two humming audio amps mystery (solved)

[g.costanza]

UPDATE...
I finally broke down and hacked an extension cord and reversed the wires. I plugged the subs' power cords into the reversed polarity extension cord. Result = the voltage coming off of the subwoofer plate amps went from 118vac to 1vac and no more hum! The wall outlet IS WIRED CORRECTLY. I even went out earlier and bought one of those fluke non contact voltage testers. That, one of those cheap 3-light outlet testers, and a DMM showed outlet was wired correctly. I also measured the voltage coming off of the chassis of the denon in both powered polarities and a Sony receiver I had lying around. The Denon measured around 50-60vac in both polarity configurations. Oh well . The Sony was 1vac in factory polarity and 70vac in reversed polarity. I'm now curious what other Denon x3300 receivers measure???
Thanks everyone for your help!


ORIGINAL POST>>>

Wondered if u guys could help with these humming subwoofer amplifiers that apparently doesn't know the words
I'll stick to the facts I know...

I bought two new JAMO powered J110 subwoofers that are near clones of the Klipsch R110/112/115 family. Same plate amps between JAMO and Klipsch versions depending on woofer size. They have 2 prong power plugs.

Denon Receiver has a 3 prong power plug.

I previously had a pair of DIY subwoofers on a single parts express BASH amp (3 prong plug) in the same spot as these new subs. Zero noise issues.

Using same single RCA coax cable connection on new subs as old setup.

JAMO subs do not hum until I connect the RCA cable going to the Denon.

JAMO subs also do not hum if I connect the headphone out jack of my iphone using a 3.5mm to RCA plug adapter.

I moved Denon and connected it to same wall outlet as the subs. No help. Still Hums.

I put a ground loop isolator on the RCA audio connection. No help. Still hums.

I tried a different RCA cable. Still hums.

I don't have "cable TV", so that can't b a hum source.

I used a FLUKE meter to check wall outlet. I get 120v from narrow HOT slot to wide slot, ground hole, and center screw. Normal.
I get 0v from wide COMMON/NEUTRAL slot to ground hole and center screw. Normal.

I used continuity setting of FLUKE and got "tone" between outer/ground ring of sub amp's RCA audio inputs JACKS to the screws, etc on the EXPOSED metal plate amps on subs.

I was then surprised to find that if I touched the screws, jacks, etc of the metal plate amp with a probe of the fluke on V~ setting, and the center ground screw on the wall outlet, I got 118v. Is that Normal? Touching the metal plate amp doesn't result in a shock. I didn't try touching the center wall outlet screw with my other hand, lol. Not sure what would happen.

The hum does increase or decrease depending on subwoofer amp volume knob setting, if that matters. When the RCA plugs are removed from the amps, I get no noise, regardless of volume setting.

Any ideas?

[Armadillo]

Plug your RCA into the amp and then short with a copper wire, the other end of the Pin and the shell. Does the hum disappear? Wriggle the RCA cable about, does the hum appear?

[g.costanza]

Just to b clear, plug rca cable into sub amp. On other end of cable, connect center pin and outer ring together? I do know already if i touch the tip of the end of the rca cable plugged into sub amp, I get a very very loud hum.

[Armadillo]

Just to b clear, plug rca cable into sub amp. On other end of cable, connect center pin and outer ring together? I do know already if i touch the tip of the end of the rca cable plugged into sub amp, I get a very very loud hum.

Yes, connect center pin to outer ring together with a jumper cable. The purpose is to detect the location of the problem. Do it I will tell you.
You hand is a signal source my friend, induced from environment static, RF etc... Please don't touch with your hand you may damage your speakers.

[g.costanza]

Ok. What u r suggesting is confusing me. I stated earlier that I get no hum from the sub amp at all unless the RCA is connected to the receiver. I do get a very very loud hum if I place my finger on the tip of the rca cable that is no longer connected to the receiver. If I slip my finger down and also touch the outer ring as well as the tip, the very very loud hum is not as loud.

[g.costanza]

I will try the wire method and report back

[Armadillo]

I will try the wire method and report back

Don't forget to wriggle the cable [while the jumper wire is connected] to detect hidden breakage. Hope the cable is good.

[technogeeky]

Detecting something like ~115 volts on the case or mounting hardware of a subwoofer may or may not be indicative of a problem. If the voltage goes away when you disconnect all of the audio inputs (in other words, a 115 volt but very low current signal is being carried along the audio wire), this is certainly a problem.

It would be a huge help to have an oscilloscope to make more definitive statement about the source of the hum (although you might be able to use the frequency detection of a multimeter).

I just repaired two different subwoofers. In my case, both had hum from mains interference. Both were ultimately caused by bad soldering joints.

