How to convert 2 channel audio output into one 3.5mm jack

Question

Hi, I'm building my own Bluetooth audio receiver using a Bluetooth module 2.1 with A2DP, so I can connect the receiver (with internal power) to any set of speakers that has a 3.5mm jack, then play music via any Bluetooth device like iPhone or PC.

I got the Bluetooth module wired up and can now be paired with any A2DP enabled device, the issues is, the Bluetooth module has output connectors for 2 audio channels: Left Positive, Left Negative, Right Positive, and Right Negative, my question is how do I connect the 2 channels to a female 3.5mm jack? do I need something like a summing box (which I don't really understand yet), or is there a simple way to solve this? I hope to keep the entire receiver small.

Thank you.

Answer 1

Actually, the bluetooth module probably has left signal, left ground, right signal and right ground. Positive and negative terminals are only seen on equipment handling balanced signals, which anything with bluetooth doesn't do.

The standard way is the following:

• left signal -> tip
• right signal -> ring
• left ground -> sleeve
• right ground -> sleeve

Or just cut the cinch connectors off of a standard $3 stereo cinch to 3,5mm jack cable. Shielding = ground, Center core = signal.

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EDIT: The above is wrong.

It seems I was wrong, and the output is actually balanced. Your module seems very similar to this one: http://www.sparkfun.com/datasheets/Wireless/Bluetooth/WT32_Datasheet-1.pdf

And in there it says the following:

AUDIO_OUT_P_RIGHT and AUDIO_OUT_N_RIGHT

Right channel audio output. The au dio output lines should be routed differentially to either the speakers or to the output amplifier, depe nding on whether or not a single-ended signal is required. Use low impedance ground plane dedicated for the audio signals.

AUDIO_OUT_P_LEFT and AUDIO_OUT_N_LEFT

Left channel audio output. The same guidelines apply to this section as discussed previously.

You could try to put both negative pins and check if that also shuts down the module (probably, given your earlier result). If not, that could be your way to do this. Alternatively, you could ignore the negative terminals, and just connect the positive ones and the ground of the board but this is a bit of a hack:

• left positive -> tip
• right positive -> ring
• ground -> sleeve

Note that this will leave you with half the signal amplitude, and possibly a degraded signal.

To do it properly, you need a circuit that converts between balanced and unbalanced, also called a balun, or, in electronic terms, a differential to single ended circuit. You can do this with transformers (http://www.jensen-transformers.com/faqs.html#faq4), there are IC's to do this, or circuits with an op-amp that require a bit more electronics knowledge, like this one: http://www.ecircuitcenter.com/Circuits/adcin_diff_se/adcin_diff_se1.htm or the stuff that's discussed here: http://www.electro-tech-online.com/general-electronics-chat/92142-differential-single-ended-output-2.html

More about the transformer way of doing this (note that they describe mono signals): http://www.jensen-transformers.com/an/an003.pdf