[Le Flaneur]

I have a MBP connected to a a Harmon-Kardon stereo receiver as a dedicated multimedia console. However, with it connected via headphone plug to RCA (the stereo doesn't have digital inputs), there's a noticeable hum. Wouldn't connected the ground on the receiver to the MBP eliminate the hum? If so, to where on the MBP would I attach the wire from the stereo receiver?

[ibook_steve]

I had the exact same problem. I ended up just using an optical connection. I have a 2007 2.33 MBP.

Steve

[Le Flaneur]

But what if an optical connection isn't possible?

[Simon]

Attach the MBP via the grounded (three prong) adapter to the same wall socket as the stereo system. That way they share a common ground. No more ground loop.

[Le Flaneur]

Originally Posted by Simon 

Attach the MBP via the grounded (three prong) adapter to the same wall socket as the stereo system. That way they share a common ground. No more ground loop.

I don't think that the stereo has a grounded plug. Should I run a wire from the ground on the stereo receiver to the third prong of the outlet (actually, an outlet strip)?

[amazing]

Just curious: Are you using the 3-prong grounded cord on the MBP power supply? I've also found that I get "better" audio if I run my MBP through my iMic--the USB interface seems to give clearer audio when run thru a board to good speakers.

http://www.amazon.com/Griffin-iMic-U...0260795&sr=8-1

[amazing]

delete double post, wonky interface problem

[Le Flaneur]

Originally Posted by amazing 

Just curious: Are you using the 3-prong grounded cord on the MBP power supply?

Yes. But the stereo doesn't have a 3-prong plug. Therein lies the problem, I think. Switching to USB --> RCA is an interesting idea.

Would this simpler solution work as well?

http://www.amazon.com/Griffin-Turnta..._bxgy_e_text_b

[Spheric Harlot]

Originally Posted by amazing 

Just curious: Are you using the 3-prong grounded cord on the MBP power supply? I've also found that I get "better" audio if I run my MBP through my iMic--the USB interface seems to give clearer audio when run thru a board to good speakers.

http://www.amazon.com/Griffin-iMic-U...0260795&sr=8-1

Well, it seems kind of obvious that if you replace the built-in $1 DAC with a $50 external interface, you should hear an improvement in the sound quality.

Using a $200 or $2000 DAC will further enhance the clarity.

I assure you, grounding has nothing to do with it.

[Spheric Harlot]

Originally Posted by Simon 

Attach the MBP via the grounded (three prong) adapter to the same wall socket as the stereo system. That way they share a common ground. No more ground loop.

Try this if you haven't.

Next, try just turning around the plug on the stereo, switching the pins in the socket. They look the same, but due to how most power supplies are built, there can and often *will* be a noticeable difference if you turn the plug around and pick up the phase on the correct pin.

Also, if your stereo is connected to a cable connection (e.g. for cable radio), that may create grounding problems, since the cable ground may be earthed elsewhere.

Disconnect that and see if it solves the hum; then you'll at least know whether it was the culprit.

[ghporter]

The stereo, lacking a separate ground conductor in its power cord, must either be completely ground-isolated, or neutral-grounded. Neither is a great option because of the ground loop problem Le Flaneur has noted. There is a way around this, though.

Assuming the stereo has a metal case, connect a wire between one of the stereo's case screws and the wall plate screw for the outlet the stereo is plugged into. The wall plate screw goes into the frame of the outlet device-which is grounded. This should ground the stereo's case and provide a suitable drain connection to eliminate the ground loop issue. Of course you must also use the 3-prong power cord with the MBP to ensure that it too is appropriately grounded.

[Le Flaneur]

Originally Posted by Spheric Harlot 

T
Next, try just turning around the plug on the stereo, switching the pins in the socket.

Turns out that they're polarized.

Get this, though: the hum has nothing to do with the MBP; it's there regardless of the input source.

[Le Flaneur]

Originally Posted by ghporter 

The stereo, lacking a separate ground conductor in its power cord, must either be completely ground-isolated, or neutral-grounded. Neither is a great option because of the ground loop problem Le Flaneur has noted. There is a way around this, though.

