<center><img src="http://pictureposter.audiworld.com/118660/audio_diagram.jpg"></center><p>I have a 2000 A4 with Symphony and Bose, and it's time to add amps (Bose amp is crapping out). I already have upgraded speakers in the front and rear doors.

I am aware that, with the Blau adapter, I can get RCA style pre-outs for the front and rear channels. I want to know if there exists a device that will take the front and rear pre-outs from the head unit and average them, such that I get one set of pre-outs for my sub amp.

Essentially, what I want is something that will do the following:

output_left = (front_left + rear_left)/2
output_right = (front_right + rear_right)/2

The above picture shows my desired setup, where the "Magic Box" block represents the device I am trying to describe.

You're probably asking, "Why the hell doesn't this idiot just tap one set of the pre-outs from the head unit for the sub amp?" My reasoning is that I want the signal going to the sub amp to attenuate when I fade the sound to either the front or rear (similar to the way the stock system works).

And yes, I have to keep the factory head unit (at least for now).

If I were to just tap the front pre-outs, when I faded to the front, the bass would overpower the mid/high frequencies. If I were to fade to the rear (which I doubt I would do frequently), the bass would drop out.

Conversely, if I were to tap the rear pre-outs, when I faded to the front, the bass would drop out. If I were to fade to the rear, the bass would overpower the mids and highs.

I've tried it before (tapping the front leads for a sub amp), and it's noticeable when fading.

Thanks.

Rear speakers are for rear seat passengers only. I usually tune my systems so that I can't hear the rear speakers in the front seat. The sub is then tuned to blend with the front speakers. The rear passengers are seating back there because they weren't important enough to sit in the front anyway.

Honestly, it is possible to do what you're thinking but I just think its a huge waste of money and doesn't make any sense. To correctly sum inputs is not cheap.

When you fade to the front your signal will be left channel biased and when you fade to the rear it will be right channel biased. You'd have to make the front and rear channel signals mono first before you did that.

Yes, I know I should be focusing on the front stage. Unfortunately, when I was younger and dumber I decided to buy Infinity Reference 5.25" components for the front and rear doors. They are "okay," but when I run only the fronts, they distort at far less than ear-bleeding volumes.

From the sound of it, it seems that what I am looking for doesn't exist in a simple, affordable package. It just bugs me because Scosche makes a high/low converter for roughly $20 that takes 4 speaker-level inputs and provides 2 line-level outputs (see link). I just wish there were something like this with 4 line-level inputs instead.<ul><li><a href="http://www.sonicelectronix.com/item_4994_Scosche+LOC-80+Adjustable+Line-Out+Converter.html#">Scosche 4x80W Input, 2 line-out</a></li></ul>

That seems kind of shady. I don't like the idea of changing the proportion of left and right channel bass as a function of fading.

Otherwise you may like to fade to front, but by doing so you'd lose all the sub (if it was running purely off the rear pre-outs, like the non Bose system))

I found several reasonably-priced 4-channel amps that provide a line output as the sum of the left channels and the sum of the right channels:

output_left = front_left + rear_left
output_right = front_right + rear_right

This is exactly what I was looking for. Here are a couple examples:

<A href="http://www.crutchfield.com/p_500MRPF250/Alpine-MRP-F250.html">Alpine MRP-F250</A>
<A href="http://www.crutchfield.com/p_136G4500/JL-Audio-G-Series-G4500.html?showAll=N&tp=115">JL Audio G Series G4500</A>


Thank you MacSub and HIS4 for your help along the way. I'm sure I'll be back with more questions in the future.