What solution do folks use for the dreaded BFD hum? - Page 9 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #81 of 324 Old 03-22-07, 10:38 PM
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Re: What solution do folks use for the dreaded BFD hum?

Have you tried properly grounding the cable line?




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post #82 of 324 Old 03-22-07, 11:45 PM
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Re: What solution do folks use for the dreaded BFD hum?

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Have you tried properly grounding the cable line?
No - I didn't realize that was something I even could do. I thought the cable company or something grounds it to earth at some point outside.

Are you saying that it is not properly grounded for sure if I am getting this hum?

How could I go about checking to see if the cable line is properly grounded and correcting it if not? Thanks.
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post #83 of 324 Old 03-23-07, 09:40 AM
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Re: What solution do folks use for the dreaded BFD hum?


Look around the back of your house for the point where the cable service enters the house. It should be where your electric meter is. You should see a so-called “grounding block” that looks like this:


The coaxial cable connects to both sides of it. Where the screw terminal is, there should be a wire that goes to the copper ground stake for your electrical service. Make sure the wire is tight and secure on both ends. Maybe even replace it to make sure the connections aren’t oxidized and corroded.

Since we’re dealing with cable TV service, a trick that often works is to use a pair of 75-300 ohm transformers, like this:


Connect the screw terminals together, and your coaxial cables on the other ends. A barrier strip is a good way to tie them together and keep the connections seperated:


You can do this inside the house, between the wall and the cable box. Check your channels after you’re done, though – sometimes it can affect the picture on the high-numbered channels, if I recall.

Regards,
Wayne



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post #84 of 324 Old 03-23-07, 09:58 AM
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Re: What solution do folks use for the dreaded BFD hum?

Thanks Wayne. I don't recall seeing any such grounding block, but I will check this out.

A few quick questions please:

1) If the grounding block is there and installed properly, and the AC outlet I'm using for the BFD is grounded properly, would that mean for certain that I would not have any hum in that situation? Or is it possible for both the cable feed and the AC outlet to be grounded properly yet still get a hum?

2) Assuming the cable tv and AC outlet is grounded properly yet I still have a hum, I'm thinking it would be best to just treat this at the BFD where the connectors are. This way I do not have to worry about any interference with my cable receiption which can have its moments as it is...

3) What are the costs of the parts involved to make the custom connectors for the BFD you mentioned? I can likely create these myself if I know exactly what parts to get and have good instructions.

Thanks!!
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post #85 of 324 Old 03-23-07, 10:25 AM
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Re: What solution do folks use for the dreaded BFD hum?


Quote:
1) If the grounding block is there and installed properly, and the AC outlet I'm using for the BFD is grounded properly, would that mean for certain that I would not have any hum in that situation? Or is it possible for both the cable feed and the AC outlet to be grounded properly yet still get a hum?
It’s still possible to get the hum. The feed could already be “infected” before it gets to your house. (Again, brucek knows more about the technicalities of ground loops that I do - I'll defer to his thoughts on this.)

Quote:
2) Assuming the cable tv and AC outlet is grounded properly yet I still have a hum, I'm thinking it would be best to just treat this at the BFD where the connectors are. This way I do not have to worry about any interference with my cable receiption which can have its moments as it is...
It’ll only cost a few bucks to try out the dual transformer trick. You can take the parts back to Radio Shack and get a refund if it doesn’t work for you. Give it a try.

Quote:
3) What are the costs of the parts involved to make the custom connectors for the BFD you mentioned? I can likely create these myself if I know exactly what parts to get and have good instructions.
From Parts Express, $1.70 for a Neutrik RCA, $2.45 for a 1/4” TRS plug (or $2.33 for an XLR), $0.60/ft. for the cable. Plus shipping. Do you have a soldering iron?

Regards,
Wayne



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post #86 of 324 Old 03-23-07, 10:27 AM
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Re: What solution do folks use for the dreaded BFD hum?

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Since we’re dealing with cable TV service, a trick that often works is to use a pair of 75-300 ohm transformers
Yeah, the problem there is two-fold. Many of these baluns pass the ground through on one side. You have to get the kind that uses no ground pass-thru. It needs to be checked with a meter. The other problem is the bandwidth. Many of these baluns only pass TV signals and don't pass the CATV full bandwidth.

The best solution would be to use a in-line transformer on the CATV cable that passed at least 1GHz and those are expensive, so the capacitive solution is the next best so as to block the DC current, yet pass the bandwidth for CATV AC signals.

