Bob The Eskimo

There are several threads on tapping into the head unit wires for a preamp signal, as well as using post amp signals to power aftermarket amplifiers. I would like to start a thread that shows clearly how/if it is possible.

I would also like to propose to the moderators that the audio electronics section of the forum be given some sub sections (i.e. installing subs, Nav section, Lighting, etc.).

From what I understand about preamp signals, there is a positive signal that transmits the music. There is also a ground or shield that is with the signal. In a standard RCA cable, the ground is actually just that, a shield arround the positive wire. This prevents or lessens any interference that may pass through the wire.

If you have looked at Monster Cables RCA wires, they have actually taken one more step with these wires. A normal RCA will have the shield connected at the head unit or source, and connected at the amp or destination. This allows signal to ground out at either end. Monster Cable has lifted the ground at the amplifier end to eliminate un needed noise to be transmited to the amp.

Having said this, I know nothing of the signal coming out of the Head Hunit of the rx8. I have an amp that accepts RCA inputs from 200 mVrms to 5Vrms. So my question is: can I connect the pre bose amp signal (+ side) under the rear window to the RCA center wire, and just ground out the negative (-)? Will this be the correct Watt and Volt to the system? Is this essentially lifting the ground from the head unit?

Or, am I completely wrong??

Any help would be greatly appreciated.

bob

mdw33333

Cactus, feel free to chime in here. I'm just feeling too lazy...

CactusGeorge

Hello Bob:
The connections from the HU appear to be differential signal, you do NOT want to connect the - lead to GND (a scenario similar to this fried the op amps of my HU).
Please refer to the following thread for more info, including a line output converter solution from mdw33333:
https://www.rx8club.com/showthread.p...threadid=23213

Bob The Eskimo

'Moving forward, I would either use a David Navone or Soundgate line level converter to isolate the differential outputs from the Soundstream. -or- I would use only the positive signal from the HU and tie the shield connection to ground. (i.e. LF+ and Gnd)."

I read that thread beofre but your statement here confused me. This is what prompted me to explore the lifted or altered ground for this system. I just remember thats how the shield in the Monster RCA cables work and used it as an example.

Differential? I am not familiar with this type of signal. Can you please tell me the what a differential signal is, and how it differs from the standard preamp signals. Also, please let me know why the positive wire can not simply be tapped and leave the ground out entierly?

CactusGeorge

Hello Bob:

In an aftermarket HU preamp signal, there is a + and a GND (shield).

The RX-8 FMS HU has a + and a - (not ground) signal that are 180 degrees out of phase--this is great for noise immunity, but aftermarket amps, in general, are not expecting this type of signal.

Something that I have not tried that may have good results, is to use the + signal from the FMS HU and ignore the - signal (so the center connection for the RCA cable is + and the shield is tied to GND).

Otherwise you have to use a line output convertor (or possibly speaker level inputs of an aftermarket amp) to utilize the differential signals.

Bob The Eskimo

Thanks for the info. eXcentric and I just had a great discussion about this. I will post my recent statement to him for all to see. I think it is a good statement.



Thank you much for that (excentric just explained to me differential as well). That clears up a lot. So as a test, we could connect the + signal to the white RCA, and the - to the red RCA, and if sound comes from both channels then we have a differential signal. If there is no sound, or it shorts the amp out, then we have a regular signal where the negative is a gound. Who has the guts to try it?

Either way, if you leave the neagative out altogether, then you should have a clean RCA signal anyway, right? There just would not be a shielding for the signal from the splice to the amp.

I understand that the mazda 6 has the same head unit as the 8. On the mazda6 forum, they have discussed doing the same thing that you did (preamp rx8 directly to the RCA jacks on the amps) about one year ago. Since the thread has not been updated with a disaterous failure, I would be willing to bet that you have the system done correctly, and they are not differential. Another bit of logic would tell us this as well. If you ground out the reverse phase signal from the differential headunit, then it would short immediately. Hence, you have already tested that it is not differential.

Another test I could to is bring one of my phase inverters from my recording studio and attach it to the - signal. If the graphical eq follows music, then we have a differential, if not, then all is well with standard RCA.

Does all of that make sense?