I have two questions for MeFi audiophiles. Electricians might be able to help too.

August 25, 2005 7:26 AM

I have two questions for MeFi audiophiles. Electricians might be able to help too.

I have two questions. The first is relatively simple. The latter is a bit more complex.

Question 1
A recent guest I had over to my apartment not knowingly placed some kind of heavy item on top of my receiver (I think it was a woman's handbag). I had the unit covered in a soft cloth at the time and did not notice any damage until days later when I went to watch a film. My concern is that my receiver (Denon 1804) now has a noticeable (to me) deviation in its top grill. Please note this diagram. Yes, the problem is only aesthetic. But it is really bugging me.

My question is in regards to removing the unit's top casing safely to fix the subtle damage caused by my guest's actions. Is this somewhat of a no-brainer exercise a novice like me could pull off? A little birdie is telling me to just leave it and move on but of course each time I look at my receiver all I see is this little bend in its top. What would those with experience in repairing this kind of damage propose I do? Again, please note that this is very slight damage. An uninformed eye probably would not notice the bend at all.

Question 2
I have just hooked a new subwoofer up to my existing system. I am now running Cerwin Vega V Series floor-standing speakers, a centre speaker, some rears and as mentioned a Cerwin Vega Classic 15" sub. Since plugging the sub in however I am getting a ground loop problem (constant monotone hum) with the sub and my receiver (Denon 1804). A call to technical support informed me that this was an issue with the earthing pin of my sub's electrical socket (the straight angled one) and that this was cause of the fault. I confirmed this too by trial and error (removing plugs and so on). The person on the phone was very helpful. He mentioned that although not endorsed by him removing the earthing pin from my sub's three-prong connector was a good way of fixing this problem. Now, I am no electrician but my mind starting thinking about this. Essentially I was thinking what is the point in hacking my brand new sub's main lead. Couldn't I just achieve the same thing using a custom-made extension cord with only a two-prong connector (e.g. one lacking an earth pin)? That is my question today. Basically would this work the same way? Also, feel free to add any other feedback on possible ways I might overcome this ground loop problem.

Yeah, I am a novice hi-fi enthusiast. Simpler explanations are always better for me.

posted by sjvilla79 to technology (22 comments total)

Edit: Now, I am no electrician but my mind started thinking about this.
posted by sjvilla79 at 7:30 AM on August 25, 2005

For the first question, you should be able to remove the screws holding the top cover in place and then slide it back and then lift it off. After you remove the power...
I'm not sure it's a good idea to have to the top of the receiver covered, it might overheat.
For the second question, go get an 3 prong to 2 prong adapter
posted by Ferrari328 at 7:47 AM on August 25, 2005

For gods sake do not unground your stereo system. Are you crazy?

Is the sub powered, and using a seperate electrical connection from the rest of the system? If so, does your amp have any switched outlets? Using one of those may help.

Try using a different outlet to plug in the sub and the other components. If you're using different ones already, try using the same one.

Removing a grounding pin on an electrical component is asking for a shock or a fire.
posted by RustyBrooks at 7:57 AM on August 25, 2005

Is there is a smooth area (no vents) to attach a large suction cup (toilet plunger) to the top of the receiver? You may be able to pull the dent up that way.
posted by weapons-grade pandemonium at 8:04 AM on August 25, 2005

Removing the ground is only dangerous if something goes wrong. It will not promote a shock, or a fire, but in doing so, you remove the safety mechanism should something happen. It's not recommended, but RustyBrooks' scaremongering is over-the-top.

The solution though, is to remove the earth in the signal cable between the subwoofer and the amplifier.
posted by benzo8 at 8:07 AM on August 25, 2005

For your ground loop problem, I would recommend buying an isolation transformer for the power to one or both devices. Removing the third prong (with an adaptor) can be a useful diagnostic technique (i.e. to check the situation before you shell out the bucks for a transformer), but is not a good idea in the long run. Likewise, removing the signal ground, while not dangerous to you, could end up damaging your equipment (i.e. fry the input amp on your receiver).

For reference, the purpose of the third prong is to provide a redundant ground that is tied directly to the chassis of the device. The idea is that if the device fails internally somehow, regardless of what's going on inside (like the hot wire contacting the chassis for example), the chassis will never be hot. In the hot wire touching the chassis example, your fuse will blow, but you will not be subjecting yourself to mains voltage if you touch the chassis.