One of them, on a very old Bose 3-2-1 subwoofer, was on the amplifier's power/ground pins where they connect to the PCB. Mechanical stress due to the amplifier chip being clipped to a large heatsink with the pins soldered to a PCB broke them free. Then, either the noise was capacitively or inductively coupled into the amplifier.

On the other subwoofer, the Phase 0° - 180° switch was found to cause the hum. There were no signs of obviously bad soldering, but when rubbing the solder pins with a plastic probe, it was easy to cause a signal which had large 60hz spikes and 20-30mV p-p of other noise.

I only mention this because there are a lot of mechanical ways for a subwoofer to damage itself, so it's important to keep an eye out for this. Having said that: for me, in both cases the hum was not increased with higher subwoofer gain (unlike your case). The hum was present with no signal input at all (unlike your case). It was a 120Hz (rectified AC, plus harmonics) hum for the Bose, and a 60Hz hum for the other subwoofer (with no harmonics).

Three facts in your case lead me to believe the problem can't be with the subwoofers:

• the hum does change volume with subwoofer amplifier gain
• the hum is absent at all gain levels when no input signal is present
• both subwoofers are new (are they?, and they both have this problem

To me, and assuming that wiggling the RCA cable and connectors around does not cause the hum to appear or disappear (meaning a mechanical->electrical problem at the inputs of the subwoofer), then the problem is surely with the receiver. The primary counter-evidence to this is that your older, 3-prong-plugged subwoofers did not have this problem. Again, to me, this just implies that your old subwoofers hid this problem by tying both devices to the same ground reference.

Some tests I would run:

• With only power and nothing else connected to the receiver, powered on, with the volume as low as possible but not zero/muted, check the audio output (RCA jack?) of various outputs (left/right analog audio, subwoofer pre-out, etc) for voltage and check for frequency with the multimeter. If there is no signal into the receiver and you see 50/60Hz (or 100/120Hz) signal output, then you can confirm it's the receiver. Where exactly and how? Who knows.
• If the above multimeter frequency check fails, increase the volume little by little until you either see a 50/60/100/120 Hz detection or no detection and you are sure it's loud enough to have heard it.
• If that check fails, hook up a function generator or a computer and generate a ~ 70 or 80 Hz tone, and make sure you can detect this with the frequency and possibly voltage settings of the multimeter.

If the problem does prove to be the receiver, it's probably going to be difficult to diagnose and fix without an oscilloscope. If you think it is the receiver, please tell us the model number.

[g.costanza]

OK.

With nothing plugged into sub I get zero hum at all sub amp volume levels.

I use a small 6" Y adapter to connect single rca to both left and right sub rca inputs. Plugging only it in to sub rca inputs, I can get the very slightest hum with sub amp volume at max. This is with my ear up to woofer cone in a dead silent room.

If I then attach the longer rca cable that would normally go to the receiver, but is left unconnected, the hum is ever so slightly louder at full sub volume, but still very low and nowhere near as loud as when the cable is connected to the denon audio receiver.

If I attach a jumper between the center pin and outer ring on the loose end of the rca cable, I'm back to having zero hum with sub amp volume at maximum.

[Armadillo]

OK.

With nothing plugged into sub I get zero hum at all sub amp volume levels.

I use a small 6" Y adapter to connect single rca to both left and right sub rca inputs. Plugging only it in to sub rca inputs, I can get the very slightest hum with sub amp volume at max. This is with my ear up to woofer cone in a dead silent room.

If I then attach the longer rca cable that would normally go to the receiver, but is left unconnected, the hum is ever so slightly louder at full sub volume, but still very low and nowhere near as loud as when the cable is connected to the denon audio receiver.

If I attach a jumper between the center pin and outer ring on the loose end of the rca cable, I'm back to having zero hum with sub amp volume at maximum.

And you forgotten to wriggle that cable while the ends is connected. !

[technogeeky]

OK.

With nothing plugged into sub I get zero hum at all sub amp volume levels.

I use a small 6" Y adapter to connect single rca to both left and right sub rca inputs. Plugging only it in to sub rca inputs, I can get the very slightest hum with sub amp volume at max. This is with my ear up to woofer cone in a dead silent room.

If I then attach the longer rca cable that would normally go to the receiver, but is left unconnected, the hum is ever so slightly louder at full sub volume, but still very low and nowhere near as loud as when the cable is connected to the denon audio receiver.

If I attach a jumper between the center pin and outer ring on the loose end of the rca cable, I'm back to having zero hum with sub amp volume at maximum.

All of these sound like the noise floor of the amplifier in the subwoofer. Everything listed in this particular reply is normal.

[g.costanza]

Ok.
I get 120v on fluke meter when 1 probe is on plate amp and 2nd probe is on center screw on wall outlet when sub is powered on and with no other connections on the subwoofer amp.

