two humming audio amps mystery (solved)

[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