Assuming the stereo has a metal case, connect a wire between one of the stereo's case screws and the wall plate screw for the outlet the stereo is plugged into. The wall plate screw goes into the frame of the outlet device-which is grounded. This should ground the stereo's case and provide a suitable drain connection to eliminate the ground loop issue. Of course you must also use the 3-prong power cord with the MBP to ensure that it too is appropriately grounded.

See my comments above about the problem not being related to the input source -- grounding the receiver as you say should do the trick, shouldn't it?

[Spheric Harlot]

Originally Posted by Le Flaneur 

Turns out that they're polarized.

Get this, though: the hum has nothing to do with the MBP; it's there regardless of the input source.

Okay, so the problem lies elsewhere.

So do you have any part of the stereo connected to an antenna input that you can disconnect to see if the hum stops?

What components do you have hooked up to what, and how? Is there a TV involved?

Did you actually try turning the plug around 180° in the socket?


It would be helpful if you'd responded to our suggestions by telling us what you've actually tried, and what the effect was, if any.

[Simon]

If you can, you should ground all the receiver/stereo equipment. Glenn has excellent advice on what to do if there's no ground conductor.

There's one exception. I've seen people get in to trouble even with grounded stereo sets when they have a tuner hooked up to cable (antenna). The problem there is that the cable ground (the outer shield) is connected to the cable company's ground which can be on quite a different potential from your local ground (I've seen reports of up to 40V). If that's the cause you'll notice the ground loop vanishes the moment you remove the cable. A possible workaround is to connect the cable shield to your local ground thus short circuiting the ground loop.

[ghporter]

As Simon says, trying to connect a ground line to the case of a receiver that has some other, not-necessarily-really-ground attached to it can be "exciting." Most of us don't have this problem though; a "good" cable connection goes through a grounding block at the customer's location so the cable's ground should be local, really-and-truly-earth-ground.

As for the hum coming or not coming from the MBP, that's irrelevant to keeping both the MBP and stereo grounded. A hum comes from an AC potential in an audio circuit. Having both devices properly grounded will prevent problems caused by poor or faulty grounding of the power system in the stereo. Those could cause a nasty shock if you connect a grounded device to a non-grounded stereo, or vice-versa.

[Spheric Harlot]

Re: cable connection

Are my posts not showing up for anybody else or what's going on here?

[Le Flaneur]

Originally Posted by Spheric Harlot 

Okay, so the problem lies elsewhere.

So do you have any part of the stereo connected to an antenna input that you can disconnect to see if the hum stops?

The only antenna input is to the TV, which is a pair of rabbit ears from Radio Shack. Still important?

Originally Posted by Spheric Harlot 

What components do you have hooked up to what, and how? Is there a TV involved?

Yeah, a Toshiba Regza 26" (but not connected to the stereo). A Philips DVD Player is connected.[/QUOTE]

Originally Posted by Spheric Harlot 

Did you actually try turning the plug around 180° in the socket?

See what I wrote above: the plug is polarized!

[Spheric Harlot]

Originally Posted by Le Flaneur 

The only antenna input is to the TV, which is a pair of rabbit ears from Radio Shack. Still important?

nope, unless it's an active antenna hooked up to a power socket.

Originally Posted by Le Flaneur 

Yeah, a Toshiba Regza 26" (but not connected to the stereo). A Philips DVD Player is connected.

I assume the DVD player is connected to the TV?

If so, the TV *is* connected to the stereo via common ground.

Are they all hooked up to the same wall socket?

Originally Posted by Le Flaneur 

See what I wrote above: the plug is polarized!

Oh, sorry. I was unfamiliar with the term.

[ghporter]

Here's a polarized, two-prong U.S. plug:

Note that one prong is wider than the other, preventing it from being inserted in the socket the wrong way. The wide blade is the neutral side, and the narrow blade is the "hot" side. One major purpose of this is to ensure that with a switched device (say a lamp), that the switch is on the "hot" side of the circuit so that with the switch off, the lamp's socket is definitely NOT powered.

I don't think there's a European equivalent of this plug.

[Spheric Harlot]

Yeah, thanks - I'd googled it.

[rytc]

Originally Posted by ghporter 

Here's a polarized, two-prong U.S. plug:

Note that one prong is wider than the other, preventing it from being inserted in the socket the wrong way. The wide blade is the neutral side, and the narrow blade is the "hot" side. One major purpose of this is to ensure that with a switched device (say a lamp), that the switch is on the "hot" side of the circuit so that with the switch off, the lamp's socket is definitely NOT powered.