I placed the fluke meter on the end of the rca that is connected to the receiver's subwoofer output that I'm getting hum from when connected to the subs. One probe clipped to center pin. One probe clipped to outer ring. I left all the sources connected to the receiver. I measured 0vdc and 0vac on the meter even if denon volume was turned up to max. I then removed the rca attached to the subwoofer output of the receiver. I switched fluke to continuity and shorted the receiver end of the rca cable and got a tone. This was just to make sure that the ends of the fluke probe attached to the rca cable plug weren't touching in the previous test.

[g.costanza]

Sorry . Nothing changes from moving the wire. I've tried a second rca cable going to receiver from sub amp. Both cause hum

[Armadillo]

JAMO subs also do not hum if I connect the headphone out jack of my iphone using a 3.5mm to RCA plug adapter.

iphone no hum. So we suspect easy problem first [other than ground loop hum]. The contacts of your RCA or the receiver is poor thereby changing the designed impedance of the input.  I don't know how bad is the rust or the mechanical spring contact of the connector. So you try to use contact cleaner to clean it first. Swop other music source as input source is a alternative idea.   

[g.costanza]

the denon receiver is 3 months old. i had no hum issue with older sub setup that i took down to install this new setup. the only difference with new subs is that they are not 3 prong powered. and i find it odd that i get 118v from the exposed plate amp "chassis", but that's y i came here to get yall's advice

[g.costanza]

Is it safe to connect a wire from the plate amps exposed chassis (one of the many screws, for example) to the center screw of the wall power outlet to see if that eliminates the hum?
Remember, this sub plate amp has a 2 prong power plug. There's no 3rd grounding pin.

[Armadillo]

the denon receiver is 3 months old. i had no hum issue with older sub setup that i took down to install this new setup. the only difference with new subs is that they are not 3 prong powered. and i find it odd that i get 118v from the exposed plate amp "chassis", but that's y i came here to get yall's advice

You should simply use a "test pen" to touch the chassis than using the fluke to touch centre pin of the outlet cause in some places, the earth and the neutral are connected. Also try to connect the 2 prong 180 degree around, to see if this occurrence disappear.

[technogeeky]

Ok.
I get 120v on fluke meter when 1 probe is on plate amp and 2nd probe is on center screw on wall outlet when sub is powered on and with no other connections on the subwoofer amp.

I placed the fluke meter on the end of the rca that is connected to the receiver's subwoofer output that I'm getting hum from when connected to the subs. One probe clipped to center pin. One probe clipped to outer ring. I left all the sources connected to the receiver. I measured 0vdc and 0vac on the meter even if denon volume was turned up to max. I then removed the rca attached to the subwoofer output of the receiver. I switched fluke to continuity and shorted the receiver end of the rca cable and got a tone. This was just to make sure that the ends of the fluke probe attached to the rca cable plug weren't touching in the previous test.

Some quick questions just to fill in information:

1. What Fluke multimeter do you have?
2. What Denon amplifier do you have?

A multimeter will be able to pick up even a tiny 120v leakage current. So being able to see 120v by touching the chassis of the subwoofer and the ground pin of a socket (through the screw) is no big deal. We know the subwoofer isn't fully grounded (no 3rd pin). This is, overall, probably a red herring.

I messed up my advice a little. It's possible (plausible) that the receiver will shutdown the output of any RCA jack that it senses is unconnected. Evidently the way these line-level systems work is:

The output impedance of the LINE OUT audio source (Denon) is usually somewhere in the 100-1000 Ohm range. The input impedance of the LINE IN audio sink (subwoofer) is usually somewhere in the 10k Ohm and up range. It's possible the receiver tries to sense this and turns off the output. Your multimeter's input impedance is probably 1M Ohm, which is so high that this sense process may fail, generating no reading.


So the test procedure would be:

1. Find out the input impedance of the subwoofer's audio input (use the resistance mode of the multimeter; connect to the center and outer pin of the input RCA jack; you may or may not need for the subwoofer to be powered; you may or may not get a sensible reading anyway; if this fails assume 10k)
2. Sacrafice a crappy RCA cable and solder a resistor of the above value (10k or whatever) between the inner and outer conductor of the cable.
3. Plug this into the Subwoofer Pre-Amp LINE OUT on the Denon
4. Take the measurement across this resistor. (Voltage is interesting to note, but really I would want a frequency measurement to see if there is 60 or 120 hz on this line)

[g.costanza]

The subwoofer 2 prong plug is polarized. Won't go in the other way. Will I damage something If I used a "cheater plug" on the subwoofer power plug that I can put in the outlet "backwards"?

[g.costanza]

Meter is a fluke 115. Denon x3300w

Can't I just measure the output of the subwoofer rca from the receiver while it is playing something to see if it is muted or not?

