Adding Auxiliary Input to a Car Stereo

Introduction

This document shows how to hack a cheap car stereo with the PT2313L audio processor integrated circuit to be able to accept an auxiliary input, say from an MP3 player.

The need for such a hack grew out of my frustration with connecting portable music players to my cheap car stereo with those crappy FM transmitters. Let's face it, they suck. If you're doing any kind of road trip you'll need to change frequencies often because local radio stations drown out the weak power of the transmitter.

My original idea was to open the stereo and look for some unused auxiliary input that I could take advantage of. I even thought of disabling the CD player by using it's audio output lines inside the stereo. Well, after opening the stereo I found no unused inputs and figured that disabling the CD player would be too much of a sacrifice, even if it's rarely used.

Pictured above is the stereo in question. It's so cheap it isn't even obvious what brand it is. Anyway, it does the job, even if that is playing scratched Dave Matthew's CDs.

Before I go any further, I'll list the tools and parts I used during this operation.

Required tools and parts

Soldering iron and solder
Sufficient tools to remove and open the stereo (screwdrivers, etc.)
A stereo earphone jack and cable (I used an old pair of earbud headphones)
A 12V power source (I didn't have this, but it would help during testing so you don't have to keep taking the stereo out to the car to test it)

Here are my old earphones. Actually they weren't even mine. A friend from North Carolina who was staying with me in Geneva forgot them when he left. Sorry Cam!

The first thing we need to do is take apart the stereo and have a look at it's internals. After taking it out of the car I brought it inside and took it apart. This was pretty easy. To really get in there I had to remove the CD player module, which was secured with four screws and a ribbon cable.

I have to admit when I took this apart I didn't really know what I was going to do. But I started looking up the part numbers of the integrated circuits that were soldered to the board. It just so happened that the first chip I looked up turned out to be the PT2313L, or 4-channel audio processor integrated circuit from Princeton Technologies. I downloaded the datasheet and had a brief look at it's inputs and outputs. I've included this information below:

The original datasheet can be found here or by searching for "PT2313L" in your favourite search engine.

I became immediately interested with the generic sounding "RIN" (PIN 6) and "LIN" (PIN 16) pins. Sounds pretty much like "left in" and "right in", don't you reckon? So the first I thought I should try was to solder the left and right channels of the earphone jack directly to these pins and the earphone jack ground to a common ground somewhere nearby on the circuit board. Let's give it a shot...

In the above photo I am soldering the earphone jack ground to a ground I found on the circuit board by using my multimeter to test for conductivity between various solder points and the metal shell of the stereo. These will definitely vary from stereo to stereo, so just poke around with your multimeter until you find a ground.

I've finished the soldering. As you can see, it's not very complicated. Here it would have been useful to have a 12V power source so I could test the stereo without going back out to the car, but the PSU I found on the street didn't power up so I was out of luck.

I put the stereo back together, making an effort to secure the weak headphone cable and solder joints by tying a knot and feeding it through a small port in the back of the stereo. Then, out to the car I went...

Not too surprisingly it did actually work. What was surprising, however, was that as soon as the MP3 player was turned on and started playing, the volume from the FM radio tuner dropped significantly. This was a definite bonus, because otherwise you'd have both the FM tuner signal and the MP3 player signal playing at the same time, which would be really annoying. I don't have any idea why this happens, but maybe it has something to do with the capacitors on the line inputs on the PT2313L...? (See the datasheet circuit.)

Update: Steve left a comment suggesting the reason for the FM volume decreasing is due to an automatic gain control circuit located after the PT2313L which controls the volume in cases where the strength of the FM signal causes variance. Thanks, Steve.

Here is a video of the stereo playing music from my Archos MP3 player. Oh Barrington Levy, you're singing about your black roses again...

If the volume of the MP3 player is really low, you can hear either static or music from the FM tuner, depending on which station it's tuned to. But otherwise, the signal is pretty clear. It was definitely better than the FM transmitter.

Conclusion

In conclusion, this experiment turned out even better than I had imagined. It was surprisingly simple and the results are pretty good considering everything. I am pretty sure something similar can be done to almost any car stereo lacking an auxiliary input. The only problem is to be able to limit or cut the signal from the original module (tape deck, FM tuner, etc.) so all you hear is the signal from your external player.

If you're attempting this yourself, keep in mind that due to the many types of car stereos out there, it may not work as easily. If you don't find the PT2313L IC, just look up the part numbers for the circuits in your stereo and try to find a place to solder the headphone jack channels.