I don't think there's a European equivalent of this plug.

That's because by definition the plug in Europe is designed such that it can only go into the socket in one direction, except in Switzerland where it could go either way around. However, in Germany, France etc etc the plug can only go in one way, this is also the case with plugs from New Zealand and Australia. Hence, the lack of an equivalent is due to the lack of a need for such an equivalent.

[Le Flaneur]

Originally Posted by ghporter 

As for the hum coming or not coming from the MBP, that's irrelevant to keeping both the MBP and stereo grounded. A hum comes from an AC potential in an audio circuit. Having both devices properly grounded will prevent problems caused by poor or faulty grounding of the power system in the stereo. Those could cause a nasty shock if you connect a grounded device to a non-grounded stereo, or vice-versa.

I'm back. The problem is that I can't find a grounding post on this receiver!! How can that be? Should I just connect the wire to one of the case screws?

[Simon]

Originally Posted by rytc 

That's because by definition the plug in Europe is designed such that it can only go into the socket in one direction, except in Switzerland where it could go either way around. However, in Germany, France etc etc the plug can only go in one way, this is also the case with plugs from New Zealand and Australia. Hence, the lack of an equivalent is due to the lack of a need for such an equivalent.

I disagree. The ungrounded Swiss plug is unpolarized, but that's also the case for the two-pronged Euro plugs.


The Schuko plug (used Germany, Austria, Sweden, etc.) is grounded and still unpolarized.


It's the French version of this plug that has a ground on one side (and actually the prong is on the socket, not the plug) that's polarized. And that's also the case for the grounded Swiss plug.


The Italians cannot polarize in any case because even if they use grounded plugs, the ground is centered.

[Simon]

Originally Posted by Le Flaneur 

I'm back. The problem is that I can't find a grounding post on this receiver!! How can that be? Should I just connect the wire to one of the case screws?

If the case is metallic there's a good chance that would work.

Are there any coaxial antenna inputs or RCA outputs? If you take on of those and ground its shield you'll probably have grounded the entire stereo set.

[Le Flaneur]

Originally Posted by Simon 

If the case is metallic there's a good chance that would work.

Are there any coaxial antenna inputs or RCA outputs? If you take on of those and ground its shield you'll probably have grounded the entire stereo set.

Thanks. I tried that out, and ran a simple wire from the center screw of the nearest outlet (with the switchplate removed) to either the coaxial antenna input or RCA output or metallic case, and there wasn't so much as a little pop heard in the sound when contact was made! No difference whatsoever. Then I substituted an older stereo receiver (NAD 7250) with a dual-prong unpolarized plug to see if it had the problem, and there was no hum -- but it had white noise of its own coming out of the right speaker.

Sounds like the original HK receiver has a bad shield inside the unit and that nothing can be done at this point, doesn't it?

[ghporter]

Originally Posted by Le Flaneur 

Sounds like the original HK receiver has a bad shield inside the unit and that nothing can be done at this point, doesn't it?

Unfortunately, your assessment seems correct... Unless you feel like rolling up your sleeves and opening up the HK receiver that is.

[tooki]

Originally Posted by rytc 

That's because by definition the plug in Europe is designed such that it can only go into the socket in one direction, except in Switzerland where it could go either way around. However, in Germany, France etc etc the plug can only go in one way, this is also the case with plugs from New Zealand and Australia. Hence, the lack of an equivalent is due to the lack of a need for such an equivalent.

Hello fellow Zürcher!

Unfortunately, as others have said, that's incorrect. The ungrounded European plug goes in either way. Europe decided to rely on double-insulation instead of polarization. (Ever notice the nested squares on the conformity plate of an appliance or power supply? That means double insulation.) Current Swiss grounded plugs are by nature polarized, but as seen above, some other European countries have unpolarized grounded plugs. Older European plugs varied.

U.S. plugs exist as unpolarized, polarized, and grounded (on grounded plugs, they may or may not bother to make one prong wider, since it is possible to forcibly remove the ground pin and create a hazardous unpolarized plug if the prong isn't widened).