Well, here's how it works:
Head units have preouts which supply unamplified voltage to an amplifier. Some headunits have 4volt preouts, some 2.5, some 2, and some even less. The lower the voltage the preout puts out the more you'll have to turn the amplifier gain up the get the same volume, but the more you turn the gain up the more distortion the amp creates. A headunit with 2 preouts probably is probably 2volts. The way you run a 4, 5, or 6 channel amp off 2 preouts is as simple as buying rca splitters

The downside is that this also reduces the voltage going to each input of the amp, so the amp has to work harder. But you should be able to have the gain at a loud enough level with minimal distortion even while using a splitter. A good rule of thumb that I have found to be true is most car audio amps begin to distort badly at max headunit volume when the gain is about 3/4-7/8 with a 2v input.

Also the splitters are typically 1 male and 2 female which would mean you'd have to run 4 rca cables to the amp with the splitters by the headunit, with the harder to find splitters like my pic (1 female 2 male) you can split your 2 rca cables closer to the amp.

An easier route would be just buy a 2 channel amp and run each channel at 2 ohms (thats running 2 of the same speaker off each channel in parallel)

Spark notes version:
Buy this^ and be careful how much gain you set.

 

Sorry I reread your op and realized i didn't answer your specific Q. I'm leaving my above post for anyone who may be wondering about a headunit/4ch amp.

If you're trying to run a 4ch amp of an ipod or zune you'd do it in the same way but again just be careful what you set your gain at because the voltage they supply is already slightly ampified for headphones. (which will help you since you're splitting it)
so you'd want this:

and the splitter posted above.

It's probably alot cheaper to do a 2ch amp powering the 4 speakers because you'll have more power available since you'd be running it at 2ohms with unsplit inputs. Just make sure the amp is rated for "2x_____watts rms at 2ohms" or 2 ohm stable. If it only says 4 ohm stable or 2 ohm stable in bridged mode and not this statement ^ then it wont work.

 

Thanks for the fantastically detailed response, can't believe I hadn't considered a splitter.

Please correct me if I'm wrong, but it is correct that running components up front with a crossover has no implication on the impedance?

If I'm speaking gibberish, I'm saying that if I go the 2 channel amp route, the 4 component speakers up front count as 2 because of the crossover design, right? I only ask because now I'm talking about running essentially 6 speakers off the 2-channel amp. Sound about right?

Also, I'll probably use my iphone primarily, which will do line out from the dock connector, so in addition to the price benefit, 2-channel is sounding (har) good.

Ok, and a fresh curveball if you would humor me: Say I want to adjust the level of the rear fill. Am I back to needing a 4-channel?

Thanks a ton!
Mike

 

Please correct me if I'm wrong, but it is correct that running components up front with a crossover has no implication on the impedance?

If I'm speaking gibberish, I'm saying that if I go the 2 channel amp route, the 4 component speakers up front count as 2 because of the crossover design, right? I only ask because now I'm talking about running essentially 6 speakers off the 2-channel amp. Sound about right?

Also, I'll probably use my iphone primarily, which will do line out from the dock connector, so in addition to the price benefit, 2-channel is sounding (har) good.

Ok, and a fresh curveball if you would humor me: Say I want to adjust the level of the rear fill. Am I back to needing a 4-channel?

To the impedance question, yes you're correct.

If you ever want to double check the ohms of a speaker or impedence you're presenting the amp with its real easy to check with a multimeter. Set it to ~20ohms and probe the wires and it'll tell you exactly what it is. If that made no sense, then nevermind it haha

This is where it gets complicated. You have 2 options here.
1. Use a 4 channel amp
-The "right" way
-have individual control of each speaker
-a little less powerful
-left and right speakers function as left and right representing the music correctly

2. Use a 2 channel amp
-The cheap "easy" way
-The right channel would be the rear speakers, the left the front(so you'd use l and r balance to fade front and rear)
-left vs right sound would get messed up though, left sounds go to front, right go to rear so side to side sounds would sound different then with headphones.