Donn Morrison Aug 18 2007

comments:

Cotty @ 2007.08.20.13:08: Once again, YOU ARE MY TECHNOLOGICAL HERO.
Michael @ 2007.08.21.07:10: You could record yourself a full-length CD of silence to play while using external input. Or did you come up with a better way of silencing it?
bjorn@jahn.se @ 2007.08.21.07:34: I did the same thing with a car tape player, i had a different chip with longer pins.
the things i did differently was:
1. Desoldered the RL legs
2. Attached one wire to the leg and one to the pcb.
3. I then used a 3,5mm female plug with "loop throug"
This meant that when nothig is connected the stereo plays as usial, but when you connect a male-male plug to the mp3 player it takes over.

There were a couple of drawbacks with this though.
1. I connected everything before the volume controle so the radio always plays at full volume when an external scourse is connected = hard to controll the sound.
2. It makes a VERRY loud rasping sound when you connect/disconnects an external device.
3. I didn't use shealded cables so the radio always picks up the generator.

The conclusion is that the sound is better than the fm transnitters but not by mutch.

donn @ 2007.08.21.07:34: michael, that's a very good idea.
dicky @ 2007.08.21.11:38: You rock man. Looking forward to pumping enya from your archos all the way up to Cham...
Bob, B. @ 2007.08.21.17:28: Did you try L/RIN 1, 2, and 3? Maybe 1 is FM, 2 is CD and 3 is AUX?
RC @ 2007.08.21.20:26: You could get rid of the (radio) background all-together with a bit more work. Get a PCB mount 1/8" headphone jack, solder a pair of wires to the chip, and a pair to the PCB as far away from the chip as possible, then cut the two copper leads on PCB with a razor. Then connect the rin/lin from the chip to the headphone jack, and the others to the switch-over connections on the headphone jack. Then find a place on the faceplate to mount the headphone jack. When you plug-in something like your MP3 player to the jack, it will disconnect the audio from the radio, but when you remove it, it will return to normal and you can listen to the radio. Personally, I bought an Aiwa unit for my car, most of which have an AUX setting and headphone jack on the faceplate, no wiring needed. Why the rest of the world hasn't figured it out, I can't imagine.
steve @ 2007.08.22.20:35: The reason the fm signal dropped in volume is most likely that there is an automatic gain adjustment circuit in the signal path after the chip you're messing with. This is used to ensure that the volume of your radio doesn't change with the strength of the recieved signal. By turning on the MP3 player, the signal has become very strong and the AGC circuit doesn't amplify the signal nearly as much. So, the FM gets quieter. Nice deductive work with the radio, BTW.
James @ 2007.08.22.21:36: You inspired me to rip apart my Ford radio, but a few hours of googling has not let me to a pinout for an ST 70010SE. Oh well... to Crutchfield I go.
andy @ 2007.08.24.07:56: thanks man, I've been wanting to do this for a while but I wasn't sure it would be doable and didn't feel like risking my deck... Anyway my deck got stolen so now I have an even shittier one so to hell with it... lol

find datasheets for almost any chip
alldatasheet.com/
imajoebob @ 2007.08.24.21:38: Nice hack. But it might have been even easier to attack the antenna instead of the chips. Since the problem with the FM transmitters - at least the name-brand ones - is interference, eliminating the broadcast signal will allow the transmitter to come booming through.

It's a simple hack. You need an antenna wire and connectors, and a simple switch. Connect the actual antenna wire to a new female connector. Run half the new wire (or a used wire from the junkyard) from the female connector to the switch. Connect the other half after the switch and to the male connector. Plug it into the radio.

When you want to listen to the MP3 player turn on the FM transmitter and throw the switch (open) to cut the antenna connection. Without the antenna the radio can't pick up any signal further than a few feet away. When you want to use the radio, throw the switch again (closed) and reconnect the antenna. You can even get matching parts from the auto parts store (or junk yard) so the switch looks like an OEM part. Put the antenna switch where they would have put the option you were too cheap to buy (rear defroster, convertible top, etc.). Or make believe it's the 70s and you're back in college and leave it hanging under the dash.

Voila! No soldering, no schematics, no real manual skills necessary. And a really clean signal.

