Adding Bluetooth to AA5 radio

[Dave A]

My neighbor asked me if I could add Bluetooth to her AA5 vintage clock radio. I choked and said I would think about it. Help me think.

My first thought is to not bother with Bluetooth and just add a 1/8" connector to the back so a phone/MP3 player could be used. Add an outboard Bluetooth receiver via the same connector if they cannot live without Bluetooth.

Now the problem. Where is a safe place to add the 1/8" shunt to a hot chassis via a SPDT switch? I want to do this as simply as possible without looking for Sams for a location to insert. Somewhere ahead of the amp is obvious. The hi/low sides of the volume control is my first guess. Variations in volume from different sources is to be expected.

Can this be done safely?

[MadMan]

https://www.youtube.com/watch?v=HHJxGh8kaT8

There you go. There is a schematic in the vid, hopefully it's of some help to you. And actually I consider the bluetooth a much safer option than a aux input jack. I mean, all I did was add an aux jack to my radio, and plugged in a usb bluetooth receiver, and it didn't ruin it. But my thinking is, it'd be safer to risk a $7 Chinese bluetooth thingy than something potentially much more expensive like a smartphone. I suppose you could just add the aux port and instruct the user to exclusively use a bluetooth receiver, and nothing else.

Yes, there is a variation in volume from the radio and the BT. However, that could be tuned by changing the (in my case) 10k resistor and also the two stereo-to-mono resistors.

Also, looking back, I just remembered that the aux input's ground is directly connected to the 120vac Line. So conceivably somebody could get zapped if they touched something on house ground and the metal jack at the same time. Not that it would be harmful, just very surprising (read: 'shocking'). I suppose you could use and audio isolation transformer if you had to. But if you keep everything inside the radio, it'll be plenty safe.

https://www.ebay.com/sch/i.html?_fro...tooth+receiver

[judge]

Here's a link to a thread I started on this topic at that other site...

In the end I used a DPDT switch - the BT had to be off while the radio was playing AM, otherwise it produced interference.

Whichever way you go, use an isolation/auto transformer. In the BT example, a 1:1 was perfect. If you add an aux-in, you'll need a 1:4. Oh, and remember to mix the stereo to mono with a couple of 2k resistors (or thereabouts).

The audio out of the BT adapter (or the terminals of the aux in), via the isolation transformer, connect across the volume control of the radio.

Personally I prefer an embedded BT adapter to an aux-in. No faffing about with a separate dongle.

[Dave A]

Thanks all for a great start with various schemes. Using the judge version, could I use the DPDT as an audio only center point for the mods? I would lift the volume ends and take them to one side of the DPDT and back on the center of the switch. Radio remains normal at this mod.

On the other side of the switch would be the Bluetooth assembly. Hopefully it becomes an input switch with the original volume control and radio amp.

I found a $11 Bluetooth board that is USB powered and not amped.

https://www.amazon.com/Niubow-Blueto...499136&sr=8-37

Then the Bluetooth side via a $1 isolation xfmr to the second side of the switch and to the volume control. 2K resistors as needed for mono and for adjusting to any given radio as the board does not seem to have volume control. The module can also take +5-35vdc from any source. Maybe even a point on the chassis? Otherwise I would glue/wire the USB adapter to the power line for simplicity.

I'm looking at building the Bluetooth side on a small board that can velcro or glue in most any open space with wires long enough to go to a rear drilled switch. I think this clears me of any hot AC floating around that cannot get back through the transformer. If it is not dangerous I will try with a spare clock radio to proof.

Or should I buy more fire insurance?

[judge]

I don't think you need to use both sides of the switch for switching sources as they would share a common terminal, you just need to switch the other line. I use the second side of the switch to switch the power to the BT module.

You will need 5V-35V DC, so you will probably need a rectifier, smoother and a 5V regulator. At least, that's what I did.

I should really draw circuit diagrams when I do these things!

I used this BT module.

[Dave A]

I'm hoping that if you go in to both sides of the volume control after the RF/IF, the interference will not be a factor. Think signal injector test. What do I know? I'm a tv guy. And the module can run on any leftover USB phone adapter laying around. I also think the adapter can handle anything you can tap. I should relearn how to draw a diagram. 7th grade was a long time ago. We are on the same page. Parts for my Frankentooth are on order. Now I have to go dig up a victim. Igor...where are you?

[Electronic M]

BT adapters and many devices have internal oscilators and digital logic lines that operate unshielded in the AM broadcast band among other bands... If it is powered and within 5' of a radio you will probably get interference from it.

[MadMan]

Can confirm, the BT module I used DOES cause nasty interference, hence why I have a switch dedicated to killing the power to the BT module. You could use a DPDT switch. I just didn't have an appropriate one handy, and used two SPDT's.

If the module uses 5-35vdc, I would imagine it has its own voltage regulator onboard. So yeah, you could rectify and smooth something in that voltage range. What do the filaments run on, in that radio?

I don't really understand the point of an isolation transformer if all the electronics are contained in the radio and not touchable in normal circumstances.

[judge]

Originally Posted by Dave A

...and the module can run on any leftover USB phone adapter laying around...

These can also throw out RF noise. That’s why I tapped into the heater string and built a little DC power supply with a 5V LDO.

[judge]

Originally Posted by MadMan

I don't really understand the point of an isolation transformer if all the electronics are contained in the radio and not touchable in normal circumstances.

Two reasons. One is to maybe equalize levels, so more of an auto transformer. The other, in my case, was to kill ground loop hum.

[Electronic M]

Originally Posted by judge

Two reasons. One is to maybe equalize levels, so more of an auto transformer. The other, in my case, was to kill ground loop hum.

Yeah if you're tapping a series string heater for power ground loop him is almost guaranteed without an isolation transformer...
I wonder if taking power from the audio output cathode resistor would have been able to circumvent that...

[MadMan]

I see. In any case, I had no interference from the samsung usb phone charger - though it is turned off when in radio mode.