Denon X3800H / Classe Audio CA-5200 - Ground Loop through RCA-cables?

[Hobolicious]

After a long period of renovation I was finally able to prioritise the downstairs living room where my modest home cinema was to be set up. After all the work was performed and everything was hooked up I was ready to sit down and enjoy all the efforts. Unfortunately I stumbled across a very annoying sound from my front speakers and my centre speaker (which are connected to my power amplifier). From my research this seems to likely be a ground loop.

I will start of by listing my overall setup:

• Receiver: Denon X3800H
  • Speakers: Dali Rubicon LCR (SL, SBL, SBR, SR), Dali Phantom K-80 (TFL, TRL, TRR, TFR)
  • Subwoofers: 2 x B&W DB1 (Both connected with SVS SoundPath RCA cables)
  • Power amplifier: Classe Audio CA-5200 (Connected with 3 x SVS SoundPath RCA cables and 3.5mm minijack used for trigger)
    • Speakers: Dali Epicon 6 (FL & FR), Dali Epicon Vokal (C)
  • TV: LG 83’’ Oled C2 (HDMI)
  • Multimedia: Apple TV 4K (HDMI)
  • Gaming console: Playstation 5 (HDMI)
  • UHD-player: Panasonic UB9000 (HDMI)
  • Ethernet cable connected from a TP-Link TL-SG108

The reason why I believe this is likely a ground loop is based on the tests I have ran:

• Sound appears as soon as one (or more) RCA cables are connected between the receiver and power amp
• The sound does not seem to be affected by connecting or disconnecting different units from the receiver (TV, Apple TV etc.)
• I have tried hooking up a speaker cable between the chassis of the receiver and power amplifier (connected behind a screw on each of the units). This lowers the sound quite noticeably, but does not eliminate it
• Sound does not appear in the front and centre speakers when the receiver and power amp are not connected with each other

One very odd thing is that at random times the sound suddenly disappears (typically after quite a few hours having been powered on), but often reappears again shortly after. This is when I have had the speaker wire connected between the chassis of both units (I have not really ran the setup without the speaker wire between the units for long enough to see if the sound disappears at some point as the sound is too unbearable without the wire connected).

All units are connected to the same power outlet (which is dedicated for the setup and was recently installed by an electrician).

I am very much a novice both with HiFi setups and troubleshooting such issues, so hopefully your suggestions and experience can help me with this very frustrating issue.

 

[TLS Guy]

After a long period of renovation I was finally able to prioritise the downstairs living room where my modest home cinema was to be set up. After all the work was performed and everything was hooked up I was ready to sit down and enjoy all the efforts. Unfortunately I stumbled across a very annoying sound from my front speakers and my centre speaker (which are connected to my power amplifier). From my research this seems to likely be a ground loop.

I will start of by listing my overall setup:

• Receiver: Denon X3800H
  • Speakers: Dali Rubicon LCR (SL, SBL, SBR, SR), Dali Phantom K-80 (TFL, TRL, TRR, TFR)
  • Subwoofers: 2 x B&W DB1 (Both connected with SVS SoundPath RCA cables)
  • Power amplifier: Classe Audio CA-5200 (Connected with 3 x SVS SoundPath RCA cables and 3.5mm minijack used for trigger)
    • Speakers: Dali Epicon 6 (FL & FR), Dali Epicon Vokal (C)
  • TV: LG 83’’ Oled C2 (HDMI)
  • Multimedia: Apple TV 4K (HDMI)
  • Gaming console: Playstation 5 (HDMI)
  • UHD-player: Panasonic UB9000 (HDMI)
  • Ethernet cable connected from a TP-Link TL-SG108

The reason why I believe this is likely a ground loop is based on the tests I have ran:

• Sound appears as soon as one (or more) RCA cables are connected between the receiver and power amp
• The sound does not seem to be affected by connecting or disconnecting different units from the receiver (TV, Apple TV etc.)
• I have tried hooking up a speaker cable between the chassis of the receiver and power amplifier (connected behind a screw on each of the units). This lowers the sound quite noticeably, but does not eliminate it
• Sound does not appear in the front and centre speakers when the receiver and power amp are not connected with each other

One very odd thing is that at random times the sound suddenly disappears (typically after quite a few hours having been powered on), but often reappears again shortly after. This is when I have had the speaker wire connected between the chassis of both units (I have not really ran the setup without the speaker wire between the units for long enough to see if the sound disappears at some point as the sound is too unbearable without the wire connected).

