two humming audio amps mystery (solved)

[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?