Car tape deck gutted for A2DP integration

[roofuskit]

Hello all, I've taken the tape mechanism out of my car's stock radio to replace it with an integrated A2DP setup for playing audio from my phone.

It now contains:
A 12v to USB adapter connected to the switched 12v and ground on the radio itself.
The 12v adapter powers an A2DP module.
I've replaced the tape head input to the board with a 1/8th jack.
And I've added a simple switch to trick the radio into thinking a tape's been inserted.

The 1/8th jack input works from the phone, though it's in need of a volume reduction.

However when I plug the 1/8th jack into the Bluetooth module it cuts the audio off.

The same thing happens when you ground the 1/8th jack to the chassis. So I'm guessing that's what's happening through the Bluetooth module, and that that audio input ground is usually isolated.

How the heck to I solve this?

Thank you so much in advance, ANY help would be greatly appreciated.

[GaspingSpark]

This is a common problem with automotive audio. You need a "ground loop isolator." You can buy them pre-made. Or build one into your project yourself using a couple of audio isolation transformers.

Search your favorite electronics supplier for a "1:1 audio isolation transformer." You would need two - one for each channel.

If you live near a RadioShack, they *might* have them in the store. The part number is 273-1374. They are out of stock online.
https://www.radioshack.com/product/inde ... Id=2103994

[roofuskit]

This is a common problem with automotive audio. You need a "ground loop isolator." You can buy them pre-made. Or build one into your project yourself using a couple of audio isolation transformers.

Search your favorite electronics supplier for a "1:1 audio isolation transformer." You would need two - one for each channel.

If you live near a RadioShack, they *might* have them in the store. The part number is 273-1374. They are out of stock online.
https://www.radioshack.com/product/inde ... Id=2103994

Thanks, I'd done some quick searching and those came up, but I wasn't sure. It looks like only the Radio Shacks in the middle of nowhere have the part around here. Odd right?

I was thinking I could probably achieve the same effect using the old tape head and the head out of my old tape adapter. At least until the parts come in.

Also, it looks like it's commonly used for phones according to the Rat shack page, think there's any chance I could salvage one from an old landline phone?

[GaspingSpark]

It looks like only the Radio Shacks in the middle of nowhere have the part around here. Odd right?

I'm actually surprised you found one that does carry it.

I was thinking I could probably achieve the same effect using the old tape head and the head out of my old tape adapter. At least until the parts come in.

Yeah, that would work. But the quality might not be the best. You just need to line them up correctly - and prevent bumps in the road from messing them up.

think there's any chance I could salvage one from an old landline phone?

You might find one in a phone. The important things are the 1:1 ratio, the impedance and the audio bandwidth.

The transformers in a phone might have a different ratio - especially if it is used to reduce voltage levels for newer electronics. Check the resistance of the coils, they should be about the same on both sides.

The impedance needs to be high or you'll lose bass frequencies. Good quality audio ones have >1kOhm impedance. Phone isolators are closer to 600Ohm. Dial-up modems usually have these transformers as well.

The transformer you use should have relatively flat frequency response over normal audio frequencies 0..20khz. The ones used in phones may not have good response over 4khz since that is where the cutoff for the phone line is. The ones in dial-up modems are said to have better frequency response.

Some more info:
http://www.epanorama.net/documents/grou ... lding.html

[st2000]

I did something similar inserting an iPod adapter into into two different car radios. I created a pre-amp-radio output and a axillary input where non existed before (i.e. connecting these together makes the radio work as always while plugging the an iPod adapter into the axillary input allows for playing music). All I did was add 1 uF (maybe it was more) caps to all the inputs and outputs to isolate the DC components of the audio devices. That worked for both types of radios in two different cars.

-good luck

[roofuskit]

I did something similar inserting an iPod adapter into into two different car radios. I created a pre-amp-radio output and a axillary input where non existed before (i.e. connecting these together makes the radio work as always while plugging the an iPod adapter into the axillary input allows for playing music). All I did was add 1 uF (maybe it was more) caps to all the inputs and outputs to isolate the DC components of the audio devices. That worked for both types of radios in two different cars.

-good luck

I actually tried the cap trick first to no avail, but I had a very limited selection of caps on hand.

I tried the tape head route, and it worked great, as long as they were perfectly lined up. I thought a bit about how much work it would be to make that a permanent fixture and also about it shaking loose on a long car ride and decided to take the half hour trek out to get the isolation transformers. (My decision was also influenced by the trip taking me past my favorite hole in the wall Italian sandwich shop, which I found out was now out of business.)

After playing with some resistor values I popped a couple 1.5M ohm resistors on the transformers to adjust for the loud input from the direct in, which help the SNR a lot. I also insulated and shielded the transformers to minimize the noise they picked up from the Bluetooth receiver and 12v adapter in the radio.

I took a lot of pictures and plan on making an in depth blog post, hopefully you'll see it on hack-a-day when I'm done.




P.S.

The ones used in phones may not have good response over 4khz since that is where the cutoff for the phone line is. The ones in dial-up modems are said to have better frequency response.

I can't wait to hunt down some old modems in thrift stores and start gutting them for future audio projects.

[redrocketwestie]

Sorry to resurrect this topic after 3 weeks, but I was wondering if you could elaborate on how you tricked your tape deck. I'm working on a similar project and I'm stuck trying to figure out what signals the stereo is expecting in the absence of documentation. Right now, everything I've tried just makes it throw errors.

It's not the same model, but even a general step in the right direction would be great.

[roofuskit]

Sorry to resurrect this topic after 3 weeks, but I was wondering if you could elaborate on how you tricked your tape deck. I'm working on a similar project and I'm stuck trying to figure out what signals the stereo is expecting in the absence of documentation. Right now, everything I've tried just makes it throw errors.

It's not the same model, but even a general step in the right direction would be great.

Well, I just watched the tape mechanism very closely as I inserted the tap, and saw which switches triggered. After the tape was inserted there was a spring loaded switch that was forced forward by a lever. I just traced the contacts, first on the daughter board and then from there to the main board. It was a simple "short to ground" signal that it looked for.

The tape mechanism in my deck was primarily mechanical. It's the first one I've ever dismantled though, so I'm not sure if it's common or not. Hope that helps!

[redrocketwestie]

Darn. That's exactly what I've been doing. I was hoping there was something I missed, but it looks like my tape deck has some bafflingly sophisticated algorithms in place (optical FETs, anyone?)

Thanks for the quick response!

[mtspl]

Hi, i would like to refresh this topic a little bit.

I`ve just finished modifying my car tape deck. Advice on this forum was extremely helpful. However, i've runned into some problems.

When i have connected audio input in place of removed tape head and played directly from my phone, low frequencies are extremely distorted and somehow ... "stiff" .
experiments with equalizer didn't gave any satisfying results. Playing this same song from cd and by this mod (doesnt matter if sound is directly from phone or the bluetooth adapter), difference is clearly hearable.

Cutting of low frequencies on the equalizer helps a little bit, but the sound is then really flat.

Another thing is that i have really a lot of noise, hearable when no music is playing. i think that is because of the bluetooth and i will try to isolate better the ground-loop transformers. hope that will help. anyways its not such a big deal compared with the first issue i have.

any ideas on the solution ?

Thanks in advance.

MM