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Discussion Starter #1 (Edited)
Getting a Pre Amp signal from your OEM Mid-line 6-CD stereo

First thing is first, I am in NO way responsible for anything attempted by the following information! This is done so at your own risk!

A few notes first about this project. First of all, the amp turn on signal is only 5 volts. This worked fine for my Kicker amp, but confirm that it will work with you amp first. If it does not, you will have to find another means of amp turn on outside of the OEM stereo. Also, I found that the signal produces a pretty good turn on/off thump if your amp is left on. However, if the amp turns on and off with the stereo, it does not thump at all. Lastly, I did this on a 2004 OEM mid-line 6-CD stereo. I can not say whether or not this will work on the Rockford Fosgate, or any other Nissan stereos.

OK, now for the fun stuff.

Step 1: Remove the stereo from the vehicle. I don't think I need to go into detail on how to remove the stereo. If you are going to do this, then I'm sure you know how to remove the stereo. If you don't, you may not want to attempt this project. ;)


Step 2: Now before you start tearing into the stereo, take a few minutes and get your RCA cables prepped and ready to go. I found them at Radio Shack, part #42-2358. This is a 6ft cable with an RCA jack (female end) on each side. You will need four total female jacks, however they come. With the Radio Shack ones you buy two sets and cut them in half giving you four jacks with long leads. The length you make them is up to you. I left mine about 24" long. You will need to strip back the jacket and prep the cable as shown. Be very careful, the wires are very thin and easy to break when trying to cut and strip. At this time you will also need to come up with a piece of blue amp turn on wire. About 24" is good for this also.


Step 3: Now that the stereo is out, you next have to open it up. Remove all the screws as marked. Then remove the top heat sink, next remove the back panel. The plugs on the top will come off with the back panel. Be sure to unplug the ribbon wire from the circuit board. Just carefully pull it up to unplug it. Also now would be a good time to plug in your soldering iron and get it warmed up.


Step 4: Now that the stereo is opened up, you will see the circuit board. There are three screws holding the board in. Remove them as marked. Also, there are two more ribbon wires that need to be carefully unplugged. Then just lift up slightly and carefully remove the circuit board by sliding it out the back of the stereo. Be careful not to damage anything on the board as you slide it out.


Now it's decision time:
There are two options at this point. This will depend on what your purpose will be with the stereo. If you will not be using the stock stereos speaker wires anymore, then you can remove the four capacitors making it much easier to install the RCA wires. But if you would like to retain the factory internal amp and use the speaker outputs then the capacitors will need to stay in place. You will then have to tap into the circuit board while the capacitors are in place, this will most likely have to be done from the back of the circuit board. This will be more difficult to do and I did not do it this way. You are on your own with this option. ;) The following instructions will be the way I did it, removing the capacitors.

Step 5: OK, now that you have the circuit board out and in front of you, you need to remove the capacitors. This is basically the point of no return. To do this you will need to find the solder joints on the back side of the circuit board for each capacitor as marked. Once you have found there locations, grab one capacitor with needle nose pliers and with the soldering iron heat up the corresponding solder joints for the same capacitor and carefully remove it. Repeat this for all four capacitors as marked.



Step 6: Now that the capacitors are all removed it's time to feed all the new wires to the circuit board. First drill a 3/8" hole in the bracket as shown. Also, drill a small hole on the side for your grounds as shown. Now feed all your wires thru the hole and put a zip tie on each side. Make certain to leave plenty of wire to get to the circuit board where you will need to solder in. You can always cut any excess off. Also, now is a good time to solder the grounds. (Note: This is all four RCA grounds twisted together.)



Step 7: Next step is to solder the amp turn on lead in place. This is the easy one. :) Carefully solder it to the tab as shown in the picture. Make sure the wire is not touching any other tab when your done.


Step 8: Now for the fun part. :) With the capacitors all removed you will see two holes inside a circle where each capacitor was located. Now you want to solder the RCA wire for each cable into the hole that is closest to the big round power supply. Note: If you have color/channel specific RCA cables, make sure you solder the correct ones into it's appropriate hole. I have marked the holes you need, as well as what channel they are, on the following picture. (**Note: DO NOT solder were I have my wires! That was my first attempt and is not the correct holes!) When you solder these wires this could be tricky. Just take your time and be patient. You want to put the soldering iron on the back side of the board and melt the solder while carefully pushing the wire into the hole from the front. Make sure not to hold the soldering iron on the circuit board for too long. This could damage the board as well as other components on the board that are near by.


