What solution do folks use for the dreaded BFD hum? - Page 8 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #71 of 324 Old 03-22-07, 01:57 PM
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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
Hmm, surprised that would work. I guess the balanced transformers (or whatever the BFD uses for those inputs) was able to nuke it. I imagine brucek can tell us how that worked – he’s the expert on these things.


Not at all. Remember, the cable service (i.e., the coaxial feed coming into your system) is grounded to earth somewhere. So is the BFD - its ground goes back to the electrical service panel, which in turn goes to the ground stake (earth).

That’s also why you still get the hum with the receiver unplugged. The ground loop is still connected through the cable feed, which ties to the cable box chassis, which connects it to the shield of the RCA cable running to the BFD and sub.
Great post, thanks. It makes perfect sense now.

My cable STB is connected to the receiver via HDMI cables. As a test I'll try removing these and some other cables to narrow things down. Or actually, better yet, I'll disconnect the cable feed from the cable box...

Quote:
Looks like what you need to do is fix that cable TV feed. It’s the problem, not the BFD.
Let's say I am able to confirm it is the cable box. You mention that one option would be to "fix that cable TV feed". How could I even go about that? Doesn't seem like there would by many (any?) options since the cable line is grounded and has to connect into the STB. Unless there is some sort of cable adapter that removes the ground?

Ya know, I just thought of something... I used to run the cable feed directly into the STB. But recently I changed it so that it runs into a UPS backup with surge protection. So the cable line feeds into the coax input for that, and then back out and into the STB. I'm now wondering if that introduced this problem... As you may recall from an earlier post I don't remember having this issue earlier so it seems to have appeared recently. I will need to play with this later today to see what happens as I switch it back the old way and so forth.

Quote:
Or maybe try Bob’s XLR-to-RCA connection scheme. I’ll be happy to make those cables for you if you like – just send me the parts and supplies.

Regards,
Wayne
Thanks for the offer Wayne! That is very kind. I'm surprised they don't make something like this that can be purchased off the shelf.

Can you please supply a list of supplies (preferably Radio Shack) for exactly what you need? I'm happy to pay you for your time on this too!

Thanks again!
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post #72 of 324 Old 03-22-07, 03:00 PM
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Re: What solution do folks use for the dreaded BFD hum?


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Ya know, I just thought of something... I used to run the cable feed directly into the STB. But recently I changed it so that it runs into a UPS backup with surge protection. So the cable line feeds into the coax input for that, and then back out and into the STB. I'm now wondering if that introduced this problem....
I never did trust those things... Something about sending a coaxial feed through a surge protector with all that AC power just seems wrong to me. Again, brucek is the expert on such things – maybe he can waylay my fears.

Quote:
Thanks for the offer Wayne!
I'm surprised they don't make something like this that can be purchased off the shelf.

Can you please supply a list of supplies (preferably Radio Shack) for exactly what you need?
If moving the cable from the surge protector doesn’t help, PM me. I’ll take a look at RadioShack, but typically their prices are outrageous. For instance, they charge $4.00 for this mediocre RCA plug. You can get far better connectors at Parts Express for well under $2.00 each. Of course, there are the shipping charges to consider, so you might be ahead with RS in the end. We’ll see.

Regards,
Wayne



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post #73 of 324 Old 03-22-07, 05:56 PM
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Re: What solution do folks use for the dreaded BFD hum?

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Hmm, surprised that would work. I guess the balanced transformers (or whatever the BFD uses for those inputs) was able to nuke it. I imagine brucek can tell us how that worked
Yeah, that works. It's know as pseudo-balanced. The common mode rejection ratio (CMRR) is quite comprimized compared to a proper balanced interface and offtime you won't have sufficient levels since both the (+)ve and (-)ve signals are expected to contain the signal with an associated maximum line level.

As you know the signal is fed into a differential amplifier at the receiving end (BFD). The CMRR advantage is fully realized when the two signal carrying conductors have identical impedances to ground (as in a proper balanced circuit). This will give the maximum noise rejection.
If we unbalance this teacup and use a paired wire core coaxial interconnect where we connect the hot signal from the source of an unbalanced circuit to (+)ve and the shield at the source end to the second wire of the pair to (-)ve, while only connecting the shield at the source end, think about what has happened. The (+)ve signal at the differential amplifier sees the source impedance of the source (i.e. 100 ohms), while the (-)ve signal at the differential amplifier sees the source impedance of zero ohms from ground. You have an impedance mismatch and the CMRR goes to the dogs. The differential amplifer still works, but its comparing partner has no signal, so the output will not be quite as high as expected.

