BFD Outputs Rumbling noise to Subwoofer

**rkruz** · **10-08-06, 08:59 PM**

Im really enjoying using the REQ. Its been a lot of fun learning and trying to set up my home theater. The BFD is a challenge though.
I cannot get a clean signal out of the BFD.
The BFD noise sounds like a rough idling engine. I'll attach a mp3 later to help explain.

The rumbling noise is present with only the BFD attached to the subwoofer (no receiver input to BFD).
The rumbling noise is very obvious even with the absence of subwoofer signal from receiver. Its a constant motor idling sound that would always be present and in fact keep the subwoofer from switching off when not in use and be very annoying if trying to watch a movie or TV. Also, when subwoofer audio is passed through the BFD, it gets modulated by the rumbling noise.

The noise will stop under the following conditions:
- Turn off all the PE filters in the preset being used.
- Switch to another present that is factory default (No PE filters).
- bypass the filters using front panel in/out
- ground isolator on BFD (both sub and BFD are plugged into same wall socket).
- Tried Balanced and unbalanced connector into BFD. Again even with no input connection the rumbling noise is present.

Seems to me to be related to the PE filter processing in the box.
My guess is the processor data is somehow getting picked up on the audio output.
I had a 6 filter config to balance my subwoofer and I switched them off 1 by 1. It was not until the last filter was turned off that the noise would halt.
To be sure, I reset the BFD and disabled the MIDI in case that was somehow involved and tried again by repeating a complete room characterization and setup but the rumbling from the BFD remained.

The attached mp3 is a 30hz tone from the receiver through the BFD and directly into the Soundblaster 24 (bypassing the subwoofer). The BFD is setup with 6 filters to flatten the subwoofer peaks. Ill include that data later.
In the supplied audio mp3 file, from 0-2s the BFD filters are bypassed by BFD front panel "in/out", and you hear a nice clean 30hz tone.
From 2s on, the BFD filters are engaged by enabling the front panel in/out switch (hear the rumbling?).

I guess my only recourse is to live without the subwoofer flattening (characterization included below) or potentially return this BFD and try another. Any thoughts or tips on next steps to figure this out?

The first image below is the room characterized when BFD is bypassed.
The 2nd image is after going through EQ process and tweaking the BFD filters to flatten as best I could. Note I lowered the desired curve to be able to just filter attenuate. The gain on the subwoofer is at 1/2 mast so I have room to turn it up.
The 3rd image is a simple block diagram of the setup.
Anything obvious Im doing wrong?

You mentioned the signal is low and next time I will beef that up, but none the less, the rumbling BFD noise is present whenver its attached to subwoofer even without an input.

Thanks again for the advise!

**brucek** · **10-09-06, 09:13 AM**

Quote:

Any thoughts or tips on next steps to figure this out?

You have a fault - return the BFD....

brucek

**rkruz** · **10-09-06, 12:56 PM**

Quote:

brucek wrote:

You have a fault - return the BFD....

brucek

All the other filters, not used for subwoofer EQ, in the bank are at factory default, so they are in the automatic hunt and kill feedback mode I guess. I wonder if they are contributing to the rumbling noise? Ive done so many experiements I have forgotten if I tried to turn off all unused filters. Thats an easy one to try and Ill do that tonight.

the experiment would be to simply listen for rumbling with all filters set to off (but "IN/Out" activited) with BFD input open and output connected to subwoofer input. Then simply activate a single filter and again listen for the rumbling and continue experiment up to the 7 filters currently programmed to tame the subwoofer. If that works, then the hunt and kill mode is the cause. If not, I will return and replace unit. Although I have my doubts about this one being defective and this could be more of a BFD design issue.

**brucek** · **10-09-06, 01:09 PM**

Quote:

All the other filters, not used for subwoofer EQ, in the bank are at factory default, so they are in the automatic hunt and kill feedback mode I guess.

All unused filters must be in the 'OF' state.

Most people use program 4 or 5.

Quote:

Although I have my doubts about this one being defective and this could be more of a BFD design issue.

No........

brucek

**rkruz** · **10-09-06, 01:33 PM**

Quote:

brucek wrote:

All unused filters must be in the 'OF' state.