[technogeeky]

Is it safe to connect a wire from the plate amps exposed chassis (one of the many screws, for example) to the center screw of the wall power outlet to see if that eliminates the hum?
Remember, this sub plate amp has a 2 prong power plug. There's no 3rd grounding pin.

There are two options:

1. Temporarily defeat (to test, be careful, do not touch the metal chassis of the Denon) the ground of the receiver by using a ground separating plug or taping up the  ground pin with thin but electrically insulating tape e.g. kapton.

2. Try and tie down the chassis of the chassis of the subwoofer to ground, as you have suggested. I don't think we can say for sure if this is safe or not. It should be safe for you, but if the subwoofer's power plug is unpolarized (e.g. you can reverse it), this has a 50/50 chance of being very bad for the subwoofer (and tripping the breaker). If it's polarized, then assuming it's designed correctly it should be fine.

[technogeeky]

Meter is a fluke 115. Denon x3300w

Can't I just measure the output of the subwoofer rca from the receiver while it is playing something to see if it is muted or not?

Yep!

Your meter also has a LoZ impedance option. You can just do the same measurement (both with and without music playing) with that setting, as the input impedance of the multimeter is about the same as the subwoofer should be (about 10k Ohms).

Your meter also has a frequency setting, so if there is indeed hum you should be able to see its frequency.

[technogeeky]

The subwoofer 2 prong plug is polarized. Won't go in the other way. Will I damage something If I used a "cheater plug" on the subwoofer power plug that I can put in the outlet "backwards"?

If you have a cheater plug, use it on the receiver instead (do not touch the metal chassis of the receiver, in case it is really malfunctioning).

There is no need to defeat the polarization of the subwoofer, and it can only lead to bad things.

[Armadillo]

The subwoofer 2 prong plug is polarized. Won't go in the other way. Will I damage something If I used a "cheater plug" on the subwoofer power plug that I can put in the outlet "backwards"?

Better to try cause I heard some places get the live and neutral the wrong way around unless you have the socket outlet tester.

[g.costanza]

I have to go to work now, but I used the continuity setting of the fluke on the ground hole and the wider NEUTRAL slot on the wall power outlet, and got a tone, which implies they r connected.

So the 2 things I want to try to test to see if the hum goes away are:
1 - Connect plate amp chassis to center screw of wall outlet with a jumper wire
2 - Reverse the power plug of the amp using a cheater-plug adapter that has the wider pin shaved down so it will go in backwards

[madires]

Sounds like a ground loop problem. The Denon's ground is referenced to mains earth while the subwoofer possibly uses neutral of the polarized plug for ground. You can check that easily by unplugging the subwoofer and measuring the resistance between the input RCA's ground and the main plug's neutral prong.

[Armadillo]

I have to go to work now, but I used the continuity setting of the fluke on the ground hole and the wider NEUTRAL slot on the wall power outlet, and got a tone, which implies they r connected.

Thanks God you are OK, cause that's  a dangerous move. What if you connected to the live and the earth with a continuity setting. BAND BOOM and blow your meter as well. But luckily you are OK. 

[g.costanza]

A few things I discovered...
I was wrong about Denon receiver. It is actually two prong. It has an iec connector on the back with two prongs in it, but the cable plugged into it end has a ground pin connection that isn't connected to anything in the iec connector.

When Denon is off and not connected to anything, it has a 40v measurement of chassis to center outlet screw. This doesn't change when simply turned on.
As stated earlier, the sub amps both have a 118v measurement of chassis to center outlet screw when plugged in to power, but not connected to Denon.
Once connected to Denon via RCA, bpth components measure 80v from chassis to center outlet screws.

[g.costanza]

I connected each of the pins on the wall plugs from each sub and the receiver to the outer rings of the audio jacks on the components using both continuity and ohm settings of the meter. In all instances I got OL, or open circuit.

[technogeeky]

If nothing is grounded, and nothing connects to the Denon or the subwoofer which is itself grounded, then you can eliminate ground loops. (Note: I am saying/assuming that you have disconnected all cables from the Denon except power, (unpowered) speakers, and the cable to connect to the subwoofer.)

I'm not sure what's left, other than radiated EMI.

Is it possible that you are using a very long cable to connect the receiver and subwoofer? Is this cable running in a chase, or otherwise very nearby and parallel to, AC power cables?

If we're really grasping at straws, are all devices which are plugged into the same electrical circuit disconnected? Usually this can be a source of conducted EMI but... usually it's only a problem in the above-audio frequency range.

Bizarre.