Buy an audio isolation transformer.
posted by aquafiend at 8:14 AM on August 25, 2005

My receiver will still be grounded though. It has its own three-prong connector. Why would removing my sub's earthing capabilities therefore be cause for concern? Isn't one component being grounded for the whole system enough?

Also, that was a good suggestion.
posted by sjvilla79 at 8:16 AM on August 25, 2005

Removing the ground is only dangerous if something goes wrong.

Doesn't this sorta go without saying?

Why would removing my sub's earthing capabilities therefore be cause for concern? Isn't one component being grounded for the whole system enough?

If your sub is self-powered, then yes, it needs to be grounded as well. If it gets power from the receiver, then it shares the receiver's ground.
posted by odinsdream at 9:10 AM on August 25, 2005

I second the audio isolation transformer idea. I have, however, set up hundreds of live shows in ballrooms and tents under many different kinds of power conditions and have lifted grounds countless times as a down and dirty solution. I've never damaged a piece of equipment due to a lifted ground, but of course, that doesn't mean it isn't within the realm of possibility. For a thorough discussion of ground loops, both video and audio, check here.
posted by Roger Dodger at 9:11 AM on August 25, 2005

Removing the ground is only dangerous if something goes wrong.

That has to be about the most retarded thing I've read in regards to electrical advice. Seriously.

I understand what you're getting at. The ground connection is not generally in use. It's a protective measure. If there is a short, it prevents you from being shocked, prevents arcing, prevents excessive heat buildup in the equipment and therefore essentially protects you from shock and fire in the event that there is a short. Removing ground will not CAUSE these things, but it will increase their severity when they happen.

One real problem with electrical safety is that equipment still works with the ground removed. This leads people to think that lifting ground is perfectly OK. It's not. In particular, it's not when you're talking about a piece of equipment that is left on for long periods of time without inspection or oversight.
posted by RustyBrooks at 9:24 AM on August 25, 2005

If you're worried about grounding, replace the outlet the sub is connected to with a GFCI. You can then leave the ground pin off safely (ask any qualified electrician, I'm not). The only problem is you can put the plug in upside down if you've ripped it out of the sub's plug. Don't do that.

As far as the receiver goes... you leave it covered with a cloth? You should check your manual. There's a 99% chance it will tell you covering the grille is bad. Of all electronics your amp is probably going to generate the most heat! Don't do that! Baking electronics will shorten their life considerably (if anything the capacitors will fail prematurely, leading to unfun and expensive repairs in the future).
posted by shepd at 10:18 AM on August 25, 2005

Have you tried plugging both into the same outlet? This can occasionally fix earthing problems, and costs nothing to try.
posted by Civil_Disobedient at 10:52 AM on August 25, 2005

Oh, and as for the receiver, just unplug it, unscrew the casing and bend it back. It's really that simple. Don't touch the power transformer inside the case--those capacitors can store a lot of juice even when unplugged.
posted by Civil_Disobedient at 10:54 AM on August 25, 2005

One thing that I'll say with regard to ungrounding: If you have a phonograph, you need to ground it, otherwise you'll get wicked motor hum. Which you don't want.
posted by klangklangston at 11:38 AM on August 25, 2005

RustyBrooks:

The ground does not help with all those things. It makes sure there is no voltage on touchable metallic parts, even if something breaks loose inside the device. It is used in some PSUs to stabilize output level, but that's it. It doesn't prevent heatup. The circuit gets just as hot if current flows to earth. There are fuses to take care of that, in your power distribution central as well as in devices that are prone to failure. And what do you mean by "Prevent arcing"? Earth is (nominally) at the same potential as neutral, providing no electrostatic shielding from the hot lead. In this case, there IS a ground connection through the receiver - subwoofer cable shield. The redundancy in grounding causes the hum. Sure, it doesn't have the thickness recommended for protection grounding, but it will tolerate way more current than the fuse.

Of course you shouldn't pull the earth plug if you don't have to, and not to delicate or high-voltage things such as computers or monitors (or phonographs).

However, if the 3->2 prong adapter solves the problem, use it. Install a ground-fault breaker to improve personal safety.
posted by springload at 5:21 PM on August 25, 2005

RustyBrooks writes "Removing the ground is only dangerous if something goes wrong.