But still, there are some advantages. You've broken the ground loop by not tying the shield to the receiving end. You've realized a bit of common noise rejection even with the poor impedance match. If there is sufficient signal to drive the BFD, then you've accomplished something.

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So basically I have a 25 foot RCA cable that goes into the AV receiver that is disconnected from the AC. Yet when I remove that end from the BFD the hum goes away. How can this be possible ??
As stated in posts above, the interconnect shields are connected to the AC case of every piece of equipment. In the case of cable, it comes in on the shield as a different potential and passes through the shields and cases of every piece of equipment that is connected together with interconnects.

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Doesn't seem like there would by many (any?) options since the cable line is grounded and has to connect into the STB. Unless there is some sort of cable adapter that removes the ground?
There are cable in-line transformer gizmos to break the cable ground. Doesn't work with satellites though (they phantom DC through the coax to power the LNA at the horn). They work for cable though.

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But recently I changed it so that it runs into a UPS backup with surge protection.
Yeah, that will be the loop. Bypass it and see what happens..

brucek
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post #74 of 324 Old 03-22-07, 06:39 PM
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Re: What solution do folks use for the dreaded BFD hum?

OK for those of you following along at home - its definitely the cable TV feed, and not the result of going through the UPS/surge protector which makes no difference.

To keep the test simple I ran the cable TV line directly into the STB. Hum was present. Disconnected it from the STB, hum disappeared.

Further, I then put a cheater plug just as a temporary test on the BFD and get no hum even with the cable TV plugged into the STB.

Based on this, do you think I would be better off with the custom built connectors Wayne can build? Or should I go with the Jensen VRD-1FF which is about $60... ? Wayne you have a PM? Thanks all!
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post #75 of 324 Old 03-22-07, 06:43 PM
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Re: What solution do folks use for the dreaded BFD hum?


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But still, there are some advantages. You've broken the ground loop by not tying the shield to the receiving end. You've realized a bit of common noise rejection even with the poor impedance match. If there is sufficient signal to drive the BFD, then you've accomplished something.
Hmm, looks like were on to something then! Don’t know why someone hasn’t tried or recommended this before (‘course, it could have been a thread I wasn’t following...)

Regards,
Wayne



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post #76 of 324 Old 03-22-07, 06:45 PM
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Re: What solution do folks use for the dreaded BFD hum?


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Wayne you have a PM?
Yup – click on my name (above the avatar) to access it.

Regards,
Wayne



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post #77 of 324 Old 03-22-07, 07:51 PM
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Re: What solution do folks use for the dreaded BFD hum?

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Wayne A. Pflughaupt wrote:
Hmm, looks like were on to something then! Don’t know why someone hasn’t tried or recommended this before
Actually this type of problem has been discussed at length a couple of years ago in AVS in a thread about using pro amps with AVRs. An engineer from QSC, Bob Lee, IIRC, showed various wiring options going from unbalanced output AVRs to balanced input pro amp inputs.
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post #78 of 324 Old 03-22-07, 08:15 PM
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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

Yup Ė click on my name (above the avatar) to access it.

Regards,
Wayne
Sorry. I meant "You have a PM."
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post #79 of 324 Old 03-22-07, 08:43 PM
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Re: What solution do folks use for the dreaded BFD hum?

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Hmm, looks like were on to something then! Donít know why someone hasnít tried or recommended this before
Because it's fraught with potential problems. We're dealing with a wacky auto servo connection that acts as both a balanced and unbalanced interface using the same jacks. Hard to say how the 4 circuits will react (2 input / 2 output).

The RCA to XLR trick in this case hinges on not activating the pin 1 to pin 3 short, because that would connect the troublesome shield of the source device to the shield of the BFD. So the auto servo level control is not activated and so the difference signal will generally be lower.

I'd be trying to find the ground loop and eliminate it if possible.

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post #80 of 324 Old 03-22-07, 08:57 PM
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Re: What solution do folks use for the dreaded BFD hum?

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I'd be trying to find the ground loop and eliminate it if possible.
burcek
Thanks. As noted above I did find the source and it is the ground in the cable TV coax into the STB.

So it looks like I have two choices:

1) This product from Jensen: http://www.jensen-transformers.com/datashts/vrd1ff.pdf or one like it. Does anyone know of any other product like this I should consider?

I do not use satellite so that's not an issue. Only potential problem is that the site says there have been some reports about certain channels not coming in.

2) Take Wayne up on his offer to send him the parts/supplies and construct the special connectors.

Which is the better route to go here folks? Advantages/disadvantages of these approaches?
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