What solution do folks use for the dreaded BFD hum? - Page 6 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #51 of 324 Old 10-18-06, 12:40 PM
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Re: What solution do folks use for the dreaded BFD hum?

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.

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post #52 of 324 Old 10-18-06, 01:33 PM
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Re: What solution do folks use for the dreaded BFD hum?

Quote:
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...

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post #53 of 324 Old 10-18-06, 02:06 PM
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Re: What solution do folks use for the dreaded BFD hum?

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.
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post #54 of 324 Old 10-18-06, 03:50 PM
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Re: What solution do folks use for the dreaded BFD hum?

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brucek wrote: View Post
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.

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post #55 of 324 Old 10-18-06, 04:01 PM
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Re: What solution do folks use for the dreaded BFD hum?

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?

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post #56 of 324 Old 10-18-06, 05:25 PM
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Re: What solution do folks use for the dreaded BFD hum?

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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........

Quote:
Will the BFD convert an XLR input to a 1/4"TRS output?
Yep, both connectors are active

Quote:
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).

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post #57 of 324 Old 10-18-06, 09:11 PM
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Re: What solution do folks use for the dreaded BFD hum?

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.
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post #58 of 324 Old 10-19-06, 07:38 AM
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Re: What solution do folks use for the dreaded BFD hum?

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.

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post #59 of 324 Old 10-19-06, 09:52 AM
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Re: What solution do folks use for the dreaded BFD hum?

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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.




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post #60 of 324 Old 10-19-06, 10:08 AM
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Re: What solution do folks use for the dreaded BFD hum?

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.)

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