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Thread: Going overboard with home made aux in?

  1. #1
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    Going overboard with home made aux in?

    This site has been a big help. I'm trying to decide how to do a wiring job and was hoping you guys could help. The FM modulator isn't cutting it, so I'm looking to add an aux in to the OEM Bose head unit in my '03 350. There's no cd changer or XM radio input port on the '03, so I need to do a hack. Basic details on what I'm going to do can be found here.
    http://www.my350z.com/forum/showthre...1&page=1&pp=20

    The idea is to cut the audio traces (left, right, ground) on the tape player's circuit board and run the upstream side into a double throw switch. RCA jacks are connected to the other side of the switch and then the switch output is connected to the downstream side of the cuts on the circuit board. So, a flip of the switch feeds the line in signal instead of the signal coming from the tape player. I also need to add a switch to short the contact in the tape player that gets pressed when a tape is inserted.

    All of this can be accomplished with one 4PDT switch. Problem is that I want to mount the switch toward the back of the center console. The audio signal is going to have to run through 4-5 feet of wire down and back through the console. I'm worried about interference and signal degradation. My idea was to use a small 12Volt 4PDT relay to switch the signals. It's not going to have to carry a lot of load, so I can probably use one that's small enough to put inside the head unit. This will keep most of the signal wires inside the head unit. I'll only have to run one 12 Volt wire through the switch in the console.

    So is this relay idea too much effort or should I just stick with a switch and use shielded cable?

  2. #2
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    Stick to a switch and use shielded cable, relays put coils right next to signal paths, so any interference from your 12V lines would be even worse through a relay... I'm not sure why you're worried about it. Your weakest link is going to be in the solder joint to the copper on the main board, that's going to be where any crappy sound is coming from.

    Also, you need to make sure your signal levels and impedance matching is correct. Impedance is less of a problem at audio frequencies but your signal levels are critical. If the traces you're talking about are before the tape pre-amp (IE. right off the tape head) then you're probably going to get noise issues no matter what you do, and your signal will need to be really, really tiny and protected from overages.

    If the traces are post-amp then you shouldn't have too many problems and they should be somewhat close to line level.. The problem is that it's difficult to tell before cutting the traces and feeding in a signal if it's pre- or post-amp..

    Good luck!

  3. #3
    MySQL Error MatrixPC's Avatar
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    no name FM modulator....

    The Relay is a very small one 0.5A (I can't remember).
    The mod above is cut into the CD line instead of tape.
    Some newer tape player won't switch to "TAPE" if there is no tape in the player or if the tape has no load. By no load I mean the tape has to spin like the "real" one.
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  4. #4
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    I'm pretty sure the spot where I'm looking to tie in is after the tape pre-amp but before the main amp. It should be line level.

    I'm trying to do the same thing that MatrixPC did, but the info I have says the place to tie in is on the circuit board. I may be able to follow the traces out to some actual wires that connect the tape player to the HU. Then I could do exactly what MatrixPC did. I haven't pulled the HU out yet and the lousy picutres that were posted on the other forum are confusing me. I wanted to have all the parts ahead of time so I only have to take the dash apart once, but a prior look may be time well spent.

    To fool the tape sensor, I could just cut the wire off a tape adapter and leave that in the tape deck. Over-riding the sensor with the switch just seemed to be a cleaner approach to me. Keep in mind that I've never used the tape player, so this whole process is just to satisfy my OCD side. It would bug me to kill the tape feature even if I never use it...

    MatrixPC's write up reminded me of something. Correct me if I'm wrong, but there's no reason to cut and run the audio ground line through a switch. I should be fine if I leave the tape ground trace/wire intact and just connect the line-in ground to it, right?

  5. #5
    MySQL Error MatrixPC's Avatar
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    Quote Originally Posted by Cambria
    MatrixPC's write up reminded me of something. Correct me if I'm wrong, but there's no reason to cut and run the audio ground line through a switch. I should be fine if I leave the tape ground trace/wire intact and just connect the line-in ground to it, right?
    If you don't run the audio ground, there "could" be ground noise and/or engine whine. In my case, I don't have noise problem and I don't have to cut the audio ground. I didn't do it cuz when I check the audio ground continuity with the HU chasic, the ground is the same.
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  6. #6
    Newbie iamag35man's Avatar
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    I've done this before on a previous car but instead of using the tape head as an input source, use the FM Demodulator. It should be a small silver metal box usually attached to the main board. It also has the FM antenna lead going directly into it. Once you find it, see if you can find a part number or brand and then google it to find the pinouts. There is going to be a line level left and right output with the metal case being a commond ground. There are usually only 4 to 10 pins so if you can't find the specs online you can probe around with a live input wire from an amp until you hear sound. Once you find the pins you can cut the trace and wire up the relay. The last time I did this I didn't have any interference issues with a relay, but it may depend on the quality of relay you use.

    I'm currently doing something simular on my 04 G35 which is basically the same radio as yours. I have the factory XM receiver installed though and I found the output/input wires from that and wired in a relay. Works great.
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  7. #7
    Newbie iamag35man's Avatar
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    Also.. If you have the relay default on the computer when unpowered, if you do have any noise, it will only be when your listening to the FM radio or tape.
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