Car Stereos: How to add a 4th pair of speakers when there are only 3 set of pre-amp outs, passive crossovers, crossover frequency

Question
Hello Brian,

i now have a system - thanks to your help earlier - which consists of 2 door and 2 rear deck speakers connected to a 4 channel amp. My head unit also had a subwoofer preamp output so i also added a single sub connected to that preamp output. The sub is powered by a separate, 5 channel amp.  Right now i am only using the sub channel on that amp. I was thinking that i could add a pair of super tweeters and use one of the free set of channels on this amp to power these.  But since i only have 3 sets of pre-amp outputs from the head unit, what would provide the audio signal to these tweeters?
Thanks for any advice.

Answer
Hi Nadeem,

Your best option in this case would be to purchase a pair of RCA Y-adapters, and split the front output from the head unit so that a second set of cables could be run to your 5-channel amp (assuming your new tweeters will be installed in the front).  This is the only way to ensure that your fader control will affect both your door speakers and your new tweeters.

You'll need to make sure that you use an appropriate crossover with the tweeters.  The built-in crossover in many amplifiers doesn't have a crossover that can be set high enough for tweeter use.  The rule of thumb for finding the crossover frequency:  find the resonant frequency (FS) of the tweeter, then double it; that's the minimum crossover frequency.  For example, if the FS parameter is 1500Hz, then you'd want to use a crossover frequency set at 3000Hz or higher.  If your amplifier doesn't let you adjust the crossover to this setting, then you'll need to use in-line passive crossovers on the tweeter wires; or pick tweeters with built-in crossovers.

Hope this helps!

Brian