[Armadillo]

So I had a case of my fujitsu notebook hooked up to a external active speakers. When on standby mode, hum appears. when I flip open the display, hum disappeared.
So I hooked up to my mp3 player, no hum.
So I hooked up to my Alien notebook, no hum.
interesting. I did an experiment measurement.
Plug into fujitsu and hooked up to my oscilloscope, I can see line frequency hum at 360 mV. Flip up and line frequency hum disappeared.
So I did a line impedance measurement. The fujitsu reads open infinity ohm, while the Alien reads 8K ohms.
I suggest you measure the resistance of the line out of your Denon.
So I am going to check my fujitsu why the output impedance became so high because it has not hum before.  Maybe the line impedance resistor came loose or there is a contact problem.

[Armadillo]

I further study it a bit;

Assuming the open circuit is 10 Mohm;
so the current would be about 360mv/10Mohm = 0.036uA.
So, the ambient stray line hum induced energy is = 130 uW [micro watts]. This kind of hum energy is very very small.

So if the resistor would has been connected; the voltage would be 0.036uA * 10Kohm = 0.00036 V or 0.36mV

The mystery is reviewed. LOL.   

[cvanc]

I put a ground loop isolator on the RCA audio connection. No help. Still hums.

Tell us more about this.  Exactly what kind of "ground loop isolator" did you use?  And how did you hook it up?

[g.costanza]

The transformer has 2 small transformers and 2 small caps. It was made for car audio, but both are DC signals so should work. I put RCA/phono plugs on either end. I tried to post a pic but iphone photos r too large for this site (really??!) and when I tried to reduce the size and use a screenshot instead, this site said it wouldn't take my "jpeg" file because it isn't spelled "jpg", lol. Guess it just wasn't meant to be, haha. Seems this site needs "updating", anyway, back to topic...

Did a lot of checking yesterday. With only the LG 55" OLED tv with 3 prong plug hooked up to denon with hdmi, which itself was hooked up to subwoofer, I got hum. Disconnecting hdmi hum went away. Nothing else was connected to tv. I put a cheater plug on the tv's plug and reconnected hdmi to denon and the hum went away. I then connected benq projector with three prong plug up to denon with hdmi. I got hum. Put cheater plug on projector and hum disappeared. Nothing else connects to projector hanging from ceiling.

My home's electric panel ground connects to cold water pipe at hot water heater in utility room. I know that at some point in my front yard the copper has been replaced with plastic at about 20 feet away from the house.

One thing that confuses me is the hum changing loudness with subwoofer volume control. Does that mean the hum originates in the denon? Ive read that the hum should b the same no matter the volume.

[g.costanza]

Grrr. Renamed jpeg to jpg...

[Armadillo]

One thing that confuses me is the hum changing loudness with subwoofer volume control. Does that mean the hum originates in the denon? Ive read that the hum should b the same no matter the volume.

The hum is picked up by the RCA cable which act like antenna. The pickup signal act as a signal hum source of, in my case 360mv@10Mohms, hence it just like a signal source, when you increase the volume, the loudness increase.

If you read my post, you should terminate it with 10Kohm and allows the pick up to drain to the common and reduce the signal pickup to near ZERO.

In this case, I would advise that you install a 10Kohm resistor to the sub-amp.

[Armadillo]

Grrr. Renamed jpeg to jpg...

This looks like a good low frequency hum pickup antenna booster.... LOL

[g.costanza]

Most of the hum devices I see r the same. A pair of transformers.

[g.costanza]

Could u possible draw me a simple “dummies” diagram about what I r suggesting? Thx!

[Armadillo]

Could u possible draw me a simple “dummies” diagram about what I r suggesting? Thx!

imagine your subamp connection. You need to open up and connect a resistor to the 2 connection of the RCA female connector, 1 to the pin and 1 to the ring.

This will make sure, any denon or others audio equipment with high source impedance you plug in, the stray signal pickup will be drain to common.
Why industrial practice not same... I don't know.. maybe denon must use denon, business practice.

Edit: Actually the correct way to think is, you are building a RCA audio line of 10Kohm, so the stray signal of the micro watts power would not be able to induce any significant voltage onto your 10Kohm properly terminated line.   

[Armadillo]

And if this work for you,

then you can take a small black box with 2 RCA connector, 1 in an 1 out, inside the box is a terminating resistor. Then you pot it with epoxy. You call it "The Hum Eater" and sell it for $150. All you need to do is to sell 100 pcs per month.

Well, why not, there are a lot of all such funny devices in the market....... like the hum transformer of yours.... hahahahaha LOL. 

[technogeeky]

Did a lot of checking yesterday. With only the LG 55" OLED tv with 3 prong plug hooked up to denon with hdmi, which itself was hooked up to subwoofer, I got hum. Disconnecting hdmi hum went away.

A-ha! So the reason your old subwoofer didn't have the hum problem is that it was grounded to the same potential was the TV, which causing (or is at least one of two participants in) the hum.

