120Hz hum in Polk Audio PSW202 or Rm6750 subwoofer
wcelsiuss
Posts: 1
For those interested, although these products and threads are old, I picked a RM6750 subwoofer from junk store - it looked like new and worked - so I was wondering why it was sent there... It had an annoying AC hum when powered - even when nothing was connected to it and volume knobs were at minimum volume. While it might not be a problem at high volumes, it was quite audible in silence and it is not acceptable in HiFi setups. I guess that its owner was annoyed and looked for other device...
It appeared that all Polk powered subwoofers share the same power amplifier board - shown below - and this board has a design defect which results in ~10mV 120Hz AC hum.
https://electronicspoint.com/forums/threads/120hz-hum-in-polk-audio-psw202-or-rm6750-subwoofer.296240/
I posted detailed description and a fix here: https://www.electronicspoint.com/forums/threads/120hz-hum-in-polk-audio-psw202-or-rm6750-subwoofer.296240/
The cause of it is incorrect board layout in the power section - there is a ground loop between common (zeroV) point of power capacitors and the ground points in the circuit (black wire to speaker).
The ground is connected by a thin wire link (JP2?) shown in the picture, and a second link to input amplifier (TDA7294 or SK...) common net and the chassis ground. As the result, ripple voltage drop on capacitor connection is applied to the input of the power amplifier. This also will result in the additional distortion in the amplifier, but distortion in subwoofers is not clearly audible (driver speaker has higher distortion anyway). This is a common mistake of un-experienced electronics engineers.
What we want to achieve is connecting the common if filter capacitors to common ground where negative feedback of TDA7294 (black wire) first, and then have this new common ground point be routed to other noise-sensitive nodes in a star-like topology.
After the modifications, the hum is not heard anymore, and I can use this box in HiFi system.
It appeared that all Polk powered subwoofers share the same power amplifier board - shown below - and this board has a design defect which results in ~10mV 120Hz AC hum.
https://electronicspoint.com/forums/threads/120hz-hum-in-polk-audio-psw202-or-rm6750-subwoofer.296240/
I posted detailed description and a fix here: https://www.electronicspoint.com/forums/threads/120hz-hum-in-polk-audio-psw202-or-rm6750-subwoofer.296240/
The cause of it is incorrect board layout in the power section - there is a ground loop between common (zeroV) point of power capacitors and the ground points in the circuit (black wire to speaker).
The ground is connected by a thin wire link (JP2?) shown in the picture, and a second link to input amplifier (TDA7294 or SK...) common net and the chassis ground. As the result, ripple voltage drop on capacitor connection is applied to the input of the power amplifier. This also will result in the additional distortion in the amplifier, but distortion in subwoofers is not clearly audible (driver speaker has higher distortion anyway). This is a common mistake of un-experienced electronics engineers.
What we want to achieve is connecting the common if filter capacitors to common ground where negative feedback of TDA7294 (black wire) first, and then have this new common ground point be routed to other noise-sensitive nodes in a star-like topology.
After the modifications, the hum is not heard anymore, and I can use this box in HiFi system.
Comments
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Excellent diagnosis and solution presented. Thank you for documenting this. I have a couple of RT2000 subwoofer amp boards with a hum so I will keep this in mind when diagnosing in the next couple of weeks. If push comes to shove, I will probably stick with step one (provided that's the issue) and see how it sounds.
I have just started repairing audio gear and have much to learn.
Cheers!Life is too short to be indirect.
