What solution do folks use for the dreaded BFD hum? - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #21 of 324 Old 10-01-06, 02:15 PM
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Re: What solution do folks use for the dreaded BFD hum?

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If I understand the root cause of ground loop hums, this seems like it will remove the voltage potential that is causing the hum.
In some cases it will. It never hurts to ground the cable at the service panel. Any difference in potential on the safety grounds at your equipment can cause a hum.

Even though the safety ground is a cold conductor, it can and usually does, develop a small potential, through mutual inductance, wire resistance and various other reasons that can be quite different at each receptacle in your house.

When you plug a power amp into one receptacle and a preamp into another receptacle, the metal cases of these two units can have a small potential difference in their safety grounds which means that this equipment's metal cases are at a slightly different potential. When you connect a single ended (RCA) cable between these two devices, a small AC current can flow in the shield because of the potential difference. This unwanted signal is in the signal loop circuit and can cause a hum. An interconnect circuit has a loop path (completed circuit) that flows through the centre conductor of the interconnect cable and back on the shield. If there is an AC signal on the shield flowing because of the ground difference potential, you'll hear a hum. Breaking the safety ground of one of the two devices removes the potential and the path for the unwanted signal flow...

Exactly the same situation can occur, except usually worse, when you introduce a new ground into the system from cable TV or a satellite. Their ground on the shield may possess a different potential than the ground in your system and current will flow in all the interconnects. Usually by centralizing and bonding all external grounds to the common house ground you're at least giving yourself the best chance of reducing this problem.

With cable, sometimes it's necessary to use an isolator device like this one. They usually work - not by magic, but by using an RF balun or similar device. They are quite safe, albeit a bit expensive.

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post #22 of 324 Old 10-01-06, 03:50 PM
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Re: What solution do folks use for the dreaded BFD hum?

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Mitch G wrote: View Post
I was reading some other stuff about ground loop hums and one suggestion was to ground your cable TV service to the same place as the power ground.
Not only is it advisable, it is a violation of electrical codes anywhere in the US to not ground the cable to the a.c. service ground. You should have a ground block at the entry point connected tot he same ground rod that your electrical service is grounded to, not a water pipe. This will likely improve your problem.

You don't have to do it yourself. Your cable provider is REQUIRED to do it. If they refuse contact your county inspector's office or state professional certification board and let them know that they are not properly grounding their installations. You'll get action.




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post #23 of 324 Old 10-02-06, 02:18 PM
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Re: What solution do folks use for the dreaded BFD hum?

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brucek wrote: View Post
Can you explain what you mean? The BFD has both balanced and unbalanced capability on the output already. If you have a hum, you could try the DC1 that was mentioned..

brucek
I was referring to lcaillo's post, the second in this thread, where he suggests this to be the proper way to cure the problem, apparently with some type of adapters or cables of adaptive nature. This would seem preferable to line source measures that utilize filters or whatever. Would this cure the problem? Apparently lcaillo believes so. Any drawbacks to this? Anyone else tried this approach?

Regarding the HumX, I agree without some data how are we to evaluate the product. Upon recieving the product, which does not include any additional data, I was left wondering if I had in fact just purchased the worlds most expensive cheater plug!

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post #24 of 324 Old 10-02-06, 02:23 PM
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Re: What solution do folks use for the dreaded BFD hum?

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Robert_Gibran wrote: View Post
I was left wondering if I had in fact just purchased the worlds most expensive cheater plug!
We need to call out the manufacturer on it if we find this to be true.
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post #25 of 324 Old 10-02-06, 02:43 PM
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Re: What solution do folks use for the dreaded BFD hum?

No hum on mine. Using phono connectors.
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post #26 of 324 Old 10-02-06, 03:06 PM
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Re: What solution do folks use for the dreaded BFD hum?

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brucek wrote: View Post
Can you explain what you mean? The BFD has both balanced and unbalanced capability on the output already.
brucek
The BFD manual states that the outputs are balanced and recommends using balanced devices. The BFD is no different from any other balanced device, in that you can ground one side and use it with unbalanced devices, halving the impedance. While this may work fine in some systems, others will have problems with it. The only certain way to avoid ground loops due to kludged unbalanced connections is to use a balancing transformer or an active device designed to modify the output configuration. Transformers are the logical choice for low frequencies like we use with subs.




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post #27 of 324 Old 10-02-06, 05:34 PM
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Re: What solution do folks use for the dreaded BFD hum?

