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120Hz hum in Polk Audio PSW202 or Rm6750 subwoofer

wcelsiuss

Dec 5, 2021
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Dec 5, 2021
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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.

polk_subwoofer1.JPG

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.

polk_subwoofer2.JPG

There is an easy fix in two steps (someone may find that first step is sufficient).
= First, solder a thick (~16-14AWG) stranded link between the common connection between the large 4700uF capacitors and the point where black speaker wire is soldered. The wire is on the green side of PCB. Ideally, it can be from the point of a pin of the capacitor with "QC PASS" label, and the wire must be as short and as straight as possible. This wire is in parallel with JP2? and reduces the resistance of this link. This will bring the hum down by ~16dB.
= Second, cut the wide trace from the common connection of capacitors to ground bracket and to other components - this trace goes from lower capacitor to the lower edge of the board. Solder a new wire (use ~22-26 AWG wire) from this bracket to the point where black wire is soldered.

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.

Unfortunately, I closed the box and did not take a picture of rework. After the modifications, the hum is not heard anymore, and I can use this box in HiFi system. I am marking it on the picture I found on Internet.
polk_subwoofer3.JPG
On the side note: The subwoofer consumes ~7 watts when it is in standby mode (LED is red). It is not great for your budget. It is desirable to switch off the AC power with common switch or connect it to a switched power outlet on the receiver to save power.
 

finkproductions

Dec 29, 2022
5
Joined
Dec 29, 2022
Messages
5
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.

View attachment 53492

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.

View attachment 53493

There is an easy fix in two steps (someone may find that first step is sufficient).
= First, solder a thick (~16-14AWG) stranded link between the common connection between the large 4700uF capacitors and the point where black speaker wire is soldered. The wire is on the green side of PCB. Ideally, it can be from the point of a pin of the capacitor with "QC PASS" label, and the wire must be as short and as straight as possible. This wire is in parallel with JP2? and reduces the resistance of this link. This will bring the hum down by ~16dB.
= Second, cut the wide trace from the common connection of capacitors to ground bracket and to other components - this trace goes from lower capacitor to the lower edge of the board. Solder a new wire (use ~22-26 AWG wire) from this bracket to the point where black wire is soldered.

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.

Unfortunately, I closed the box and did not take a picture of rework. After the modifications, the hum is not heard anymore, and I can use this box in HiFi system. I am marking it on the picture I found on Internet.
View attachment 53494
On the side note: The subwoofer consumes ~7 watts when it is in standby mode (LED is red). It is not great for your budget. It is desirable to switch off the AC power with common switch or connect it to a switched power outlet on the receiver to save power.
I don't know if it's the same with my polk audio psw10 but I have two of them and they hum even without anything plugged to it. I'll have to do what you did. But I'm a noob and I don't want to mess that up. I think I understand but can you show me more precisely what I have to do? I have to solde a wire between two points. But in one of your picture I see only one soldure. Thank you very much in advance! The hum is so annoying you're very kind to propose a solution!
 

wcelsiuss

Dec 5, 2021
4
Joined
Dec 5, 2021
Messages
4
I don't know if it's the same with my polk audio psw10 but I have two of them and they hum even without anything plugged to it. I'll have to do what you did. But I'm a noob and I don't want to mess that up. I think I understand but can you show me more precisely what I have to do? I have to solde a wire between two points. But in one of your picture I see only one soldure. Thank you very much in advance! The hum is so annoying you're very kind to propose a solution!
Unfortunately I don't have better pictures, but here is the explanation of the problem and solution.
Originally, the "common ground" is connected to the conductor shared with center tap of the power transformer before it reaches the power capacitors. Because of that, ripple current flowing through this segment from transformer to capacitors C1 and C2, causes voltage drop on it, and this small hum AC voltage is injected in feedback loop of the power amplifier. It is amplified and applied to the speaker, causing audible hum. The gain of the amplifier is around ~10, so the hum is quite audible. It looks as shown in schematic. I am not showing the values of R1 and R2, but Gain=R2/R1 in this setup.
Schematics_Original.JPG



The goal is to rewire the rectifier to let the center tap first reach connection between capacitors C1 and C2, and then connect feedback to 0V without sharing wire/PCB trace segment. Thus, the hum will not be injected into feedback loop, and hum will disappear. The diagram below shows it. This is a design mistake and is common for junior engineers.
Schematics_Fixed.JPG



This is the "star topology" fix which may be sufficient for most.

