Using a cheater 3-prong to 2-prong adapter

[uscpsycho]

I find that odd too. My only guess is that the AVR has better isolation/filtering than the external amp(s) and so avoids the problem? You could try the cheater on the external amp to see if that make the problem go away. Then if an external amp is available with a 2 prong plug, that would be a work-around.

That seems like a plausible assumption. But the external amp is getting its audio signal from the AVR so you'd think it's also filtered. If the AVR has better filtering it's possible the filtering is only done for its own speaker outputs and the any pre-outs (which feed the external amp) aren't filtered. Actually, that might make sense because you want the audio to an external amp to be unaltered so maybe it's unfiltered too.

I'll try putting the cheater on the amp and see what happens. That's a great idea. If it works replacing it with a two-prong amp seems easy enough.

Electrically, the same as the extension cord, but with in wall wiring. Add a new receptacle next to the existing #2, but bring power to it from the circuit that supplying receptacle #1. [Bringing power directly from the box supplying receptacle #1 would be safest, but I would think that any box on that circuit would work. Although perhaps it's best to be take the safer route.]

This is easy in theory but involves a lot of work so not exactly easy to do. I proposed this to the handyman and he gets it but doesn't want to do it. I think partly because he'd be putting holes in walls that he spent most of yesterday patching.

I would be curious what the AC voltage is between the ground pins on the outlets in question.


I would also be interested in the megohms between the ground pin on each unplugged plug and its neutral (the wider blade).

I've got a multimeter that confuses the hell out of me. If you can tell me how to test this I will.

2) GFCI will not isolate the ground any kind of AC hum or interference. It only monitors that the current flow between the hot and neutral is less than 5 milliamp difference. Above that it trips.

I know it's hard to read from the other posts but the GFCI isn't here to help with the hum. The cheater plug does the job perfectly. The GFCI adapter joined this party to live between the existing outlet and the cheater plug to reduce some of the risk that was introduced by using the cheater plug. I was hoping the GFCI adapter was enough to put this matter to rest but I was advised that permanently eliminating ground from the projector is not a good solution.

3) It's hard to read from other posts but it appears that one outlet is on one phase and the other is on the other phase of a two phase system and there is a ground potential difference. Place a voltmeter from the hot to the neutral and from the hot to the ground. Check the voltages. Then do the same with the other outlet. You want the readings to be the same.

It was suggested that the two outlets might be on different phases but I have not confirmed. Is your suggestion about using a voltmeter a way to figure out if they are on the same phase (instead of the counting technique suggested above) or is this an additional troubleshooting test? Can I do this with my multimeter? If so please explain how.

Have you tried just changing the Ethernet cable?

Are any of the items a simple, 2-prong, unpolarized plug? If they are, try flipping the plug around.

I have tried two different ethernet cables one shielded and one unshielded.

I know some of the 2-prong plugs are unpolarized. I will try flipping them but this is going to be a bigger headache than you imagine! I've got several power strips with lots of plugs and it's like Tetris getting everything to fit, so flipping these guys is gonna be quite the operation!

 

[wwhitney]

I've got a multimeter that confuses the hell out of me. If you can tell me how to test this I will.

If your multimeter has a selector dial in the middle, the setting you want is in an area that says V~ (the ~ may be under the V). Avoid the area that has V and straight lines (possibly one dotted), that's for DC. Set the meter on 200 or 600 V~ (that's the range of values you are measuring, I think 200 is enough for 240V as it just means hundreds of volts, but maybe you need 600).

You should have two insulated probes with the ends bare. Don't touch the bare ends, just handle the insulated part. If you go to a regular receptacle and stick one probe in the long slot (neutral) and one in the short slot (hot), you should get a reading around 120V (somewhere in the 115V - 125V range). You may have to wiggle the probes a little in the slots to get good contact; if the number just flashes briefly and then disappears that's likely due to poor contact.

Once you have that technique working, if you still have the extension cord end from receptacle #1 in proximity to receptacle #2, you can test directly if they are on the same leg. Put one probe in #2 hot (narrow slot) and one probe in #1 hot (via the extension cord). You should get either 0V or near 240V (again, 230V - 250V). 0V means the two receptacles are on the same leg (or, conceivably, that they both have hot/neutral reversed); 240V means they are on opposite legs (and neither has hot/neutral reversed). The breaker counting method should also work and give the same answer.

If you got around 120V, that would mean one or the other has hot/neutral reversed, but not both. In general to check for hot/neutral reversal (assuming the grounding pin is connected properly), put one probe in the grounding slot (a different shape, so you may need a different angle on the probe to get good contact) and one probe in either hot (narrow) or neutral. G-N should be 0V, G-H should be around 120V. H/N reverse would show up via those numbers being swapped, G-N on the receptacle showing around 120V.

Personally, I don't think the problem is likely to be caused by either opposite legs or H/N reversed, I think the problem is noise on the EGC and inadequate filtering/isolation in the external amp. But that's at best a slightly educated guess, so the alternatives are worth ruling out.

