93 W124:can I feed pre-outs from aftemarket head to OEM amps? - PeachParts Mercedes-Benz Forum

G-Benz · #1 09-05-2007, 05:46 PM

I searched and read the vast amount of info from the threads on this forum. This is what I've gathered so far:

OEM speaker setup is 2-ohms, not 4 like most car systems. That explains why the head unit seems unusually hot when I send a direct signal to the front speakers using the head unit amp section.

Fader in the console is basically useless. I tested the leads and found nothing that would send a signal to the rears...must run speaker wire to the rear.

From the Becker website, I was able to splice the needed power leads including telephone mute, antenna on, and the dimmer signal...doesn't fade dim when you twist the rheostat by the gauge cluster, but it has a night/day display when you switch the headlight switch on/off. Satisfied with that outcome, I soldered and heat-shrink wrapped that bundle.

Antenna lead is plugged into the master unit in the trunk...went out and bought an extension. Must run that cable in the same manner as the rear speaker wire (if I go the head unit-powered route).

Eyeballed the two output amps bolted to the other side of the master receiver...at least that's what I think they are.

All of the threads I've read discuss using aftermarket output amps or using the head unit to power the speakers and bypass the OEM amps altogether...why is this?

I'm thinking I can send the seperate front/rear pre-outs from the head unit to the pre-ins on the two amps. Seems to solve the 2-ohm issue, and the speakers are already connected.

Does anyone know how to tap into the OEM preamp sections?

iwrock · #2 09-08-2007, 09:54 PM

You can use a hi low converter to convert the preouts into speaker level outputs.

I did this in my 400E. I used the stock amp to feed power to the speakers. Sounded OK and got loud at only like half volume on the headunit.

G-Benz · #3 09-09-2007, 01:01 AM

Head unit has both pre and speaker-level outputs.

I've noticed that bass output is fine when the volume is cranked up, but abyssmal at low volumes...seems as though he OEM setup had a loudness contour.

As I said earlier, the 2-ohm load is what worries me...the head unit heat sink gets abnormally hot in a very short period of time when I use the head unit amp directly.

So I wondered if it was feasible to use the head unit pre-outs into the stock amps instead?

iwrock · #4 09-09-2007, 01:10 AM

Quote:

Originally Posted by G-Benz

Head unit has both pre and speaker-level outputs.

I've noticed that bass output is fine when the volume is cranked up, but abyssmal at low volumes...seems as though he OEM setup had a loudness contour.

As I said earlier, the 2-ohm load is what worries me...the head unit heat sink gets abnormally hot in a very short period of time when I use the head unit amp directly.

So I wondered if it was feasible to use the head unit pre-outs into the stock amps instead?

Yes,as I said before you can do it that way using a hi low converter. In my 400e I had the stock amplifiers wired up to the aftermarket headunit. It lacked bass at lower volumes,as its the active sound system where its supposed to get louder the fater you go.

Zeus · #5 09-09-2007, 08:08 PM

Quote:

Originally Posted by G-Benz

As I said earlier, the 2-ohm load is what worries me...the head unit heat sink gets abnormally hot in a very short period of time when I use the head unit amp directly.

So I wondered if it was feasible to use the head unit pre-outs into the stock amps instead?

You're right about worrying about the 2ohm load!

If you can send a preout into the stock amp, that should work? I don't think impedance will be a factor then as you are not using the headunit's power output section to drive the speaker - so no need to match impedance.

Only question is how to tap into the stock amps with your RCA (?) preouts...can you pull an amp and put it on the bench and see how to do it?

G-Benz · #6 09-10-2007, 09:43 AM

Quote:

Originally Posted by Zeus

You're right about worrying about the 2ohm load!

If you can send a preout into the stock amp, that should work? I don't think impedance will be a factor then as you are not using the headunit's power output section to drive the speaker - so no need to match impedance.

Only question is how to tap into the stock amps with your RCA (?) preouts...can you pull an amp and put it on the bench and see how to do it?

Picking up car tonite..had to get EGR replaced so it can pass inspection.

I will try to bench-test each amp to see if I can tap into the pre-in sections of each amp. Not too concerned about RCA connections, as I will modify the cable runs where needed...besides, I didn't see anything but large plug harnesses...I will have to test each lead seperately to see which works.

If I am successful with this venture, I will post pics...

Zeus · #7 09-10-2007, 01:21 PM

Quote:

Originally Posted by G-Benz

Picking up car tonite..had to get EGR replaced so it can pass inspection.

I will try to bench-test each amp to see if I can tap into the pre-in sections of each amp. Not too concerned about RCA connections, as I will modify the cable runs where needed...besides, I didn't see anything but large plug harnesses...I will have to test each lead seperately to see which works.

If I am successful with this venture, I will post pics...

Cool, let us know how it goes, I'm interested to see if this will work! Thx.

SkiWithG · #8 09-21-2007, 08:21 PM

G-Benz - any update on your progress? I'm comtemplating the exact same thing and would welcome the opportunity to learn from your adventures.

Thanks,
Glenn

Monomer · #9 09-21-2007, 08:54 PM

The High-low is known as a direct-box.

converting higher level signals into balanced or line-level.

the stock setup is like that because it's 2/2 - the right side has two speakers wired in parallel and the left is the same. The best way to handle this is to run new wire to the fronts and the you have 4x4ohms speakers and it'll allow you to zone your sound. - This is what I did on my w126, along with installing JBL "GTO" speakers.

G-Benz · #10 10-29-2007, 02:47 PM

Quote:

Originally Posted by SkiWithG

G-Benz - any update on your progress? I'm comtemplating the exact same thing and would welcome the opportunity to learn from your adventures.

Thanks,
Glenn

Haven't had a free weekend up until now to actually work on the system...car passed emissions after three tries.

I have back seat and trunk panels removed to gain access to wire routes and amp(s) for testing. So far I have found the antenna lead extension I purchased was TOO short!

This will be a SLOOOOWWWW process. Will take pics and update as I progress...