Adding Bluetooth to Eurorack

 

I have a new bluetooth headphone that I am testing, and I thought it would be nice if I could hook it up to my Eurorack. I therefore ordered a 4.1 Bluetooth Transmitter module which arrived the next day. The module is very small and came with a sample circuit (shown below). As you can see on the picture to the left, the module is very small, with the soldering points less than 1mm apart. In order to make this manageable I used the cut off pieces of my resistors as little extensions. I also added one to the on-board switch called "PAIR".

Most of the connections are as displayed in the diagram below, although I used a 1K resistor instead of 330 Ohm as I don't need the LED to be very bright. This has the added benefit that the on-board red LED can be seen shining through the yellow LED. I used a 3 position switch for the "PAIR" and "CONNECT" buttons, with the central position being normal operation and the left and right triggering one of the two options.

I left out the 100uF capacitor, although whether you need something depends on your Eurorack's power supply (I'd recommend at least 100nF up to 10uF. 100uF is definitely overkill). As per comments online I connected AGND to PGND (you can see the ground wires in the picture on the left as they are floating above the circuit).

I added a 100k dual gang potentiometer between the audio in and the module. Although I intended it for the output of the headphone amplifier I wanted to allow Eurorack modules to be plugged directly into this module as well, and having a volume control made this more likely to prevent issues. I should probably have added a separate resistor divider network, as currently I suspect you can still destroy the module if you have the knob all the way to the right and feed it a 10Vpp Eurorack signal.

The input jack is stereo, which means that if you use a mono patch cable one side will be silent. You can fix this by having a special mono to stereo patch cable. If you want to build this module specifically for Eurorack you may want to use a mono jack and feed the signal to both IN_L and IN_R, or you may want to add two input jacks, one for IN_L and one for IN_R.

It was surprising how easily this bluetooth module hooks up to devices. The moment I turned it on it immediately hooked up to my amplifier, and after switching it to pair and turning my headphone on it connected to that as well.

For the power input I just use a 2 pin header, which I wire to one of my PICmicro programming headers of which I always have one or two in my rack. If you don't have something like this, you can use a 16-pin header, or create a custom power cable that extracts the 5V from the power bus. If you prefer to keep a 10 pin header you will need a 78L05 (possibly with two capacitors and a diode) to provide the 5V from the 12V.