The question arises: How many speakers can be put on an 8 ohm amp?
That’s a tricky question. The answer is, as many as you want, theoretically speaking. No, that’s not a joke, the explanation follows.
Just to be clear, when we refer to an 8 ohm amp, we are talking about a typical stereo amplifier with two outputs. The kind that make up 99% of commercial 8 ohm amplifiers.
An 8 ohm amplifier is called such because typically it performs very well with an eight ohm load attached to it. When dealing with most commercial ceiling speakers, you will find that the majority have an 8 ohm rating for audio program material (AC). There are a few out there that are rated at 16 ohms and I know of none that are 4 ohms at the time of this writing.
Let’s say you want eight speakers in a large classroom. You purchase eight of the finest ceiling speakers you can find (otherwise known as the SP-230N). You hire a tech to put them in and taking the easy way out, he wires them in parallel. That means he took the speaker wire pair (+ and -), went into the first speaker and then jumped from that speaker straight to the next speaker, then to the next and so on. This gives you four speakers per channel (rated at 8 ohms each). Since your tech took the easy way out he has just loaded your amp with 2 ohms on each channel. What effect will that have on the amplifier? Most amplifiers will either overheat or shutdown as soon as the volume is turned up. “What? But you said…” I know, at the beginning of the article I said you could hook up as many as you wanted, but it has to be done properly. Almost no amplifier on the market is made to drive much below 4 ohms. Remember, the lower the ohm rating the harder the amplifier has to work to supply the current, this always causes heat and eventually a shutdown or damage.
In order to do this properly so that your classroom audio system sounds great and you are not overworking your amplifier, a series-parallel configuration must be used. For an illustration of this, please refer to our “Tech Tips” section of our website, click on the “Multiple Speaker Diagram”. Basically you take two speakers wired in series to make a 16 ohm pair, then wire those two pairs in parallel, this brings the load back down to eight ohms. So now you have the ideal load of 8 ohms, but you have a total of eight speakers in your classroom. This has many benefits. One big one is that your classroom audio system will actually be louder for the same power. More speakers makes your system more efficient. Also, audio dispersion is a bit better.
This parallel-series configuration can be used to add more and more speakers as long as the load is balanced back to 8 ohms. However, if you need more than eight speakers, it is probably time to consider a 70V system. The practical drawbacks start to become a big factor over eight speakers.
So really, the answer is eight – practically speaking.