All units are connected to the same power outlet (which is dedicated for the setup and was recently installed by an electrician).

I am very much a novice both with HiFi setups and troubleshooting such issues, so hopefully your suggestions and experience can help me with this very frustrating issue.

I suspect that the ground loop is being caused by something connected to the receiver.

Just connect the power amp to the receiver. Unplug everything else from the receiver except the power cord, and then connect up the units one by one.

The most common offenders are Internet cables and TV cable systems.

 

[Hobolicious]

I suspect that the ground loop is being caused by something connected to the receiver.

Just connect the power amp to the receiver. Unplug everything else from the receiver except the power cord, and then connect up the units one by one.

The most common offenders are Internet cables and TV cable systems.

I disconnected everything from the receiver except for power cord, speaker cable and RCA cables running between the receiver and power amplifier and the sound still persists (meaning I disconnected trigger cable towards power amp, the speaker cable connected between the chassis of the two units, ethernet cable, and HDMIs for TV, PS5, Apple TV and Panasonic UB9000, I also disconnected both RCA cables from subs just to be sure). I was unsure if I should disconnect all the speaker wires from the receiver, but I would be very surprised if they had any impact on the issue (for information I am running Klotz LY240 AWG12 speaker wires for all speakers and they have FosPower banana plugs for the connection to both the receiver and power amplifier).

After that I disconnected all the RCA cables between the receiver and power amp and the sound from listening distance went away (you can hear a faint sound if you put your ear up against the fronts and centre speaker, but I am uncertain I this is the same problematic sound I typically would hear when everything is hooked up).

With the above, do you still think it can be possible that any of the other connected units are causing the issue? Or have I now isolated the issue to the three unbalanced RCA cables running between the receiver and power amp?

It is a shame that the speaker wire that I connected between the chassis of the two units did not fully eliminate the problem (just reduce the sound, but unfortunately not enough for it to be within tolerable levels).

I feel really out of depth with the situation at this point. I am uncertain if the easiest fix is just to try three RCA Ground Loop Isolators on the three RCA cables running between the two units, however, I don't know if this would impact the overall performance of the system. This would obviously not cure the root cause.

 

[PENG]

I disconnected everything from the receiver except for power cord, speaker cable and RCA cables running between the receiver and power amplifier and the sound still persists (meaning I disconnected trigger cable towards power amp, the speaker cable connected between the chassis of the two units, ethernet cable, and HDMIs for TV, PS5, Apple TV and Panasonic UB9000, I also disconnected both RCA cables from subs just to be sure). I was unsure if I should disconnect all the speaker wires from the receiver, but I would be very surprised if they had any impact on the issue (for information I am running Klotz LY240 AWG12 speaker wires for all speakers and they have FosPower banana plugs for the connection to both the receiver and power amplifier).

After that I disconnected all the RCA cables between the receiver and power amp and the sound from listening distance went away (you can hear a faint sound if you put your ear up against the fronts and centre speaker, but I am uncertain I this is the same problematic sound I typically would hear when everything is hooked up).

With the above, do you still think it can be possible that any of the other connected units are causing the issue? Or have I now isolated the issue to the three unbalanced RCA cables running between the receiver and power amp?

It is a shame that the speaker wire that I connected between the chassis of the two units did not fully eliminate the problem (just reduce the sound, but unfortunately not enough for it to be within tolerable levels).

I feel really out of depth with the situation at this point. I am uncertain if the easiest fix is just to try three RCA Ground Loop Isolators on the three RCA cables running between the two units, however, I don't know if this would impact the overall performance of the system. This would obviously not cure the root cause.

I would try to help you troubleshoot, but need to be clear on the following:

1) You refer to "sound", what kind of sound? Is it hum, or hiss and if hum, does it sound like 60 Hz, 120 Hz, 180 Hz, or 240 Hz?

2) When you said disconnected everything except power cord, RCA cables, speaker cables, did you mean 3 pairs of speaker cables, and 3 RCA cables all connected?

3) Were the AVR-X3800H and the CA-5200 connected to the same wall outlet directly, without any other gadget such as isolating, noise suppression, UPS, power conditioners etc.?

4) If no to 3), have you tried plugging the CA-5200 to the same outlet, just for troubleshooting purposes.

