Need help with BFD hum...

**mlmore** · **04-22-07, 10:06 AM**

I know this has been covered before, but I am still confused. Here is my situation:

I have all my HT gear plugged into a Furman power conditioner. Based on some tests using REW, I decided to move my sub from the back of the room to the front corner. I built a 40' interconnect using a balanced cable with unbalanced connectors (Canare RCA). So the sub is the only component not plugged into the Furman. At this point I have no problems.

Now, based on all the excellent knowledge on this forum, I add a BFD to the signal chain. I terminated one end of the interconnect with a 1/4" jack and now I have the dreaded hum in the sub - and to a lesser extent in the mains. I unplug the 75 ohm cable and most of the hum goes away, but not entirely. But, I turn on the Xbox 360 and the hum comes back in full force.

So I have a few questions. First, why does adding the BFD to my system introduce the hum? I would think that by using a 40' unbalanced interconnect I would have heard the hum w/o the BFD. Is there a preferred way of terminating the 1/4" jack? I only soldered the twisted pair and left the conducter on the shield off.

I wonder if I should try grounding the BFD and/or the cable coming into the house. Any thoughts/recommendations?

**brucek** · **04-22-07, 10:26 AM**

Quote:

I built a 40' interconnect using a balanced cable with unbalanced connectors ............
I only soldered the twisted pair and left the conductor on the shield off..........
I terminated one end of the interconnect with a 1/4" jack .........

Can you expand on these statements. Do you mean the source is a legitimate balanced output and you unbalanced it. Specify connector type on either end (i.e. RCA or XLR or 1/4" TRS or 1/4" TS) and wiring of same.

brucek

**mlmore** · **04-22-07, 01:16 PM**

I used a Silver Sonic BL-1 cable (l have a bunch of this stuff laying around) and terminated it with Canare RCA connectors. After I purchase the BFD, I removed the RCA on the source end and replaced it with a mono 1/4" connector from Radio Shack (I don't know the difference betweeen TS and TRS). This was the 40' cable that goes from the BFD output to the subwoofer. I also built a short interconnect that goes from my Denon AVR3805 to the BFD.

Since the BL-1 cable is a balanced cable, I didn't need the foil shield wire for the RCA or 1/4" so I ignored it and only soldered the twisted pair to the connectors. I have a feeling this is not the preferred way of making an interconnect for a balanced device like the BFD.

**brucek** · **04-22-07, 01:35 PM**

Well, the BFD has the ability to operate balanced or unbalanced, dependant on the connectors used.

TS and TRS for 1/4" plugs and jacks means Tip, Sleeve and Tip, Ring, Sleeve.

I still don't know what pins you have connected up, so it's hard to tell you how to solve the problem.

The best way to proceed would be to establish whether the ground loop is between the receiver and BFD or the BFD and sub amplifier. To establish this, remove just the Input connections to the BFD and tell me whether the BFD and sub together still hum.

brucek

**Wayne A. Pflughaupt** · **04-22-07, 03:22 PM**

Quote:

First, why does adding the BFD to my system introduce the hum?

It isn’t, really. It’s a ground loop that is typically traced to a CATV or satellite feed that’s not properly grounded to your home’s electrical ground rod.

Quote:

I would think that by using a 40' unbalanced interconnect I would have heard the hum w/o the BFD.

If your sub had a grounded plug, you would have.

Quote:

(I don't know the difference betweeen TS and TRS).

TRS (tip/ring/sleeve) is also known as a stereo plug. TS (tip/sleeve) is aka a mono plug – the one you said you have.

Quote:

Since the BL-1 cable is a balanced cable, I didn't need the foil shield wire for the RCA or 1/4" so I ignored it and only soldered the twisted pair to the connectors. I have a feeling this is not the preferred way of making an interconnect for a balanced device like the BFD.

It’s not the way to make any cable for line-level use. The shield should always be connected. This may well be the source of your problem. But even if it’s not, go back and do your cable right: Tie both of the twisted pairs together and solder them to the center pin of the RCA and 1/4” mono plug, and the shield to the sleeve.

Regards,
Wayne

**mlmore** · **04-22-07, 04:18 PM**

Thanks for the replies. Wayne, I'll go back and terminate the cable as you suggested, w/ the drain wire going to the sleeve. But the cable was working fine w/o the BFD. I will have to check, but I think either the short interconnect between the amp and BFD is the problem or I need to ground the BFD.

