my friend is interested in knowing the answer to this before he purchases an amp

ps. please dont respond with "2 channels" i already used that one on him haha

 

Generally speaking (in real simple terms).......a 2 channel amp.would power two speakers and a 4 channel amp would power four speakers.

 

There's a lot of variables with many different amps. There really is no clear cut answer minues what pnthr30 has stated.

 

First, what exactly is a "channel" and are we dealing with "Stereo" or "Surround". Most likely, your HU does not do Dolby ProLogic II(x) on the fly, although some do (See the new Calrion HUs), so I'm going to assume you're working in Stereo (2 channel) mode. Basically, stereo is two independent channels that each one recieves information, be it a duplication of the original channel, or seperate information all together (this becomes more complicated in digital/surround modes)

In short, most two channel amps are designed to power two speakers, or through monobridging you can still power both speakers. Basically, you could do just your door speakers with a two channel, or you could do door+rear speakers. A four channel amp would be best to power all the cars speakers, plus subs.

 

I don't know if I would suggest powering all four interior speakers and a sub with one four channel amp......I guess it could be done, as some of the more expensive/complex amps are capable of doing so, but it is not typical (or the most efficient/productive way to use an amp) IMO. If you wanted to power all the interior speakers (4 speakers) as well as a sub, I would suggest either a five channel amp or multiple amps (four channel amp for the interior speakers and either a mono amp or a two channel amp for the sub). Admittedly, there are flaws in every "rule", or school of thought, but to keep things simple, this is what I would suggest.

 

A solution to end all problems would unfortunately, as the previous posted mentioned, be a five channel amp. A GOOD quality one of these, making enough power to power all speakers at rated RMS would run you a quite a bit of money. I'm thinking somewhere between 500-1000. I don't have any brand names, or model numbers handy as I've never used a five channel, but that'd be the best. Either that or seperate amps, which in ittself becomes a pain.

 

How much of a problem? Besides tha pain of wiring up two amps (although if you have a good aftermarket head unit it isn't all that hard) there isn't much more. Maybe you say it's a pain b/c you need a power distributor to power both of them. But having two amps really isn't that much different from having one.

 

A two channel amp and a four channel amp rated at the same wattage will power four speakers just the same. However, a two channel amp powering four speakers would be running at 2 ohms, so make sure your amplifier is stable at 2 ohms (that means not sony). I'd recommend you stick to a four channel amplifier, it's just a lot cleaner to have one speaker/one channel.

 

I agree, having two amps is not a big deal (I currently have two). The only difference is that you have to run a two sets of RCA's and get a distributor (not a big deal). Otherwise it is pretty much the same as one (one power cord, one ground, one remote wire).

 

The resistance the speakers pose on the amp is dependent on the way the speakers are wired and what resistance the speakers are themselves. It is not written in stone that 4 speakers to a 2 channel amp will have a resistance of 2 ohms.

In car audio, really the only difference between powering 4 speakers with a 2 channel vs. 4 channel amp is that there is no independent adjustment of the volume between front and rear speakers. That is it, nothing else. There is no problem in running 4 speakers off a 2 channel amp, other than not being able to fade the music to the front and rear. Remember, music is recorded in stereo, not surround.