I am trying to put together a decent sound system in my 69 chevelle. I have 6x9s in the kick panels and 6x9s in the rear. They are powered by a 4 channel amp. Today I got a cheap set of 8 inch subs at a garage sale. They are in two separate boxes and I was wondering how to wire another amp to power these subs.
Thanks for the help
steve

Steve,

I need to know what you present spec's are on the stereo (output) and what the spec's are on the input of the first (one installed) and second (the one you want to install) amp. You need to know if the Stereo can drive both Amps, as it may start to work and then burn out later. It may seem to work, but if you have a mismatch (impendance mismatch), it may not work for long. Also, you need to look at how much Power you will be pulling with both amps, and what your Alternator (or generator) can put out. This tied in with your harness, in case you have increased your Alternator output, but you still have the old wiring harness. So, I guess you have a few factors here to look at.

Fred.

You can use your existing amplifier to drive the subs in combination with the 6x9's, without adding an additional amplifier, by using a few relatively inexpensive components.

Lots of variables here. A few things needed:

1) Impedance (Ohms) of subwoofers - 4 or 8 (each).
2) Impedance (Ohms) of rear 6x9's - 4 or 8 (each).
3) Output power per channel (in 4ch. mode) of your existing amplifier.

If your amp is 50W/ch. or better you should be OK. It's not going to give you molar-rattling/mirror vibrating bass, but it's a start.

If your speakers are NOT the same impedance, DO NOT try this. "4's with 4's are OK, or 8's with 8's, but not a 4ohm with an 8ohm.

Look here and click on the "6db/oct." crossover box.
http://home.earthlink.net/~j.delke/id1.html

What you will be doing is adding a capacitor (non-polarized) in line with the + lead of your 6x9's, and an inductor coil/choke in line with the + lead of your subwoofers.

You will then parallel connect a sub to the existing L.H. rear 6x9, and do the same with the other sub (Yes, there will be 2 speakers connected to one output of the amp).

I'd recommend components for about 100Hz for the subwoofer choke, and 130 - 200Hz for the 6x9s.

No, this will not short out your amplifier. What is happening is the capacitor is blocking frequencies from 130Hz and down from the 6x9's, and frequencies 100Hz and higher from the subs. The amplifier will not see a double load, as only 1 speaker will be "working" at this crossover point.

(Technically the amp will see a double load temporarily at this point, but for all practical purposes it will be so small it will handle it just fine).

These components should be available at a GOOD car stereo shop. Just be sure to get NON-polarised capacitors with a 50 volt or better rating.
OR - If you're not interested in this, let me know and I'll detail a second amp install.

you can get power splitters that allow you to run larger wire to the battery and ground and then it splits the signal to 2 or 4 smaller wires (depending on the block)

forgot to add you need to make sure your stereo has an extra set of RCA outputs

thanks for all the help. My stereo only has one RCA output , is this a problem or can I just use splitters.Also how many times can you split splitters? Is it possible to spit twice to power two 4 channel amps?

I take it you're meaning to split the RCA output?

Most decks have enough "oompah" to allow one or two splits. This can be accomplished with an inexpensive RCA "Y" cord. Doing this poses another dilemma however.

By simply splitting the RCA output, you'll be sending full range audio to your subwoofer amplifier. Subwoofers don't reproduce higher frequencies very well at all, and this will "muddy" your bass response. At this point you have a few choices:
1) Use an amplifier for the subs that has a built-in crossover section (at the input/pre-amp level) for subs (130-ish Hz and lower)
2) Use any amplifier and purchase an "active" electronic crossover box to send the bass/mid/highs to your 4 channel unit, and sub freqs to the sub amp.
3) Use any amplifier and install the inductor/chokes described above on the subwoofer speakers.
3b) Optional, but a good idea: Still install the capacitors on the rear (or front) 6x9's to cut the sub-bass from them. You have dedicated speakers for this now and it'll take some load off your 4 ch. amp.

thanks for all the help