**mlmore** · **04-22-07, 09:00 PM**

Ok, I found out that the connection from my amp to the BFD is causing the hum. I rebuilt the interconnect as Wayne suggested. I connected a ground wire from the BFD to the chassis of my amp. The hum is still there. It gets considerably worse if the xbox is turned on. There is no hum when I take the BFD out of the loop. Any ideas?

**brucek** · **04-22-07, 10:22 PM**

Quote:

This was the 40' cable that goes from the BFD output to the subwoofer.
I also built a short interconnect that goes from my Denon AVR3805 to the BFD.

Quote:

I think either the short interconnect between the amp and BFD is the problem or I need to ground the BFD.

Quote:

I found out that the connection from my amp to the BFD is causing the hum

I am truly confused. When you say amp, do you actually mean receiver? Can you tell me if the hum goes away when you remove the cable to the input to the BFD and leave the cable that goes from the BFD to the subwoofer amp?

brucek

**Wayne A. Pflughaupt** · **04-22-07, 10:56 PM**

Quote:

Any ideas?

Disconnect the CATV or satellite feed (see post #5).

Regards,
Wayne

**mlmore** · **04-23-07, 07:15 AM**

brucek,

Yes, I mean the receiver. When I remove the cable to the input to the BFD and leave the cable that goes from the BFD to the subwoofer amp the hum goes away.

Mike

**mlmore** · **04-23-07, 08:37 AM**

Wayne,

Here's what confuses me. Without the BFD, the CATV produces no hum. As soon as I hook the receiver to the BFD and the BFD to the subwoofer...major hum. If I disconnect CATV, most of the hum goes away, but if I then turn on the Xbox, all of the hum comes back (even with CATV unplugged).

**bobgpsr** · **04-23-07, 09:09 AM**

Quote:

mlmore wrote:

Wayne,

Here's what confuses me. Without the BFD, the CATV produces no hum. As soon as I hook the receiver to the BFD and the BFD to the subwoofer...major hum. If I disconnect CATV, most of the hum goes away, but if I then turn on the Xbox, all of the hum comes back (even with CATV unplugged).

Is the XBOX linked to the net via a cable modem (or through a router to the modem)? If so it is the same problem as the CATV. You need to ensure that there is a very good high quality ground to the cable where it enters the house. Should be a double F connector lightening arrestor there and it needs to be tied to ground and also the to the same ground as used for the house power breaker box.

Even then a lot of us have problems with BFD hum. I solved mine by connecting just the the + and - lines (twisted pair part of the twisted shielded pair cable used for interconnect) on the XLR plug that is used to feed the input of the BFD. I connected the interconnect shield (with the - of the twisted pair) at the RCA plug used for the AVR output but let the interconnect shield float at the BFD input. This is like using only the Tip and Ring of a TRS connector.

Code:

RCA plug to AVR                                     XLR plug to BFD

             _ <-- shield
            (  ) 
XXXX--------------------------------------XXXX  2 +
            (  )       ^
            (  )       |
          ------------------------------------XXXX  3 -
______| (_)       ^                                  _____  1 open
          |_|         |  <-- twisted pair

**brucek** · **04-23-07, 09:18 AM**

Quote:

When I remove the cable to the input to the BFD and leave the cable that goes from the BFD to the subwoofer amp the hum goes away

Good. I suspect the pseudo balanced cable show below will solve the problem. Sorry for my poor photoshop skills.

Use 1/4" TRS plug (sometimes called a 1/4" stereo plug) and wire it as shown using your (two wire and shield cable.)

On the RCA side, the (+) hot wire is connected to the pin and the (-) ground wire plus the shield are connected to the case of the RCA.

On the 1/4" TRS BFD side the (+) hot wire is connected to the TIP and the (-) ground wire is connected to the RING. The shield is left unconnected on this BFD side with no connections to the 1/4" TRS connector.

Now you have the hot signal feeding the balanced positive BFD differential amp and the ground low signal feeding the balanced negative BFD differential amp. The shield is connected along the cable to provide RF protection. The offending (hum producing) raised potential imbalance between the receiver and BFD is now not allowed to flow between the case of the Receiver and BFD.

Give it a try. This should work in your case.

Be sure to use a regular three prong connection on the BFD power cord. No cheaters allowed.

Name: Unbalanced to balanced RCA to TRS.jpg
Views: 263
Size: 30.9 KB

Name: Unbalanced to balanced RCA to TRS.jpg
Views: 263
Size: 30.9 KB

brucek

**mlmore** · **04-23-07, 11:31 AM**

brucek,

Thanks for the excellent diagram. Couldn't I just use my 1/4 TS plug and solder the (-) to the sleeve?

I guess I'm going to get a lot of practice with the ol' solder iron.

