and it seems that half the guitar world won't do simple things because they are worrying too much about breaking their stuff whilst others are fearlessly trashing their gear through too much experimentation. As a crap musician but competent technician I'll try to lay out some basics as simply as I can. If this gets a reasonable reception I'll try to build this up into a series of hopefully helpful articles. Please let me know if you think I've made any mistakes and ask any extra questions if I've not covered something or failed to explain it clearly. I'll leave it a while and then try to update my article to include all your ideas.
This first article is about the ohm thing; how do I match speakers to amplifiers, will it improve the sound and will it destroy my amplifier? How can you get to be really loud?
The Ohm Thing
Ohms are about resistance. How easily will electricity flow through something and how much current will it draw. Everything electrical has a resistance, light bulbs have ohms, electric ovens have ohms and even drummers have ohms. Lots of ohms means that electricity has trouble getting through and less ohms mean that electricity storms through. Sweaty drummers have fewer ohms and are easier to electrocute. Single loudspeakers usually have eight ohms, though you can buy them with four or with sixteen. (Just in case of confusion I ought to explain about impedance. Speakers are often described as having an impedance of eight ohms, this is because some of their ohms come from speakers being electromagnets. This is more complicated than you need in a musicians guide so for us ohms , resistance and impedance are the same thing.)
How Many Speakers Can I Safely Connect
OK if you connect two speakers in parallel (more on this later) then there are two routes that the electricity from the amplifier can go through and there will be twice as much power right? Three speakers and three times as much, four speakers, five Actually this is pretty much what can happen but before you get carried away you should think about the poor amplifier. A 100W amplifier is still only a 100W amplifier. If it is a solid state (transistor) amp it will bravely try to go on pushing more and more power through your speakers Transistor amps will try to deliver double the power every time you halve the ohms. Eventually they will run out of steam. What happens next depends upon the detailed design. Most will have protection and will just limit their own power/distort or blow a fuse. (never go to a gig without spare fuses) With many of them the power supply will be inadequate for the power demand so they only develop their max power into a speaker with the right number of ohms. This means you are wasting your time trying to squeeze more power out. The worst case if you try to add too many speakers is that the output transistors will overheat and fry. FET,s are less likely than silicon transistors to do this, but if you understood that sentence you don't need me to explain.
If you have a valve amp then the situation is that you must match the output to the speaker and fortunately you have a transformer inside the amp to do this. The deadly thing for valve amps is too high an impedance, too many ohms. Well designed valve amps should have multiple tappings on the output transformer and you really should try to match the ohms on the amp and the speakers.
As a general rule most amplifiers will deal with four ohms with no bother although depending on the power supply they may be happier with eight. If you have two speaker cabinets of eight ohms then wiring them up in parallel will give you four ohms and your amp should deal with this happily. The problem would come if you tried to plug in three cabs. Now you have three speakers with only two and a bit ohms, you could be demanding three times the power from your poor amp. Don't do this unless you have calculated the overall impedance and are happy that your amp can cope. If in doubt contact the manufacturer and find out if the amp will cope with low impedances (anything less than four ohms).
If you have a combo amp with a built in speaker and an outlet socket for an extension speaker things can get a little more complex. On some combos the socket has a switch so that the internal speaker is disconnected (my Peavey does this). This means I can ignore the ohms of that speaker and just use the Peavey as if it were a head. If your combo leaves the speaker connected when you plug in a lead (which you can tell as the sound will still come out of that speaker) then any speaker will be in parallel with the internal speaker and you need to work out the ohms of the two speakers combined to make sure it doesn,t go too low. Fortunately manufacturers don't usually provide a socket for an extension speaker if the amp won't take it so you can be pretty confident about plugging in your extra speaker. Just like with a head amp don't just plug in in two extra speakers as with the internal speaker this makes three and you could damage some amps.
Having too many speakers will only damage amps at high volumes. In any practical situation the damage is almost always caused by too much power. Turn the volume right down at first until you know everything is working and turn the volume up a little at a time listening for any distortion you are not expecting.
There are ways of connecting more than two cabs to an amp by using series connection though you will need special leads to do this. Before you try this you should read the section on series and parallel. In any case you should ask yourself why you want to do this for an instrument amp.
Will This Be Louder
Yes probably. But it is not completely straightforward. Loudness is subjective because we are more sensitive to some frequencies of sound than others. New speakers may give a smoother sound and seem quieter. Secondly speakers vary quite a lot in their efficiency, that is in how well they turn the electricity from your amp into sound. In practice it is quite possible for the most efficient speakers to make 10 watts sound like 100 watts do from less efficient speakers.
