Fixing hissings sounds from XS3868 BL audio module hooked up to a pam403 audio a

[MikeLemon]

Hello,

I've this XS3868 audio bluetooth module:

http://www.ebay.com/itm/1PCS-XS3868-Bluetooth-Stereo-Audio-Module-OVC3860-Chip-Supports-A2DP-AVRCP-New-/252459433746?hash=item3ac7c13f12:g:es8AAOSwGXtXhcXB

 connected to this PAM8403 circuit:

http://www.sunrom.com/media/content/846/PAM8403-schematic.gif

on my own PCB

Now testing the Audio amp standalone it works fine just hooked up to my smartphone but after soldering the BL A module and playing music and audio from there every time I interact with my phone to play it and while playing music it makes that hissing sounds but about 5-6 seconds after I stop playing the music it suddenly stop and I assume the problem is in the audio bluetooth module what do you suggest I do?

Thanks.

[Buriedcode]

Its quite common with bluetooth modules for several reasons.

1. Bluetooth modules generally have an SOC integrating output amplifiers (analogue), the DAC, processor, RF parts etc.. and so are often noisy.  This is why many have differential outputs, so noise from the other parts that makes it onto the output lines can be cancelled out.  Solution:  Use differential output if it has it, your module doesn't.

2. Sharing the same power supply. Bluetooth modules can create a fair bit of noise on the power lines. Solution try powering the amplifier and BT module separately.

3. Ground loops.  With both amplifier and BT module powered, they obviously share a common ground.  But also the bluetooth module has a 'common' output for the audio lines (AGND). Which is used as a ground reference from the amplifier.  Differences between this ground and actual ground can cause ground loops.  Solution: Separate power supplies for both modules, with the only lines between them AGND, L and R (so the only ground they share is AGND). An even better idea would be to use isolation transformers, but that may be a bit extreme.

4. Just deal with it.  I have noticed a tiny bit of background noise when I interact with my phone, and, like yours it stays there for a few seconds, before the music is clear again.  I suspect this is actually firmware, because bluetooth modules also output this 'noise' out the I2S port, which is purely digital.

[MikeLemon]

Well thank you Tip 3 is quite easy I'll take that!

About tip 1 what kind of buffer and how should I wire that?

2 the AMP uses 5V from a SMPS and BT module 3.3V from a linear voltage regulator but still sharing the same ground but I don't think that matters.

4 that'd be annoying I want it to be professional yet the design has to be simple quite small and thin.

[Buriedcode]

About tip 1 what kind of buffer and how should I wire that?

I don't think that particular module/chip has differential audio out, its single ended, with both left and right channels sharing a common ground - AGND.  Modules like the CSR8645/8670 have L+,L-,R+,R- with built in drivers meaning they can be connected directly to the drivers in headphones, but require a differential to single ended circuit if they are to be used for systems with a common ground (like headphone jacks).  Your module is more convenient, as it doesn't require conversion, but this also means noise will be higher as there is no way to cancel it out.

2 the AMP uses 5V from a SMPS and BT module 3.3V from a linear voltage regulator but still sharing the same ground but I don't think that matters.

Thats good, sounds like their power supplies have enough filtering between them.  But when you saying 'sharing same ground', is this just AGND? or the ground of the power supplies? or both?  The reason I suggested separate power supplies is because with one power supply derived from the other, you still have a common ground.  This is as well as the AGND for the audio. This means you have two grounds, one for power, and one for audio.   I was suggesting you try and use two separate power supplies, with two batteries, just for testing, to see if that cures it.  If you can only have one power supply, you can get isolated power supply converters for this purpose.  This effectively eliminates the common (power) ground between BT module and amp.

4 that'd be annoying I want it to be professional yet the design has to be simple quite small and thin.

I believe  the manufacturers of BT dongles have already done the best they can.  Some are noisy, because they were designed and manufactured to a price point - you can't expect to pay $5 for one and get crystal clear sound. But getting towards $20-$30 and there are some that are pretty damn quiet.  Avantree is a popular brand.

As an alternative, which may open up a can of worms (ie: they are a bit of a sod to work with) the CSR chipsets can be remarkably good for audio, given they use the built-in Apt-X codec.  The higher quality is only really noticeable with headphones, and they can still suffer the problems off crosstalk, noise, beeps etc..

Found a link, this guy also recommends isolating power supplies!
http://www.penguin.cz/~utx/hardware/Bluetooth_receiver_Enjoy_BT-AU01/#hack_to_silence_idle_noise


Good luck.