What solution do folks use for the dreaded BFD hum?

[Ayreonaut]

I'm sorry to report that lifting the ground did not help. I desoldered the ground from the sleeve in the TRS connector plugged into the CleanBOX II, and the results were unchanged. It still generated a loud hum. Wierd.

The hum I'm trying to get rid of is very low level. I can hear the hum start when I rotate the receiver volume dial up from "---" (muted) to "-70" dB. Could it simply be the noise floor of the sub pre-out of my Denon AVR-1905?

I did come up with something that reduced the level of the hum by -10dB, so that its pratically inaudible at the listening positions now.

I had the BFD input set to "-10 dBV" as instructed. But I had to set my reciever's sub pre-out to the minimum -12dB and it still clipped the input of the BFD with "The Haunting" in DTS at reference level. So I switched the BFD to the "+4 dBu" setting and was able to increase the receiver's pre-out level to -2 dB without clipping, and dial back 10dB on the amp. So now the hum is being amplified 10dB less, and its barely noticable. It doesn't bother my wife at all, and I only care because I tend to obsess.

[brucek]

I desoldered the ground from the sleeve in the TRS connector plugged into the CleanBOX II, and the results were unchanged. It still generated a loud hum. Wierd

Then it's not a ground loop. That's what this exercise was supposed to remove. The hum is in the signal from the receiver. No amount of gizmos will remove that unfortunately...

brucek

[Josuah]

Well, my DMC-1 and MPS-1 are both plugged into the same outlet, and if I completely decouple the grounds between the two, I have crazy noise. I found that depending on the arrangement of XLR pre-outs to amp modules, the amount of noise (and the channels, since different channels are going to different modules now) would vary. I had to decouple only some of the XLR connection grounds.

So, I didn't have a ground loop either, it would seem, based on extensive troubleshooting on my own, with AV123, and from discussions with a Furman engineer. However, I did have noise that I was able to remove by lifting the ground on selective interconnects.

[Ayreonaut]

brucek wrote:

The hum is in the signal from the receiver. No amount of gizmos will remove that unfortunately

Note that I'm talking about two different hums.

1. Without the CleanBOX II there is a very quiet hum. It might be the receiver.
2. When the CleanBOX II is introduced, there is a very loud hum. I don't know how this is being generated.

The quiet hum may be generated by the receiver, but I wanted to try the isolation transformer to see if it would be eliminated that way or not. Unfortunately, the isolation transformer is introducing another problem altogether and I am no closer to a definitive diagnosis of the original problem.

[Ayreonaut]

I may try the original CleanBOX instead of the CleanBOX II.
It converts RCA inputs to XLR outputs.

Will the BFD convert an XLR input to a 1/4"TRS output?

[brucek]

When the CleanBOX II is introduced, there is a very loud hum. I don't know how this is being generated

Makes no sense to me........

Will the BFD convert an XLR input to a 1/4"TRS output?

Yep, both connectors are active

I may try the original CleanBOX instead of the CleanBOX II.

I think they're the same except the Cleanbox introduces some gain with an active stage (requires plugging in an AC adapter).

brucek

[Josuah]

Inserting the CleanBox II seems to be exactly what I experienced when I completely disconnected the processor and amp grounds from each other. I had to leave ground between at least one channel, or else everything was kaka.

Have you tried rearranging which outlets your receiver and BFD plug into? Trying combinations of different outlets and same outlets might do something. It didn't do anything for me, but you might try it.

One thing I was doing before with a Yamaha RX-V1400 and Outlaw Audio 7100 and Alesis DEQ-830 was to wire the chassis together using 12awg copper and then stick the last end of the copper into the ground of an empty outlet on the same power strip used by all three components. That reduced my noise by a few dB.

[Ayreonaut]

I'm thrilled to report that I eliminated the receiver hum completely.
No, I did not solve the CleanBOX II mystery.

I messed around with the BFD filters some more. I used the old fake lowpass trick by inserting a filter at 1.00 kHz, 120/60 octaves wide, down -30 dB. This obliterated whatever noise the receiver was generating, all of which was above the subwofer band.

The CleanBOX II will go back. It never made sense to me either, Bruce.

[brucek]

by inserting a filter at 1.00 kHz, 120/60 octaves wide, down -30 dB. This obliterated whatever noise the receiver was generating, all of which was above the subwofer band

Interesting that your receiver is outputting a noise around 1000Hz.

When I look at your filter (in green) I see it has almost no effect on your existing filters. That's good.
The phase (in red) only effects the crossover area by about 5 degrees, so that's good.

Too bad you had to go to so much trouble to track down the problem, but that happens sometimes.




brucek

[Ayreonaut]

The noise isn't centered on 1.00 kHz, it seems to be made up of many frequency components and I couldn't track them all down by ear. The broad cut was my attempt to kill all components of that noise wherever they were in one fell swoop, and it worked.

Thanks for showing me the phase too. (Note that my filter width is two octaves and it looks like you simulated for one.)

[brucek]

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. ....

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....





brucek

[Ayreonaut]

brucek wrote:

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?

[brucek]

I used the attenuation to help shape my house curve

Great..good idea..

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

Agreed.

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......

brucek

[lovingdvd]

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...

[lovingdvd]

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!

[Wayne A. Pflughaupt]

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

[brucek]

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

[bobgpsr]

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.

[lovingdvd]

brucek wrote:

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.

[Wayne A. Pflughaupt]

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.

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

[lovingdvd]

Wayne A. Pflughaupt wrote:

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...

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.

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!

[Wayne A. Pflughaupt]

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.

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

[brucek]

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.

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.

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.

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

[lovingdvd]

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!

[Wayne A. Pflughaupt]

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