Most people use program 4 or 5.

No........

brucek

I can try a simple experiment tonight. I can simply connect the output of the BFD to the soundblaster 24 input and use banks 4 or 5 and turn off all the inputs and look spectrally and listen audibly for the rumbling noise in my home recording studio (different then the home theater im EQing but great for high resolution recording and has some JBL references that get low enough to reproduce the rumbling). Then try turning on one by one the subwoofer filters while looking and
listening for the rumbling noise. Ill let you know.

I fight these noise pick up issues all the time in my work as Im a Electronics design engineer. Not usually at audio, but my experience is that these noise pickup problems are not failures perse, but a design issue related to proximity of analog to digital circuits or inadeguate filtering or shielding etc. But I will return the unit for replacement if turning off all unused filters doesnt and using bank 4 or 5 doesnt tame the rumbling noise

**brucek** · **10-09-06, 02:23 PM**

Quote:

my experience is that these noise pickup problems are not failures perse, but a design issue related to proximity of analog to digital circuits or inadeguate filtering or shielding etc

Agreed, and the BFD is a very inexpensive device that doesn't enjoy circuits with creative pathways for noise immunity, but it does a very passable job in this low frequency application. The noise floor also isn't anything to write home about, but subwoofers aren't victims of the bandwidths involved.

Evidenced by the thousands of satisfied users of the BFD who have not witnessed any low frequency noise from their systems, I think it's safe to say you have a faulty unit or you have it in the feedback destroy mode......

brucek

**rkruz** · **10-09-06, 03:51 PM**

Quote:

brucek wrote:

Agreed, and the BFD is a very inexpensive device that doesn't enjoy circuits with creative pathways for noise immunity, but it does a very passable job in this low frequency application. The noise floor also isn't anything to write home about, but subwoofers aren't victims of the bandwidths involved.

Evidenced by the thousands of satisfied users of the BFD who have not witnessed any low frequency noise from their systems, I think it's safe to say you have a faulty unit or you have it in the feedback destroy mode......

brucek

I think in this case, the coupling need not be a low frequency signal, but could be the frequency content contained in the rise time or rep rate of a digital signal and appearing in the <100hz range. Its good to know there are so many that dont have this problem so hopefully I do have a unit with the odd ground connection failure from a poor solder joint or something. The first thing to try to eliminate the possibility of the automatic filters causing the problem which I have not done yet.
thanks again

**Wayne A. Pflughaupt** · **10-09-06, 04:16 PM**

Quote:

I fight these noise pick up issues all the time in my work as Im a Electronics design engineer. Not usually at audio, but my experience is that these noise pickup problems are not failures perse, but a design issue related to proximity of analog to digital circuits or inadeguate filtering or shielding etc.

Have you tried re-locating the BFD? Maybe you have it next to an amplifier or some other component that’s generating noise directly into it. We used to see this problem installing pro audio – certain brands were susceptible to picking up noise, so we had to be careful where we placed them in the rack.

Regards,
Wayne

**rkruz** · **10-10-06, 12:50 AM**

Quote:

Wayne A. Pflughaupt wrote:

Have you tried re-locating the BFD? Maybe you have it next to an amplifier or some other component that’s generating noise directly into it. We used to see this problem installing pro audio – certain brands were susceptible to picking up noise, so we had to be careful where we placed them in the rack.

Regards,
Wayne

LOL. Great point!. I thought the same thing during the night and was late to work repeating the experiment and this time moving the BFD about 8 feet away from the subwoofer. But it did not make any difference.

While doing this 2nd experiment, I did conclude that its not noise the BFD is creating, its actually modulating the input signal. So if I put in a nice clean 30hz for example and then switched the BFD inline, the clean 30 hz begins to sound like a rough engine idling. If I bypass the BFD the tone is clean again. In my case I had 6 filters programmed and also turned off all the other unused filters in both banks. I then turned off each of the 6 filters I programmed and when turned off, would change the rumbling sound a bit, but the rumbling was always there. I tried different filters but it didnt matter...each filter was a bit unique in the modulation. Every filter modulated the input tone. I also tried moving the tone to 80 and 100 and the same thing. While it seemed to modulate the tone less at these higher frequencies the rumbling was still there.

