Ok so here's a problem that's been bothering me for quite some time and I think after much hair pulling and extensive testing I've located the actual cause of the problem but I am unable to find a suitable solution at this time and would like some help please.
I am using a DFPlayer Mini MP3 Player in close proximity to RF which the MP3 player is picking up and reacting to. These MP3 modules are very simple things. They use a dedicated YX5200 MP3 decoder IC with on board DAC and Serial Comms and the audio from that is fed into an 8002A Audio Op Amp.
The problematic wire seems to be the Op Amp's Negative Speaker Wire Output which also feeds back into the negative input on the op amp via a 68K resistor. No doubt this is feedback for the Amplifier Gain.
If RF gets anywhere near this Speaker Negative wire, a buzzing / humming starts to appear and gradually increases in strength and begins to loop over and over. When the RF is removed, this buzzing carries on looping round and round like an echo but it starts to decay.
This suggests to me that the negative output is picking up the RF and looping it round and round through the feedback back into the Op Amp input.
What I've tried to Resolve the Problem Myself:-
First of all, I should point out that when I built my project, the MP3 gave me a great deal of difficulty to begin with. Until now I haven't understood where the problem actually lay so please don't mock my silly ideas. I'm still learning lol.
So when I first built my project, the Speaker Negative output from the Op Amp has always given me trouble. No matter where I connected it or how I connected it, I would get dreadful buzzing so I left it free floating and just used the Speaker Positive output from the Op Amp. And that got me some audio out but that came with an equal level of buzzing / humming noise.
Next I decided to try shielding so I put my project in a metal enclosure. That cured a little of the noise but plenty still remained. So I added layers of tinfoil between the enclosure lid and the PCB which dropped the noise considerably but some still remained.
Then I decided to us a DC blocking capacitor in series with the Speaker Positive which cleared up all but a distinct audible buzz at a certain frequency which I couldn't get rid of at all and this is when I decided to investigate the problem in more depth.
As turns out, ignoring that Speaker Negative wire was a BIG mistake and all it does is pick up RF and feed it back through to the amplified audio signal over and over and over.
So stupid me now has a problem of my own making of sorts when I should have just dealt with the issue on day one and found a way of dealing with the output from the Op Amp properly!
Lesson learned there....
Further Research Into the Problem and Possible Solutions:-
Now that I understand what the problem actually is, I can stop stabbing away in the dark and hoping I get lucky and actually do something about it.
Having done some reading, it seems I need to use RC filters at the input of the Op Amp to keep the RF at bay but I must confess, I've no idea where to start.
Ideally I'd like to modify the MP3 modules I'm using but due to their small size, that doesn't look like it's going to be a simple task and will require some seriously steady handed soldering!
The other option is to get rid of the MP3 modules and source some MP3 decoder chips and build my own MP3 circuit with filtered Audio Op Amp. I'm happy to go that route but I have no idea how to go about filtering the RF from the Op Amp.
I've read some about it and it says to use RC filters to keep the RF out of the Op Amp's input but I've no idea where to begin if I'm honest. It's all going round in my head and I'm trying to make sense of what I need to do.
All I need to do is get a good audio signal from the MP3 file into the AF input on the transmitter without the RF affecting it.
I did find this article on filtering Op Amp input which looked promising until I noticed that the Images that go along with it aren't in the article or on the server anymore
Any help I can get on this I would greatly appreciate!!!!
Also another issue I have is that I can't find any MP3 decoder ICs to buy at a reasonable price anywhere. The YX5200 seems to be obsolete. The next version seems to be the YX5300 but again, you can't buy those IC's anywhere either. I notice at one point Atmega did their own MP3 decoder IC but that's also obsolete.
Perhaps my search powers are beginning to fail me but I can't find any cheap MP3 ICs anywhere.
All google brings up are ones on eBay for silly money, ones that are obsolete or it brings up complete MP3 modules. What's going on? Is MP3 obsolete or is there some new fangled technology I'm not seeing?
So my only choice right now seems to be to buy MP3 Modules and butcher the IC's from them.
All help greatly received!!!