What solution do folks use for the dreaded BFD hum? - Page 5 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #41 of 324 Old 10-06-06, 01:50 PM
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Re: What solution do folks use for the dreaded BFD hum?

The cable that goes from the receiver to the ART is an RCA to TS. It has the center pin of the RCA connected to the TIP of the TS plug, and it has the shield of the RCA connected to the SHIELD of the TS plug. This grounds one side of the input transformer in the ART.

The standard remedy for this is to float the input side of the transformer with a cable called an XLR ground lift adapter. It is a cable with XLR's on both ends with the sleeve connection lifted on one end, and so the signal goes to the transformer which is now floating without the ground loop passed through.

You can make your own single solution with an RCA connector and a TRS connector and a piece of shielded wire.

Of course we don't have a schematic of the ART, but it's a good quess...

The schematic would be like this:

What solution do folks use for the dreaded BFD hum?-lift-copy.jpg

Make any sense?

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post #42 of 324 Old 10-06-06, 02:08 PM
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Re: What solution do folks use for the dreaded BFD hum?

I don't have it with me but there is a schematic on the back of the package of the CleanBOX II. On the input side, the tip and sleeve are connected to the transformer. On the output side, the tip and ring are connected to the transformer.

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post #43 of 324 Old 10-06-06, 03:05 PM
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Re: What solution do folks use for the dreaded BFD hum?

Quote:
On the input side, the tip and sleeve are connected to the transformer
Mmm, doesn't make sense to me, since it accepts balanced inputs, it would have to have the tip and ring connected to the input of the transformer and the sleeve to transformer ground.....

Take a picture of it so I can see....

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post #44 of 324 Old 10-06-06, 04:29 PM
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Re: What solution do folks use for the dreaded BFD hum?

I won't have a chance to scan it, I'm leaving for vacation tomorrow morning and I have to go home and pack tonight. But I remeber what it looks like pretty well, I was studying it very hard last night.

It looked something like this:

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post #45 of 324 Old 10-06-06, 05:07 PM
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Re: What solution do folks use for the dreaded BFD hum?

When you return you'll have to take a picture and then we'll sort it out because that makes very little sense with regard to the ability to feed a balanced signal to the device - there is no ring (-ve) connection on the input?......

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post #46 of 324 Old 10-06-06, 08:46 PM
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Re: What solution do folks use for the dreaded BFD hum?

Despite what I said, I took a "break" from packing to scan and post the CleanBOX II schemaitc. Here 'tis.


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post #47 of 324 Old 10-07-06, 06:56 AM
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Re: What solution do folks use for the dreaded BFD hum?

Oh well, call me surprised. They are only allowing unbalanced inputs when the information says, "Connections, 1/4" balanced/unbalanced jacks".... I would take that to mean the input or output would accept balanced or unbalanced.

OK fine, the input accepts unbalanced and you can use the RCA to 1/4" TS plug connected from the receiver to the ART that you used before.

On the output you can use the balanced 1/4" TRS cable except you need to lift the shield on one end. You can make your own cable or modify the one you have (if it doesn't have molded connectors). I've never seen TRS ground lift adapters - only XLR ground lift adapters..

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post #48 of 324 Old 10-17-06, 01:17 PM
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Re: What solution do folks use for the dreaded BFD hum?

I went back to Sam Ash today to get a cable that I can mod if necessary.

While there, we hooked up my CleanBOX II to make sure it was working properly.
We used an RCA-to-1/4"TS adapter cable for input, and a 1/4"TRS cable for ouput, just like I have at home. No problem at all.

Why am I having this problem at all? Am I haveing to lift the ground because of a problemm with my BFD? Do the other people using the CleanBOX II with the BFD have to lift the ground too?

Tonight I'll try my new cable with the ground lifted from the CleanBOX II and see if it works.

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post #49 of 324 Old 10-17-06, 02:34 PM
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Re: What solution do folks use for the dreaded BFD hum?

Quote:
We used an RCA-to-1/4"TS adapter cable for input, and a 1/4"TRS cable for ouput, just like I have at home. No problem at all.
Why am I having this problem at all? Am I haveing to lift the ground because of a problemm with my BFD? Do the other people using the CleanBOX II with the BFD have to lift the ground too?
No, you don't have a problem with your BFD. You established that when you answered my question and said, "There is no hum at all if the receiver is not connected to the input of the CleanBOX."

This says the ground reference of the BFD and the Carver amp are close and not creating a hum producing ground loop. When you hook up the Receiver, the hum begins. This is where the potential difference exists. This problem requires we convert the unbalanced signal of the receivers output to balanced (using the ART), so that we are then able to lift the line-level ground (completely safe, as opposed to lifting an AC safety ground).

Note, we couldn't lift the ground on a single ended unbalanced signal, since the ground shield itself is required for proper signal flow. This is the beauty of balanced signals (that the ART is supplying) - we are able to lift the line-level ground because the signal is carried on two difference signals from the transformer. Sometimes you get away without lifting the ground on a balanced signal, and sometimes not. In your case it would seem not.

Lift the shield between the ART and the BFD and you should be fine..........

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post #50 of 324 Old 10-17-06, 02:56 PM
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Re: What solution do folks use for the dreaded BFD hum?

Thanks 1,000,000 Bruce. I'll let you know what happens.

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