5) Have you tried the process of elimination, that is, have nothing but the power cords connected, no speaker wires, no RCA interconnects, then proceed with connecting one RCA interconnect at a time, and so on, until you hear that "hum" (assuming it is a hum sound for now)? If it is caused by one or more of the RCA interconnects, then it will narrow things thing for solution.

1) is important, you need to know for sure if it is a ground loop "hum", just "sound" doesn't tell us that.

First of all, connecting a wire "behind screws" of two preamp, power amp never, or rarely work, but the myth continues on the internet. That's because without seeing the schematics and wiring diagrams of both units, it would a hit and miss thing. Thing about it simply, if that works, why wouldn't manufacturers tell you right there in their manuals and even provide such ground screw in the first place. Such chassis ground connections do exist in many old gear, mostly those that have phono preamp built in, but the vast majority don't provide such things, yet people will look for those other "screws" not really designed for the purpose. So I would suggest you don't waste time on that one, as you have tried already (yes in some cases it could be effective).

 

[Hobolicious]

I would try to help you troubleshoot, but need to be clear on the following:

1) You refer to "sound", what kind of sound? Is it hum, or hiss and if hum, does it sound like 60 Hz, 120 Hz, 180 Hz, or 240 Hz?

2) When you said disconnected everything except power cord, RCA cables, speaker cables, did you mean 3 pairs of speaker cables, and 3 RCA cables all connected?

3) Were the AVR-X3800H and the CA-5200 connected to the same wall outlet directly, without any other gadget such as isolating, noise suppression, UPS, power conditioners etc.?

4) If yes to 3), have you try to plug the CA-5200 to a the same outlet, just for troubleshooting purposes.

5) Have you tried the process of elimination, that is, have nothing but the power cords connected, no speaker wires, no RCA interconnects, then proceed with connecting one RCA interconnect at a time, and so on, until you hear that "hum" (assuming it is a hum sound for now)? If it is caused by one or more of the RCA interconnects, then it will narrow things thing for solution.

1) is important, you need to know for sure if it is a ground loop "hum", just "sound" doesn't tell us that.

First of all, connecting a wire "behind screws" of two preamp, power amp never, or rarely work, but the myth continues on the internet. That's because without seeing the schematics and wiring diagrams of both units, it would a hit and miss thing. Thing about it simply, if that works, why wouldn't manufacturers tell you right there in their manuals and even provide such ground screw in the first place. Such chassis ground connections do exist in many old gear, mostly those that have phono preamp built in, but the vast majority don't provide such things, yet people will look for those other "screws" not really designed for the purpose. So I would suggest you don't waste time on that one, as you have tried already (yes in some cases it could be effective).

Hello PENG, and thanks for the post. I will try to answer to the best of my ability:

1) As English is not my mother tongue, do you potentially have a video showcasing the difference between the "hum" and "hiss" sounds? Is there any easy way to measure the Hz of the sound? Or maybe an easy way for me to record it and upload it somewhere for you?

2) When I said disconnected I meant that they only things still left in the receiver (Denon) were 3 RCA cables running between the receiver (Denon) and power amplifier (Classe), power cord from the receiver (Denon), 8 sets of speaker cables running from the receiver (Denon), power cord from power amplifier (Classe) and 3 sets of speaker cables running from the power amplifier (Classe). As I also stated, I removed the 3 RCA cables running between the two units (Denon and Classe) to see if this eliminated the sound issue, which it did.

3) Yes, they are connected to the same wall outlet (which in turn is on a dedicated power circuit for the setup. The only things connected to this are the power amplifier, receiver, TV, PS5, Panasonic UB9000. Apple TV, TP-Link Hub and 2 x B&W DB1 subwoofers).

4) What did you mean by this? If I have tried connecting the Classe to a different power socket than the Denon (within the same power circuit)? Or potentially if I have tried connecting the Classe to a power socket of a different power circuit while still keeping the Denon in the power socket of the dedicated power circuit? I believe I tested both of these a while back without any luck, but can certainly try again if you think I should.

5) I have not removed the speaker cables, but everything else. Do you think the speaker cables could have any impact on the matter? If I were to remove all the speaker cables I assume by this you meant on the receiver, but still leaving the speaker cables running from the power amplifier to the fronts and centre (seeing this is where the sound issue is occurring)? For information I did, when I had removed everything except for power cords of the two units and the in total 11 speaker cables, try to connect RCA cables in isolation between the Denon and Classe, as well as in combination (meaning two of them and all three), and the sound seemingly was occurring and of similar level in all combinations and when single RCAs were connected alone.