Christopher Thompson/eagleapex@gmail.com @ 2007.08.25.09:16: I looked at the IC PDF and it seems that in your head unit you don't have a tape player. But the IC has a RIN3 and LIN3 for tape playback. If you used these instead would it pipe through, or not because you can't select that from the faceplate? Would then the RIN2 and LIN2 for CD player work when not playing a CD? I wish I had a car so I could answer these questions. Good job though. Great hack.
donn @ 2007.09.04.02:26: Christopher: There is no way to select the input on the faceplate and those inputs on the IC had no connections.
jeffgette@gmail.com @ 2007.09.04.17:26: Where would I connect the wires on the circuit board of a 2000 Ford AM/FM radio? I can't find it online anywhere.
jonathan.fillion@gmail.com @ 2007.09.07.10:04: I did the same mod on my Tercel '98 stock radio. I actually cut the trace between the tuner and the pre-amp IC (can't remember what it was though), and diverted it on the front of the unit to a dual input 1/8' jack (ie, when nothing is plugged in radio goes in, when I plug something it takes over).

I had to use shielded wires to prevent coupling between the radio and my input. If anyone wants more info drop me a line.
anonymous @ 2007.09.08.22:44: Cool mod I was thinking about doing it but googled it first and came across this.
Jamantste@yahoo.com @ 2007.09.12.12:54: I just did this to my Pioneer DEH-1800. I just dove in and founr that it had two posts labeled LCHAN and RCHAN. I soldered on the ends and crossed my fingers. It works great. Good volume and no distortion at all. Best of all if you plug in a source it just takes over and cuts out whatever is on the station. I got lucky there. Thanks for paving the way Donn.
arv @ 2007.09.14.14:27: I just did this on my Alpine TK-426. The 4CHAN and the 7CHAN on higher Quads/channel (per, on the TK series) are much more crisp and allow only my music to come apump-pump-pumpin through. (Without audio amplification I might add.)

I also like jonathan fillon's cut-the-trace solution. Your next mod, if you choose to accept it donn, is to install a little kill-switch for the tuner signal, then you just keep your existing input which is working nicely. also use a switch, double-plus good!
zod records @ 2007.09.23.09:42: Nice work fellas!
sam @ 2007.10.03.19:08: Ff you wanted to, instead of playing over the FM line maybe you could make a CD that has one big audio track on it that is silent, then play over that?
AlbertoCattanR @ 2007.10.07.20:30: Just did this on an Eclipse 54400 and it worked GREAT!!! Thanks a lot and greetings from Bogota, Colombia!
Alexander Else @ 2007.10.14.06:29: Great work. This inspired me to do the same to my JVC car stereo. I documented most of it on flickr.

http://www.flickr.com/photos/aelse/sets/72157602412975798/

I'm happy that i didn't blow up my car, my stereo or myself in the process.
Dan Lee/danlee_58@yahoo.com @ 2007.11.05.19:21: I have been planning to do this myself. I plan to start on the older AM/FM/CD unit in my truck. I can replace that cheaply, if the mod doesn't work. I also want to do it on my Delco unit in my '04 Pontiac, after I practice on the truck.
I purchased two #35RAPC4BH3 Switchcraft 3.5mm Stereo Input jacks. These are break-before-make type and will disconnect the internal audio signal when a 3.5mm stereo plug is inserteed. I'll mount the jack in the front panel, and wire it to the circuit board, after I cut the traces on the board. I'll let you know how it works out.

kzarrrrr@yahoo.com @ 2007.11.08.21:48: well thanks for your info. i was able to hook up a plug for my phone/mp3 player thanks to your info. my chip was different for a pioneer, but i followed your info and looked up the chip set for my stereo and sure enough found it. only thing i did wrong was that there where 4 in-lines, side to side and front and back. so i just switched the wires from the back line to the front speakers (only 2 speakers in my truck).

i found that the sound is just as loud in my case no matter how ho high or low i turn the volume so to get rid of any sound other then my phone/mp3 player, i just turn down the nob and turn up the sound on my player. just wish i could play it louder, but it's still loud enough to hear fine.
Manuel @ 2008.01.20.18:50: I did modify my JVC KD-S5050 and it worked like a charm. I found out that it had the Philips TEA6320 Sound fader control circuit mounted on it. So I just googled for the datasheets and that was it. I connected the jack to IAR (input A right source) and IAL (input A left source). I chose those ones coz they're the ones connected to the cd player, so now I just popped in a cd with a dummy mp3 and voila, no interference at all and a crystal clear sound. The input would ONLY work if there's a cd playing, and stops for about half a second when de cd reaches the end of the last track and begins again, but this is really a SMALL price to pay. Thanks a lot Donn.
reko_tomo@yahoo.com @ 2008.02.27.22:04: i've tried my 1990 car stereo SONY RX-1700
which has no cd changer option
by cutting the wire from cassette's
head unit (which goes to the board).
there are 3 wires, for LEFT-RIGHT&GROUND line.

so i put a dummy cassette, then connect the
cut wire to my nokia cellphone
with stereo male cable, then i play the mp3
from the cellphone.

it worked, but my problem is : distorted sound.
1. if i turn-down at lowest volume on the cell,
and turn-up my car stereo volume, i got
a very noticable hiss sound noise, and
the distorted music still there but reduced.