There was also another problem: the aluminum front panel is also ground terminal for the audio source, and it is also connected to this noisy shared segment. The noise will be applied to audio input, but because the signal amplitude there is high, it is not too noticeable. The second fix is to disconnect the ground wire from the original place and connect it with a wire to the center of the "star" - which actually looks like a star on PCB. This would reduce additional hum which comes from input connectors. The second wire mentioned in original post does that.

Note also that in your case the hum may be caused by other reasons, especially if it appeared recently and was not there originally. The most common case are dry capacitors C1 and C2 - with aging, electrolytic capacitors lose their capacitance and may require replacement with fresh ones. You may need a capacitance meter to confirm that, or ask somebody skilled to check it for you...
 
Last edited:

finkproductions

Dec 29, 2022
5
Joined
Dec 29, 2022
Messages
5
Thank you very much! I think I'll give it a try an study your explanation. I don't want to blow anything! I have two of them and the hum is very noticable and annoying. Thanks again, it's very appreciated. I will write back if it work!
 

wcelsiuss

Dec 5, 2021
4
Joined
Dec 5, 2021
Messages
4
Good luck, make sure you understand what you are doing! I have no idea about the design of your model, it may be different. As I said, if the hum is very noticeable and it was not there before, the most probable reason are bad (dry) power capacitors (there big cylinders). I was surprised by the fact that their voltage rating were lower than the actual voltage on them (which is yet another mistake of the design). If they are 10 or more years old, it may be better to order a couple new capacitors on Amazon or eBay and replace them as a starting point.
PS: observe polarity signs when you are soldering new ones.
 

finkproductions

Dec 29, 2022
5
Joined
Dec 29, 2022
Messages
5
Thank you! I'll check the capacitor too.. it's weird I only have your awser in my e-mail but can't see it here with good quality diagrams. Like it's gone. Weird, at least I have the e-mail! Thank you!
 

finkproductions

Dec 29, 2022
5
Joined
Dec 29, 2022
Messages
5
Unfortunately I don't have better pictures, but here is the explanation of the problem and solution.
Originally, the "common ground" is connected to the conductor shared with center tap of the power transformer before it reaches the power capacitors. Because of that, ripple current flowing through this segment from transformer to capacitors C1 and C2, causes voltage drop on it, and this small hum AC voltage is injected in feedback loop of the power amplifier. It is amplified and applied to the speaker, causing audible hum. The gain of the amplifier is around ~10, so the hum is quite audible. It looks as shown in schematic. I am not showing the values of R1 and R2, but Gain=R2/R1 in this setup.
View attachment 57725



The goal is to rewire the rectifier to let the center tap first reach connection between capacitors C1 and C2, and then connect feedback to 0V without sharing wire/PCB trace segment. Thus, the hum will not be injected into feedback loop, and hum will disappear. The diagram below shows it. This is a design mistake and is common for junior engineers.
View attachment 57726



This is the "star topology" fix which may be sufficient for most.

There was also another problem: the aluminum front panel is also ground terminal for the audio source, and it is also connected to this noisy shared segment. The noise will be applied to audio input, but because the signal amplitude there is high, it is not too noticeable. The second fix is to disconnect the ground wire from the original place and connect it with a wire to the center of the "star" - which actually looks like a star on PCB. This would reduce additional hum which comes from input connectors. The second wire mentioned in original post does that.