 

[Hobolicious]

1) As English is not my mother tongue, do you potentially have a video showcasing the difference between the "hum" and "hiss" sounds? Is there any easy way to measure the Hz of the sound? Or maybe an easy way for me to record it and upload it somewhere for you?

I tried uploading a recording from my phone of the noise both with and without a cable between the two chassis (to hear the difference in sound level). Not perfect, but maybe it can give a better impression of the sound.

 

[Hobolicious]

I tried uploading a recording from my phone of the noise both with and without a cable between the two chassis (to hear the difference in sound level). Not perfect, but maybe it can give a better impression of the sound.

Replies on another forum seem to think this is not a ground loop going by the recorded sound. Here are parts of the messages:

"That's not the atypical sound I'd expect from a system as you usually hear a deeper hum with feedback from earthing issues.

It sounds almost like a digital sound is leaking into the system somewhere as this is a much higher frequency.

It seems that you have some electrical noise which maybe a high-pass filter isn't doing its job correctly and removing the signal"

Suggestions from another person:

"That noise isn't ground loop, it's something else.

If you connect the speakers directly to the AVR does the noise happen?
If you connect a different source to the amp does it happen? You could even use a phone, or an MP3 player, or your TV. Anything with an audio out - you might need a 3.5mm to RCA adaptor but amazon have them."

 

[TLS Guy]

Replies on another forum seem to think this is not a ground loop going by the recorded sound. Here are parts of the messages:

"That's not the atypical sound I'd expect from a system as you usually hear a deeper hum with feedback from earthing issues.

It sounds almost like a digital sound is leaking into the system somewhere as this is a much higher frequency.

It seems that you have some electrical noise which maybe a high-pass filter isn't doing its job correctly and removing the signal"

Suggestions from another person:

"That noise isn't ground loop, it's something else.

If you connect the speakers directly to the AVR does the noise happen?
If you connect a different source to the amp does it happen? You could even use a phone, or an MP3 player, or your TV. Anything with an audio out - you might need a 3.5mm to RCA adaptor but amazon have them."

I have a suspicion that the ground plane of that Classe amp is not what it should be. Unfortunately that is not that uncommon in these exotic high priced amps.

You need to disconnect the TV and everything else. If just the receiver and amp are connected via RCA cables and only the two power cords and speakers are connected then that is the problem.

One thing you might try is to connect the turntable grounding pin of the receiver to a reliable ground, as the problem might be the receiver grounding. I am pretty sure this is a ground loop, but it may be a grounding error internal to one of the units. I have run across this.

 

[PENG]

Hello PENG, and thanks for the post. I will try to answer to the best of my ability:

1) As English is not my mother tongue, do you potentially have a video showcasing the difference between the "hum" and "hiss" sounds? Is there any easy way to measure the Hz of the sound? Or maybe an easy way for me to record it and upload it somewhere for you?

I thought you have done your research, but anyway, try this one:
Mains hum - Wikipedia

Wiki included 3 audio clips, 50,60, 400 Hz. People will say 50, 60, but you will more likely hear 120 and 240 Hz.

2) When I said disconnected I meant that they only things still left in the receiver (Denon) were 3 RCA cables running between the receiver (Denon) and power amplifier (Classe), power cord from the receiver (Denon), 8 sets of speaker cables running from the receiver (Denon), power cord from power amplifier (Classe) and 3 sets of speaker cables running from the power amplifier (Classe). As I also stated, I removed the 3 RCA cables running between the two units (Denon and Classe) to see if this eliminated the sound issue, which it did.

But my question the is, did you try to connect them back one at a time, it is possible that only one is causing the trouble, so this is a troubleshooting step.

3) Yes, they are connected to the same wall outlet (which in turn is on a dedicated power circuit for the setup. The only things connected to this are the power amplifier, receiver, TV, PS5, Panasonic UB9000. Apple TV, TP-Link Hub and 2 x B&W DB1 subwoofers).

If it is easy enough to try a different outlet, it would be good to do, just in case the outlet may be wired wrong.

4) What did you mean by this? If I have tried connecting the Classe to a different power socket than the Denon (within the same power circuit)? Or potentially if I have tried connecting the Classe to a power socket of a different power circuit while still keeping the Denon in the power socket of the dedicated power circuit? I believe I tested both of these a while back without any luck, but can certainly try again if you think I should.

Sorry I had a typo, fixed now.

