Nowadays most of the phone manufacturers are now skipping 3.5mm audio jack. This makes little tricky to connect to old school external speakers which needs aux input. Either you will need to buy an adapter USB to aux or an bluetooth receiver which can be connected to these speakers.

I had received stereo speaker as free gift when I had bought new laptop. It is powered by connecting to any USB port which outputs 5V 1amp and needs to connect its audio pin to audio jack.

As fas as input is concerned its limited to wired one, so I thought to extend it to make it wireless with the help of bluetooth headset. So here we will be converting a wired stereo speaker to bluetooth speaker.

I am using a bluetooth headset (which was I found it in my junk stuff :) , right now its battery is dead).

I choose this bluetooth headset, because both needed similar power 5V 1amp input (300mAh for headset). So in case you don't have similar specs devices; you might need to use different power source or use simple voltage and current divider circuit to power speaker and bluetooth headset.

Also make sure the speakers has builtin amplifier as the output from bluetooth headset won't drive the speakers. You can buy separate class-D 2 channel amplifier to drive the speakers.

Supplies

1. USB powered speakers

2. Bluetooth headset, should be stereo.

3. (optional) class D 2 channel amplifier.

4. (optional) USB power source as per specification of speakers and amplifier.

5. Four 1k - 10k resistors (all of same value, for coupling)

6. Prototype board (to put above resistors)

7. Couple of wires

8. Soldering wire and iron

9. super glue

Step 1: Disassemble Bluetooth Headset and Identify the Power and Audio Connections

After disassembling the bluetooth headset, I figured out its power input and audio output connections. These should be very easy to identify on other bluetooth headsets as well; just check where battery has connected (that will be power input) and the speaker output where the left and right speakers are connected.

Please identify whether the audio output has common ground between left and right channel, by measuring the resistance between L+ and R+ OR L- and R-. If resistance is zero then its common grounded. You don't need to follow below step.

In my case for bluetooth headset, both right and left channel has separate ground. We need common ground as the USB speaker's audio input has common ground. To make common ground you will need to put resistive or capacitive coupler between left and right channel. (You can search about it).

There are some complex schematics to achieve it, but I found simpler resistive coupler to make it work, and use input power's ground as common ground. Simply get four same valued resisters (between 1k to 10k) and connect them to each L+, L-, R+, R-. And then connect resisters of L+, L- together to get left audio output and vice versa for right audio output.

Below is the link which I have referred on StackExchange (thanks to StackExchange )

Using diodes to mix 2 audio signals into one set of speakers (Thanks to GetFree and Majenko for posting question and answer)

Step 2: Disassemble Speakers and Identify the Power and Audio Connections

Disassemble the one of the speaker to revel its circuit board.

There should be connections made to input the 5V DC from USB cable. And there should be audio input connections on it which should be connected to 3.5 mm aux cable.

Step 3: Make Connections !

1. Connect two wires on circuit board of speakers where the 5V input is fed. And connect these wires to input power to bluetooth headset. Make sure you maintain the polarity.

2. Connect the resistive coupler to L+, L-, R+, R- audio out pins to bluetooth headset. (In case your bluetooth headset has common ground, still I would suggest to put some resistance between similar range 1k- 5k to L and R output as it must be amplified output from bluetooth board, and it might fry out the components on speakers audio input)

3. Connect the couplers output (L and R) to L and R on speakers circuit board.

4. Connect input power's (-ve) terminal as ground for audio input . (It might give some audible white noise in case the input DC source has some noise. I am yet to figure out correct solution to it. But still current solution is functional)

5. Now inspect all connections with multimeter using its short circuit test to check whether you have not made any unwanted short circuit connections :)

6. Put back the speaker's back cover and attach the bluetooth headset, I have kept it external so that I can use its controls. Also it has mic so that I can use it for calls as well.

7. Power it ON and pair it with phone !! Enjoy the wireless bluetooth speakers !