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In Reply to: RE: dc hum remedy posted by leadbelly on June 23, 2009 at 19:21:01
Lift the ground on the plug and see if the hum goes away. This happened to me when I thought I did not have a ground loop. Permanent solution for me was a cheater plug on the amp, plugged into a circuit with a GFCI on it. Much safer than a 3 prong plug, plugged into a non-GFCI outlet.Cut-Throat
Hi Cut-Throat,
I would encourage you to pursue another solution rather than lifting the safety ground. I have written an article that may help you.
Dave
If you think this is un-safe you're mistaken.I've researched this carefully and thoroughly and verified it with an Electrical Inspector.
Replacing Receptacles to Meet the NEC
"A grounding-type receptacle that is GFCI protected without an equipment grounding conductor is a SAFER installation than a grounding-type receptacle with an equipment grounding conductor (if GFCI protection is not provided). This is because the GFCI protection device will clear a ground-fault when the fault-current is 5 milliamperes (+ or – 1 milliampere), which is less than the current level necessary to cause serious electric shock or electrocution"
Cut-Throat
Edits: 06/25/09
Hi Cut-Throat,
Please read the article. I make a couple of points that you may find interesting. One is that a loop in itself is not bad, it depends what is on the loop. The point is that something is causing the ground loop current - go after that. Another is that there are a couple of ways to effectively deal with a ground loop (other than isolating the safety ground.)
Dave
Dave,
I tried to read your document, but quickly got lost by page 4. I will save the document and study it in a long Minnesota Winter. I don't want to redesign the amplifiers and pre-amplifiers that Welborne Built, I just want to listen to some music without hum.
I had my Electrical Inspector friend over and he assured me that if I lifted the grounds and added a GFCI to the circuits that it was safer than not lifting the grounds on a regular circuit.
If you know a better way that you can state simply, give it go. But I don't intend to re-design the amplifiers that Welborne built as I am unqualified and unmotivated.
Thanks!
Cut-Throat
Hi Cut-Throat,
I would very much like to make the article as simple as possible. Your feedback and questions will be appreciated.
Dave
nt
Cut-Throat
As a former member of the IBEW and their excellent apprentice program, I have to agree with Dave. Here's why.
Notice that GFCIs have a grounding conductor, if it was true that it was safer without it, it wouldn't have one. Here's what the grounding conductor does. If a winding wire would come into contact with the frame of a motor or transformer, it would trip the equipment fuse, or in worst case the circuit breaker.
Without a grounding conductor and it's path to ground, the appliance is energized at that voltage (whatever that may be), but no current flows due to an open circuit, so the GFCI doesn't trip. You come along and touch the energized frame, and your body is the source to ground. If the GFCI works, and they have known to malfunction, you at best are given a mild shock. People (especially children, and people with underlying health issues) have been known to receive a fatal shock from as little as 6ma. That is well into the 5ma +/- 1 ma.
If the appliance had a grounding conductor, hopefully you wouldn't be touching the frame at the time of malfunction, and the appliance wouldn't form a booby trap waiting to happen.
The GFCI just provides an added measure of insurance, if it detects a current imbalance of 6ma or more it SHOULD trip, which is less energy than it takes to blow a fuse or trip a breaker. However, without a grounding conductor your body would be the probable conductor of that current, and it provides no preventative protection, that a grounding conductor would provide. So a GFCI with a grounding conductor provides the safest condition.
With all my projects, I have never had a ground loop problem, and if I did I'd fix it, rather than lifting the ground. Sorry Cut-Throat, but lifting the ground in equipment without a double insulated case is just bad advice.
twystd
The hum may not bug some folks, but it bugs me. Read the following. Written when most audio was 2 prong plugs.
Figure 1. Ground loop involving line cords.
Both the preamp and power amp are equipped with 3 wire line cords and 3 prong plugs. (Note: this is almost unheard of in home audio equipment. This example is given for illustration purposes only). The green wire in each line cord is connected to the chassis of each unit. The ground prong of each plug connects to the local power ground. The two chassis' are also connected together by the shields of the audio cable. The audio cables are much longer than indicated in the diagram.
The shaded area shows the ground loop. As you can see there is a continuous loop from the preamp chassis to the power amp through the audio cable shields, from the power amp through the grounding wire of the line cord to the wall socket, and from the wall socket to the preamp chassis.
The two shields of the stereo audio cable are held so close together for most of their length that they rarely contribute any significant interference.
What can be done?
In the case of Figure 1.
You might try grounding the two chassis' together with very heavy wire in an attempt to short out the induced voltage. The added wire will just create another loop unless you place it physically very close to the audio cable. Even if you do that all you will succeed in doing will be to reduce the volume of the hum but it will still be audible.
You might also try bringing the line cords close together and close to the audio cable but most likely all you will accomplish will be to increase the hum. You will place AC carrying conductors into close proximity with the audio cables thus inducing more hum than you are getting rid of.
The only sure way is to break the loop. Cutting the shields on the audio cable rarely works with unbalanced inputs. There will always be a potential difference between the two chassis' even though they are grounded to the power line ground. The reason is that the grounding wire gets a current induced in it because it is in such close proximity to the current carrying conductors of the line cord. If this were a real situation, the only solution would be to somehow disconnect the power line ground from one of the chassis'. This is not approved by electrical inspectors and fortunately doesn't need to be done in home stereo equipment.
Cut-Throat
.
