Hi all,
Im using a pam8403 2x3w module audio amplifier and BLK-MD-SPK-B bluetooth module receiver (datasheet here). for this thread sake i've made a simple schematic to show you the problem im having. last time i built this schematic (last summer) this noise was not ocurring.
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Looks like RF picked up by the amplifier input (or possibly other amplifier terminals). Not sure if this will ever work satisfyingly on a breadboard. Series resistors (e.g. 1 - 2k) directly at the amplifier inputs may help, also small SMD capacitors (10 - 100 pF) shorting the inputs against the a nearby ground pin.
The datasheet indicates that the speakers output from the bluetooth id differential, in probably means that it isn't referred to ground. It is OK when connected to earphones. The datasheet also recommends to use 4 serial capacitors if connected to an amplifier.
Try first to add those 4 caps.
If you still have problems try to connect the power of bluetooth to one battery and the power of the amplifier to another battery, the connections between the 2 circuits will be only 3 wires that go to your amplifier inputs.
Try also to connect earphones directly to the bluetooth to check if you get good sound there.
https://obrazki.elektroda.pl/6591333400_1430654382.gif
Hi,
In my eyes most likely it is a GND reference problem.
To your amplifier (is it a class D?) You have one power gnd connection. Here you may expect high current peaks.
This causes voltage drop. If this shift your analog GND, thn you hear this as noise.
The same most likely is with your bluetooth circuit.
Here you have a power GND line carrying pulsed currents.
You either need very solid, low impedance GND lines from each device to a common star point
On a breadboard this is almost impossible.
Even better is to use a differential analog signalling between bluetooth circuit and amplifier.
Simplified it is possible to use a signal_gnd as reference...
Klaus
thanks for the reply, i've tried that and no effect.
with two diferent power supplies (both lipo) the "data sound" stops completely and just a small "ch" sound remains. but, for some reason, when using two diferent power supplies the sound stutters. with the 4 caps it just makes it lower but its always there.
I assume you tried to connect earphones instead of the amplifier and found that it works well.
The range of bluetooth is 1 meter, I guess you tried to get very near to the transmitter.
If you get no sound with batteries and some sound with power supply then it can be low RF reception which improves with earthing. Otherwise you have invented a clever device that can tell between battery power to mains power.
Knowing that the bluetooth works well is a good start.
If the amplifier works well on its own then we know that it's the interfacing that doesn't work.
"using only amplifier direct to phone through cable and using speakers i get only the small "ch" sound." In this test you did is the phone connected to the input of the amplifier? If not do you have any source of sound that you can connect to the input of the amplifier to test if you can hear that sound well in the speaker? It can be MP3 player or portable radio.
If you establish that the amplifier works well then we will have to move on to the interconnection.
There is the possibility that the output of the bluetooth is digital (PWM) and it interferes with the oscillator of the amplifier causing distortions and hiss.
Put in series to each input a resistor of 4.7K to 22K and from each input of the amplifier to ground cap of 0.01uF to 0.1uF. This will be a low pass filter. If it helps then it will be needed to refine the filter.
Other possibility is that the signal level is far too high. Replace the capacitors of the filter with resistors of 1K and test it.
Stereo audio output can directly drive 40mW @ 32Ωspeeker without the need for DC-blocking capacitor
In my first post I asked you to try 2 separate batteries and your answer was that the sound stops.
Now you telling that with 2 separate batteries and 100 ohm it works perfectly.
Another option is to have an isolation inverter that is powered by your single battery and produces 5V for the bluetooth. This is an easier solution.
Hi,
On an internet page according the bluetooth module i found:
So it seems there already is an amplifier installed.
Maybe is class D. Do you know?
Especially the " without the need for DC-blocking capacitor" tells me that there might ba a problem with SPKN signal, as said in post#6.
Klaus
Hi,
instead of transformer you could also use a stereo OPAMP.
differential OPAMP circuit with three identical 10k resistors and two 20k resistors.
The schematic (of a standard four resistor circuit) you can find in the internet. Just replace the single_resistor_to_GND with one 20k to GND and another 20k to VCC.
(you may change values. But mind to use 3 equal plus 2 equal with exactely twice the value of the others. )
Klaus
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