What solution do folks use for the dreaded BFD hum? - Page 7 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #61 of 330 Old 10-19-06, 10:35 AM
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Re: What solution do folks use for the dreaded BFD hum?

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The noise isn't centered on 1.00 kHz, it seems to be made up of many frequency components
I guess you could get creative with more than one filter and create a brick wall. We had a thread a while back where Sonnie and John created a crossover out of filters. It was quite well done. I can't find it. Either way, you say the problem is solved now with the single filter. ....

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Note that my filter width is two octaves and it looks like you simulated for one
Ahh, good catch. OK, here it is with 120/60. A bit more of an effect to your existing filters I would think. The single filter adds ~-4dB at 100Hz and about 6 degrees of phase at the 80Hz crossover area. You might want to do an REW sweep sometime to see the actual effect that this single bad boy causes....





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post #62 of 330 Old 10-19-06, 10:46 AM
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Re: What solution do folks use for the dreaded BFD hum?

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brucek wrote: View Post
The single filter adds ~-4dB at 100Hz and about 6 degrees of phase at the 80Hz crossover area.
I used the attenuation to help shape my house curve.
My crossover is at 60 Hz, that bit of phase shift is no big deal right?
Should I add or subtract a foot from my receiver time delay to compensate?

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post #63 of 330 Old 10-19-06, 12:42 PM
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Re: What solution do folks use for the dreaded BFD hum?

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I used the attenuation to help shape my house curve
Great..good idea..

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My crossover is at 60 Hz, that bit of phase shift is no big deal right?
Agreed.

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Should I add or subtract a foot from my receiver time delay to compensate?
Neither. The foot you add to sub speaker distance (when you use a BFD) is to compensate for the full 1 msec delay the BFD offers. A change in phase as small as this filter creates around the crossover would likely have negligable effect and could only be esatablished by doing a response check and see if you have any new dips or peaks around the 60Hz area......

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post #64 of 330 Old 03-21-07, 09:31 PM
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Re: What solution do folks use for the dreaded BFD hum?

Interesting reading. I'm trying to eliminate my hum and would like some advice please...

I have my BFD and sub plugged into the same AC outlet and I get a low hum out of the sub.

Now, I am CERTAIN that the BFD is causing the hum and not my receiver, because if I remove the BFD from the chain (go straight from my receiver to my sub with the RCA) then there is no hum.

What is the cheapest and simpliest way to resolve this issue? Is it really necessary to purchase a $40-50 part for this? I do not want to use a cheater plug for safety reasons.

This product looks pretty basic http://www.jensen-transformers.com/ci1rr.html but I'm not sure how much it runs.

Also - I read in another thread that someone connected a wire between the BFD grounding screw and a screw on their amp/pre-amp and solved the problem. I don't have that option as the sub is the only other piece of equipment near the BFD. However I was wondering if I could run a wire from the BFD grounding screw to a screw on the metal backplate on my HSU sub. Do you think that backplate is grounded? If so would this work? If there is any risk of shock or damage to the equipment trying this than I'd want to stay clear. Thanks!

Edit: Just found the pricing for the Jensen - over $100 wow, that rules that out...

Last edited by lovingdvd; 03-22-07 at 09:16 AM.
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post #65 of 330 Old 03-22-07, 09:08 AM
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Re: What solution do folks use for the dreaded BFD hum?

Also just to follow up from my post directly above - I'd like to clarify a few things please...

I have this hum even when my receiver is OFF.

If I unplug the INPUT RCA going into the BFD (from the receiver) then the hum stops. However if I take this same plug and go directly into my sub, I do NOT get any hum (even with the BFD still plugged into the same AC wall outlet as the sub).

I also notice that once in a while the hum gets a tad louder (again with the receiver and all AV components off or even unplugged0 and the first light on the output LED array of lights blinks a bit.

So it does not seem like this is an AC grounding loop. Rather the BFD is generating this hum internally. I do not remember having this issue at all once I first set this up.

The hum is quite low but enough to be audible and also enough to keep the sub woofer on all the time instead of going into its passive mode when the receiver is off.

Has my BFD gone bad? Or is all this par for the course, and this a perfect candidate to fix using the Jensen transformer or similar solution to resolve this?

Thanks!
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post #66 of 330 Old 03-22-07, 09:45 AM
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Re: What solution do folks use for the dreaded BFD hum?


Quote:
If I unplug the INPUT RCA going into the BFD (from the receiver) then the hum stops. However if I take this same plug and go directly into my sub, I do NOT get any hum (even with the BFD still plugged into the same AC wall outlet as the sub).
There ya go – that means the problem is a ground loop between the BFD and other components in your system. This is probably because you have it plugged into a different circuit on a different service leg as the rest of the system. Your best bet would be to move the BFD to where the rest of the electronics are.

