I’ve been doing some tinkering with the RN-52 Bluetooth Audio module from Roving Networks, in prep for building a portable wireless speaker system, & thought I’d share my designs.
Initially I was having some issues with RF noise on the audio output from the RN-52, as I was only using the outputs single-ended. The module didn’t like this treatment, with all the RF whine coming straight out of the speakers.
To fix this issue I have used a pair of jellybean LM386 audio power amplifiers, running in differential input mode. This solves the high-pitched whine when the audio is enabled, & also allows the module to directly drive a set of 32Ω headphones at a reasonable level.
In Eagle I have designed a simple board, routing only the audio output, serial TTL & command mode pins out, along with the supporting power supply circuitry to operate from 12v DC.
![RN-52 Breadboard](http://www.experimental-engineering.co.uk/wp-content/uploads/2015/01/IMG_20150128_230738-300x225.jpg)
Above is the current incarnation of the circuit on the breadboard. The RN-52 is on the left, audio power stage in the centre & headphone output on the right.
![RN-52 Breakout](http://www.experimental-engineering.co.uk/wp-content/uploads/2015/01/IMG_20150128_230731-300x225.jpg)
The bluetooth module on a breakout board. I was cheap in this case & etched my own board. I’m not paying Sparkfun, (as much as I like them), an extra ~£10 for a small PCB with the pins broken out. Much cheaper to spend 15 minutes with the laser printer & the iron, & do a toner transfer PCB.
As this board is single sided, I added a ground plane on the underside with copper foil, to help with the RF issues. Breadboards really aren’t all that good at rejecting noise induced when there’s a 2.4GHz transceiver mounted on them.
![LM386 Amplifier](http://www.experimental-engineering.co.uk/wp-content/uploads/2015/01/IMG_20150128_230719-300x225.jpg)
The LM386 audio power stage. The differential inputs from the module are capacitively coupled with 1µF electrolytics. This setup remarkably reduced the noise on the output. I left these at their default gain of 20, as I’ll be connecting another high power amplifier stage to drive large speakers.
![RN-52 Eagle Layout](http://www.experimental-engineering.co.uk/wp-content/uploads/2015/01/RN-52-SCH-300x200.png)
Here’s the circuit laid out in Eagle, ready for PCB.
![RN-52 Eagle PCB](http://www.experimental-engineering.co.uk/wp-content/uploads/2015/01/RN-52-PCB-265x300.png)
And here’s the PCB layout. Only one link required for the +5v line from the TTL serial port.
As always, the Eagle PCB & Schematic layout files are available at the bottom of the article.
*Update 29-01-15*
Rerouted a few things:
- Moved the audio power stage to the +12v rail to improve sound response. – As the LM386 has a max input voltage of 12v (absolute maximum 15v), a regulated supply is recommended. The LM386-N4 variant has a higher voltage range, up to 18v. This should be suitable for an unregulated supply.
- Removed 1µF coupling capacitors to reduce distortion & amplifier hiss. The capacitors appeared to cause some instability on the amplifier, causing random distortion. Removing them has cured this. No signal hiss has also been reduced to a very low level.
- Reversed input polarity on input of one of the amplifiers – this appears to produce better audio.
- Added PWR.EN header to allow connection of power button. Saves hassle of cycling power to the board when the RN-52 goes into sleep mode.
Improved PCB & Schematic layouts.
![RN-52-SCH-v3.6](http://www.experimental-engineering.co.uk/wp-content/uploads/2015/01/RN-52-SCH-v3.6-300x200.png)
![RN-52-PCB-v3.6](http://www.experimental-engineering.co.uk/wp-content/uploads/2015/01/RN-52-PCB-v3.6-265x300.png)
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