I just got a pair of Sony MDR-NC20 headphones. The noise canceling feature works much better than I was expecting. Although I have yet to try them on an airplane, it does a great job of blocking the window A/C unit near my desk at work.

Here's the "problem" -- while at work I plug the empeg in with the AC adapter and attach the headphones to the back of the empeg (MK2A) using a stereo RCA to 1/8" stereo miniplug adapter. With these new headphones I hear a very low hum when the player is paused and during quite periods of songs. This does not occur if --
(a) the noise canceling circuitry is turned off,
(b) the headphones are detached from the empeg, or
(c) other non-NC headphones are attached.

One more "clue" ... if I touch the metal housing of the empeg the hum totally disappers (it sounds perfect) until I stop touching it. Is this some sort of ground loop? I welcome any suggestions on what I should do to correct this (short of keeping my hand on the empeg at all times). Does anyone else have these same headphones, and if so do they experience the same problem? Thanks in advance. ...Mark

Buy one of those grounding straps that go around your wrist and hook to something (your empeg)? Perhaps this isn't the solution you're looking for...

Matthew

Greetings!

It sounds like your noise cancelling headphones, usually with their own power supply or battery, is a slightly different ground from your empeg. If it is annoying, you could use one of the car install noise filters (RCA ended, no need for power or grounding) to give you some isolation. It should be enough to clear it up for you, since it sounds like a very low volume noise. Either that, or run a ground connection from the headphones (or you) to the empeg so that they share ground.

Yup, you're definitely hearing a ground loop. Try:

- Messing with plugging the headphone powersupply and the player powersupply into separate outlets (or closer outlets if they are already in separate outlets).

- Making sure everything else is unplugged from the empeg besides the headphones and the power supply.

- Try different powersupply for the empeg or the headphones.

The power source on the headphones is an AAA battery, so not much can be done there. I tried switching from a rechargeable to a fresh alkaline AAA, as well as changing the outlet the AC adapter was plugged into, but noticed no change. I will bring in my spare AC adapter tomorrow and try that. One thing that I discovered that does help is to keep the serial cable attached between my laptop and the empeg. This significantly reduces the hum, but does not eliminate it completely.

The ground loop RCA filter looks promising. Hopefully I will not have to resort to connecting its ground cables to me with electrodes in order to isolate it properly. My thanks go to Matthew, Paul & Tony for your insightful suggestions. I'll let you know what ends up working. ...Mark

I didn't realize that the headphones didn't have a grounded power supply.

Question: Are you sure of the following things?

- There really is no hum when the noise-cancelling feature is deactivated?

- The headphones do not produce the same hum when used with other pieces of consumer stereo gear?

> Question: Are you sure of the following things?
>
> - There really is no hum when the noise-cancelling
> feature is deactivated?

In a quiet environment the hum definitely goes away when the NC circuit is switched off and they are attached to the empeg at a paused state (or quiet passage).

> - The headphones do not produce the same hum when
> used with other pieces of consumer stereo gear?

I have connected them to my Rio 500 player, which runs off a AA battery, and I hear no hum with NC active.

...Mark

using a stereo RCA to 1/8" stereo miniplug adapter

I take it that this is something you bought from the RadioShack (or similar)? That is, is it a reputable adapter, or is it possible that it's broken? (I'm not sure whether being from RadioShack is reputable or not....)

What are you using as the AC adapter for the empeg? I seem to remember that someone changing his adapter and his noise going away.

What kind of power supply do you have? Is it one of the sunpower "brick" units with two separate cords coming out of it (one from each side)?

If so, see if using an alternate power supply makes the hum go away.

Problem solved!

Tony was on the right path in this case ...
I had been using the AC adapter that came with the player (Model No: NP12-1S1210), made in Taiwan with an output of: DC 12v DC 1A and + center, so following Tony's advice I dug up a different adapter which still met the spec. In this case a POTRANS "brick" unit (as he described), made in China with an output of 15v DC 1.7A and + center. That was the ticket ... zero hum with NC active. Maybe it is a better quality power supply, or maybe the added voltage plays into it, but it works and that is all that really matters. You have all been very helpful, thank you so much. ...Mark

P.S. The RCA to 1/8” plug adapter is not from Radio Shack. It came bundled with a PC sound card ( Sound Blaster AWE32 Gold ), and appears to be of reasonable quality. I have yet to see a similar adapter for sale, short of using a gender changer. If anyone knows of a source I would be interested in a spare. This one is bound to break as a result of its daily use. Ideally I will get around to building a sled based box in the future.

RadioShack sells a (2)RCA -> 1/4" phone jack adapter for $5. If you're like me, you've probably got a dozen 1/8"->1/4" adapters lying around, and they don't wear out much, so that show work just fine, I think.