What solution do folks use for the dreaded BFD hum?

**Wayne A. Pflughaupt** · **03-22-07, 06:45 PM**

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Wayne you have a PM?

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

Regards,
Wayne

**bobgpsr** · **03-22-07, 07:51 PM**

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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.

**lovingdvd** · **03-22-07, 08:15 PM**

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Wayne A. Pflughaupt wrote:

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

Regards,
Wayne

Sorry. I meant "You have a PM."

**brucek** · **03-22-07, 08:43 PM**

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Hmm, looks like were on to something then! Don’t know why someone hasn’t 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.

burcek

**lovingdvd** · **03-22-07, 08:57 PM**

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brucek wrote:

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?

**lcaillo** · **03-22-07, 10:38 PM**

Have you tried properly grounding the cable line?

**lovingdvd** · **03-22-07, 11:45 PM**

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lcaillo wrote:

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.

**Wayne A. Pflughaupt** · **03-23-07, 09:40 AM**

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

**lovingdvd** · **03-23-07, 09:58 AM**

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!!

**Wayne A. Pflughaupt** · **03-23-07, 10:25 AM**

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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.)

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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.

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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

**brucek** · **03-23-07, 10:27 AM**

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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...

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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).

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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.....

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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..

brucek

**lovingdvd** · **03-23-07, 11:10 AM**

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Wayne A. Pflughaupt wrote:

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).

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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!

**brucek** · **03-23-07, 12:32 PM**

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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....

brucek

**lovingdvd** · **03-23-07, 12:38 PM**

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brucek wrote:

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!

**brucek** · **03-23-07, 12:55 PM**

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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.....

brucek

**lovingdvd** · **03-23-07, 01:19 PM**

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brucek wrote:

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.....

brucek

Nope, no analog whatsoever.

Wayne - as a side note it sounds like your plan is to create a cable that goes directly from the BFD to the sub input. However if possible I'd prefer to make just an adapter piece that I can plug my existing cable into. This would provide more flexibility in the future if I move components around (not locked into a specific cable length).

**brucek** · **03-23-07, 01:39 PM**

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as a side note it sounds like your plan is to create a cable that goes directly from the BFD to the sub input

Is the ground loop not between the BFD and the source equipment? Why are you not trying this cable mod between the receiver and the BFD? This a unbalanced to balanced modification....

brucek

**lovingdvd** · **03-23-07, 01:58 PM**

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brucek wrote:

Is the ground loop not between the BFD and the source equipment? Why are you not trying this cable mod between the receiver and the BFD? This a unbalanced to balanced modification....

brucek

Good point I was thinking about this the wrong way because I was picturing modding the connecting between the BFD and the sub. When it really needs to be between the BFD and the receiver...

Which brings up a very important point. My RCA cable run from the receiver to the BFD is 25 feet and under permanently installed carpet. No way for me to switch that out. So I'll definitely need some sort of coupler/adapter that I can plug into the end of this cable (currently at the BFD) and then plug that into the BFD (replacing the connection between the receiver and BFD is not an option).

**Wayne A. Pflughaupt** · **03-23-07, 02:24 PM**

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lovingdvd wrote:

Wayne - as a side note it sounds like your plan is to create a cable that goes directly from the BFD to the sub input.

What brucek said – you want this between the receiver and BFD. No point doing it from the BFd’s output – that won’t help.

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My RCA cable run from the receiver to the BFD is 25 feet and under permanently installed carpet. No way for me to switch that out. So I'll definitely need some sort of coupler/adapter that I can plug into the end of this cable

I don’t know of any decent female RCA connectors, so I’d just make a short cable with a male RCA and an XLR (or 1/ 4” TRS), and use a coupler to connect that to the cable you’re using now.

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So Wayne if you'd be so kind as perhaps to provide some detailed instructions

Basically, all you need to do is connect the RCA’s center conductor (signal +) to the XLR’s Pin 2 terminal, and the RCA’s shield (-) to the XLR’s Pin 3. Using a TRS, it would be (+) to tip, and (-) to ring. See the bottom diagram here, only the black wire would go to Pin 3. Do not add the jumper between Pins 1 and 3

If you need more detailed instructions, let me know.

Regards,
Wayne

**lovingdvd** · **03-23-07, 02:50 PM**

OK - I'm confused already!

I'm not sure what the difference is between XLR vs. TRS and how/if it matters which to use.

Currently my RCA line from the receiver goes into a Radio Shack plug that has the 1/4" pin on one end and the RCA female on the other. Is this a XLR, TRS, or neither?

Also in my custom cable thingie what is going to serve as the female end that my existing RCA feed from the receiver will now go into?

I'm also confused about that bottom diagram you reference in that link. That link shows a XLR but when talking about using a TRS you refer to it even though it is a XLR.

I'm also confused about this custom piece of cable. It sounds like a need to by a RCA cable with a female end on one end that my existing RCA would feed into, then cut off the other end and solder that into the XLR / TRS?

If its not too much trouble can you provide the short list of Radio Shack part #s I'll need, and exactly what wires I'll be connecting to what terminal # in each part #? Think of it as a "anti-hum custom cable creation for dummies guide"

Once I know what to get I'll pick up the parts on my way home today and give it a shot, and let you know how it works out! Thanks again!!

**Wayne A. Pflughaupt** · **03-23-07, 10:45 PM**

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I'm not sure what the difference is between XLR vs. TRS and how/if it matters which to use.

It doesn’t matter which you use:

TRS (tip/ring/sleeve, aka “stereo plug”)

XLR male (input)

XLR female (output)

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Currently my RCA line from the receiver goes into a Radio Shack plug that has the 1/4" pin on one end and the RCA female on the other. Is this a XLR, TRS, or neither?

That would be a TS (tip/sleeve, aka “mono plug”).

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I'm also confused about that bottom diagram you reference in that link. That link shows a XLR but when talking about using a TRS you refer to it even though it is a XLR.

When I said “or TRS” that meant a different connector, not a different name for an XLR. Either connector can be used for a balanced input. See Pg. 19 of the BFD manual for some details.

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I'm also confused about this custom piece of cable. It sounds like a need to by a RCA cable with a female end on one end that my existing RCA would feed into, then cut off the other end and solder that into the XLR / TRS?

You could certainly do that.

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If its not too much trouble can you provide the short list of Radio Shack part #s I'll need,

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