Here's a ISO-MAX VRD-1FF Cable TV Ground Isolator ground break solution that you would put at the STB as the cable enters. It should do the trick I would think. A bit expensive, but that's what it takes...

Quote:
1) If the grounding block is there and installed properly, and the AC outlet I'm using for the BFD is grounded properly, would that mean for certain that I would not have any hum in that situation? Or is it possible for both the cable feed and the AC outlet to be grounded properly yet still get a hum?
Correct. The further you are away from the point of grounding, the more difference potential can build across the long distance (distance adds impedance).

Quote:
2) Assuming the cable tv and AC outlet is grounded properly yet I still have a hum, I'm thinking it would be best to just treat this at the BFD where the connectors are. This way I do not have to worry about any interference with my cable receiption which can have its moments as it is...
Or treat the cable as I mention above.....

Quote:
3) What are the costs of the parts involved to make the custom connectors for the BFD you mentioned? I can likely create these myself if I know exactly what parts to get and have good instructions.
At the CATV frequencies, it's best to buy off the shelf.
Even if you attack it at the audio level (i.e. transformers after or before the BFD), it has to be done to pass the low frequency signal of a sub without attenuation. A transformer that can pass 10Hz would be the Jensen variety you mentioned earlier. Pricy though..

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post #87 of 324 Old 03-23-07, 11:10 AM
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Re: What solution do folks use for the dreaded BFD hum?

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Wayne A. Pflughaupt wrote: View Post

From Parts Express, $1.70 for a Neutrik RCA, $2.45 for a 1/4” TRS plug (or $2.33 for an XLR), $0.60/ft. for the cable. Plus shipping. Do you have a soldering iron?
Yes, I have a soldering iron and most of the basics (electrical tape etc).

Quote:
brucek wrote:
At the CATV frequencies, it's best to buy off the shelf.
Even if you attack it at the audio level (i.e. transformers after or before the BFD), it has to be done to pass the low frequency signal of a sub without attenuation. A transformer that can pass 10Hz would be the Jensen variety you mentioned earlier. Pricy though..
Would this apply to the approach Wayne is talking about with the custom cable, and I'd have to worry about whether the sub frequencies were getting mucked with? Or is the custom cable/connector approach just a matter of isolating the ground that doesn't really make any changes to the frequencies itself (my understanding)?

Thanks!
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post #88 of 324 Old 03-23-07, 12:32 PM
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Re: What solution do folks use for the dreaded BFD hum?

Quote:
Would this apply to the approach Wayne is talking about with the custom cable, and I'd have to worry about whether the sub frequencies were getting mucked with? Or is the custom cable/connector approach just a matter of isolating the ground that doesn't really make any changes to the frequencies itself (my understanding)?
The latter statement is correct, but realize the cable approach isn't guaranteed to work in all cases. Most people would rough that sort of idea up first and if it worked in their case, then they'd spend some money and make it nice....

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post #89 of 324 Old 03-23-07, 12:38 PM
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Re: What solution do folks use for the dreaded BFD hum?

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brucek wrote: View Post
The latter statement is correct, but realize the cable approach isn't guaranteed to work in all cases. Most people would rough that sort of idea up first and if it worked in their case, then they'd spend some money and make it nice....

brucek
Here's a quick update... I check outside and there is a grounding block that is grounded into a cooper pipe. Looks like the telephone is also tied into this pipe. The cooper pipe runs inside the house and into the electrical panel. So it sounds like the cable is grounded properly.

As far as solutions go - I did find a cable isolator similar to the Jensen except it is only $10 and rated up to 1000Mhz instead of 1300Mhz like the Jensen.

However I'd much prefer to try with the custom cable / adapter approach as I don't like the idea of inserting anything before the STB. The only issue the ground loop is causing is with the BFD.

So Wayne if you'd be so kind as perhaps to provide some detailed instructions I'd like to give this a try first. If that doesn't work then I'll go to the Jensen or similar isolation transformer for insertion at the STB. Thanks!
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post #90 of 324 Old 03-23-07, 12:55 PM
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Re: What solution do folks use for the dreaded BFD hum?

Quote:
However I'd much prefer to try with the custom cable / adapter approach as I don't like the idea of inserting anything before the STB. The only issue the ground loop is causing is with the BFD.
If you still use any analog cable channels, I suspect there's a few hum bars in the video. The digital channels wouldn't give a hoot.....

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