5) I have not removed the speaker cables, but everything else. Do you think the speaker cables could have any impact on the matter? If I were to remove all the speaker cables I assume by this you meant on the receiver, but still leaving the speaker cables running from the power amplifier to the fronts and centre (seeing this is where the sound issue is occurring)? For information I did, when I had removed everything except for power cords of the two units and the in total 11 speaker cables, try to connect RCA cables in isolation between the Denon and Classe, as well as in combination (meaning two of them and all three), and the sound seemingly was occurring and of similar level in all combinations and when single RCAs were connected alone.

Not likely, but you have already found the noise was caused by 1, or 2 or 3 or the RCA interconnect anyway so you can focus on finding out why, and the fix.

One quick thing to try is to use a RCA to XLR cable, no they are not the same as using XLR to XLR, but if you use the right cable that are wired correctly, it will help, may be enough to fix the noise issue if it is in fact caused by ground loop. Examples of the RCA to XLR that you can try are the following:

Benchmark RCA to XLRM Adapter Cable for Analog Audio - pin 3 to RCA sh - Benchmark Media Systems

or for the cheap ones:

Monoprice 6ft Premier Series XLR Male to RCA Male Cable, 16AWG (Gold Plated) - Monoprice.com

the connection may not be as good as Benchmark's but I know they do wire the pins 1 and 3 correctly.

 

[Hobolicious]

I have a suspicion that the ground plane of that Classe amp is not what it should be. Unfortunately that is not that uncommon in these exotic high priced amps.

You need to disconnect the TV and everything else. If just the receiver and amp are connected via RCA cables and only the two power cords and speakers are connected then that is the problem.

One thing you might try is to connect the turntable grounding pin of the receiver to a reliable ground, as the problem might be the receiver grounding. I am pretty sure this is a ground loop, but it may be a grounding error internal to one of the units. I have run across this.

What would typically be deemed as «a reliable ground» to connect to from the receiver?

And you still believe this to be a ground loop issue after listening to the recorded sounds I added above?

 

[PENG]

What would typically be deemed as «a reliable ground» to connect to from the receiver?

And you still believe this to be a ground loop issue after listening to the recorded sounds I added above?

That's ground loop caused noise. Try the Monoprice cable I linked and see if that helps. If not, return it, or keep it anyway as it may still do better than using RCAs, and then try one of those signal isolators.

It is also possible something has gone wrong with the power amp. How's sound quality anyway aside from the noise.

 

[highfigh]

What does "the speaker cable connected between the chassis of the two units" mean?

 

[PENG]

About the frequency being higher than 50 or 60 Hz:

Guitarcenter has an useful article on this exact point:
How to Fix Hum, Buzz and Other Noise in Your Audio Cables | GC Riffs (guitarcenter.com)

Types of Noise

Strictly speaking, pretty much all that buzzing, chirping and clicking that finds its way into your audio falls under the general category of EMI (Electro-Magnetic Interference). This is generally broken down into the sub-categories of 60Hz hum, RFI (Radio-Frequency Interference, which is just EMI where the source lies within the radio frequency range) with everything that doesn't fall into those two buckets considered just general EMI. It's all annoying and sometimes difficult to track down.

60Hz hum isn't always at 60Hz, but can occur at varying harmonics of that frequency. Which is a good thing, because it can help you determine the source and cause of the problem. If the hum is actually at 60Hz, it's likely due to faulty cables, poor shielding or an audio cable being close to a strong magnetic field, like a transformer-based power supply. If the hum is an octave higher, at 120Hz, or at one of the higher harmonics of 60Hz, it's almost always some sort of ground loop problem.

The clip you recorded sound more like 200 Hz, are you in Europe, or other countries that has 50 Hz mains?

 

[TLS Guy]

Replies on another forum seem to think this is not a ground loop going by the recorded sound. Here are parts of the messages:

"That's not the atypical sound I'd expect from a system as you usually hear a deeper hum with feedback from earthing issues.

It sounds almost like a digital sound is leaking into the system somewhere as this is a much higher frequency.

It seems that you have some electrical noise which maybe a high-pass filter isn't doing its job correctly and removing the signal"

Suggestions from another person:

"That noise isn't ground loop, it's something else.

If you connect the speakers directly to the AVR does the noise happen?
If you connect a different source to the amp does it happen? You could even use a phone, or an MP3 player, or your TV. Anything with an audio out - you might need a 3.5mm to RCA adaptor but amazon have them."