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I was referring to lcaillo's post, the second in this thread, where he suggests this to be the proper way to cure the problem, apparently with some type of adapters or cables of adaptive nature. This would seem preferable to line source measures that utilize filters or whatever. Would this cure the problem? Apparently lcaillo believes so. Any drawbacks to this? Anyone else tried this approach?
Robert,

I'm don't see where lcaillo's post refers to adapters or cables. He said the proper way to eliminate the problem is with a transformer - and he's correct. I backed this up in my post where I suggested any of the in-line solutions suggested in the Guide would work by using transformers or differential amps. You do have to be careful to not purchase a transformer that is so cheap that it suffers a poor low frequency response.

Either the DCI-ALHI or the Ebtech Hum Eliminator would be fine. Personally, I like Jensen transformers. Some people just make their own with Jensen transformers or they purchase them in a box like this or this.
Jensen supplies this type of proper spec sheet. Why can't the others? I suppose that's why Jensen products are so expensive.
MarkerTek for $120.....or mono MarkerTek for $95...............

brucek

EDIT: I should be mentioning the Jensen isolator for subs which has better low frequency response.
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post #28 of 324 Old 10-03-06, 12:04 AM
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Re: What solution do folks use for the dreaded BFD hum?

Bruce,
The HumX patent application appears to be number 20040264712 which can be seen at:
http://appft1.uspto.gov/netahtml/PTO/srchnum.html

In addition to the BFD I also have a hum problem with an HDMI connection between my Panasonic projector and Denon receiver. I have not seen any signal path solutions for HDMI.
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post #29 of 324 Old 10-03-06, 04:55 AM
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Re: What solution do folks use for the dreaded BFD hum?

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Dundas wrote: View Post
Bruce,
The HumX patent application appears to be number 20040264712 which can be seen at:
http://appft1.uspto.gov/netahtml/PTO/srchnum.html

In addition to the BFD I also have a hum problem with an HDMI connection between my Panasonic projector and Denon receiver. I have not seen any signal path solutions for HDMI.
The HDMI connection is just one of the ground paths that exist in your system. Look for a solution elsewhere by improving the ground at some point or isolating one. Chances are that the problem is something else and breaking that connection is just one of several that relieves the problem.




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post #30 of 324 Old 10-03-06, 08:51 AM
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Re: What solution do folks use for the dreaded BFD hum?

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Upon recieving the product, which does not include any additional data, I was left wondering if I had in fact just purchased the worlds most expensive cheater plug!
Quote:
The HumX patent application appears to be number 20040264712 which can be seen at:
The patent documentation shows the device to be exactly as I indicated before. It uses the standard method of inserting (parallel back to back diodes) in series with the safety ground. Using the fact that the flow of current that causes hum is usually quite low, it is easy to see that if you place a device in series that inhibits that flow until a small breakdown voltage is reached, then no current will flow and the hum problem is solved.

A single diode has a breakdown of 0.7volts and two would have a breakdown of 1.4volts, etc. You need two sets in parallel with opposing polorization, since this is required to pass AC current.

This is the standard principle used in marine galvanic isolators to block AC and DC currents from reaching a boat connected to shore power. It eliminates corrosion of the boat hull etc.

Anyway, this is great as long as those diodes are rated to pass a steady state current long enough to trip a breaker on the hot line. A 120voltAC dead short can pass enormous current. It can easily be a few hundred amps depending on your service. The breaker should trip fairly quick. It must trip before those diodes blow. Most high current diodes are the variety that require bolting to a heat sink to achieve their rating. But then, they must pass that current for long periods of times. The diodes in the HumX only need to pass that current long enough to trip a 15 or 20 amp breaker. Hopefully the diodes that are jammed into that little device are spec'd to achieve that. I'd feel confident if it had a UL or CSA sticker on it. Either way, the HumX is sure better than a cheater plug.

You also have to realize that if the BFD is connected to a device that has a safety ground, then the shields of the interconnects will pass the current in the event of a BFD failure and trip the breaker. In fact, that's long been a trick to eliminate ground loops and its resultant hum. Choose a central device that everything is connected to (such as a processor), and then cheat every other device except the processor. The theory being that the interconnects will provide the path to safety ground through the one device that is safe. This is a horrible idea though, since some unsuspecting person may have the interconnects pulled off and a fault may occur and electrocute them. Don't use this method.

The best and safest method is to plug in the three prong plug to the wall and if there is a hum, solve it at the line level.........

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