Parallel, Series
Series/Parallel
Power Ratings
Impedance Ratings
How to wire multiple speakers together in a cabinet can be confusing.
When combining multiple speakers in a cabinet, the Rules are simple:
The speaker impedances should all be the same, and
The speaker power ratings should all be the same.
Of course, at some point you may have to break the rules, and that is what we are going to investigate.
Impedance (Z):
Impedance is sort of like resistance, only different, because impedance is determined by frequency. But for our purposes, we will pretend that impedance and resistance are the same thing.
Amplifier output power is rated at a specific impedance; for instance, a Marshall amp is often rated at 100 Watts at 16 Ohms. A Fender amp might be rated at 40 Watts at 8 Ohms.
Speakers have two important ratings, Impedance and Power Rating. As an example, a Jensen P12Q is rated 40 Watts and the impedance is (usually) 8 Ohms.
Series:
Speakers wired in series look like this:
The positive lead from the amp is connected to the positive terminal of the first speaker. The negative terminal of the first speaker is connected to the positive terminal of the second speaker. The negative lead from the amp is connected to the negative terminal of the second speaker.
The impedance of the two speakers in series is additive, so 8 Ohms + 8 Ohms = 16 Ohms. The impedance of this cabinet with two identical speakers in series is 16 Ohms.
The power rating of speakers in series is also additive, so we have 25 Watts + 25 Watts = 50 Watts. The power rating of this cabinet with two identical speakers in series is 50 Watts.
Parallel:
Speakers wired in parallel look like this:
The positive lead from the amp is connected to the positive terminals of each speaker, and the negative lead from the amp is connected to the negative terminals of each speaker.
The reciprocal of the impedance of speakers in parallel is equal to the sum of the reciprocals of the individual speakers, so 1/Z = 1/8 + 1/8 = 2/8 = 1/4. The impedance of this cabinet with two identical speakers in parallel is 4 Ohms.
The power rating of speakers in parallel is the sum of the individual power ratings. so this cabinet’s power rating is 50W + 50W = 100W. The power rating of this cabinet with two identical speakers in parallel is 100 Watts.
Series / Parallel:
When you have more than two speakers, it is possible to wire them in series / parallel, which looks like this:
In this cabinet, speakers A and B are wired in series, as are C and D. The A+B pair are wired in parallel with the C+D pair. So the combination of all four speakers is wired series / parallel.
The impedance of the speakers in series add up, so A+B = 32 Ohms, and C+D = 32 Ohms. A+B are in parallel with C+D, so the sum of their reciprocals is 1/32 + 1/32 = 2/32 = 1/16. The reciprocal of 1/16 is 16, so the impedance of this cabinet of four identical speakers wired in series/parallel is 16 Ohms.
The sum of the power ratings of all four speakers is 25 X 4 = 100. The power rating of this cabinet of identical speakers wired series / parallel is 100 Watts.
Breaking the Rules:
Speakers with Different Power Ratings
Got it? Good. So far we have obeyed the two rules of speaker combinations. But what happens if we have two speakers that we want to use in a cabinet, but they have different power ratings? Let’s say we have one speaker that is 50 Watts at 8 Ohms and one speaker that is 25 Watts at 8 Ohms? Like this:
In this case, the power rating of the cabinet is the lowest power rating speaker times the number of speakers, so 25 Watts times 2, or 50 Watts. The power rating of this cabinet is 50 Watts, and it doesn’t matter if they are wired in series or parallel. In this example, the amplifier can deliver 100 Watts into 8 Ohms, but the cabinet is only rated at 50 Watts. Uh, oh. Thus the red “X”. Not good.
Speakers with Different Impedances:
What if we have two speakers of the same power rating but different impedances?
Something like this:
First, the cabinet power rating is still 100 Watts, two 50 Watt speakers. The total impedance of the cabinet is 12 Ohms, which isn’t good, because our amplifier wants to see 8 Ohms. The additional problem is that the 4 Ohm speaker will pull 66.7 Watts from the 100 Watt amplifier, while the 8 Ohm speaker will pull 33.3 Watts. The total is 100 Watts, but the 4 Ohm speaker is exceeded in its power rating. Uh, oh. Another red “X”.
The calculation is: Power(A) = Total Power X (Impedance of A/ Total Impedance)
Power(A) = 100 X (8/12) = 33.3 Watts
Power(B) = 100 X (4/12) = 66.7 Watts
Here is an extreme example: Let’s say you have a Marshall stack with four speakers wired in series / parallel. One of them is blown, so you replace it, but you only have a 10W speaker, when the others are all 25 Watts:
Remember the formula for speaker power ratings is the lowest power rating times the number of speakers. So, in this case, 4 X 10 = 40, the cabinet with the replaced speaker can only handle 40 Watts. And you have a 100 Watt amplifier. Not good.
Here’s the rules again:
The speaker impedances must all be the same, and
The speaker power ratings must all be the same.
It seems that every time we break the rules something bad happens. It is not always true, as in this example:
In this case, the cabinet power rating is the lowest rated speaker times the number of speakers (25 X 2), which is 50 Watts. The amplifier can deliver 40 Watts, so the cabinet can handle it, even though one speaker is only rated at 25 Watts. Cool.
