Continuing on my quest to get good audio from my phone to my car's aux in I started playing around with bluetooth receiver modules. The ElectroDragon PKB-PCBA caught my eye as a fairly easy to implement solution that offers potentially good quality for the price. It uses the OVC3860 IC which I've heard good things about and also features a SGM4917 headphone amplifier IC which appears to be a fairly handy part with both integrated click/pop suppression and charge pump circuitry to generate it's own negative rail for producing a ground referenced output. Many of the other similar boards I've seen use a basic opamp with a single supply and bulky capacitors at the output to get rid of the resulting DC offset. I believe this is the main reason for their poor performance.
As with all of these Chinese made modules English documentation is sparse at best for the PKB-PCBA and ElectroDragon support was utterly useless as well when I tried to ask for more specifications. Fortunately whoever designed the board had easy implementation as a goal and pretty much all you need to do to use this thing is just feed it power and get your audio output from it. I don't know the exact schematic of the board but it probably looks similar to this application circuit for the BLK-MD-SPK-B which looks fairly solid to me.
If I wanted a more audiophile grade solution I could try to lay my own PCB using a Roving Networks / Microchip RN52 Bluetooth Audio Module with a TPA6138A2 from TI driving the output, but I don't have the equipment or experience needed to take measurements for optimizing the layout/performance of such a design and it's really just too much work for me. My approach of using a highly integrated module saves a lot of time and still yield respectable performance.
Note that I tried the Griffin Technologies BlueTrip Aux (it is one of few commercial products low profile enough to plug-in inside my car's center compartment) before pursuing a DIY solution and found that the BlueTrip outputs a low volume signal with completely non-existent bass. My music simply sounded so terrible that I returned the BlueTrip immediately after testing it. The funny thing is that I was contacted by another BMW owner about my Amazon review who had the same issue with the BlueTrip so I know it isn't just me.
I did some basic bench testing with the breakout board I made below before moving onto the full design.
I found it to draw about 80mA of current total when driving a 32Ω pair of headphones at listening volumes and was not able to measure any DC offset at the output with my equipment. Output voltage level hovered around 0.7Vrms at max volume which is pretty good. The sound quality seemed fairly good through my headphones - there was an audible noise floor but I think this was due to my testing power supply being tied to earth ground. We'll see how it fares in the car on the full design.
As mentioned previously all you really need to do to implement the PKB-PCBA is feed it stable power and give it a jack for output. The 4.7uF output capacitors on the ElectroDragon wiki application circuit are almost definitely not necessary as I did not see any harmful DC offset at the output.
My main concern regarding sound quality would be the possibility of ground loop noise within the car, as the ground throughout this circuit is tied to both the car power ground and stereo ground. If this becomes a huge issue, I may need to use a ground loop noise suppression cable at the output or implement an isolated DC-DC converter for power like I did previously to power a CMOY.
EAGLE Project Folder
An LM2937ES-5.0 IC from Texas Instruments takes in the nominal 12V directly from the car's cigarette lighter jack and provides the 5V power that the PKB-PCBA needs. This is a fixed 5V LDO regulator in a TO-263 form factor which uses the PCB as a heatsink. The LM2937 is "ideally suited for automotive applications" according to the TI datasheet and will "protect itself and any load circuitry from reverse battery connections, two-battery jumps and up to +60V/−50V load dump transients." A 10uF ceramic cap bypasses the input of this regulator and a 220uF low ESR electrolytic cap and 1uF ceramic cap keep the ESR at the output of the regulator within the stable region.
You can see I've also put in a DC power jack for the 12V from the car and a little slide power switch for turning off the unit when it's no longer needed.
R5/R10 add some series resistance to the output of the PKB-PCBA to provide short circuit protection; I'm not really sure if they're necessary as most headphone amp ICs should have this integrated, but they don't really harm the performance any since the input impedance of a car stereo is so high.
Mouser Part #
|PKB-PCBA||Nice little bluetooth receiver module you can buy from ElectroDragon based on the OVC3860 IC. I do not know of a US supplier that stocks these so shipping usually takes a while.|
|LM2937ES-5.0||5V fixed TO-263 variant of the LM2937 LDO regulator from Texas Instruments|
|STX-3120-3B||Basic 3-pin stereo jack from Kycon. It's a very cheap part but the 5-pin version is used in the Objective2 so I know the quality is fairly good.|
|103-12100-EV||Tiny little SPDT slide switch from Mountain Switch that serves as the power switch in this circuit.|
|RXK221M1EBK-0815P||A 220uF low ESR electrolytic capacitor from Lelon that serves as the power reservoir for the circuit|
|MF1/4DCT52R8R06F||Metal-film output series resistors- these aren't really necessary but help provide some short circuit protection as I'm not sure if this is integrated into the SGM4917 IC|
of the other little parts in this circuit I got from Tayda Electronics
or somewhere on eBay.
-2.1mm ID / 5.5mm OD DC power jack (Tayda A-4118)
-1206 X7R SMD Decoupling Caps can be found around on Mouser or Tayda
-1KΩ 0805 SMD RLED (seems like a high resistance for a LED resistor but the LED is still pretty bright actually)
-Blue or Red 1206 SMD LED - I had some superbright ones laying around from Sky-Macau
-For the cigarette lighter plug the Philmore units from Minute Man Electronics are pretty good quality in my opinion
Unfortunately I couldn't really find any particular enclosure that this would perfectly fit inside of so I simply made the board perfectly square with holes for standoffs at each corner for screwing it down to some sort of block. The whole board only measures 5 x 5 cm so it can easily fit almost anywhere in the car.
I soldered down the smaller SMD parts first and then did all of the through-hole parts in one pass. I stuck a tiny little heatsink on the ground plane for cooling the LM2937 but it isn't really necessary as I only measured 85mA of current draw from a 12V power source while driving headphones.
Soldering wires to the cigarette lighter plug was pretty tricky for me especially since the wire I'm using is so thick, but I managed to get everything in there.
Fully setup - nice and neat under that center compartment. Notice how there isn't a whole lot of space height-wise in there which is why I used right angle connectors for the power and 3.5mm aux jacks.
Setup: from Galaxy Nexus 3.5mm and A2DP to 2008 BMW 328xi stock stereo
The overall sound is better than the sub $50 receivers I've tried from Amazon so far - it's still a little muddier than just using the aux input directly but the frequency response is quite good. The bass response in particular is noticeably better with the receiver so my phone likely just doesn't do a good job with this from its 3.5mm out. I don't hear any audible noise floor or ground loop noise; my car may have audio transformers on the aux input to deal with this, but this could potentially be an issue in other vehicles.
One annoying thing I've noticed is that my phone seems to still think it's connected for a while after I've already turned off the car and receiver, so it'll keep "playing" until it finally notices it's been disconnected.
Overall if you want a simple and good quality bluetooth music receiver this is worth a shot if you're an experienced DIYer. The parts involved are fairly cheap and it's a good learning experience as well. The PKB-PCBA performs remarkably well especially given its price point, and I'm sure that a higher end design could do even better. Sadly I haven't seen many particularly good car powered bluetooth receivers on the market besides the TuneLink Auto which is a bit too tall to fit under the center compartment of my car.
I've shown here you can get pretty decent results with only cheapo parts; I'm only disappointed that I could never find a good commercial product to suit my application, as it wasn't even all that difficult to put together a fairly well performing DIY solution.