BMW aftermarket sub to stock system

A friend of mine just bought a new 5-series BMW and he'd really like to fill out low end without any factory alterations.  Through my research, every source reccommends the use of a speaker level converter (post factory amplified) to supply the sub amp with a line level signal.  The factory "front-stage" amp has to receive line level signal from the head unit and at line level current, i feel, it would be alot cleaner if I split that signal to the aftermarket sub amp.  Does anyone have any info or experience with this????

Most likely if there is a line-level signal going from the head unit to the factory amp, it is done just by wires and not RCA connections.  Very very few factory systems use RCA connections to go from head unit to amplifier.  But of course if it's there I'd make good use of it if I were you.

Personally I don't know if the difference in sound quality between using the low level or the high level signals would be all that apparent because this is just a sub amp, not a 4 channel amp.  And from what it sounds like, your friend just wants to add bass to the system, not necessarily blow everyone away with huge amounts of amplification.  Again this is just my own personal opinion.

Assuming the sub and amp are gonna be in the trunk of the vehicle, I'd just tap in to the rear speakers and use a line out converter to bring it down to a low-level, RCA type connection to input to the sub amp.  Overall it's a much simpler install and it keeps (almost) all the wires back in the trunk.  You'll still have to grab the +12 volt turn on from up at the head unit, unless of course there's a factory amplifier mounted in the trunk somewhere.

But like I said, if the RCA connections are already there, USE THEM!

There aren't any actual "factory" RCA type connectors.  What I had planned on doing was splicing into the actual rear signal wires (entering the factory amp located in the trunk) and hardwiring the "new" line out RCA's to the sub amp.  I've done some coax splicing in the past, but from checking out the factory amp, there are only regular, single-conductor inputs.  Could it be that there is a two stage amplification in the BMW system (first at the head, then at the amp)??? anyone know? I don't mind the little extra work to avoid some of that speaker level distortion that factory systems are renowned for.  Thanks for the responses "wvsquirrel" and "esmith69".

If you're lucky and the factory preamp voltage and impedence is within the limits of your amplifier, you can technically just hook up positive and negative to corresponding wires in a RCA cable assembly, and then plug this in to your aftermarket amplifier.  Only problem with this is that sometimes those factory systems use a single negative wire for all 4 speaker preamp outputs.  I guess it would work just using this same wire for ALL of the negative signal wires in each of the 4 RCA cable assemblies but there's always the potential for noise problems.  If done properly, I don't think it'd be a problem.  But I doubt you're gojnna be lucky enough to have the factory head unit's output signal match up perfectly to the aftermarket amp.

Many of the factory amplified systems use a high-level (amplified) output signal from the head unit to the factory amplifier, where more juice is added to the signal.  I don't know for sure if this particular vehicle has this kind of system or not.  My guess would be it does NOT, but it's certainly a possibility.  One way you could be able to tell is by just hooking up any old test speaker to one output channel coming from the deck, and see if you get any sound.  If you either hear nothing at all, or just a very very faint volume, then it probably just has low-level outputs.  However, if it is loud enough that you can hear it without having your ear next to the speaker, then it probably has amplified (high level) outputs.

If  you are able to determine that the factory head unit is amplifying the output signal, keep in mind that this means it will be impossible for you to avoid using a non-amplified signal for the aftermarket amp's input.  You can run it through filters, converters, EQs and all sorts of other stuff to try and clean it up a bit, but it's never going to be the same as a low-level output.

Now whether or not you would even be able to tell much of a difference between an aftermarket sub amp using low-level inputs and an aftermarket sub amp using high-level inputs, is something you need to decide on your own.  Personally I don't think you'll be able to tell a difference between the two especially considering you're not looking to build a competition-type system, just something to fill in the low frequencies.

If you hook up an LOC "after" the factory amplifier then you'd be basically using the amplified signal and bringing it down with the LOC.  Virtually the same thing as tapping in to the rear speaker wires.

If you have to end up using a LOC, I'd recommend you avoid using the ones that are built in to many aftermarket amplifiers, and instead use a good external one like the one in that crutchfield link above, or if your budget permits, one from david navone. 

VERY good point auex, I don't know why I failed to mention that.  I know that is the case for the mustang mach audio systems as well as many others of course.

Actually the first time I ever worked on a mach 460 system, it was for adding on a 2-channel amp and subs, and I made the rookie mistake of tapping in to the highs and not the lows.  I DID use a speaker popper until I found the wires that made the rear speakers pop, but at the time I did not know that there were also tweeters in the back, and it turns out that was what I was popping.  So of course when I got everything all hooked up there was barely any bass coming out of the subs.  And to think, all that time I could have just tapped in to the two rear amps for the bass signal!

Sorry I digress...I was going to ask, if there is no factory subwoofer what would be the best way to go about picking the best speaker outputs to tap in to?  Do you just have to listen to each individual speaker and decide which one is getting sent the most bass? Or is there some other, more scientific way to figure it out?  Let's assume for the purposes of my question that the vehicle's front and back speakers are the same size, and that no wiring information or schematics are available to consult.  Any ideas anyone?