Cheers, Wayne

 

[Sponsor]

 

[Reach4]

I've got a multimeter that confuses the hell out of me. If you can tell me how to test this I will.

For measuring the ground_to ground_voltage, you can go to a lower voltage setting -- maybe a 20 volt scale. A 3-wire extension cord can help bring both grounds within reach of the meter probes.

 

[wwhitney]

For measuring the ground_to ground_voltage, you can go to a lower voltage setting -- maybe a 20 volt scale. A 3-wire extension cord can help bring both grounds within reach of the meter probes.

If you get a non-zero reading between EGCs from different locations, that's a sign of significant problem. I can only think of 3 possibilities:

(0) One of the receptacles is mis-wired and actually has the hot or neutral on the grounding pins.

(1) It's a real reading and there's a voltage difference. That implies current is flowing on the EGC somewhere in between, because V=IR, and if I = 0 then V = 0. In normal operation the EGC should not be carrying any current.

(2) It's a spurious reading because the meter is high impedance and the EGC is disconnected between the two points, and the disconnected segment is capacitively coupled to the hot conductor in its cable. The disconnection point should be found and reconnected.

Cheers, Wayne

 

[Stuff]

(1) It's a real reading and there's a voltage difference. That implies current is flowing on the EGC somewhere in between, because V=IR, and if I = 0 then V = 0. In normal operation the EGC should not be carrying any current.

A few milliamps of current on the EGC has been allowed for electronic light switches. Some audio folk have pointed to these devices for a causing hum.

Otherwise, yes, there is a wiring problem somewhere and is a safety issue that is more important to fix than the hum issue..

 

[uscpsycho]

I find that odd too. My only guess is that the AVR has better isolation/filtering than the external amp(s) and so avoids the problem? You could try the cheater on the external amp to see if that make the problem go away. Then if an external amp is available with a 2 prong plug, that would be a work-around.

Just when this couldn't get any weirder...

I discovered that my receiver does not have a grounded 3-prong plug. I didn't even check it before, I just assumed it did. So it makes sense that the speakers connected to the receiver have no hum because there is no ground. Maybe the receiver was designed this way to ensure there is never a ground loop problem?

The external amp does have a grounded three prong plug. So the ONLY devices with 3-prong plugs are the projector and the external amp. Now it makes a little more sense that when those two are connected (with two baluns and a receiver between them) I get ground loop hum only from the two speakers connected to the external amp.

I tried putting the cheater plug on the external amp instead of the projector and I still have the hum, but it's not as loud as without the cheater. I also tried that Hum Xterminator I mentioned earlier with the external amp and it still doesn't fix the problem.

So now I am thinking that even if I replace my external amp with one that only has a 2 prong plug I will still have the ground loop hum issue. Correct? Or is there some logical explanation for why a pure two-prong external amp won't have the problem even though this is having a problem even with the cheater plug?

 

[wwhitney]

I tried putting the cheater plug on the external amp instead of the projector and I still have the hum, but it's not as loud as without the cheater.

That is a very interesting symptom. That suggests the hum is from some sort of coupling to the ground (EGC), and that it's a stronger coupling if the external amp has an EGC connection (some coupling in the external amp), but there's coupling elsewhere as well.

Just to double check, the HDMI cable is just carrying video to the projector, right? And there's no speakers connected to the projector?

I also tried that Hum Xterminator I mentioned earlier with the external amp and it still doesn't fix the problem.

You could try the Hum Xterminator at the projector and (temporarily) the cheater at the external amp. Since the projector is seemingly the source of the hum.

Cheers, Wayne

 

[wwhitney]

A few milliamps of current on the EGC has been allowed for electronic light switches.

Yeah, 0.5 ma per device was allowed for a while. I think the UL standard has changed to discourage or disallow those, after the NEC changed to require a grounded conductor (neutral) at most switching locations. That way the switches have no excuse for using the EGC as a circuit conductor.

But I don't think you're going to be able to see the voltage difference from a couple ma with a multimeter. Maybe I'm mistaken.

Cheers, Wayne

 

[Reach4]

But I don't think you're going to be able to see the voltage difference from a couple ma with a multimeter. Maybe I'm mistaken.

You could. If mV was not picking up, you would switch to mA.

 

[uscpsycho]

That is a very interesting symptom. That suggests the hum is from some sort of coupling to the ground (EGC), and that it's a stronger coupling if the external amp has an EGC connection (some coupling in the external amp), but there's coupling elsewhere as well.

Just to double check, the HDMI cable is just carrying video to the projector, right? And there's no speakers connected to the projector?


You could try the Hum Xterminator at the projector and (temporarily) the cheater at the external amp. Since the projector is seemingly the source of the hum.

Cheers, Wayne

One HDMI cable is carrying video from the AV Receiver to the transmitter balun and another HDMI cable is carrying video from the receiving balun to the projector. That is all. There is no direct HDMI connection between the projector and anything in the AV closet.