One thing that confuses me is the hum changing loudness with subwoofer volume control. Does that mean the hum originates in the denon? Ive read that the hum should b the same no matter the volume.

If the hum changes with volume, it means that the hum is in the signal path. So if that's all we knew (e.g. disregarding everything we've learned over this thread except: 1) there's hum, 2) it changes loudness with gain) then we can say that the hum isn't coming from coupling into the voltage/current supplied to the subwoofer amplifier integrated circuit inside. In other words, because the hum changes with volume, we can be sure it's somewhere in the signal path at least as early as the audio level input circuitry of the subwoofer, or earlier.

This is easier to understand in the reverse situation. If the hum is always a constant level, no matter the gain, then it can not possibly be coming from the input signal -- because the input signal scales with the gain, because that's the purpose of an amplifier.

[technogeeky]

I think we're at the point that we could propose a solution, though I'm not sure what is best. I suspect there are other solutions that could be undertaken (I'm not sure what Armadillo is proposing that you do to fix things, or if he's just explaining input impedances...). Both of these solutions carry some risk. Solution 1 carries some risk to humans, so it should probably not be preferred. Solution 2 only carries risk to the subwoofer.

If anyone else has a better idea, by all means let's hear it!

• Permanently defeat the grounding of the TV.^[1]
• Add grounding straps to both the amplifier and subwoofers, and ground them.^[2]
• anything else?

^[1]: This is probably a bad idea, since if something does go wrong then the entire chassis of the TV, along with the metal mounting bracket, could become live with 120V mains. But you typically don't actually touch the television during normal use, so it would only be a risk to people who attempt to move/remove it -- and only if there is failure of some component in the TV which puts 120V on the chassis.

^[2]: This may not actually work (either the amplifier or the Denon may not connect their chassis to any electrical circuit). This may also fail: if the subwoofer is incorrectly or poorly designed, they may be assuming that the chassis is not grounded to any external potential. Depending on how bad this gets, this may cause damage to the subwoofer.

[cvanc]

Grrr. Renamed jpeg to jpg...

Hmmm.  Can you post clearer pictures of both the front and the back?  Or maybe hand-draw the schematic of this?

I suspect this may not be what you think it is.  But need more info to be sure.

[Armadillo]

If the hum changes with volume, it means that the hum is in the signal path. So if that's all we knew

There are 2 types of hums that we are looking at here.

Type 1. Is the Ground Loop hums, the loudness somewhat stay about the same.

Type 2. The other is the induced hums, that will get amplified. [this is what OP said he is confused].

Thus in your proposed solution of 1, 2 and 3, can you please explain what are you trying to abate type 1 or type 2 and how it is done, technically speaking. I ask because  I don't understand your methods 1, 2 and 3.

Also, OP said, when he plug in the RCA cable to the sub-amp [the other end dangling], hums appear, and if he unplug it, hums disappear. Can you please explain how your propose 1, 2 and 3 are related to this happening?

Please clarify. thanks.

[technogeeky]

If the hum changes with volume, it means that the hum is in the signal path. So if that's all we knew

There are 2 types of hums that we are looking at here.

Type 1. Is the Ground Loop hums, the loudness somewhat stay about the same.

Type 2. The other is the induced hums, that will get amplified. [this is what OP said he is confused].

Thus in your proposed solution of 1, 2 and 3, can you please explain what are you trying to abate type 1 or type 2 and how it is done, technically speaking. I ask because  I don't understand your methods 1, 2 and 3.

Please clarify. thanks.

Under this system of classification, this is a type 2 problem. At least that's my working guess. The ground loop is created in the TV -> HDMI -> Denon receiver system. And the RCA cable that is fed to the subwoofer contains the noise already.

So the solution I'm proposing is to either: stop anything from being ground referenced (the TV, in solution number one) OR ground reference everything to the same reference (connect the chassis of the Denon to ground, and connect the chassis of the subwoofer to ground).

In both cases, all three devices will have only one possible reference. In the first case, it's the neutral pin. In the second case, it's the ground pin.

Also, OP said, when he plug in the RCA cable to the sub-amp [the other end dangling], hums appear, and if he unplug it, hums disappear. Can you please explain how your propose 1, 2 and 3 are related to this happening?

I didn't see this test result because I didn't read carefully, but I don't think it's particularly meaningful. An open (not shorted) RCA cable is going to pick up noise and amplify it. It's akin to using the subwoofer as an environmental probe. If you mean to say that he took an RCA cable, shorted one end of it, and hooked the other end up to the subwoofer and it hums -- then yes, all of what I said doesn't make any sense and there's something wrong with the subwoofer.

[g.costanza]

One test I did do early on was plug the rca into the subwoofer and short the other end. No hum at all.