**brucek** · **04-23-07, 11:33 AM**

Quote:

Couldn't I just use my 1/4 TS plug and solder the (-) to the sleeve?

Absolutely not. That would provide a path for the case of the receiver to connect to the case of the BFD. You need the TRS plug. The only connection is then to the input differential amp of the BFD..

brucek

**mlmore** · **04-23-07, 11:36 AM**

Of course!!! I guess I'm a little slow for not thinking of this, but the xbox does have CAT5 connected to a router connected to a cable modem.

Should Comcast have grounded the cable for me, or is there an easy way for me to do it. There is a splitter after the cable enters the house with a grounding screw on it. Can I simply run a wire from the splitter to ground in the breaker panel?

**mlmore** · **04-23-07, 11:44 AM**

Should I make the BFD-sub interconnect the same way?

thanks for your help. I will let you know if it works.

**brucek** · **04-23-07, 01:05 PM**

Quote:

Should Comcast have grounded the cable for me, or is there an easy way for me to do it. There is a splitter after the cable enters the house with a grounding screw on it. Can I simply run a wire from the splitter to ground in the breaker panel?

Normally the cable company will provide one of these grounding blocks as the cable enters the house near the service panel and then attach a wire to the panel. You could do it yourself though. The grounding blocks are available at Radio Shack. Here's a picture. you can see where the wire goes with the screw to tighten it.

Quote:

Should I make the BFD-sub interconnect the same way?

No, you've broken the loop with the BFD input cable. That should be suffice.

brucek

**Wayne A. Pflughaupt** · **04-23-07, 06:05 PM**

Quote:

Wayne,

Here's what confuses me. Without the BFD, the CATV produces no hum. As soon as I hook the receiver to the BFD and the BFD to the subwoofer...major hum. If I disconnect CATV, most of the hum goes away, but if I then turn on the Xbox, all of the hum comes back (even with CATV unplugged).

No mystery there – that’s the way it always works. The problem isn’t evident until you introduce a component with a grounded plug into the signal chain. That’s what completes the ground loop.

I’d try to verify - and correct if needed – the CATV ground before trying the psuedo-balanced input cable. We generally recommend that as a last resort. Your cable company should take care of the grounding, if it’s a problem. As brucek noted, it should be connected to your electrical service panel, or even better, ground rod.

Regards,
Wayne

**tdamocles** · **04-23-07, 07:32 PM**

Quote:

Wayne A. Pflughaupt wrote:

Your cable company should take care of the grounding, if it’s a problem. As brucek noted, it should be connected to your electrical service panel, or even better, ground rod.

Regards,
Wayne

The problem here is......to get the cable company to admit that there is a grounding problem.. The techs that they send are usually clueless....They will probably blame the other equipment....

Good luck.....

**mlmore** · **04-23-07, 09:12 PM**

Well, the receiver has a grounded plug, the xbox has a grounded plug, I never had the problem before. I've always plugged everything into the Furman to avoid the ground loop problem. I still don't understand why the BFD causes the problem.

I knew I should have paid more attention in my EE class...anyway, the solder iron is heating up. I'll report my findings. Thanks again.

**mlmore** · **04-23-07, 10:25 PM**

Made the pseudo-balanced cable...still have hum. I guess I should try the cheater plug to see if that eliminates the noise - just as a test.

**MakeFlat** · **04-23-07, 11:21 PM**

A good reference is here:

http://www.jensen-transformers.com/an/an003.pdf

**Wayne A. Pflughaupt** · **04-23-07, 11:40 PM**

Quote:

mlmore wrote:

Well, the receiver has a grounded plug, the xbox has a grounded plug, I never had the problem before. I've always plugged everything into the Furman to avoid the ground loop problem. I still don't understand why the BFD causes the problem.

Well, the Furman doesn’t have anything to do with it, that’s for sure. As far as why none of the other grounded components didn’t cause the hum, that’s pretty strange. Brucek?

Regards,
Wayne

**brucek** · **04-24-07, 06:55 AM**

Quote:

that’s pretty strange

Yeah, that differential amp should be able to reject most of the small amount of noise between the ground and hot signal. (Hopefully mlmore is sure he doesn't have any continuity between the RCA shield and the sleeve of the TRS cable he made

)

I suppose the next solution would have to be transformer based. I would also solvethe cable TV ground problem too as suggested already.

First I would establish that it is indeed a ground loop by testing a cheater on the BFD. Of course the cable just tested should have safely accomplished the same result, so I'm somewhat confused.

Here's the transformer based solution.

brucek

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