Integrating a Bluetooth Aux Input into a 1980s Car Radio

[a1eks]

Hello everyone,

I've joined this forum to ask a few questions about adding an aux input to an old car radio from the 1980s.

Recently, I found this brand-new car radio, which I've tested, and it works perfectly. However, even though I thought about installing it in a vintage car, I'd like to add a Bluetooth aux input. (Yes, I know there are cassette converters with Bluetooth, but I wanted to avoid those).

It has simple switches for changing between AM/FM and FM STEREO. What's also interesting is that it comes with electrical schematics in the user manual (a cool feature that is now rare).


I've noticed that the FM and AM radios are separate in the schematics, so I was wondering if it would be possible to sacrifice the AM radio and activate the aux input by pressing the AM/FM button?

So, here's where I need help. If I interpret the schematics correctly:

A & B are the outputs of the two channels of the FM tuner
A' & B' are the two channels of the AM tuner
A" & B" are the inputs to the SP800 amplifier
So, I was thinking of cutting off the power to the AM tuner, using it to power the Bluetooth module, and connecting it to A' & B'. Would this work? Do you have any better ideas? What would be the best place to connect the left and right aux channels?

Thanks!

[tooki]

At first glance, I’d agree with your analysis.

[shabaz]

That looks like a plan that would work. It looks like all power is cut off to the AM side if the trace at the red arrow is broken, and you could connect that red location instead to your Bluetooth module as you say, or if you prefer, leave the Bluetooth module powered so that the user doesn't need to keep pressing Play again on their phone (if they swap between AM and FM for instance), and in that case, use that red arrow location to directly power a low-current relay, which could be double-pole, connecting the audio from the Bluetooth module to those audio connection points (blue arrows). You could unsolder the capacitors there to remove any audio impact of the AM portion further.

[jwet]

The circuit is clever really - probably the lowest cost solution.

You might consider putting the BT module's outputs to the left of C37 and C37 and disconnect the circuitry there (diodes and caps further left).  Though the BT module likely has output caps, they might not be rated to the voltage that might be present.  Using a small relay as the last poster recommended would be the most conservative but probably isn't necessary.

Good luck- my bet is that it will work great.

[adinsen]

I installed a bluetooth module in a 1980's Blaupunkt SQM48 Paris recently by simply cutting the stereo connection to the master volume control and feeding the signal from tape and FM/AM radio through a bluetooth module. It works great and the module is plenty sensitive enough to work even when built into the unit.

This is the module I used: https://www.ebay.de/itm/175823702418

[kripton2035]

I would try to get a bluetooth 5 module at least, to have a better audio quality . even if in a car you have a lot of noise.

[adinsen]

I would try to get a bluetooth 5 module at least, to have a better audio quality . even if in a car you have a lot of noise.

HiFi and quality are relative terms. IMO most 1980's car radio amps and the associated speaker installations don't deserve BT5 

[a1eks]

Thank you, everybody

I have some doubts now, as this is the radio with an old common ground system.

1. Does that mean I need to find a Bluetooth module that uses a common ground for aux in and for the voltage supply? Is that really the issue, or is it usual for the input audio ground and the power supply on such modules to be common?

2. Currently, the car has a newer stereo system, and the previous owner updated the speaker wiring system, so each speaker (all four of them) has two wires connected directly to the stereo deck. 

I know that when common grounding is present, one must add an isolation transformer, but in my case, I thought of connecting all the negative wires to a single point. Is that fine? Would that create ground loops and noise issues? 

[jwet]

The speaker outputs need to be driven by a bridge configuration- this connection quadruples the output power available and eliminates big output coupling caps.  With a 4 ohm speaker load, P max singled ended would be about 15W, bridge tied load could be over 50.     However, the stereo inputs are always  ground referenced line inputs (in simple automotive audio stuff, fancier stuff uses balanced lines but not here).  Isolation transformers are necessary when you cross ground domains/imbalances.  This is the case where there are two different power supplies to equipment that don't share a ground.  AC currents flow in the ground connection and make a mess of things.  Power your BT module from a quiet point in the radio, perhaps where the AM radio used to get its supply (unswitched) and ground to a low impedance point on the radio.  Much of the noise would come from any voltage fluctuations on ground caused usually by current running through less than low impedance ground connections.  If you find a place in the radio with a quiet analog rail, power it from there with some additional bypassing and ground to the fattest ground you can- this ground may move relative to car ground but won't move relative to the radio's ground reference.  It should work fine.  You'll likely also likely to need a 5v source for your BT module, this may be available in the radio but it wouldn't be difficult to add a linear.

[adinsen]

The speaker outputs need to be driven by a bridge configuration- this connection quadruples the output power available and eliminates big output coupling caps. With a 4 ohm speaker load, P max singled ended would be about 15W, bridge tied load could be over 50.     

Only, this particular radio does not have a bridged output amp.

You'll likely also likely to need a 5v source for your BT module, this may be available in the radio but it wouldn't be difficult to add a linear.

The module I used can be powered from any unregulated DC 7-30V

1. Does that mean I need to find a Bluetooth module that uses a common ground for aux in and for the voltage supply? Is that really the issue, or is it usual for the input audio ground and the power supply on such modules to be common?

I'm pretty sure all BT modules has common negative ground for in/out and power.

2. Currently, the car has a newer stereo system, and the previous owner updated the speaker wiring system, so each speaker (all four of them) has two wires connected directly to the stereo deck. 

This amp can only drive two speakers. I suggest you fit a separate power amp hidden under the dash if you want to keep the four speaker system. The separate amp would be bridged as jwet commented.

I know that when common grounding is present, one must add an isolation transformer, but in my case, I thought of connecting all the negative wires to a single point. Is that fine? Would that create ground loops and noise issues? 

You don't need isolation transformers as there are no balanced signals here and you'll be fitting the BT module inside the unit connected to the same ground.

[a1eks]

2. Currently, the car has a newer stereo system, and the previous owner updated the speaker wiring system, so each speaker (all four of them) has two wires connected directly to the stereo deck. 

This amp can only drive two speakers. I suggest you fit a separate power amp hidden under the dash if you want to keep the four speaker system. The separate amp would be bridged as jwet commented.

I thought so too, but when I looked at the wiring manual for the same radio, it shows that it can be wired to 4 speakers. This makes sense, as the car was originally fitted with 4-ohm speakers.

It is true that there are two outputs from the amp, and two wires coming from the radio (white and green). However, in the connector, the white wire is hardwired to the brown wire, and the green wire is hardwired to the blue wire.

[JustMeHere]

Back in the day they'd put something like then Nintendo TV adapter between the antenna and the radio.  It wasn't for Bluetooth but for CDs.  Same concept.