CM5908 To Aftermarket Amp

[Julien]

My car hasn't had a factory radio in it since the 90s. In it's place is an aftermarket deck, amp wired to the factory premium speakers.

I'd like to go back to a factory deck but wouldn't mind running my aftermarket amp instead of splicing a factory unit back in.

Since the Alpine CM5908 has 4 dedicated outputs can I wire from the back of the radio to the high-level in on an aftermarket amp? Any reason this wouldn't work as well or better than the stock amp? I'd ensure all 4 wires go straight to the amp with no common grounds or faders (don't think the CM5908 used one anyways)




Thanks!

[Julien]

From what I gather I have 3 different ways to do this:
1. Use the existing high level in on my alpine amp. Would this cause noise though?
2. Get an LC6 LOC behind the factory radio and run rcas to the back of the car where my amp(s) live
3. Go High level to RCA and see if the alpine amp can handle it. In my e46 HK setup it did.

Anybody?

[robrez]

bump for you. A lot of us are wanting to dust-off our old CM5908s these days.

[bmwman91]

Running the speaker-level outputs from the HU to an amplifier is not inherently a problem, but there are some requirements in order for it to work well (aka no alternator whine or ignition noise).

Each speaker-level output channel on the HU is a pair of wires, and neither one is a "ground." They form a bridge-tied load, with each effectively having the same signal on it with one inverted versus the other and both centered at ~6.5VDC. The ideal way to use this is to run both signal lines to the amp with some twisted pair wire (CAT5/6 is actually great for this). However, you will need an amplifier or level converter that can take in a differential signal like this. Most amps' RCA inputs just connect the ring to ground, which is obviously a bad thing when connecting it to the output of a speaker amplifier lol. So again, you need something specifically saying that it can use the pair of speaker-level lines. Many amps out there have both speaker-level and pre-amp inputs. This setup has the advantage of being pseudo-differential, so any EMI induced into the twisted pair of wires can be rejected at the amp, if it is configured to do so.

If you do not have an amp that can handle a BTL type signal pair, you can ruin a single one of the speaker-level output lines to the amp. You will, however, absolutely need to splice it into a quality shielded RCA cable as close to the HU as possible. Running a single-ended audio signal in a car without good shielding is asking for lots of noise issues. You can leave the shield at the HU end disconnected since it will ground at the amp, and you do not want the shield to end up acting as a ground loop between the front & rear of the car.

The main issue, IMO, with speaker-level outputs going to pre-amp inputs is that most pre-amp input stages are designed expecting a <5Vrms input voltage, and speaker-level outputs are generally going to be 9-10Vrms at high volume. So the volume knob on the stock HU will only really be useful through 25-50% of its range, depending on the specific hardware in the amp. That is kind of annoying. You are unlikely to damage the amplifier by running too high an input voltage, but it will probably clip/distort like crazy if you crank the volume too high.

Lastly, there is the LC6i and stuff like it. I used to use one of those, and it was fine. I ran the full pairs of speaker-level outputs to it via CAT6 cable. Later I modded a factory HU to have speaker level outputs, so I sold it.

[bmwman91]

Also, forget 99% of the RCA cables out on the market, they are almost all trash. The shielding is crap. Build your own from this stuff (this is what I used for my system). The shielding has >94% optical coverage and is braided versus wrapped. EMI has not been an issue for me, at all, and it is extremely durable.

(I have zero affiliation with Canare or Performance Audio, it's just where I got my stuff from last time)

2-condutor (twisted pair) cable:

https://www.performanceaudio.com/canare-l-2t2s-2-conductor-microphone-cable-black.html

4-conductor (2 twisted pairs) cable:

https://www.performanceaudio.com/canare-l-4e6s-star-quad-microphone-cable-black.html

Solder-on RCA plugs:

https://www.performanceaudio.com/canare-f-10-deluxe-clamp-rca-plug.html

My setup:
(see post 100 for the cable I built for my install)

https://www.r3vlimited.com/board/showthread.php?t=339298

[Julien]

Thanks for the great tech BMWMAN91! Afaik some alpine amps can take balanced differential signals directly to their RCAs. I did it on my e46

[bmwman91]

That is cool. IMO, the ideal setup is running shielded twisted pair wire to a balanced input that can handle speaker level voltages (so that you can retain full use of the volume control knob lol). The standard STMicro quad channel power amp ICs that all head units use will add in a little noise and distortion, but it is so far below the noise floor of a running and/or moving car that it is immaterial.

[muppetman342]

Any luck with this? I'm currently wiring in a Kicker Key180 to the stock CM5908 and man is it giving me trouble. Amp goes right into protect mode on power up, then power cycling it brings it out. Currently using the power antenna wire as the remote wire and wondering if that's causing a problem.

Disconnecting the amp from everything but power still gives me this problem.

Tried connecting the amp directly to the battery with nothing else connected and just putting the remote onto the positive terminal of the battery, and this seems to make it come on the first time, leading me to think that the power antenna lead is doing something odd. DMM shows 12V on it with power on so

[muppetman342]

Whelp the antenna lead out of the CM5908 was only getting 11.5V and apparently that was making my amplifier unhappy. Ran the remote wire to the radio power instead and everything seems to be working now. Hope this helps someone