Portable bluetooth amplifier speaker! noise problem, tips needed

[kaning]

after many time of searching and trying to do it following your instructions i got really confused.



this is the configuration you are talking about right? can you help with values?

 

[KlausST]

Hi,

Sorry for confusing you.

Please look for a "differential amplifier circuit"
Then take the standard circuit with four resistors.

If you found this...
Then there is usually one resistor connected to GND.
Replace this resistor with two resistors of twice the original value.
Connect one of these to GND and the other to VCC.

If it is not clear now, then i draw a schematic...Let me know.
But not within the next 11 hours...

Klaus

 

[kaning]

Hi,

I've made a small schematic to make sure i got it.



now the same circuit for the right amplifier input. this way signal ground pin from amplifier is not connected. am i missing something? feel free to complete the schematic.

 

[Vbase]

You got it right Kaning, connect the GRD of the amp to the battery - .
You can try it with any op amp on one channel and if you are happy do the other channel.

 

[KlausST]

Hi,

The schematic is correct.

One thing i recognized: Now it is inverting. If this is a problem you could interchange the inputs...

Klaus

 

[kaning]

it didnt work :/ im using tl072cp. actually it got lower but because the overall output sound also got lower but the same "bad sound" effect there. i also changed the inputs..

 

[Vbase]

Have you got 100 ohm resistors across the outputs of the bluetooth? If not, try it.

 

[kaning]

HI,
yes, i also tried, but the resistor makes the output sound even lower wich makes the maximum volume very low...

Meanwhile i started searching for the other two options. I couldnt find any isolation inverter like you said (isolation transformer) for such low values. i find a lot with 5v output but not with low input voltage (lipo battery). I found some audio transformers for you guys to check.
option 1
option2
or like this for bigger current. i totally dont know the maximum current of the output bluetooth signal.
option3

if you could point some isolation transformer, it would be nice to try both.

- - - Updated - - -

here is a image from the bluetooth datasheet with a board circuit using this bluetooth module and amplifier. what means the grounds from the amplifier being diferent from the rest of the circuit?



- - - Updated - - -

here is the pcb, if helps..

 

[Vbase]

I thought that the options of audio transformers or inverter are difficult this is why the advice of Klaus is so attractive.
I should have remind you to use an op amp that work at 4V supply, the TL072 needs 8V minimum and it is likely to be the reason that the sound is very low.
Please don't put aside this option before Klaus looks at it and gives his opinion.

From the transformer you selected option 2 looks most attractive option 3 is big without any advantage.
If you have an old radio or other electronics that you use for salvage of components, you can use any salvaged audio transformer for the experiment on one channel.

 

[kaning]

i'll try to find one!!
have you checked the schematic?

 

[Vbase]

The amplifier in the circuit you sent is working on the same battery. The ground of the amp is separated on the PCB but I'm sure it is joined to the - of the battery.

 

[KlausST]

Hi,

As Vbase said , the OPAMP is not suitable here.
Look for a 5V rated RR out opamp.

It's not clear what the output of the bluetooth is. Maybe it is classD, then you need a low filter at the output.
For a first try you can use two 1nF capacitors per channel. One from out to In-, the other from GND to In+.

Do you have a scope?

Please take anew picture of your circuit, so we can see all connections.

Klaus