If you have a power amp with a 3 prong plug and a pre-amp with a 3 prong plug you have a ground loop. Whether your hearing is good enough to hear it or it doesn't bug you is another topic.
I have 104 db Klipshorn/Altec speakers, and I can hear hum on them without the grounds lifted. I have studied this problem for 5 years and unless you use isolation transformers (which are not cheap and may compromise other parts of your system) lifting the ground is the ONLY way to totally eliminate the problem. If there was a cheaper more effective way to completely eliminate ground loop hum, I would have done it long ago. Other than using a passive pre-amp (which I own, but doesn't sound as good)
The other bottom line is an Electrical Inspector will tell you that a circuit with GFCI protection and a 3 prong device with the ground lifted is a safer situation than a non GFCI circuit with a 3 prong plug intact. So before you chastise a lifted ground with GFCI protection, please lobby everyone to install GFCI protection on their stereo circuits. I am not sure why lifting the ground raises hackles on people like it does, but it certainly does!
If we really wanted to be completely safe we wouldn't want to use electricity at all. Or even think about driving a car! Let's keep lifting a ground in perspective.
Cut-Throat
Do you really think all of us that don't have ground loop problems, are just putting up with it, or have defective hearing?? I guess your equipment is the summit of perfection, and if it has problem so does everyone else's.....jeeze!
You've talked to an electrical inspector that has proved the Peter Principle, that you get promoted to your own level of incompetency. It depends if the fault happened while I was in physical contact with the device, then a GFCI would be best, as it would trip before a fuse would blow, or circuit breaker would throw. However if the fault happened while the device was operating, but I wasn't in physical contact, I'd prefer the three wire system, as it would trip as soon as the fault happened, without me providing the path to ground, and suffering the 6ma shock.
Without the three wire system, a device under fault just sets at it's faulty electrical potential, until presented with a path to ground, and that could be you. This is why the three wire system was adopted in the first place. With the GFCI, hopefully you'd be exposed to only the theoretical 6ma short duration shock, and you don't have a pacemaker, underlying heart condition, or your three year old didn't stick his tongue on it!;-)
twystd
Not only is the 3-prong system necessary for safety-- the GFCI units SOUND BAD...... very bad.
In a practical system, we're going to have 3-prong wiring on every component.
It is up to the installer to arrange all of these 3-prong connections AND WIRING LENGTHS and PLACEMENTS so that ground-loop problems are minimized.
Equipment is not safe, nor is it necessarily going to sound "better" with grounds "lifted".
Keep the grounds, and learn how to use them properly.
---Dennis---
Hi Dennis,
"Keep the grounds, and learn how to use them properly."
AMEN....
Dave
nt
Cut-Throat
Which says that a 2 prong appliance with a GFCI on it is safer than a circuit with a 3 prong plug without a GFCI.
That is a fact, not my opinion. - Per the NEC
Maybe you misunderstood. The outlet has a ground on it! - The GFCI is installed on the circuit at the circuit breaker. If any voltage leaked to the chassis, the GFCI trips immediately.
Cut-Throat
I'm not aware that the NEC advises the use of a cheater plug. I believe the 2 prong appliances, refer to an appliance with a double insulated case, which are the only appliances than can be legally manufactured with a 2 prong plug. Of course it has been several years since I've studied the NEC code book, and things could have changed.
When you state "If any voltage leaked to the chassis, the GFCI trips immediately" is not accurate. If the chassis is not grounded, and it won't be without a grounding conductor, the chassis is energized at that voltage (could be several hundred volts). This voltage just stays there until provided with a path to ground. If you touch the energized chassis, your body then becomes the path to ground, and hopefully the GFCI trips immediately.
I really don't want to be argumentative, and I will agree that what you are doing is reasonably safe, IF the GFCI functions properly (even though there have been documented cases of 6ma being fatal). In the apprenticeship program we were taught that GFCIs were not absolutely safe (of course nothing is), but the grounding conductor should never be lifted.
Maybe I'm just too careful, but I prefer birth control and an "overcoat". Besides I like the idea that with a grounding conductor you have the preventative safety feature as well.
I also think that any piece of equipment that generates a ground loop isn't properly designed, and it is best to address that, rather than putting a band aid on it. I also suspect that the sonics might be improved with better grounding practices, and the lower noise floor that usually comes with it.
I'm not saying that what you are doing won't work, or that it is horrendously unsafe, but that it is not best practice, from either a safety or sonics point of view. I think the best approach would be to fix the ground loop. The cheater plug and GFCI is however an easy and effective "fix", but should only be used as a last resort. What really worries me is, I've seen some recommend lifting the ground, and don't even mention a GFCI, to your credit, you are not guilty of that.
twystd
Replacing Receptacles to Meet the NEC
"A grounding-type receptacle that is GFCI protected without an equipment grounding conductor is a SAFER installation than a grounding-type receptacle with an equipment grounding conductor (if GFCI protection is not provided). This is because the GFCI protection device will clear a ground-fault when the fault-current is 5 milliamperes (+ or – 1 milliampere), which is less than the current level necessary to cause serious electric shock or electrocution"
Cut-Throat
GFCIs just give electricians something to do.
I have tried this very combination--cheater plug into a gfi device--and every other combination in the course of my experiments: no real effect. Thank you for your suggestion. How did your unknown ground loop problem manifest itself?
I had a slight hum (but still annoying on soft passages) coming from both speakers. It was not as loud as most ground loops that I have heard, so I didn't expect it to be a ground loop. But the noise went dead silent after lifting the ground.
Cut-Throat
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