Solved: How does a NJM2073 audio amplifier works?

[Frazatto]

I "finished" upgrading some vintage pc speakers and it does all that I wanted to, except that the power is disappointing now that I'm using a bluetooth receiver instead of the original audio jack. I wander if I could still take this project a little further and fix this issue.

Well, the original circuit uses a NJM2073 on each box, I already looked at the datasheet but I can't understand how it works exactly.
https://datasheetspdf.com/pdf-file/606742/NewJapanRadio/2073D/1

Poking the board with the multimeter, V+/Gnd is constant ~7V, I was expecting this to change as I turn the volume knob, but it doesn't.
The volume knob is  a 2.4K pot, at least that is what I get measuring the pins as I turn it.
All other components are resistances and capacitors.

It would be VERY nice if I could just change the pot for a lower resistance one and miraculously get what I want hahaha
So, is there hope for my little project?

[magic]

Per the datasheet, the feedback network is fully internal, you have no access to the IN- input of the actual amplifier, and the INPUT- pin is used to ground the feedback network (through an external capacitor - see application examples).

You can't change the gain of this stage. (Well, you could decrease it, by adding a resistor in series with the capacitor. It's not clear if the amplifier would remain stable.)

If there are no opamps in there before the NJM2073 then you will need to add one, assuming that the bluetooth receiver itself can't be set for louder output.

[Frazatto]

Per the datasheet, the feedback network is fully internal, you have no access to the IN- input of the actual amplifier, and the INPUT- pin is used to ground the feedback network (through an external capacitor - see application examples).

If there are no opamps in there before the NJM2073 then you will need to add one, assuming that the bluetooth receiver itself can't be set for louder output.

Hum....that is a shame.
No, the bluetooth receiver doesn't have any regulation that I can see either (documentations is scarce though).
https://www.aliexpress.com/item/1005001830804948.html?mp=1

[Frazatto]

You can't change the gain of this stage.

No...wait, this makes no sense....
How does the volume knob works as intended than???
There are no other ICs there or.....hummmmmm

Alright, I think we are on to something, there is a transistor on the other side of the board:
https://html.alldatasheet.com/html-pdf/54742/FAIRCHILD/S9014/404/1/S9014.html

I can't quite follow the trail though, it jumps around too much.
Is it pre-amplifying the signal itself before the IC than?

[magic]

A volume knob simply attenuates the signal, or passes it unchanged if set to maximum.

A transistor may be doing just about anything. No way to know without understanding the surrounding circuit.

[Zero999]

Per the datasheet, the feedback network is fully internal, you have no access to the IN- input of the actual amplifier, and the INPUT- pin is used to ground the feedback network (through an external capacitor - see application examples).

You can't change the gain of this stage. (Well, you could decrease it, by adding a resistor in series with the capacitor. It's not clear if the amplifier would remain stable.)

If there are no opamps in there before the NJM2073 then you will need to add one, assuming that the bluetooth receiver itself can't be set for louder output.

Reducing the gain is clearly shown in the data sheet.
https://www.mouser.com/datasheet/2/294/NJM2073_E-275459.pdf

True no minimum gain is given for stability, bu that can be fixed by increasing the gain at ultrasonic frequencies.

[Frazatto]

Good news everybody!

Observing the board, I noticed a 33ohm resistor in parallel with the pot, it become a simple test.
If removing the resistor decreased the volume, it means the pot was just attenuating the signal.

But it didn't, it has a very adequate power output now.
I guess it means the pot drives the pre-amplification transistor.....

Thanks for your help   o/