If that doesn’t get it, look to your cable TV service or sat dish. The CATV service and the dish should be grounded outside to the ground stake at your electrical service.

Regards,
Wayne



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post #67 of 330 Old 03-22-07, 10:00 AM
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Re: What solution do folks use for the dreaded BFD hum?

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So it does not seem like this is an AC grounding loop.
Sounds exactly like an AC grounding loop.

Re-read your post where you say:

If I unplug the INPUT RCA going into the BFD (from the receiver) then the hum stops

Fairly clear indication that there is a potential difference between the Receiver and the BFD. This is standard fare and can be confirmed by testing with a cheater at the BFD. Does the hum go away. yes? then you have a ground loop.

Even though the safety ground (a.k.a. third prong) is a cold conductor, it can and usually does, develop a small potential, through mutual inductance, wire resistance, etc., that can be quite different at each receptacle in your house. In addition, various other reasons (such as introducing a new ground from cable or a satellite) can cause a safety ground potential difference between chassis.

For example, when you plug a receiver into one receptacle and a BDF into another receptacle, the metal cases of these two units can have a small potential difference in their safety grounds which means that these equipment's metal cases are at a slightly different potential. Since internally the AC ground is connected to the DC ground, this potential will be on the shield of an interconnect. 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... Breaking the safety ground is dangerous though and not recommended. (See the advantage of balanced XLR circuits where you are able to safely lift the shield and the circuit remains intact on the (+)/(-) lines?).

Anyway, the solution is to either find the ground loop (which is between your receiver and BFD) or safely use a transformer device that allows the signal to pass but breaks the shield. The device you indicated would achieve that end. Expensive though.

Again. First prove that its a ground loop with the cheater on the BFD. Then remove it and find your solution. Try unplugging you cable.

brucek

Last edited by brucek; 03-22-07 at 10:01 AM. Reason: oops, Wayne beat me to the draw.....
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post #68 of 330 Old 03-22-07, 10:07 AM
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Re: What solution do folks use for the dreaded BFD hum?

Quote:
Wayne A. Pflughaupt wrote:
There ya go that means the problem is a ground loop between the BFD and other components in your system. This is probably because you have it plugged into a different circuit on a different service leg as the rest of the system. Your best bet would be to move the BFD to where the rest of the electronics are.
I had the same issue and a cheater plug adapter would fix the hum (not perferred). The BFD was plugged into the same power strip as the AVR. And the sub (PCU) was on the same house circuit as the AVR.

But I also had intermitent shield/ground connection problems with the RCA to phone plug adapters I was using to connect unbalenced connections to the BFD. My solution was to custom build RCA to XLR cables and experiment with different ground/shield connection possibilities with the XLR wiring. Problems were solved when I used just the + & - connections (leaving the shield/ground open) for the input to the BFD from the AVR. Did have to hook up both the - and the shield/ground for the run to the sub. Cheater plug no longer required and the XLR connections are very reliable.
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post #69 of 330 Old 03-22-07, 12:21 PM
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Re: What solution do folks use for the dreaded BFD hum?

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brucek wrote: View Post
Sounds exactly like an AC grounding loop.

Re-read your post where you say:

If I unplug the INPUT RCA going into the BFD (from the receiver) then the hum stops

Fairly clear indication that there is a potential difference between the Receiver and the BFD. This is standard fare and can be confirmed by testing with a cheater at the BFD. Does the hum go away. yes? then you have a ground loop.
brucek
Thanks. Here's the part though that is most confusing... I actually unplugged my AV receiver from the wall, but the hum did not go away! 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 ?? I actually removed the power cord from the back of the AV receiver so I know I had the right plug unplugged. If this was a grounding loop, wouldn't the AV receiver need to be plugged in?

Also a couple side notes to consider - my cable box and dvd player all have just two prongs. Wouldn't it need 3 prongs to have the ground loop issue? Also as a test I plugged my sub and the BFD into a power string, then plugged the power strip into the same AC outlet that my equipment rack is powered by. This made no difference in the hum. I also tried plugging this into various outlets and also made no difference.
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post #70 of 330 Old 03-22-07, 12:45 PM
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Re: What solution do folks use for the dreaded BFD hum?


Quote:
But I also had intermitent shield/ground connection problems with the RCA to phone plug adapters I was using to connect unbalenced connections to the BFD. My solution was to custom build RCA to XLR cables and experiment with different ground/shield connection possibilities with the XLR wiring. Problems were solved when I used just the + & - connections (leaving the shield/ground open) for the input to the BFD from the AVR.
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.

Quote:
Also a couple side notes to consider - my cable box and dvd player all have just two prongs. Wouldn't it need 3 prongs to have the ground loop issue?
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.

Looks like what you need to do is fix that cable TV feed. It’s the problem, not the BFD. 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



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