Car Stereos: 4-way amp RCA output, subwoofer amp, rca output

Question
Hi Brian, I saw your answer to Andrew about installing 4-way and a mono amp together, and it was very detailed. However, I have a few more doubts.

In my current system I have a pretty simple pioneer mp3 deck with 4 pre-amp channels and a 4-way amp that I installed as follows: the rear speakers are hooked up to the amp's front channels and a sub on it's bridged rear channels. The front speakers are connected to the line level outputs of the deck itself. This way, the "fader" function of the deck controls the amount of "balance" between the normal speakers and the subwoofer. It was my way of doing it the cheapest way possible and going around the missing subwoofer control. I know it's far from perfect, but it worked for me.


I just bought a new deck with built in gps, dvd, etc, but unfortunately it doesn't have a subwoofer output.

With the new deck, I want to keep the 4-way amp for the 4 speakers and add a more powerful mono amp for a more powerful subwoofer. The 4-way amp has an RCA output on it (which I presume is still a low level output right?). So I guess I can attach this output to a mono amp. My doubt is here: the 4-way amp has built in crossovers, so if I filter out the lowest frequencies from the 4 channels to avoid distortion on the normal speakers, would it also filter out the low frequencies at the RCA output of the amp, or leave it untouched, so that i can still use for the sub amp? And also, in this set up, the "fader" would control the balance between the front speakers and rear speakers together with the sub, right? Thanks for your attention and looking forward your answer.

Answer
Hi Andrei,

You might want to check your amplifier's owner's manual for specifics, but in most cases, the RCA output on a 4-channel amp works like this:  it's a low-level output that combines the front and rear inputs, and won't be affected by the amplifier's crossover and bass boost settings.  That makes it ideal for an additional mono subwoofer amp, which isn't surprising, since that's by far the most common use for RCA outputs on a multi-channel amplifier.  

In short, the low-level output from the 4-channel amp shouldn't be affected by the fader control, or by the amp's high-pass filter settings.

Hope this helps!

Brian