"That has to be about the most retarded thing I've read in regards to electrical advice. Seriously.

"I understand what you're getting at. The ground connection is not generally in use. It's a protective measure. If there is a short, it prevents you from being shocked, prevents arcing, prevents excessive heat buildup in the equipment and therefore essentially protects you from shock and fire in the event that there is a short. Removing ground will not CAUSE these things, but it will increase their severity when they happen.

"One real problem with electrical safety is that equipment still works with the ground removed. This leads people to think that lifting ground is perfectly OK. It's not. In particular, it's not when you're talking about a piece of equipment that is left on for long periods of time without inspection or oversight."

Rusty, I suggest that you haven't a clue about what you talk. If you wish to call me, or my adice, "retarded", that's up to you, but you can't argue with reality. I said, specifically, that it wasn't recommended, and I offered an alternative solution. But the things you talk of are just wrong - a piece of ungrounded equipment is no more dangerous than a piece of grounded equipment in ideal conditions, however long you leave it connected for. The ground doesn't affect heat, current flow or any other damn thing unles something goes wrong. In that situation, the ground offers the current a route to earth, which it will take. This increases the current to a point that the fuse blows and voila! - you're safe. If the earth has been removed and the equipment fails in such that live runs to the case (no longer earthed), it wil do nothing whatsoever until offered a route to earth - ie: when you touch the case - at which point the current will flow though you and you will be shocked until the current blows the fuse (if ever). Claiming that removing the fuse will cause death, overheating, or pasta to cook tough is totally incorrect and does not answer the question with correctly.

In the situation of a ground loop, you want to break the loop in some way. Ungrounding the equipment is one way but it is not recommended. Adding a ground transformer is a good way of solving the problem, but it could be expensive and isn't the only solution. Cutting the ground connection on the signal cable between the equipment and its receiver is another solution, which I originally suggested. It's not the only answer, but it's an answer, whereas "are you crazy?" is not...
posted by benzo8 at 6:54 PM on August 25, 2005

Cutting the ground connection on the signal cable between the equipment and its receiver is another solution, which I originally suggested.

Can you buy leads already made like this though? I don't want to do this myself.

Buy an audio isolation transformer.

Would something like this work right out of the box? Can you suggest any good models to look out for?

As far as the receiver goes... you leave it covered with a cloth? You should check your manual. There's a 99% chance it will tell you covering the grille is bad.

I always wait a couple of hours before covering the unit. Mostly though I only cover it when it's not in use for extended periods of time. There's no harm in covering a unit that's not still warm from use is there? I wouldn't think there would be.

So, I'm still not really wiser as to fixing my ground loop problem. I've rung a couple of dealers today and they've just said they always just pull the problem device's earth pin. You can imagine how this just made me more confused! Anyway, I'll ask again who here is the most confident in their reply. I really appreciate all the comments but I'd just like to know what's worthwhile pursuing and what's not. My email address for additional private feedback is svillarosa at hotmail dot com.
posted by sjvilla79 at 10:28 PM on August 25, 2005

Before ungrounding the unit, try plugging everything into the same outlet.

Ungrounding will probably work. It's not the best solution. It probably will never matter. But there is a small chance that if there ever was an electrical short you would touch the casing and get a lethal shock. The line about "it's fine until something goes wrong" is like saying "you should drive without seatbelts, since --hey-- it doesn't matter unless you get into a wreck."

THe difference is that auto wrecks happen all the time whereas electrocution from ungrounded appliances is more rare. But you are most certainly increasing the chances of injury. It may never matter, but I still don't think it's a good idea if you can solve the problem easily with other means.
posted by Rhomboid at 12:52 AM on August 26, 2005

Well it seems logical to me to cut out all possible risk of causing any damage, problem, or whatever else might happen as a result of de-earthing my sub's main connector. I appreciate the comparison you made, Rhomboid. Thanks for the clarification.

I also agree with you on finding a better option. I've really not much idea where to start with this though, The stores I rang today just gave me disappointing feedback. They even went as far to suggest that using an isolator on the ground loop wouldn't even work. Doesn't this just contradict a lot of what has been said here? Are there even isolators for specific use in the home environment? Perhaps I'm looking in the wrong places and talking to the wrong people. I'm starting to wish I never bought this new subwoofer now. Perhaps I should just pay an electrician or hi-fi tech guy to sort this for me. Sigh. I think I'm losing sight of the bigger picture.
posted by sjvilla79 at 4:39 AM on August 26, 2005

Several people are confident about their replies. I'm one, and here is mine.