2. if i put 1/3 volume on my cell,
then the hiss reduced, but more music distorted.

3. if i put more than 1/2 volume on my cell,
then i got a completely distorted music.

i just wonder, is there anyway i can do
to normalize all this hiss and distorted sound ?

anykind of respond will be greatly appreciated,
and i'm very sorry if this is not the right place
to put my question.

thank you in advanced.

regards,
reko

dan @ 2008.03.20.20:01: man i just did this for a JVC KD-G700, it was a LOT harder to distinquish the chips but my EE friend and I found a schematic for the actual player and had to connect leads coming from the chip on the bottom of the board.

end result? works beautifully! I love this cd player (the eq options are first rate) and I'm so glad I can use it for my zune now too.

be forewarned, the absolute hardest part was opening this jvc cd player, you'd think it would be a lot simpler than it was

anyways thanks so much for putting this up!
Jesse @ 2008.04.13.12:15: Just did this on my JVC KD-G200 (AM/FM/CD) and $15 Skull Candy earbuds. I had no prior experience or knowledge of electronics so this is a real easy project. The hardest part was actually taking the head unit apart and retrieving the data sheet (the player looks fairly old and all of the components inside had their serial numbers almost completely faded away). Thanks to Google and some of the testimonials posted on this page! Specific thanks go out to Donn Morrison, Alexander Else, and Manuel because their testimonials were most helpful to my situation.

With the JVC KD-G200, all you need to do is work the bottom off of the head unit. I ended unscrewing the whole thing which was completely unnecessary. It took a few hours of trial and error to look up all the chips on the board. I noticed Manuel had JVC stereo which used a "Sound fader" chip that matched a few numbers of a chip on my board. I Googled it and found I had the Philips TEA6320T Sound fader as well. Having not soldered since high school and having never worked with wires, I went to work. The uncolored copper wire is the ground by the way. I soldered the right wire to the pin 17 IAR (Input A Right) and the left wire to the pin 16 IAL (Input A Left) directly on the chip (these are my CD inputs). I chose the pin 2 GND (Ground) as my ground for both channels.

How does it work? Awesomely. Sound comes in very clear, although a little low. I'm not sure if this is due to this hack job or if it's my Archos 604 which is notorious for lower volume. Anyway, I just pop in any CD and then plug in my player. The CD volume fades and my device is picked up. No sound from the CD player makes it through even at high volume. I highly recommend this as a cheap and fun alternative to buying a new player as it can save you around $100. Not to mention the bragging rights you gain amongst your friends from having successfully hacked your car stereo.
chern05@yahoo.com @ 2008.05.21.16:08: WARNING this experiment could end up costing you fortunes, like it did to me. since i didn't have a portable battery to test it i had to plug it into the car during the procedure. while tinkering with the wires it shorted somehow and busted my mp3 player, CD player and car electronics.

Don't use expensive player to test it out.
COST = $300
prout4072@hotmail.com @ 2008.06.07.12:01: I did it on a corolla 97, like jonathan filion, i had to find the radio tuner and plug directly at the output of it, hopefully, in was written directly on the board. If somebody need help doing this, email me I'll be glad to help you out. The difficult par was to dissassemble every single part of the radio because the connections were under everything. Did not used shielded wire and sound quality is great!
Dan D. @ 2008.07.02.18:02: Nick hack.
Marko @ 2008.07.16.15:38: I also managed to transform my Pioneer Deh-1600 into aux in car radio. Everything works swell. I connected to RCH and LCH pins (which are very near the Tuner box), and both grounds together on nearest GND. I looked for PM4009A datasheet but no luck.
I also noticed some FL+ FR+ and RL- RR- pins but fortunately I left them alone.
Cheers man!!
patrick-@hotmail.com @ 2008.07.22.22:31: oi eu sou brasileiro gostaria de saber se o cd palyer kewood kdc 218s usa este circuit integrad PT 2313 l por favor me responda!!!!

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