Bluetooth Stereo From Unused Phono Input

For this project, I will be implementing Bluetooth into my old stereo by making use of the Phono input which remains unused since I set it up.

What is Phono?

Phono is short for Phonograph. A phonograph is essentially just a mechanical device for the recording and replay sound, common product names being the gramophone, turntable & record player. When a record is being played, a small stylus follows the groove along a recorded disc, causing minute vibrations up the arm of the stylus. These vibrations are then amplified further by apparatus in the device, before being sent to the output of the record player. This signal is very low voltage, so in order to make it feasible to work with, stereos with the PHONO input use a separate dedicated amplifier chip to step up the signal to a voltage that the main amplifier circuitry can work with. This is why if you've ever accidentally (or experimentally) tried to play a non-phono output into the phono input, it will be much louder (possibly damaging) than if you used the appropriate input.

Step 1: The Subjects

The stereo I used in this project is a Luxman Stereo Integrated Amplifier LV-121 I received as a gift over a year ago. I scavenged the Bluetooth module from a Bluetooth speaker (an XC5227 Portable Bluetooth Speaker) I had lying around in my house since it was just collecting dust. It isn't the best one around, and I have better speakers that I use, but this one works fine since I just need it for its Bluetooth function; not portability anymore.

Before taking anything apart, it is always a good idea to find a datasheet/service manual in case there is anything dangerous inside that you wouldn't normally be aware of. A service manual will also help you keep track of your parts as you are disassembling something.

Step 2: Speaker Disassembly

This part was pretty simple; All I had to do was remove a few screws & some hot glue and I had full access. The Bluetooth board is very basic, with external connections just to the USB charger, battery, and speakers. There aren't really any hazards to look out for when disassembling something this small, due to the low operating voltage. The most damage you could probably do is short the battery, and even so, doing this would not cause long term damage, so long as the short circuit path is removed quickly (i.e. If you simply poked both terminals with a screwdriver, NOT if you formed a solder bridge across the terminals).

Since this is just something I wanted to scrap for parts, I could get away with the SASRAS method (See A Screw, Remove A Screw). This method is very fast if you don't plan on putting the thing back together, but lacks organisation. In this situation where there are minimal parts, putting it back together would've been simple enough anyway so there was nothing to worry about.

Step 3: Stereo Disassembly

[ [ ALWAYS POWER OFF & UNPLUG YOUR DEVICE BEFORE SERVICING TO REDUCE CHANCE OF ELECTROCUTION ] ]

Taking apart a stereo is usually very simple, but it is always important to have a system you can rely on. If you have something you just want to scrap for parts, you can usually get away with See A Screw, Remove A Screw (SASRAS). This method is great for speedy disassembly, but can make it difficult to put something back together due to the lack of organisation. In my situation when disassembling something such as a high quality stereo, it is best to use the OSIB method (Outer Shell to Inner Body). This system is essentially viewing your subject as an onion, where you peel back one layer at a time. For this stereo, you would start with removing screws from the top cover, placing it aside, then the screws of the back panel, and placing that aside. This should give you plenty of access to the internals (for this project). For anything past here, it is a good idea to consult the service manual of the product you are disassembling. This is the safe way to avoid any potential damage to you (i.e. electrocution) and the product (i.e. bending/snapping of components, PCB strain, etc).

Step 4: Implementation

The Bluetooth board has 3 ports:

SPEAKER (+ & -)
BATTERY (+ & -)
CHARGE (+ & -)

In order for the Bluetooth module to work, it needs 5V to operate. I tried to find a 5V regulator near the power supply section, but only found a 15V regulator. This was still simple enough to work with, since I just needed to add a voltage divider to get my 5V. The formula for a voltage divider is:

V(out) / V(in) = R(2) / (R(1) + R(2))

I attached a rather crude voltage divider to the 15V regulator, loosely hanging in the air (This is a bad idea for a number of reasons). I then connected two wires from the voltage divider to the Charger terminals of the Bluetooth board. This meant that whenever the stereo was turned on (and thus the regulator), the battery would be constantly charging (NOTE: If you consistently apply power to a battery for a long period of time, the maximum charge capacity will eventually deteriorate). I tried to operate the Bluetooth board using just a 5V supply, but this wouldn't work, so I left the battery connected.

After some trial and error of what works best, I decided that applying the Bluetooth signal before the Phono pre-amp sounds the best (This way the sound is being amplified appropriately). I then soldered two wires from the speaker terminals to 2 capacitors at the Phono input terminals (I would've soldered straight to the terminals but they wouldn't stay on there properly).

Step 5: Final Thoughts / Notes

Some things did not go according to plan when making this. Some such things include:

The Bluetooth board could not be powered solely by a 5V rail.
A switch was needed to connect the battery to the board, so that it was not constantly on.
There was no 5V regulator to make things easier.

Overall, I think this project was very successful.

If you want to attempt something like this and would like some help, don't be afraid to contact me :)

If you would like more clarification on an explanation I gave, leave a comment and I'll make things more clear.