I'm reverse-engineering the analog frontend of an SPDIF-to-analog converter. I'd like to increase its output level before feeding the audio (AUX) input of a Bluetooth speaker, which seems to amplify it less than other audio inputs.
This is the schematic I obtained for left channel (right channel is identical):
The audio signal comes from a MS4344 DAC, a chip with the same pinout as the CS4344 DAC, then goes through an SMD capacitor of an undefined value, then it seems to go directly to the RCA jack, but also to the collector of a BC848B NPN transistor, whose base is connected to 5V via a resistor.
I'm not sure what's the function of transistor. It's parallel to the load; is it meant to "take" a fixed amount of current? Maybe it can be altered to provider a higher output level?
By the way, this schematic is rather different than the suggested connection diagram (page 11 of datasheet). So I'm not sure what the designers had in mind.