How Loud Can I Go
If you play gigs then you only need to match the volume of the other instruments if your band is to sound balanced. The best band in the world will sound rubbish if they are not properly mixed. In practice the critical point for most bands comes when your sound matches the drummer. Any louder and the drums will need to be miked through the PA to match you. If your PA can deal with this then you are in a whole new ball game. You might as well get the extra sound for all of the instruments through the PA which will give you the chance of having someone mix you properly and achieving a balanced sound. Keeping the onstage sound down to manageable levels and letting the PA do the work makes sense at every level
Even fairly modest combos will produce over 100dB of sound which is more than enough to damage your hearing. Do you really need to be louder? Is hearing important to musicians? I think you know that one. Adding extra speakers should be about the quality of sound for most of us not just a mad pursuit of power.
How Safe Are The Speakers.
So far this has all been about protecting the amp from too few ohms (too many speakers). Now I want to look at matching the speakers to the amp. There are many ways to damage a speaker but the amp only has two ways. It can cook the speaker by passing too much power for too long or it can pop the speaker by pushing it right out of its frame. Most instrument speakers and professional PA speakers are designed so that they can not be pushed out of their frames but hi-fi speakers often aren't. Don't play guitar through your hi-fi unless you want an excuse to buy new speakers. Very low notes do demand that the speaker cones have to move further though, so using bass or keyboards with guitar speakers at high volumes can also cause problems.
Burning out your speakers is much more likely. You have to match the power output of your amplifier to the maximum power handled by the speakers. Power is measured in watts and you need to make sure that you are dealing with real watts. Real watts are RMS watts, ignore anything which says music power, peak watts, PMPO or anything other than RMS. RMS measures the maximum output your amp will produce without distortion. Being a guitarist of course means you will want distortion or overdrive. This effectively adds to the power your amp is generating in fact with distortion your amp will generate up to 1.414 times the RMS output. If you want your speakers safe then it means you need at least one and a half times the power handling in your speakers. This means that a thirty watt amp should have speakers that can handle at least 45Watts and a 100w amp should be matched with at least 150W speakers. In practice I always try to go for double the power handling, ie 200W speakers with a 100W amp. It is absolutely safe to use big speakers with a small amp though: 200W speakers are fine with a 10W practice amp.
Series And Parallel
Ok this is the ever so slightly technical bit. Speakers have a positive and a negative terminal. This is so that we know which way they go when we apply an electrical current to them from the amplifier. If we connect the positives together and the negatives together then they are connected in parallel. The positives are always connected to the tips of jack plugs and are usually marked with a + or a red dot on the speaker itself.
Connecting speakers in this way has a number of effects:
You increase the volume of sound.
The impedance is reduced below that of the lowest speaker.
The amplifier will have to supply more current.
The power handling (the watts) are doubled
To calculate the new impedance you need to use a bit of algebra and know the impedance of all the speakers, R1 ,R2, R3 etc. to calculate the new overall or total impedance Rtot the formula is
If you had two speakers and they are both 8ohms the calculation is:
1 = 1 + 1 + 1
Rtot R1 R2 R3
So Rtot =4ohms
Fortunately we usually only connect up speakers with the same ohms in parallel so that if you divide the ohms of a single speaker by the number of speakers in parallel then this gives you the overall impedance. So three 8ohm speakers in parallel would be 8/3 or 2.6667 ohms. Anything more complicated and you have to do the sums.
The positive on one speaker is connected to the negative on the next one. They make a daisy chain. The plug is connected to the positive on one speaker and the negative of the other, pin to positive still.
Series connection also makes changes:
The ohms increase
The sound level decreases
The current from the amp decreases
The power handling is doubled.
Calculating the ohms is easy, just add them together. For example two 4ohm speakers give you 8ohms.
The final method of connecting speakers that you will come across with musical instruments is used as a useful way of connecting four speakers without reducing the ohms too much. Effectively you are just combining the two ways we have already looked at and it is called series/parallel. This is the classic 4x12 connection. First connect pairs of speakers in parallel. Then connect the two pairs of speakers in series as if they were single speakers. (sorry about this but I lost the diagrams that made everything clearer try the Celestion website and look in Dr Decibels Secrets).
If you have a couple of 2x12 cabs and you want to connect them together then this would be a good way to go about it. If the internal wiring is in series then no problem, if they are internally wired in parallel, and they probably are, then you need your repair shop to make a lead up to connect them in series.
If you want to calculate the impedance you need to do it in two parts, first caculate the impedance of each parallel pair then add them together. In practice if all the speakers are the same then the ohms will be the same as for a single speaker.
This is a really useful way of doing things because you get double the sound of a single speaker without using any extra power from the amplifier or having to drive a really low impedance speaker. The power handling is four times that of a single unit. The disadvantage? You have to pay for and carry four speakers at a time.
There it is then, I hope this is fairly clear and if you are one of life's experimenters you have a reasonable chance of getting the sound you want without too many costly mistakes with your gear. I'd really like any comments or questions. Good luck.