It is a ground loop. I should have asked you before. What country are you in? Which country did you source your equipment from? It is possible that one or both of your units are not set up correctly for your country, especially if you are in one that does not have neutral.

 

[TLS Guy]

This thread reinforces the fact, that the country of residence should be listed below the membership name on every post. This should have happened long ago.

 

[Hobolicious]

About the frequency being higher than 50 or 60 Hz:

Guitarcenter has an useful article on this exact point:
How to Fix Hum, Buzz and Other Noise in Your Audio Cables | GC Riffs (guitarcenter.com)



The clip you recorded sound more like 200 Hz, are you in Europe, or other countries that has 50 Hz mains?

It is a ground loop. I should have asked you before. What country are you in? Which country did you source your equipment from? It is possible that one or both of your units are not set up correctly for your country, especially if you are in one that does not have neutral.

Sorry for not adding that piece of information as I initially planned to do so. I am indeed located in Europe (more specifically Norway).

As for equipement, I bought the Denon directly from a local HiFi store that are located across the Nordics (called HiFi Klubben), and the Classe was bought second hand, but also originally bought from the same chain (HiFi Klubben).

 

[Hobolicious]

What does "the speaker cable connected between the chassis of the two units" mean?

Copper of the speaker wire is fastened behind a screw on each of the chassis of the two units (to touch bare metal). This was to see if the Classe would find ground through the speaker wire instead of seemingly doing so through the unbalanced RCA cable(s).

 

[highfigh]

Copper of the speaker wire is fastened behind a screw on each of the chassis of the two units (to touch bare metal). This was to see if the Classe would find ground through the speaker wire instead of seemingly doing so through the unbalanced RCA cable(s).

Do you have a multimeter? Measure the resistance between each piece of equipment. If you're powering these from different outlets, don't. Use the same outlet and a power strip- doesn't need to filter the power, it just needs to come from the same outlet to make sure there's as little resistance as possible on any power cord connection.

If you're using a subwoofer, disconnect that, too. If the hum only shows up when that's connected, try connecting it to the same outlet as the AVR and power amp- if it stops, use a different outlet for the sub or reassign that outlet in the breaker panel.

Are your outlets grounded, or ungrounded?

 

[Hobolicious]

That's ground loop caused noise. Try the Monoprice cable I linked and see if that helps. If not, return it, or keep it anyway as it may still do better than using RCAs, and then try one of those signal isolators.

It is also possible something has gone wrong with the power amp. How's sound quality anyway aside from the noise.

From what I can hear the sound quality seems fine (except for the obvious issue with the troublesome noise).

In regard to the Monoprice they don't seem to be available locally (mainly seem to be available in the US?). I did find the Benchmark cables in a store, but it is obviously a hefty price for cables. They are about 80 USD per 6ft cable. I am also a bit uncertain if 6ft is just a tad short (7ft would probably be better as a minimum), but might be just enough.

Do you know if there are any decent brands typically available in Europe (either Norway or UK, as I am heading over there in a months time)? I assume RCA Female to XML Male adapters are a no-go (and would not replace my current RCA cables if you believe they might be causing the issue)?

From a very quick search locally I find for example (outside of insanely expensive cables):
- Cordial CFU 3 MC
- Roland RCC-10-RCXM
- Adam Hall K3TM
- There also seem to be some Klotz RCA to XLR (Supreme/Superior or AT-CM0300) cables available

 

[Hobolicious]

Do you have a multimeter? Measure the resistance between each piece of equipment. If you're powering these from different outlets, don't. Use the same outlet and a power strip- doesn't need to filter the power, it just needs to come from the same outlet to make sure there's as little resistance as possible on any power cord connection.

If you're using a subwoofer, disconnect that, too. If the hum only shows up when that's connected, try connecting it to the same outlet as the AVR and power amp- if it stops, use a different outlet for the sub or reassign that outlet in the breaker panel.

Are your outlets grounded, or ungrounded?

I don't have a multimeter available, but potentially my father-in-law might. They are powered from the same outlet.

As for the subwoofers, I did disconnect the RCA cables towards the receiver from both subs with the problematic sound still persisting. I did however forget to remove the power cords from the power circuit (the subs are connected to two different outlets than the one the receiver and power amp are connected to. However, all of the outlets are connected to the same dedicated power circuit for the HiFi setup).

All outlets are newly installed and are grounded.