Sony SRS-D25 subwoofer – fix for hum/buzz
First created 19 January 2022.
I've owned this Sony 2.1 subwoofer PC speaker system since 2007. In recent years, it had developed a background mains hum. Adjusting the volume control does not affect the volume of the hum.
It was likely to be a bad capacitor issue given its age.
The main unit was easy to dismantle by first removing the 6 recessed screws. The front panel can then be separated. Remember to unplug the blue ended ribbon cable to improve access.
The power amplifier board is clearly visible. Visual inspection does not reveal any bulging or leaking electrolytic capacitors.
Unfortunately, the board cannot be removed because the audio leads passing through the rear of the case are hot glued in place !
I didn't like the idea of trying to remove the glue or cut all the audio leads to extract the power amplifier board.
The 11.4v AC secondary winding from the transformer is connected to the circuit board shown below. I measured 13.5v AC across the blue wires into the PCB. The bridge rectifier visible next to the 2 pin socket suggests the board requires a regulated 16-19v DC supply.
I connected a bench power supply to the PCB and the hum immediately disappeared. The SRS-D25 was able to operate down to 11v DC. It cut out just above 10v DC.
I decided to bypass the existing transformer and simply power the SRS-D25 from a new external DC power adapter. I didn't have any spare 15-20v DC power adapter, so I used an 'off the shelf' 12v @ 2A switched mode power adapter.
I cut the primary and secondary leads off the transformer and joined them together. Then installed the modified lead into the SRS-D25.
Finally, I cut the AC mains plug off the end of the cable and fitted a female barrel connector for the new external 12v DC power adapter.
At normal listening volume levels, audio seems fine. When I wind the volume up to unacceptable levels, I haven't noticed any audio 'clipping'.
I also measured the DC voltage across the main subwoofer unit. At 18v, I measured 50-60mV. For 12v, there was only 32-40mV. ie. Negligible DC bias.
Some notebook 19v power adapters from around 10 years ago didn't have a 3rd 'sense/data pin. I may try and find a discarded one to use with the SRS-D25. Toshiba used 19v power adapters with regular 2.5 x 5.5mm barrel plugs. In the mean time, the 12v power adapter seems to work just fine.
I've also observed when the SRS-D25 is plugged into my Dell laptop, there is a very high pitch buzz or chirp when the volume control on the SRS-D25 is at Maximum setting. There is no noise when the audio cable is unplugged from the laptop.
I tested 5 different 12v power adapters – 3x 2Amp plug-in-wall, and 2x 5Amp brick style. Three adapters produced a noise. For one particular plug-in-wall adapter, the noise got louder when I placed my hand on it or if I brought the adapter closer to the laptop. Two other plug-in-wall adapters were silent.
I have discovered this video which shows the PCB can be partially removed.
Non-English Youtube video showing inside of SRS-D25
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