The tv is wall mounted. I have put it on a cheater plug as of now. I think it’s entire chassis is plastic (lg old55b6) although I assume the metal wall mount could b a hazard as it screws into metal threads.

I have a very complicated home entertainment system. To sum this up...

I have a tv in the living room (where the subwoofer and Denon are) that has a 3 prong plug. It connects to Denon with hdmi and also connects via coax cable to ota (over-the-air) antenna on roof. The ota antenna has a 4 port amplifier that feeds 4 sources.

I have a projector in the same room with a 3 prong plug that connects to Denon with hdmi.

I have a tv in the bedroom with 3 prong plug that is connected to the ota antenna on roof via coax cable. 

I have a tv in kitchen with 2 prong plug that is connected to the ota antenna on roof via coax cable.

Having any of these 3 pronged displays plugged in creates hum in the subwoofer. I assume the hum from bedroom tv gets through coax cable.

I also have a ota tuner box in the living room that sends ota broadcasts from the ota roof antenna to the projector via an hdmi that goes through the Denon. It has a 2 prong plug. It’s unplugged so I’m not even sure what affect it may have on hum.

The living room tv, the Denonr receiver, a living room Apple TV, a bedroom Apple TV, a second bedroom Apple TV, and a pc/media server are all connected via Ethernet to a router and a modem for the internet which is supplied by a cable company. So while I don’t have “cable”, I do have the wire coming into my home. It has a coupler/joiner just outside the house with a screw down for a ground wire. It runs completely through my attic to the other side of my home where the breaker box is at and grounded to a water pipe where the breaker box is grounded.

[Armadillo]

I didn't see this test result because I didn't read carefully, but I don't think it's particularly meaningful.

Vital hints;

[technogeeky]

I didn't see this test result because I didn't read carefully, but I don't think it's particularly meaningful.

Vital hints;

Yes, your highlighting points out that this test is not helpful (in this case); it is just measuring environmental noise. The subwoofer is amplifying stray mains noise, and the longer the cable, the more of this it will be able to pick up. If you touch the center pins (and perhaps also the outer jacket), the subwoofer will also amplify all of the noise picked up by your body.

[Armadillo]

If you mean to say that he took an RCA cable, shorted one end of it, and hooked the other end up to the subwoofer and it hums -- then yes,

Yes, your highlighting points out that this test is not helpful (in this case);

For one, it is certainly helpful to you to correct your own mistakes of what you have not read and in spite failed to understand and appreciate.
It is certainly not helpful if you don't bother to read, and we need to remind you and back tracks what have been said and done.
It is obvious to me that you are grasping with the subject of the ground loop noise that you don't quite understand and needs to be pointed out of the 2 types we are dealing with.

The test conducted that you think is not helpful, was to ascertain that it is from the type 2 noise. [Edit: And now that you knew and acknowledged the type 2 hums, you belittle the test conducted?  ]
You have no means and methods to differentiate the two and less understand the types per se other than judging belittle others to elevate yourself.   
Cheers!   

If you don't understand the types, do you think I should say you are not helpful in my earlier post or belittle your ideas inspite of not understanding the types just to make you understand the kind you are?

[g.costanza]

I just want to repeat from earlier post...

“With only the LG 55" OLED tv with 3 prong plug hooked up to denon with hdmi, which itself was hooked up to subwoofer, I got hum. Disconnecting hdmi hum went away. Nothing else was connected to tv.”

I’ll also add that nothing other than passsive speakers were hooked up to Denon receiver.

And the same held true for a 3 prong projector on the ceiling connected to Denon with hdmi. Hum if plugged into power. No hum when unplugged.

AND YET ANOTHER DISCOVERY...
I should have checked further earlier because that old partsexpress bash amp I was using on a diy subwoofer for the last year in this exact same system where I had NO HUM...it has a 3 prong plug but there is no ground pin inside the iec connector! It’s actually 2 prong. Now I’m really confused why this new sub is humming!

[g.costanza]

OK. Another test...

Plugged subs and denon receiver together with rca. Plugged both subs and receiver to same outlet. Power on and I get hum. Not ear blasting hum, but about 10bd over ambient room noise. Clearly audible. Nothing else connected to Denon. Not even passive speakers. Nothing.
NOW, I'm on bare feet on tile floor. If I touch Denon chassis, the hum gets louder. If I release the denon and walk over and touch sub's plate amp, the hum gets quieter. Hmmm..

[technogeeky]

If you mean to say that he took an RCA cable, shorted one end of it, and hooked the other end up to the subwoofer and it hums -- then yes,

Yes, your highlighting points out that this test is not helpful (in this case);

For one, it is certainly helpful to you to correct your own mistakes of what you have not read and in spite failed to understand and appreciate.
It is certainly not helpful if you don't bother to read, and we need to remind you and back tracks what have been said and done.
It is obvious to me that you are grasping with the subject of the ground loop noise that you don't quite understand and needs to be pointed out of the 2 types we are dealing with.