Before I try another BFD I might look into buying one of the other units the Room EQ supports. I dont have a lot of hope another BFD will be an improvement in what Im hearing.

Thanks again for the great advise!

**Wayne A. Pflughaupt** · **10-10-06, 01:13 AM**

Hmm... Well then, I vote with brucek – return it for another one. This is the first any of us have heard of this problem, so I’d try another BFD before you move on to something else. Keep us posted.

Regards,
Wayne

**rkruz** · **10-10-06, 01:45 AM**

Quote:

Wayne A. Pflughaupt wrote:

Hmm... Well then, I vote with brucek – return it for another one. This is the first any of us have heard of this problem, so I’d try another BFD before you move on to something else. Keep us posted.

Regards,
Wayne

I think now it might be a low signal issue. When I play with it on my bench and lower the signal to the lowest levels the BFD displays I can hear the rumbling. Its not as apparent as I dont have a true subwoofer here. I do have some more power to send to it from the AVReciever so I will try that next with the big subwoofer.

Are the levels your sending to your BFDs generally provide level indication on the from panel? Its about 1 LED (-40) strength or less that I hear the rumbling with the filters turned on. if I can get it to 2 LEDs level or -30 then the tone is pretty solid without rumbling.

**brucek** · **10-10-06, 08:25 AM**

Quote:

Are the levels your sending to your BFDs generally provide level indication on the from panel?

Generally yes, to obtain the best S/N you want to take full advantage of every bit available in the BFD. The bit resolution defines the dynamic range of any digital system and so the signal to noise ratio is maximized by using as much of the dynamic range as possible. If the range from your weakest signal to the strongest signal was only half the required maximum input level, you would be robbing yourself of dynamic range and subsequently worstening your signal to noise level.

But in my tests, output signals remain linear even with extremely low input levels (an inherant problem with ADC's) across the full audio band and doesn't distort until you're slightly above the red clipping led.

Again I have to say you appear to have a fault. Perhaps you could exchange it for a new unit.

brucek

**rkruz** · **10-10-06, 06:57 PM**

Quote:

brucek wrote:

Generally yes, to obtain the best S/N you want to take full advantage of every bit available in the BFD. The bit resolution defines the dynamic range of any digital system and so the signal to noise ratio is maximized by using as much of the dynamic range as possible. If the range from your weakest signal to the strongest signal was only half the required maximum input level, you would be robbing yourself of dynamic range and subsequently worstening your signal to noise level.

But in my tests, output signals remain linear even with extremely low input levels (an inherant problem with ADC's) across the full audio band and doesn't distort until you're slightly above the red clipping led.

Again I have to say you appear to have a fault. Perhaps you could exchange it for a new unit.

brucek

what is the extremely low input level at? What is indicated on the BFD front panel indicators at this level?

**Wayne A. Pflughaupt** · **10-10-06, 07:17 PM**

I’d say “extremely low” would be hardly ever seeing more than the first bar or two on the meter light up. What kind of readings are you getting?

Regards,
Wayne

**rkruz** · **10-10-06, 07:40 PM**

Quote:

Wayne A. Pflughaupt wrote:

I’d say “extremely low” would be hardly ever seeing more than the first bar or two on the meter light up. What kind of readings are you getting?

Regards,
Wayne

well the rumbling Im referring to occurs when I cannot see a meter indication but can hear the tone. At that point it has the engine idling rumble. WHen it gets to 1 to 2 bars then the tone is even as it should be. So I guess it has something to do with the low level signals that most folks would not hear in a room eq routine.

With no input, I dont hear anything now. Before I thought it was outputting noise but now I think it was modulating the 60hz ground loop hum. But with a cheater plug that is gone and its very quiet. But when inputing a subwoofer low level tone, I can hear the rumbling. I suppose one should expect to get some output aberration when the input analog signal is just hitting the S/N threshold to be digitized. I'll play again as soon as I can and try to get a nice clean SPDIF signal into the reciever.

