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Hi all does anyone have or know where I can obtain plans to build an adapter run an mp3 player, 1/8" stereo plug into the antena port of an fm car stereo? It also obviously needs antenna input to finction as a pass through. I know you can buy these things for about $50 so should be able to build for less. I tried the wireless version and didn't like the quality. Thanks.

Hardwiring would be very simple if the radio has a mechanical volume control instead of a stepper electronic type.

If mechanical, a 1/8" jack with integrated cancellation contacts can be wired to the volume control, in such way that if there is no plug inserted, the radio works normally; and when inserting the plug, the audio fed to the final stage comes from the jack and the tuner audio is disabled.

Miguel

They are called shorting jacks and come in minature and subminature phone (not phono) sizes. Very common item at most electronic outlets. Expect to pay about $4-$5 for the set.

You cannot simply tap into the antenna port of the radio with a 1/8" jack and feed in audio. You need an RF modulator of some sort. The wireless versions you speak of I have found to be very lacking. There are some modulators that get wired in-line with the antenna but you still have to tune a transmitter (modulator) and FM tuner to the same frequency.

If you have a cassette you can do one of two things; get a cassette signal injector or modify the radio so that you can switch the cassette in and out. Most of the cassette stereos I've dealt with in cars are "modular" systems. The base radio is the same whether it has the cassette or not. The cassette has a "line level" amp to its heads that then feeds the amplified signal to the main board of the radio. In cassette mode you can switch to the external source by disconnecting the cassette and feeding the signal from the MP3 player or whatever to the audio input normally used by the cassette module. In my case I had to pop in a cassette to get the audio to switch over. The cassette ran but the audio was from an external source. I found this lacking as well and soon unmodified the radio.

Well, if you already have the FM transmitting (wireless) gadget, then its most of what you need (audio mixer and FM modulator and carrier frequency generator). Try removing its antenna and coupling it (antenna output) to the car antenna via a series connected capacitor. Find a few antenna connectors and shield it in a metal box. I expect it would sound the same though as audio quality is dictated by the performance of the audio section of this gadget.

I suggest the poor audio quality might also be in part the loss of quality experienced by MP3 compression. You may have never noticed it with ear buds but connect it to a good amp, and the quality loss becomes more evident (especially to young ears). Try ripping a familiar track at several different sampling rates 96k, 128k etc and play them back in the car. Can you hear the difference. (I doubt I could but some can). It may yet sound different on your home reciever so you can't really compare fairly.

Your best option is to upgrade the radio to one with line level inputs, either RCA or 1/8" phono jack. If you want to do it without RF modulation, that's your best choice aside from tapping into the amp internal to the radio as Jollyrgr suggested or using a casette interface.

Poor quality sound is most likely due to the audio bandwidth limitations placed on the reciever by the inherent design of stereo FM, usuaully cutoff below 50 hz and above 15 khz. The 15k cutoff is so the 19k stereo pilot signal can be detected to recreate the original stereo signal. As for the 50 hz cutoff, ???. Another reason could be that FM radio signals are highly processed before transmission, getting a boost in the low and high ends of the audio range, then are reprocessed back down to their original levels in the reciever. If the signal from your MP3 player didn't get conditioned in the same way or was sent out "flat", the reciever's processing would muddy the sound quite a bit! I reminded one of my local audio installers of this when I went in looking for a good reciever with a 1/8" CD input jack on the face, and he said he wondered why the add-on CD players that used the antenna jack for input sounded like crap! (this was WAY back when. )

Jolly, perhaps you tapped into the linelevel prior to the tape eq board? Or the standard eq was built into the preamp itself? Shoulda sounded fine otherwise.

My advice is try to find the specs on just how much FM conditions the signal before broadcasting and build that circuit. Insert it between the MP3 player and the FM transmitter. If that doesn't work, either upgrade the head unit or add an amp that accepts other line inputs.

Good luck.

CeaSaR

I used to have one of the FM transmitters, RCA brand. It was garbage. Wasn't temperature compensated - the transmission frequency would change based on how warm it was outside. It would like to center itself inbetween stations, and sometimes it would be .2MHZ off what it was set to! And the audio quality sucked, BAD! Very prone to clipping on strong signals, and it was very quiet!

You have a few options:
#1 - see if your car's factory head unit has an optional cd player/tape deck attachment. You might get lucky and have an aux input you can directly plug into. Honda's from the 90's (don't quote me on that) stereos (panasonic, actually) have an mini-din aux input on the back. You can't just wire it up though, it needs to "talk" to the auxillary device. My original head unit from my 99 acura integra has this. There are 2 ways to get around this:

Get a cheap auxillary unit, disconnect the audio outs from it and wire it up to the audio outs of your MP3 player. The other option is to get a 'box' that has a chip that emulates an auxillary device and just has two RCA jacks on it.

#2 - get a new stereo with an aux input! I have a Wal-mart VR3 180W peak head unit that will read MP3 CD's, has a USB port on the front (though its kind of flakey and for some reason cuts songs short on me), a SD port, and a 1/8" AUX input on the front. I think the whole unit and a honda wiring harness was around $85 or so. Piece of cake to install, too. I know its not the best thing out there (Max volume is "45"?) but I love the thing!

Thanks for all the replies unfortunately none are practical for my situation. It is a Sony stereo with electronic volume control, not mechanical, no inputs on the rear. I was looking for plans to build the modulator, but I may have to purchase one instead or as one of you suggested get a stereo with an 1/8" stereo input (which are abundnt now it seems).