The test conducted that you think is not helpful, was to ascertain that it is from the type 2 noise. [Edit: And now that you knew and acknowledged the type 2 hums, you belittle the test conducted?  ]
You have no means and methods to differentiate the two and less understand the types per se other than judging belittle others to elevate yourself.   
Cheers!   

If you don't understand the types, do you think I should say you are not helpful in my earlier post or belittle your ideas inspite of not understanding the types just to make you understand the kind you are?

I'm sorry that anything I said made you feel unimportant. This was certainly not my intention!

You should feel free to chastize me about any subject, especially when I am incorrect. I welcome it. This is even more important in a case like this, when I don't have anything near a full understanding of this type of problem.

Having said that, I do have to rely on my own ability to read quickly and pick and choose what information I think is genuinely useful and trustworthy. I also think it's pretty clear that I do understand the distinction between what you label as Type 1 and Type 2 ground loop noises (as I mentioned this distinction in my very first post in this thread).

[technogeeky]

OK. Another test...

Plugged subs and denon receiver together with rca. Plugged both subs and receiver to same outlet. Power on and I get hum. Not ear blasting hum, but about 10bd over ambient room noise. Clearly audible. Nothing else connected to Denon. Not even passive speakers. Nothing.
NOW, I'm on bare feet on tile floor. If I touch Denon chassis, the hum gets louder. If I release the denon and walk over and touch sub's plate amp, the hum gets quieter. Hmmm..

As both Armadillo and I have mentioned, it would be a good idea to test and report back the output and input impedances of the Denon's SUBWOOFER OUT and the subwoofer's INPUT RCA jacks. You probably don't have an impedance meter, so a multimeter in resistance mode will work. My expectation (as I mentioned in my first post, I think) is that the Denon should read something like 1k ohms from RCA shield to RCA pin. The subwoofer inputs should read something like 10k ohms from RCA shield to RCA pin. In the event that the subwoofer does not read something like 10k (and it reads something like 1M or 10M, or OL), then the solution Armadillo mentioned is the easiest and safest modification to attempt to fix the problem: you solder a 10k resistor across the RCA shield and RCA pin at the subwoofer (preferably on the inside).

[g.costanza]

Could u please tell me how to do this? Power on?

[madires]

I'd would also investigate how the signal ground ties in with the power supply for each device involved.

[Armadillo]

@technogeeky;

  Thanks, We are good.

[technogeeky]

Could u please tell me how to do this? Power on?

Power off, though it shouldn't matter for the subwoofer. It might matter for the receiver (if there is, say, 100mV on the output it might disrupt your multimeter's resistance reading)

[g.costanza]

Ok Will try that.

I did discover that with subs on and rca attached, but not connected at receiver, I was getting slight hum (subs are completely silent wen powered on and no rca attached). If I ran a wire from the center wall outlet screw (ground) to the metal of the amp chassis, the amp went completely silent. What does that mean?

So then, with receiver and sub connected with rca cable, I did the same thing with the denon. I touched the center wall outlet screw to the denon metal chassis (which is on another outlet across the room). Rather than the sub going completely silent as before, there was a very loud hum. What does that mean?

THANKS EVERYONE FOR THE HELP! Electricity is still like magic to me, lol 

[g.costanza]

OK. I broke down and reversed the power cable on the sub by hacking into an extension cord and reversing the wires. Result...sub is dead silent. I'm only measuring 1vac from the back amplifier plate to the wall outlet center screw whereas I was getting 1vac before.

[Armadillo]

, the earth and the neutral are connected. Also try to connect the 2 prong 180 degree around, to see if this occurrence disappear.

OK. I broke down and reversed the power cable on the sub by hacking into an extension cord and reversing the wires. Result...sub is dead silent. I'm only measuring 1vac from the back amplifier plate to the wall outlet center screw whereas I was getting 1vac before.

I thought you had cheater plugged it already!. 

[g.costanza]

No. I had put a cheater plug on the lg oled tv to lift it’s ground but didn’t reverse its power plug polarity. That help reduced hum.  I was hesitant reverse any component’s power plug cuz so many ppl in various places were saying it was dangerous and could damage something. The subs are brand new and the $1000 Denon is just a few months old. It was my last resort. Sadly the Denon’s chassis reading is 50-60 volt AC irregardless of rotating its power plug. Which results in a very slight hum. The OLD Sony received I had lying around and lacks a lot of the features I need, measured 1 volt ac in factory plug orientation. This resulted in dead silence when connected to the sub. No hum at all. The Sony measured 70 volts ac in reversed power polarity. Too bad for the new Denon