**brucek** · **10-10-06, 08:04 PM**

Quote:

what is the extremely low input level at? What is indicated on the BFD front panel indicators at this level?

When I tested it, I went to 10mvpk and it was still quite linear - certainly no LED's indicating...

Quote:

So I guess it has something to do with the low level signals that most folks would not hear in a room eq routine

I suggest not. The BFD does not produce any audible rumbing noise with or without filters...

Quote:

I suppose one should expect to get some output aberration when the input analog signal is just hitting the S/N threshold to be digitized

I doubt you'd hear it at that signal level. The S/N is about 94dB.

brucek

**rkruz** · **10-10-06, 10:48 PM**

Quote:

brucek wrote:

When I tested it, I went to 10mvpk and it was still quite linear - certainly no LED's indicating...

I suggest not. The BFD does not produce any audible rumbing noise with or without filters...

I doubt you'd hear it at that signal level. The S/N is about 94dB.

brucek

no the BFD doesnt produce any rumbling on its own. What its doing is modulating the input signal to create the rumbling sounds. Really noticeable with a CW input to the BFD of 30 hz or so. Once the input tone gets above a certain threshold, then it becomes a clean tone out of the BFD. Maybe it wont be noticeable with soundtracks. Ill set it up and give it a try.

**brucek** · **10-10-06, 11:06 PM**

Quote:

What its doing is modulating the input signal to create the rumbling sounds

Because you have a fault. Why would you not simply return it and get a unit that didn't do this?

brucek

**rkruz** · **10-10-06, 11:19 PM**

on the level display, I set the input power so that I just light the 1st -40db lamp. But when I increase the power over 10db more (anywhere from 50 to 100hz) the lamp level indicator does not move to reflect a >10db input change. This is the same on both channels. Is that a clear indication of a fault or is this typical of these BFD units?

**Wayne A. Pflughaupt** · **10-11-06, 02:44 PM**

Hmm... Not sure what you mean by “increased the power over 10 dB from 50-100 Hz.” Typically what you do is increase the receiver’s subwoofer output level (in the receiver’s menu). You arre driving the BFD and your sub from a receiver, I assume?

There is some debate around here about how the meters respond to different input signals, but aside from that the levels you’re feeding the BFD are way too low. That may or may not have something to do with the problems you’re having, but we can’t tell for sure until you get the input level up. Have you tried our recommended level-setting exercise, using a DVD from a high-output action flick?

Regards,
Wayne

**rkruz** · **10-11-06, 05:35 PM**

Quote:

Wayne A. Pflughaupt wrote:

[f sans ms,verdana,arial] Hmm... Not sure what you mean by “increased the power over 10 dB from 50-100 Hz.” Typically what you do is increase the receiver’s subwoofer output level (in the receiver’s menu). You arre driving the BFD and your sub from a receiver, I assume?

There is some debate around here about how the meters respond to different input signals, but aside from that the levels you’re feeding the BFD are way too low. That may or may not have something to do with the problems you’re having, but we can’t tell for sure until you get the input level up. Have you tried our recommended level-setting exercise, using a DVD from a high-output action flick?

Regards,
Wayne [/font]

I didnt word it well. I increased the power level (at a fixed CW) by more then 10db and the input level did not change from -40db indication on the BFD front panel. I did this at a couple different frequencies in the subwoofer range and got the same result. I could hear the audio SPL increasing as I increased the power but did not see the meter increase accordingly. Im just trying to get a good description of a failure so that I can easily and in good conscious exchange the unit.

No, I have not tried the recommended level setting excercise, but I will. I assume its easy to find onsite.

thanks again

**Wayne A. Pflughaupt** · **10-11-06, 06:00 PM**

Scroll down to the “Setting Input Level" section of the BFD Guide.

What’s CW?

Regards,
Wayne

**rkruz** · **10-12-06, 01:02 AM**

Quote:

Wayne A. Pflughaupt wrote:

Scroll down to the “Setting Input Level" section of the BFD Guide.

What’s CW?

Regards,
Wayne

sorry...CW = continuous wave. A radio frequency term to specify a carrier without modulation. Probably not commonly used for audio stuff. thanks for the level tip.

Advertisement