Note also that in your case the hum may be caused by other reasons, especially if it appeared recently and was not there originally. The most common case are dry capacitors C1 and C2 - with aging, electrolytic capacitors lose their capacitance and may require replacement with fresh ones. You may need a capacitance meter to confirm that, or ask somebody skilled to check it for you...
I open my amp just now and I think it's the same has you.. but like I said
Unfortunately I don't have better pictures, but here is the explanation of the problem and solution.
Originally, the "common ground" is connected to the conductor shared with center tap of the power transformer before it reaches the power capacitors. Because of that, ripple current flowing through this segment from transformer to capacitors C1 and C2, causes voltage drop on it, and this small hum AC voltage is injected in feedback loop of the power amplifier. It is amplified and applied to the speaker, causing audible hum. The gain of the amplifier is around ~10, so the hum is quite audible. It looks as shown in schematic. I am not showing the values of R1 and R2, but Gain=R2/R1 in this setup.
View attachment 57725



The goal is to rewire the rectifier to let the center tap first reach connection between capacitors C1 and C2, and then connect feedback to 0V without sharing wire/PCB trace segment. Thus, the hum will not be injected into feedback loop, and hum will disappear. The diagram below shows it. This is a design mistake and is common for junior engineers.
View attachment 57726



This is the "star topology" fix which may be sufficient for most.

There was also another problem: the aluminum front panel is also ground terminal for the audio source, and it is also connected to this noisy shared segment. The noise will be applied to audio input, but because the signal amplitude there is high, it is not too noticeable. The second fix is to disconnect the ground wire from the original place and connect it with a wire to the center of the "star" - which actually looks like a star on PCB. This would reduce additional hum which comes from input connectors. The second wire mentioned in original post does that.

Note also that in your case the hum may be caused by other reasons, especially if it appeared recently and was not there originally. The most common case are dry capacitors C1 and C2 - with aging, electrolytic capacitors lose their capacitance and may require replacement with fresh ones. You may need a capacitance meter to confirm that, or ask somebody skilled to check it for you...
I open the thing.. I read your answer on repeat and I still don't know whtlat to do. Amp looks like the same has you. I have to sold a wire between where and where? Sorry I'm a noob.. I wanted to study in electronic when I was young.. but I'm a soundman finally and the hum of my two polkaudio annoy me..
 

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wcelsiuss

Dec 5, 2021
4
Joined
Dec 5, 2021
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Yes, the first fix is to make this black link (as you show) with the relatively thick wire (~1mm dia). The point which looks as a "star" is actually noise-less ground point (as shown in the schematics). Make sure that it is isolated wire, so you do not short any other traces.
The second fix is to cut the trace segment from the lower capacitor (on your picture) going to the metal bracket for the screw at the edge of the board and making a new connection of this bracket to the "star" with another wire link. The latter link may be any ~0.4-0.8mm wire. As you see you really only rewire the connections, changing the path through which the ripple current flows.
Note that this fixes annoying hum (which was not very loud before). If you have really loud hum, you may need to check these big capacitors. By the way, I see that your capacitors are 4700uF 50V (mine was 25V - and the voltage there is ~30V). You can have a look here:
for some info, it is easy to check with oscilloscope, but I guess that you don't have any capacitance meters. If you have AC voltmeter, you can try to measure AC voltage between capacitor pins. If you see that it is greater than ~1V AC, the corresponding capacitor may need a replacement. Good luck.


Links_both..JPG
 

BrokenEmpire

Mar 13, 2023
1
Joined
Mar 13, 2023
Messages
1
I realize this is an older thread, however, I just wanted to send a thank you to Wcelsiuss. I recently performed this modification to my RM6750 sub and it worked perfectly. Thank you very much, it's so much nicer without that annoying hum.
 

TUATARA2

Mar 11, 2024
1
Joined
Mar 11, 2024
Messages
1
Yes - Just did the stated mod to my PSW202.
Cleared up a hum - still a pretty sloggy and dull subwoofer
On board amp seems ok
The two main power caps are off-brand 4700uF 50v and read : C15 - 4402uF & C14 - 4422uF.
Within the 20% tolerance range but kinda weak

Thinking of changing them but wondering if it would make a difference?
Any body had success with that?
 
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