My friend is selling a rockford fosgate 4 ch amp. I dont know the model number but it is rated at 62.5w rms X 4 @ 4 ohms and 125W rms X 4 @ 2 ohms. What I need in a mono sub amp but will this amp work?

to drive 1 sub.. no... not unless it states it can be bridged down further to 1 chan,... but generally you would want at the most a 2chan amp to run 1 sub.. ideal would be a mono amp..... the only exception to this would be if you are running a dual voice coil sub.. you might be able to pull off the 4 ch. amp although i dont think it will be very effecient...

If you got a sub with dual voice coils, then yes, you could use your 4 channel amp and bridge it down to two channels....so depending on your sub, yes.

or ypu could run your 2 front speakers with channels 1 and 2 and then bridge channels 3and4 together to run you sub. this would give you the best over all sound if you have replaced your stock speakers.

How do you bridge speakers? Is that where 1 neg lead(left side)goes to the other pos(right side)and vice-versa?

Good call....that would be the best way to go, as long as you have a crossover on the sub.

you do not bridge spearkers you bridge amplifiers. you basicaly a running 2 channels as one to get apx twice the power. you would run the pos of one channel to the pos of the sub and the neg of the other channel to the neg of the sub. each amp is diffrent but that is a good start. some amps have a switch so it knows that it is being bridged and then others want you to run the wires a prescribed way.

ok i have a bit of an update, the amp is rated at 250 watts RMS x 2 at 4 ohms in bridged mode, will this be enough power to drive 12'*? I will probably just use it to power my other speakers (not my subs) but i was just wondering if it would work well. thanks

get a dual voice coil woofer and bridge the amp. run one bridgeed channel to the first coil and the the other brigded channel to the second coil. you will get a lot more power from that amp this way. you could run 2 subs with 245 watts each but you can run a better subwoofer this way.

well i wold be using 2 subs because i already have them. Would this amp provide enough power to drive 2 infinity 12 inch subs? Im probably not going to do it but for now im just brainstorming...