Step 9: Now the hard part is done. Make sure to double check all your new connections. Now put the circuit board back into the stereo making sure to put all the screws back in and plug all of the ribbon wires back in. Finish reassembling the stereo and take it out and test it in your truck. Remember to make sure you have your airbag light plugged in on the front panel before you turn on the truck or the airbag light will come on. Once you confirm that everything works you can finish installing it back into the truck and your done! Cool huh. 8)
 

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And 10 stereos later...I might have one that's not fried...

Nice post for those with skills enough to accomplish this.
 

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As a technical note on desoldering: When removing the capacitors from the circuit board use of a solder sucker or solder wick to remove the unwanted solder will make the job easier, plus the CB holes are 'empty' when it is time to solder the new wires.

Overall a good how-to on performing the project. Well written and documented!
 

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Discussion Starter #6
bestatchess said:
Is there any advantage to this over just using an LOC? Are you bypassing some kind of built-in equalization or something? What signal level are you getting after the bypass?
Well I'm sure most agree that an aftermarket HU is the way to go, but I just like the OEM look. I think there are many advantages, but that's my opinion. :) With my LOC I was getting a hiss at low volumes. The sound quality was terrible, it just sounded like the crappy stock stereo only louder. lol It would distort at low-medium volumes and there did appear to be some type of built in equalization that this eliminated. I don't have a test tone CD so I didn't measure pre amp voltage, but judging by my amp gain settings, I would say this is in the area of just a standard 1.5-2v pre amp signal. All I can say is it sounds sooooo much better than the LOC and if you want to keep the OEM look, this is about as good as you can get. Plus it only cost me $2.00 for the RCA leads (on close out at Radio Shack) and a few hours time. And much of that time was trial and error, and taking pictures. ;)
 

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I log on to this site at least three times each day and I somehow missed this thread!! thanks for the writeup!!
 

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bestatchess said:
Is there any advantage to this over just using an LOC? Are you bypassing some kind of built-in equalization or something? What signal level are you getting after the bypass?
When I tried to use a loc I was getting 4 to 5 volts with the volume low out of the rca's. It was weird.
 

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You need to connect electrolytic capacitiors in series with the taps otherwise you will have DC coming out of the RCA outputs. If you connect to an amplifier that does not have a electrolytic capacitior on it's input, you could fry the "Pre-Amp" IC in the HU. The ICs in a radio running on 12 volts, all have DC voltages on all the pins. You must use a capacitor as a DC block. Connect the + terminal of the cap to the IC. 10uf or greater will be good enough.

Also use smaller diameter coax to minimise stress to the PCB traces and Vias.

Note:

You cannot try to tap on the "other" side of the coupling caps already in the HU. There is DC on both side of the Cap.
 

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Discussion Starter #11
thajjar said:
You need to connect electrolytic capacitiors in series with the taps otherwise you will have DC coming out of the RCA outputs. If you connect to an amplifier that does not have a electrolytic capacitior on it's input, you could fry the "Pre-Amp" IC in the HU. The ICs in a radio running on 12 volts, all have DC voltages on all the pins. You must use a capacitor as a DC block. Connect the + terminal of the cap to the IC. 10uf or greater will be good enough.

Also use smaller diameter coax to minimise stress to the PCB traces and Vias.

Note:

You cannot try to tap on the "other" side of the coupling caps already in the HU. There is DC on both side of the Cap.
Can you explain this in laymans terms with more detail? I guess I'm confused as to why I measured A/C voltage? Also what is an IC? I am by no means a electronics guy and don't understand most of what you said. All I know is that it works great the way I have it. And if you can prevent someone else from potentially having a problem that would be great.
 

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IC means integrated circuit.

In the typical HU, when you need to connect two audios ICs together, you need a coupling capacitor to block the DC biasing voltages from one IC from bothering the other. The main reason for this is due to the fact that you usually only have a single supply voltage like 5-12 volts to power the IC. The ICs audio pins will have 1/2 the power voltage on them. The same goes for when you want to feed an audio signal from an IC to the outside world. There will be say 2.5-6 volts DC offset on the input/output pins of the IC. The audio signal "rides" on top of this DC offset voltage unless you block it with a capacitor. In your case the amp you are sending the signal to probably has input coupling caps blocking this DC voltage. Be careful not to drop your RCA connectors on a ground point. You might pop your HU.