I tried your suggestion of the Hum X at the projector and the cheater at the external amp and it worked! Hum is gone. But this is probably still not the right solution since the cheater is still in play. I wonder if a second Hum X on the external amp would accomplish the same as the cheater? I don't love the idea of two of these dongles at $85+ per Hum X but $180 out the door is probably cheaper than just about any of my other options.

 

[Reach4]

One HDMI cable is carrying video from the AV Receiver to the transmitter balun and another HDMI cable is carrying video from the receiving balun to the projector. That is all. There is no direct HDMI connection between the projector and anything in the AV closet.

I tried your suggestion of the Hum X at the projector and the cheater at the external amp and it worked! Hum is gone. But this is probably still not the right solution since the cheater is still in play. I wonder if a second Hum X on the external amp would accomplish the same as the cheater? I don't love the idea of two of these dongles at $85+ per Hum X but $180 out the door is probably cheaper than just about any of my other options.

The extension cord sounds pretty cheap.

I don't know what it costs to extend to a new outlet.

 

[wwhitney]

I tried your suggestion of the Hum X at the projector and the cheater at the external amp and it worked!

And I assume removing the cheater at the external amp causes the hum to come back?

Then your options seem to be (a) try a second Hum X on the external amp, that's a gamble (b) get an external amp that has a two prong plug or (c) extend the circuit serving receptacle #1 to a new receptacle next to receptacle #2. There might also be (d) get an external amp with better internal filtering/isolation so it doesn't pick up the hum, but that's somewhat speculation.

Cheers, Wayne

 

[uscpsycho]

The extension cord sounds pretty cheap.

I don't know what it costs to extend to a new outlet.

Extension cord is cheap but I can't have one permanently running across my house from the AV closet to my theater.

Extending the outlet definitely costs more than $200 between electrician, painter to patch and materials.

And I assume removing the cheater at the external amp causes the hum to come back?

Then your options seem to be (a) try a second Hum X on the external amp, that's a gamble (b) get an external amp that has a two prong plug or (c) extend the circuit serving receptacle #1 to a new receptacle next to receptacle #2. There might also be (d) get an external amp with better internal filtering/isolation so it doesn't pick up the hum, but that's somewhat speculation.

Cheers, Wayne

Yep, removing the cheater at the amp with Hum X at the projector brings the hum back. I already tried that when I first got the Hum X. Since the cheater worked alone with the projector, I thought the Hum X would work even better but it only reduced the hum instead of exterminating it.

I have other smaller amps that only have 2 prong plugs (not polarized) which I use for other parts of the house. I plugged one of those guys in place of the current amp without the Hum X at the projector. Lo and behold there was no hum, but the speakers connected to that amp are not the same ones that are in the theater. I don't think that makes a difference.

Not sure why the 3-prong amp + cheater only works with the Hum X but the 2-prong amp works without the Hum X. Weird.

A small two channel amp with a two prong plug is cheaper than two Hum X so I think this is the solution. Does it matter if I get one with a polarized plug or not?

 

[Reach4]

Does it matter if I get one with a polarized plug or not?

No.

 

[uscpsycho]

The company that makes my external amp has a scaled down version of it. I called to ask if that one has a 2 prong or 3 prong plug. Tech support thought it was a weird question so I explained what was happening and the guy told me to forget about buying a new amp and that it was totally fine to use the cheater plug on the amp I have. This is it.

Should I trust that advice? I know the cheater was a bad idea to use on the projector. Also a terrible idea on the amp?

 

[wwhitney]

Not sure why the 3-prong amp + cheater only works with the Hum X but the 2-prong amp works without the Hum X. Weird.

You could try a 2 wire extension cord, then the cheater on your existing amp. On the off chance the proximity of the cord's EGC to the receptacle is enough for the noise to occur (I'm stretching here).

Should I trust that advice?

No. A piece of equipment with a 3 pronged plug may have exposed metal parts connected to the ground pin. A piece of equipment with a 2 prong plug should have no exposed metal parts, or the live components should be "double insulated" from the metal parts (not clear on the exact details of that).

So you want something designed to only require a 2 prong plug, rather than just defeating the ground connection on a device that has been designed to have a ground connection.

Cheers, Wayne

 

[DIYorBust]

Does anyone else think the outlet should be tested with a voltmeter and pulled ot check the wiring configuration?

 

[uscpsycho]

Just want to thank everyone who helped me with this and post a final resolution for the benefit of anyone who has a similar problem in the future and sees this thread.

I bought the stepped down version of the external amp that the buzz was coming from. The new amp has a polarized two-prong plug and there is no hum or buzz when I use it. I am not using the Hum Exterminator or the cheater plug. I am so grateful to whoever came up with the idea of trying an amp that only has a two-prong plug! Brilliant! And thanks to everyone for pushing me to properly solve this instead of using the cheater plug as the permanent solution.

I feel like this was a total team effort. I should have clicked the Like button more than I did. I owe you all a beer!

Stay safe!