M-Audio BX8 2002, Strange Hum From Studio Monitor Tweeter!

Thread Starter

SEGACDX

Joined Oct 12, 2018
15
I've got a pair for M-Audio BX8's the original ones from 2002. https://img.audiofanzine.com/images/u/product/normal/m-audio-bx8-3304.jpg

So I've been trying to solve this problem forever and put in on hold many times over a few years, I've done everything I can think of and nothing seems to solve the problem.

I thought it may be bad caps or and it may be but I've tired different caps with no luck, It seems the problem I've narrowed down is for now with the toroidal transformer making the tweeter side of the amp pick up hum/buzz from the mounting plate.

I own two of these from the same year 2002 in the same line of monitors, so I've been using the good one to compare, I thought it might be a grounding problem and it might be, so I added grounding to places missing on the unit with the tweeter hum/buzzing problem and while I did manage to make it match in terms of where the ground should lead to, testing the points on the board where it should've had ground and didn't before are fixed. It could be some kind of ground loop or something but I am not sure.

Here's the important bit, if I move the transformer away from the casing it's normally attached too the humming/buzzing goes from audible buzzing in the listening position, back down to normal levels of noise, gentle static if you put your ear up to it just like the good unit.

If anyone knows what may be causing that hum to come back only when it comes closer to the plate it's normally attached to it would be a big help.

I'll put some photo's to help clarify but if all else fails I may just have to leave it disconnected form the case when installing it back into the speaker box since it works fine other than the very annoying loud humming coming out of the tweeter when connected to the case.

I put some photo's and a similar schematic down below, I can add more if needed of the boards in detail if needed.
 

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Thread Starter

SEGACDX

Joined Oct 12, 2018
15
Didn't realize I could not edit my post to add more images, I've added some photo's of the boards, not the best but useable. I'll add or redo any photo's here if they are not clear enough since I can change them without editing my post. I hope someone can help me track down the problem.
 

MrChips

Joined Oct 2, 2009
23,511
This sounds like a tough nut to crack and requires some in-depth detective work.

I am going to hazard a guess that the hum/buzz is low frequency 60Hz hum?
You have two speakers, one bad and one good. Does the construction and circuitry look identical?
Do the two transformers appear to be identical?

When you move the transformer away the buzz diminishes.
Sounds like 60Hz AC inductive pickup on the tweeter electronics.
Is there another location where you can mount the transformer?
Would a ferromagnetic shield between the transformer and the electronics help to reduce the buzz?
 

andrewmm

Joined Feb 25, 2011
1,462
As @MrChips says, this is going to be "interesting to find"

Check the easy ones first,
does it matter which way up the toroid is, or where the leads come into / out of it in relation to the rest of the board, It should not matter , but.

Does the good one by some chance have a plastic washer under it between the base and the torroid ?

I'm wondering, would be able to localise things,
the noise could be coming in on the power supplies or on the input / pickup.

Do the different cones have different amplifier circuits ?
If so , and they are AC coupled , Im wondering if you could short to ground the input to the amp that has the problem,
if the noise is on the input, the noise will go, if its on the PSU, it will drop but still be there.

We are assuming the noise is "mains pickup" i.e. 50 or 60 Hz, depending where you are.
Does that sound about right ?

If its at another frequency, we need to look in another direction, such as motor boating,

Does the noise change as the input is changed in amplitude ?

BTW: I'm envious. great little monitor speakers,
I used to have some great LS35A and Quad ELS63's, but I'm death in one ear now, and tinnitus in the other, so any audio sounds as "good" as the next. Serious envy ,,,
 

Thread Starter

SEGACDX

Joined Oct 12, 2018
15
This sounds like a tough nut to crack and requires some in-depth detective work.

I am going to hazard a guess that the hum/buzz is low frequency 60Hz hum?
You have two speakers, one bad and one good. Does the construction and circuitry look identical?
Do the two transformers appear to be identical?

When you move the transformer away the buzz diminishes.
Sounds like 60Hz AC inductive pickup on the tweeter electronics.
Is there another location where you can mount the transformer?
Would a ferromagnetic shield between the transformer and the electronics help to reduce the buzz?
Both of them are more or less identical with very minor differences, like different branded caps and a different grounding on the sheading on the bottom circuit which I changed to match so the grounding was connected to the same places. The transformers are the same. I thought maybe the mounting plate is making some kind of inductance and pasting it into the transformer, I don't have any sheading I could add, but maybe you know of something like a household item I could use to test it.

I can make a short video of what is happening if that helps identify the problem?
 

Thread Starter

SEGACDX

Joined Oct 12, 2018
15
As @MrChips says, this is going to be "interesting to find"

Check the easy ones first,
does it matter which way up the toroid is, or where the leads come into / out of it in relation to the rest of the board, It should not matter , but.

Does the good one by some chance have a plastic washer under it between the base and the torroid ?

I'm wondering, would be able to localise things,
the noise could be coming in on the power supplies or on the input / pickup.

Do the different cones have different amplifier circuits ?
If so , and they are AC coupled , Im wondering if you could short to ground the input to the amp that has the problem,
if the noise is on the input, the noise will go, if its on the PSU, it will drop but still be there.

We are assuming the noise is "mains pickup" i.e. 50 or 60 Hz, depending where you are.
Does that sound about right ?

If its at another frequency, we need to look in another direction, such as motor boating,

Does the noise change as the input is changed in amplitude ?
If I rotate the toroid it still makes the same sound on both sides, They both have a rubber gasket under the toroid, connected to the mounting plate by a nut and bolt through the center. It's a Bi-amped Unit so both Woofer and Tweeter have amps to drive both, I live in a 60HZ region, If I change the volume it will get louder or quieter but will still be there. I hope that helps.
 

