Originally Posted by JRock3x8
so theoretically if I purchase and install side speakers for the front sides then I might have to buy DIFFERENT speakers when I expand to all 4 sides?
When a speaker has low impedance - like 4Ohm - the amp is required to supply more current (measured in Amperes - characterized as a larger or heavier draw) in order to keep up. If the amp is not able to keep up, it can overheat, or produce a distorted signal. Most amps in receivers are good with 8Ohm loads, and okay with 6Ohm loads, but are usually not recommended for 4Ohm loads. This varies with manufacturer, but most common consumer-grade stuff is not characterized as 4Ohm-stable. The cases where this comes into play are when the playback level is elevated, especially for an extended time - the heat builds up.
So let's say you have true 8Ohm speakers that are easy for your amps to drive. Now you hook up two sets to the same amp terminals - just running two red wires into the positive amp output and two black wires into the negative amp output. This results in parallel configuration, and effectively halves the impedance. Now for any given playback level, the amp is required to produce twice as much current as it did before. This puts you back in the area of driving 4Ohm speakers, even though your speakers are 8Ohm individually. With most amps, this wouldn't be recommended. Still, I'm going to to it with mine, at least at first. I'll be keeping an eye on it to see if it seems like a problem. One of the reasons I'm willing to give this a try is that my speakers are so sensitive. 2.83V should get me about 93dB or 94dB of output from each of them. That means that the amp wasn't going to be working hard at all to drive one - I figure it should be okay driving two. The other variable is playback level; I plan to listen close to reference, but you don't plan to listen that loud - that works in your favor when trying to run two speakers on the same amp channel.
The other way to wire the speakers, series, looks a little weird. You would connect the red wire from one speaker (let's call it "Speaker A") to the positive output of the amp. THen the black wire to Speaker A doesn't get connected to the amp - it gets connected to the red wire to "Speaker B." Then the black wire from "Speaker B" gets connected to the negative output of the amp. This means that the amp doesn't have to produce double the current as it did when you wired them in parallel. Instead, the voltage needs to double. That means you have to adjust the trim for those output channels in your processor, and you are more likely to run out of headroom and end up with a clipped, distorted signal. Again, high sensitivity and low playback levels can both mitigate the effects.
What do you do if you find that you have a problem? Well, the trouble isn't necessarily one component or the other; it's the way they interact. So you have to decide. If you want to continue to use all the speakers, you have three options.
1 - replace the speakers with higher impedance, (or perhaps higher sensitivity) versions.
2 - replace the amp with a more powerful version
3 - find a way to split the signal from the processor to two amps and drive the speakers separately
You won't want to consider option three, because it will mean jumping from a moderate home theater receiver to a the minimum a mid-range receiver with pre-outs and then some splitter cables and an extra set of amps. It's a significant jump in equipment cost and complexity.