Help adding Bluetooth audio receiver to HiFi amplifier - Page 1
Forum section: electronics / projects

author: RogerClark
author_rank: Contributor
timestamp: February 04, 2017, 12:27:24 am
content: I've been updating all my old Hifi gear by fitting Bluetooth audio receivers in them, however I have an issue with processor noise being generated by the receivers getting into the audio input.The dongle I've been using is like thishttps://www.aliexpress.com/item/3-5mm-Stereo-Audio-Music-Speaker-Receiver-Adapter-Dongle-USB-Bluetooth-Wireless-Audio-Adapter/32775535858.html?spm=2114.01010208.3.127.ORmkap&ws_ab_test=searchweb0_0,searchweb201602_3_10065_10068_10000032_119_10000025_10000029_430_10000028_10060_10062_10056_10055_10000062_10054_10059_10099_10000022_10000012_10103_10102_10000015_10096_10000018_10000019_10000056_10000059_10052_10053_10107_10050_10106_10051_10000053_10000007_10000050_10084_10083_10000047_10080_10082_10081_10110_10111_10112_10113_10114_10115_10000041_10000044_10078_10079_10000038_429_10073_10000035_10121-10050_10111,searchweb201603_10,afswitch_4,single_sort_2_price_asc&btsid=cc2a626e-0390-4931-ab6d-0cee6144e427Inside it looks like thishttp://img.mysku-st.ru/uploads/images/01/85/60/2015/11/02/4202d8.jpgUSB is not connected, it only uses 5V from the USB connectorI remove both the USB connector and also the jack socket and connect directly to the 5V supply inside the amplifier and into one of the audio inputs e.g. the Tuner input.Before you tell me there is an impedance missmatch between the dongle's audio output and the amplifiers inputs.Yes I know, but it generally seems OK.However, what I do seem to experience is a problem where the processor chip, creates noise which seems to be injected back into the supply and manifests its self as hisses and pops etc if the audio played is in a quiet section.One thing that seemed to improve this, was to put a 1000uF capacitor across the 5V input of the dongle, and to put a 10 ohm resistor in series from the amplifiers 5V supply.I also tried putting a choke in the incoming 5V line and that helps a little but the frequency of the noise is in the audible range, so a ferrite choke doesn't seem to help.I have noticed that running from a completely separated isolated USB power supply does help a lot.But I can't work out quite why this would be, because the internal supply in the amp I'm having the problem with, is 12V and I have to regulate down to 5V using a 7805, and there is no shortage of current.Can any suggest any other methods to prevent this processor noise (which seems to be injected into the supply), from getting into the audio.I guess this may be an earthing issue as well. The dongle has shared GND between power and audio output.Perhaps somehow I need to have separate grounds

author: RogerClark
author_rank: Contributor
timestamp: February 04, 2017, 04:48:19 am
content: Just to update my own post...I tried putting a choke in the 5V input, but that didn't make any difference to the interference / noise.I also tried a separate signal GND, but again that didn't make any difference.I also tried another RC filter on the 5V input, consisting of a 470uF capacitor and a 12 ohm resistor, which should give a filter with 3dB at about 30 Hz if my calculations are correct, but that didn't help.I tried powering the receiver from a USB charger and this significantly reduced the noise,So I've ended up putting a separate encapsulated switching PSU inside the case (as there is loads of room), and powering the receiver from that. (and connecting the GND to the 0V on the PSU)It seems like total overkill, but I can't seem to find any other way to do this, just using the various DC supplies available inside the amp.I'm still interested however, in anyone's thoughts about how the noise is getting onto the audio, and how using a separate switching PSU seems to fix the problem.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 04, 2017, 05:24:37 am
content: You have a ground loop. It's too bad that Bluetooth audio modules with digital output are not that common, since that would almost completely eliminate ground loop issues and give better audio quality.

author: RogerClark
author_rank: Contributor
timestamp: February 04, 2017, 06:13:48 am
content: Can you think of a way to eliminate the ground loop ?I've tried connecting just with one ground to the power input, I've tried 2 grounds (Power and signal), and also tried just having a signal ground.On this particular amp (Yamaha DSP-A5 , around 15 years old), it didn't seem to helpI did the same mod on a Kenwood amp and it was more successful, and putting the 1000uF across the power input on the Bluetooth receiver, and a 10 ohm in series with the supply, got rid of most of the noise. Though using a separate switching PSU would reduce the noise further.As these amps are quite old, one of them only has analogue inputs.The Yamaha has multiple optical inputs. so I guess finding a Bluetooth audio receiver with optical output would also cure the problem.But looking at the price differential, its cheaper for me to simply use a encapsulated switching PSU to provide isolation.Edit.I just noticed I can get isolated DC to DC power modules on eBay very cheaplyhttp://www.ebay.com/itm/DC-DC-B0505S-1W-Isolation-Power-Module-5V-To-5V-For-RS232-485-CAN-IO-80-20mHz-/331933432018?hash=item4d48c638d2:g:T~cAAOSwLN5Wjii3I think I'll order a few as they look like they could be another solution to this problem, (assuming they do what they claim)

author: MHbach
author_rank: Newbie
timestamp: April 13, 2017, 10:40:48 am
content: I fixed the problem. I soldered a short and thick cable between the ground of the AMP Platine and the Receiver Platine.All Grounds needs good connection without resistance.The ground via the Mini-Jack wasn't enough.