- High pitch noise independent of speaker volume
- Only happens in Bluetooth, PC and AUX modes.
- Temperature dependent. As the unit warms up (after 10 min), the noise gets lower but never disappears. The colder the room, the worst.
I captured this picture with a spectrum analyser app:
You can see, it detected a peak around 4.4 kHz. This peak starts with higher frequency and amplitude when the unit is cold. As it warms up, it rolls down until maybe 1 or 2 kHz.
I investigated the problem. The sound amplification electronics are in the subwoofer box. It seemed like there was a problem with a switching power supply that steps down 20V from the main PSU to 5V for other parts of the circuitry. This noisy supply is controlled by a AOZ1210 IC. If one blows or apply heat to this IC, the noise frequency changes very noticeably.
I observed with an oscilloscope the supply output but no noise was being injected. This made me think, the noise wasn't being produced at the output but could be inductively coupled to other part of the PCB.
After this I decided to return the speakers, since, this was a manufacturer problem, not mine.
The replacement
The replacement speakers luckily worked fine. There was no noise at all.
As for the serial numbers, this new unit was manufactured actually before the defective one.
Defective unit SN: 441405044429
New unit SN: 441405042325
I try to visually inspect the PCBs of both units but couldn't notice any differences. The PCB revision was the exact same one, so no layout changes. The codes written in the AOZ1210 package were also the same ones.
The defective:
The new:
(The IC and the supply circuitry are located in the bottom right corner, next to the antenna).
The remote
Just as a remark, in my first unit the remote needed to be pointed exactly to the receiver in the speaker. It was very tricky but now I feel it's better. (BTW, the battery was more than OK).
Bottom line
I couldn't really tell what was the reason of the problem. but if you are in the same situation as I was, return them.