If you have a high-end system with an outer connector in the cable and two cables inside, one signal wire and one ground wite, things are different. Then you can try cutting the shield at the subwoofer.

But let's presume that this is not the case, that he cable to the subwoofer does not have a shield separate from the signal ground. That means, it consists of one wire inside another one. On both ends, the outer connector is connected to the chassi, which is in turn connected by the third prong to earth. The receiver outputs an electrical signal representing the sound. It does this by applying a voltage between its own chassi and the cable's inner conductor. The subwoofer has its own amplifier, which reads the voltage between its chassi and the inner conductor.

Now there is a problem: Since the chassis of the two devices are grounded separately, there is a big loop in the ground conductors, that picks up magnetic fields. Since this loop includes the outer connector of the cable, a current flows through it, producing an AC voltage between the receiver and the subwoofer. The receiver outputs a voltage between its ground and inner connector, and the subwoofer reads the same voltage between its ground and the inner connector. But there is a voltage difference between the grounds of the two devices! And it fluctuates at 60Hz! The subwoofer sees this as a 60Hz sound on top of the music you're trying to feed it.

Now, you could do as other people say and cut the outer connector of the cable. That's quite drastic. The reference grounds of the two devices will be connected only through the third prong, through mains protection wiring. There is no longer a loop picking up magnetic fields, so the situation might be somewhat better. However, the ground connection is still quite long and may pick up interference. It's not certain, or even likely, that the two grounds are equal even with the cable outer snipped. A small constant difference between the two grounds, or a small temporary difference from a surge, does no harm to the system. But there could be a big difference, triggered by the failure of another device on the same circuit, lightning or something else. An event like that could trash the input of your subwoofer. In short, cutting the outer of the cable is not recommended at all. An audio isolation transformer solves this problem. It breaks the ground loop and makes sure that the inner connector (the signal) is referenced to local ground. The relative ground potentials of the to devices are no longer important. Beware, however, that transformers tend to pick up hum by themselves. Also, the transformer will filter away some of the lowest-frequency sound, which might not make it such a good choice for a subwoofer.

Lifting the ground of the subwoofer (using 2->3 prong adapter) makes sure that the ground potential is the same as in the receiver. That should solve the problem and is what I'd try first.

Regarding electrical safety: It's very rare that a hot wire comes in contact with the chassi. If it does, the protection earth is there to short the chassi to ground. In an instant, a fuse will blow due to the high current. If you lift the ground, you no longer have that recommended protection. Nonetheless, there IS a connection to earth. Through the signal cable outer, remember? This cable shorts the chassi to the receiver chassi, which is in turn shorted to mains earth. Protection earth wires are prescribed to be of a certain gaugue, and the signal cable is not that thick. Still, I cannot imagine that it would blow before the fuse does.

Lastly, the best protection you can get is a ground fault breaker. It monitors the current through the hot and the neutral, and as soon as they don't match, it breaks the circuit. It's very efficient, and saves you in a situation when you come between the hot and protection earth, or between hot and a water pipe. Get one of those and try the 2-prong adapter. If it works, don't worry too much. The breaker is as likely to save you from accidents as the third prong ever was.
posted by springload at 10:17 AM on August 26, 2005

springload, that was a great, well-written reply to my queries. I'm going to take all this information and try a few things. Again, thanks to you and the others who shared their knowledge. Great stuff. I'll post back once I get something working.
posted by sjvilla79 at 8:28 PM on August 26, 2005

Well, that was fast. I found out where/what the fault was. It was my antenna connection at the wall causing the issue! It would appear I need to isolate the antenna instead. Either that or I could just leave it. I mean my system isn't complex and it's not hard to pull the plug on the antenna whenever I want to use it.

Yeah, I'll casually look at perhaps getting an antenna isolator. I might even go with some of the methods discussed here (I'll probably want to watch television soon enough for example).

Things are sweet now though. Don't you just love happy endings? Whoop!
posted by sjvilla79 at 9:17 PM on August 26, 2005