Some designs use very few coupling capacitors, but the ICs are specifically designed to use the DC offset on the output of one IC to bias the input of the attached IC. Other designs use very tiny ceramic chip capacitors for coupling. GM HUs do this everywhere. You can only use a small value coupling capacitor if the input impedance of the following stage is very high.

As far as measuring AC, if you use a cheapy voltmeter set to the AC scale, it blocks DC when measuring AC. If you use a True RMS voltmeter, it will measure the DC offset as well as the AC signal. You will see a "signal" voltage even with the volume muted.

Another BIG thing to mention is that you have exposed an internal IC to a world of hurt by not putting some form of transient protection on each out put. As a minimum I would add two diodes and a 100 ohm resistor plus the blocking cap.

The schematic would be like below:

IC +
_|_
/\ 1N4002
+ |
IC Output ---| |-------+---\/\/\/\/-----o Pre-Amp Out
10 UF | 100 ohm
_|_
/\ 1N4002
|
IC Gnd
 

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Discussion Starter #13
OK, First I appologize for being so ignorant, but I'm still not following you 100%. I think I understand what your saying, but the materials and how to install them I'm still not following. Like I said I'm not an electronics guy so I don't under stand the schematic you made. As for my meter, it's a radio shack brand digital multimeter. It is about 15 or more years old and is nothing fancy at all. And will adding all this stuff have any effect on the output or sound quality?
 

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Here's a response that I got from one of my electronics-savvy friends regarding post #10:

If you recall back to...uh... whenever you learned about capacitors, you should remember that they're basically two conductive (metal) plates that don't touch. That's right, it's like a wire with a hole in it. A DC current won't flow because there's a big hole there. An AC voltage will induce current flow on the other plate. Why? Because current flowing thru a wire induces a magnetic field around it. This changing magnetic field magnetizes the other plate which induces current flowing there, too. So a cap blocks DC and works like a short to AC.

So why do you want one in a car stereo? DC can cause unwanted noise which sounds like crap when it gets thru to the amp and gets amplified. The caps filter that out and leave the sweet, sweet AC signal. That's what often happens in these circumstances, but the guy in post ten is warning that too high a DC signal applied to an IC could damage it. Those small things (ICs) are fragile and work within pretty tight tolerances. Try applying an extra volt or two to the input on your CPU and see how well it fares. If those are filter caps - which it looks like they are - in addition to causing noise in your speakers, too high a voltage there could break the IC in the next stage. I expect you would care about this more than a bit of noise. Bottom line.... caps live in your stereo for a reason. To offset the lack of caps the tech guy says stick a small (10?F) cap and a 100? resistor in series in place of the caps. The cap will still block the DC, so I'm not sure why he threw in a resistor. Also, be sure to NOT accidentally connect one of those wires to a ground. That would probably put a pile of voltage on your input circuit and burn something. To avoid that, connect a diode from ground to the resistor and another from the resistor to the IC. Diodes are one-way and should keep anything from grounding when it shouldn't.

"You can only use a small value coupling capacitor if the input impedance of the following stage is very high."
This refers to impedance matching between the two stages. Look it up on the wikipedia.

So the lesson for today is if you have to remove caps don't let your circuits get too much DC voltage on them to blow them up and don't short anything to ground (causing an excess DC voltage, which blows up your circuit).
 

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steves

Your friend is confusing all the various things that a capacitor is used for and jumbled them together for this one use. The other error is that a capacitor is subject to an electric field not a magnetic one when a voltage is connected across it. The Impedance of a perfect capacitor is infinite at DC and a short at infinite frequency.

The job of the capacitor in this specific case is to block the DC voltage on the output pin of the audio IC. If you do not do this and you connect a resistive load like some amps have or short the output, you could draw excess current out of the IC and damage.

The series resistor and the two diodes is to provide a simple surge protection circuit if a transient gets coupled into the coaxial cable from the amp or during installation. ICs are very sensitive and cannot handle voltages outside what they were designed to work with on their pins. If a positive transient comes in, the top diode conducts and dumps it into the + supply. If a negative transient comes in, the bottom diode conducts dumping it to ground. The resistor limits the amount of current the diodes will be subjected to.