For your setup youll only have 2 channels so youll never have a front or rear output. To fade front to rear and keep the left and right channels left and right and not front and rear you'd need a 4 channel amp because you would use the gains to adjust front and rear and your iphone to adjust left and right.

-one other plus about getting a 4 channel amp vs 2 ch is typically 2 channels are used for subwoofers (a little more powerful, less adjustements). 4 channels are used for highs/mids so they usually have some type of high pass filter (google it) or frequency range adjustment.
If you post the speakers you have or are thinking about buying I would be happy to recommend an amp as well

Spark notes version: get a 4 channel amp

 

Gold stars. So many gold stars. :*

I am completely ok with less power, and I didn't realize I'd lose proper stereo sound if I went with a 2ch. So 4 channel it is!

I haven't started shopping at all yet, but if I had to guess, I'd say I'm ok with investing $500 in the full system.

If I was to run off and go try this on my own at this point, I'd probably start with:
- 4ch amp
- Components (if these exist in the right sizes)
- Compact full-range (You think?) for rear fill.

re: Components, I notice clearwater sells a 8"/1" set, but I don't see anything in similar sizes at Crutchfield (not that I would order from there necessarily, prices always seem too high)

 

Ok, I'm not sure how you're planning on mounting the speakers, if it were me I'd probably do 6.5's in the doors like factory and then mount tweeters in the top panel. so you'd need a component set for the front. I've had good luck with rockford fosgate, kicker, and polk audio. If I had to recommend one it'd be this http://www.crutchfield.com/p_206DS62/Kicker-DS6-2.html?tp=106 but honestly there are tons out there that would sound good.

For the rear, if you could get 6x9's in boxes that would be ideal. This is slightly simplified if you want more detail just ask!: In order for you to have audible bass, you're going to need to put you're speakers in some type of enclosure. The door speakers are not quite ideal because they use the door for their box and 1 its not sealed in the least, 2 it's too big for the speaker. But it'll sound alright regarding everything above 200 hertz. The bass is going to have to come from the rear, and the larger speaker you can fit the better. 6x9's are the largest common size for speakers (not subs) so that's what I would go for. Although I can't think of an elegant solution to fit 6x9's... So basically what do you plan on?

For a better understanding of hertz and what frequency is what, put on some half decent headphones and look up ____hz test tone on youtube and you can hear the different frequencies. This is useful in tuning your amp and understanding speaker/amp specs.

Here's a 4 channel amp I think would fit the bill:
http://www.crutchfield.com/p_500MRPF300/Alpine-MRP-F300.html?tp=35782
Honestly, I've had great luck buying new and used amps off ebay but your experience may differ haha.

 

I checked out the clearwater system you said(and saw its for 06-11) and realized I should ask what year miata you have? I was referring to a 90-97 miata above as far as speaker placement

 

Ah! Sorry, this is for an '04 MSM. Here's the clearwater system that I've seen folks talk about*:
http://www.clearwateraudio.com/cwc8.html

*usually they're talking about the BOSE-compatible model though, bleh. But same sizes, otherwise.

I have a little experience building enclosures, typically for subs though, but I was hoping to do something really conservative for the rear. Ideally I would mimic the NB wind-blocker style enclosure. I don't think I'll be able to use the stock one once I move my harddog over from my NA to the MSM, but it works well enough that I would be willing to sacrifice some SQ for something compact that doesn't require me to add too much additional weight. I'm totally prepared to compromise on the rear fill and try to get the best SQ out of just the front woofer/tweeter combo.

And like you, I've had great success with prices on ebay in the past. All the HUs, amps and speakers have come from there in the past 10 years

Thanks again, you've gone above and beyond.

-Mike

 

Sure no problem, love to help other enthusiasts out when I can!
Yeah, I think with that setup you'd be doing quite well even with no speakers in the rear haha, with a good amp to drive the components you should be real happy. Good luck to ya and stop back to let us know the results!