Hi, new to the forum!
So, I'm working on a bluetooth speaker and have been doing the proof-of-concept stuff recently and I'd like some help diagnosing an issue.
From the 240V wall I have a PSU that provides 30V (max 1A) to:
A voltage divider dual PSU (Image of rough scheme attached) using 4700uF Caps and 1k 2W resistors which feeds +/- 15V to one of those chinese LM1875T boards.
I bought the board, and replaced all the components with genuine parts (TI LM1875T NOPB & ELNA caps).
Hooked it up to an Up2Stream Pro bluetooth/wifi board and a small speaker in a quick enclosure to test.
It worked initially - good level of output with a clean sound. However I stupidly scoped the output wrong shorting the output to ground.
After this, on power-up, the speaker will play for 1-2 seconds and cut out - measuring the voltage across the V+/ G / V-, I find the PSU has shifted to one side:
(28V+/ G / 2V-) instead of (15V+ / G / 15V-), and this naturally stops the amplifier doing it's job.
Thinking I'd damaged the LM1875T board, I soldered up a copy, exact same components but new. Connecting this new board yields the same result.
Diving a bit deeper I found the voltage shift doesn't need an input to occur - scoping the input from the Up2Stream shows it working normally. Simply powering up the LM1875T board causes this shift.
I also found disconnecting the speaker prevents this issue from happening - with no load connected to the output, the voltage is normal (15V+ / G / 15V-) on the PSU.
Checking the speaker with another amp shows it working normally.
Lastly, I measured all components in the dual PSU and amp boards and they all seem normal.
At this point i'm really confused and was wondering if anyone has the intuition/experience to see whats going on here.
Many thanks in advance!
Adam