The capacitors value is set by how low a frequency response you want on the pre-amp output. The series cap and the load Impedance on the amp input, forms a high pass filter. If the input Impedance amp is high, then the cap can be small in value. 10 UF should be OK if the amp has an innput Impednace >1000,000 ohms.

You do not want to try and drive an amp with a low input Impedance, like speaker level inputs. The IC will not be capable of driving anything lower than say 1,000 Ohms.
 

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When I said I was getting voltage out of the rca's that was with a line out converter. If you have a LOC and have a voltmeter try and check . I bet you will have several volts at your rca/line level out. This will be at almost all times even at a low or muted volume. As far as the mod goes mine is working flawlessly since it was done at least a year ago. I been away from the forums and just checked back recently. As far as adding a capacitor wouldn't that cause a loss at low frequencies? Regardless I'm happy with mine however if it were to go out I would pick up one of the newer double din DVD players. I like the idea of being able to hook up a reverse cam to it. And frankly the FM radio is not bright enough along with the lack of mp3 on my oem.
Since we got some brains in the house I have a question. I have a dirt bike that has a 12v ac power supply for the tail and brake lights. I just converted my rear fender/brake/tail lights to a compact LED light/fender set up. I need to convert the AC to DC (for the led's)so I will be making a bridge rectifier. Do you know what value diodes I need for the LEDs? I think it was a total of 10 LED's so the draw should be minimum. Its a led so I think "dirty" DC will work. Any ideas? I was going to solder the four diodes and the cap and make a small mold and fill it with resin or silicone to keep it waterproof and insulated. I figure it will be a total of 2 cu inch or smaller. Send me a email if you can recommend a wattage diode. [email protected] Thanks and sorry for the off topic.:cheers:
 

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Discussion Starter #17
Ok, so what I'm getting from all this is that these are basically just safety precautions and not "really" needed as far as the function goes? Now from my years of installing stereo's it's always been a well known fact that you can damage any stereo by letting the positive of an RCA cable touch ground when the power on. So does this mean that even high end aftermarket stereo's don't even have this protection? And I checked my meter, and it will only read the voltage in which it's set at. So if I put the meter on A/C then test my 12v battery I get 0. So this means that I am in fact getting an A/C voltage at the preamps. Also, Flush brings up a good point. Won't a capacitor limit the frequency range? Sorry to be so dense with all this. :)
 

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The cap is absolutely needed if you cannot guarantee that the amp you are going to connect the HU to has a DC blocking cap on it's input. All amps "should" have caps on their line level inputs. Certainly the speaker level inputs of an amp usually do not, but the pre-amp would not drive such a low impedance anyway. If someone was to accidently connect the IC pin to a speaker level input of an amp it could be bye-bye IC.

Adding a cap will hurt the low frequncy response, IF the capacitor value chosen is too low. If the cap value is large enough, the low frequncy roll off will be below 1 Hz. The cap value is chosen based on the input impedance of the load, in this case an amplifier.

If you want to measure an AC voltage riding on top of a DC one for THIS specific case of an audio IC, measure the signal twice with the DVM to DC and then AC settings.

All car HUs, PCMs ...etc.. have some form of surge suppression on ALL the wires connected to them. Cars are nasty sopurces of transients and it is easy to pop electronics if they are not protected.
 

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You telling me your dirt bike dynamo + rectifier/regulator + battery is putting out AC? I need more info on what you setup is.

Where is your battery? Normally a battery is the big capacitor "filtering" the AC output of the dynamo + rectifier/regulator. A 3 phase dynamo on a bike will generate lower "ripple" on the output of the rectifier versus "home" single phase AC.

Do you have a rectifier?

Do you have a regulator?
 

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On the OEM line level out put I'm all good. Before the mod I had the constant voltage out of the LOC line side.
As for the bike project the bike I have is a dual ac, dc type set up. It has a charge coil/generator(100watts), regulator(12v) and a rectifier(dc). As weird as it sounds it works like this. One side sends 20w to the regulator then to the rectifier. This is the charging circuit for the battery for starting, horn and turn signal and flasher. The other side, 80w goes to the regulator then to power all other lights headlight 35w, marker 5w, tail and brake light.
 
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