MrChips

Joined Oct 2, 2009
23,511
For magnetic shielding, any piece of sheet metal that will stick to a magnet, such as a small baking pan or canned food container.
 

MrChips

Joined Oct 2, 2009
23,511
If I change the volume it will get louder or quieter but will still be there. I hope that helps.
Let's study this a bit more carefully.
Is there a volume level control on the speaker?

Let's start at the bottom.
Assuming you have level controls on the amplifier and on the speaker,
with both controls set to 0 what sound do you hear?
With the amp level at 0, what sound do you hear as the speaker control is turn up slowly?
 

Thread Starter

SEGACDX

Joined Oct 12, 2018
15
Let's study this a bit more carefully.
Is there a volume level control on the speaker?

Let's start at the bottom.
Assuming you have level controls on the amplifier and on the speaker,
with both controls set to 0 what sound do you hear?
With the amp level at 0, what sound do you hear as the speaker control is turn up slowly?
There is only a volume on the speaker itself it's not connected to an interface to make sound, If I have it at 0 with the toroid off the plate as far as it can go it's faint static, with it next to it at 0 it's making the buzzing, if I turn it up to 100 it will be much louder, If I take the toroid away from the mounting plate it will become just normal static again.

I made a short video to explain a bit better.

 

djsfantasi

Joined Apr 11, 2010
7,689
Can you temporarily place a sheet of steel between the toroid and the case? Or the toroid and the switches/wiring? Or both? Steel framing gussets from Lowe’s or Home Depot could be used to test.
 

Thread Starter

SEGACDX

Joined Oct 12, 2018
15
Can you temporarily place a sheet of steel between the toroid and the case? Or the toroid and the switches/wiring? Or both? Steel framing gussets from Lowe’s or Home Depot could be used to test.
I'll give it a try, Right now I am trying to use a steel can with a hole drilled into it to mount it with the toroid inside it and it seems to be working, the strange thing is, if I rotate it into the right position the buzzing stops and if I rotate it a bit more it comes right back. So maybe it needs to be mounted in a very specific way.
 

Thread Starter

SEGACDX

Joined Oct 12, 2018
15
I'm not going to say yet that I've solved it but I think I am onto something here, So looking at both units the toroid is mounted in a specific rotation, with the other good unit it also does have the buzz if you put your hear right up to it at full volume, but now the previously bad buzzing unit, with the toroid set into the position I used while listening to it up to my ear is now has no buzz at all at full volume and only a tiny bit when set to 0. Which is reverse from the other one where the buzz is only heard at full volume and almost inaudible, I didn't even know is was there on the good unit. So maybe I'll do the same modification when I have another steel can of the right size.

I find it really strange that the rotation of the toroid adds or subtracts the hum/buzzing, so there is some fundamental property of how these transformers work I do not understand.

I did test them both and they work just fine but I'll have to put in a few hours or a day or two of use to see if the problem is solved.

I'd like to thank everyone so far for being so helpful, I really appreciate it, I tired EEV-Forums first and got no reply's and remembered this forum being helpful on a few problems I've had so thank you all for the help.
 

andrewmm

Joined Feb 25, 2011
1,462
I'm not going to say yet that I've solved it but I think I am onto something here, So looking at both units the toroid is mounted in a specific rotation, with the other good unit it also does have the buzz if you put your hear right up to it at full volume, but now the previously bad buzzing unit, with the toroid set into the position I used while listening to it up to my ear is now has no buzz at all at full volume and only a tiny bit when set to 0. Which is reverse from the other one where the buzz is only heard at full volume and almost inaudible, I didn't even know is was there on the good unit. So maybe I'll do the same modification when I have another steel can of the right size.

I find it really strange that the rotation of the toroid adds or subtracts the hum/buzzing, so there is some fundamental property of how these transformers work I do not understand.

I did test them both and they work just fine but I'll have to put in a few hours or a day or two of use to see if the problem is solved.

I'd like to thank everyone so far for being so helpful, I really appreciate it, I tired EEV-Forums first and got no reply's and remembered this forum being helpful on a few problems I've had so thank you all for the help.
These sort of things are crazy to decode,
Agreed, the orientation of the torroid "should" make no difference, but I know from past experience , it can do ,
I've never explained why ,
 

MrChips

Joined Oct 2, 2009
23,511
You may have noticed on some tube amplifiers the output transformer is mounted away from the AC Line transformer and rotated at an angle. They have a reason for doing that.
 

Thread Starter

SEGACDX

Joined Oct 12, 2018
15
You may have noticed on some tube amplifiers the output transformer is mounted away from the AC Line transformer and rotated at an angle. They have a reason for doing that.
Maybe it's the same reason they may use squared transformers on most of the newer studio monitor amplifiers, since you can see the ends are shielded and there is only one or two ways it can be mounted. Still I am happy to have solved my problem for now.
 

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Thread Starter

SEGACDX

Joined Oct 12, 2018
15
One thing I've been thinking about is could I get something like Copper Tape used for EMI/RFI shielding and use that to wrap the toroid to cut down the buzz and hum without having to use a steel food can with a hole drilled into it, Do any of you think that would work or would that make it worse? I've seen from toroid's wrapped in copper foil tape when looking into sheading online.
 

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MrChips

Joined Oct 2, 2009
23,511
One thing I've been thinking about is could I get something like Copper Tape used for EMI/RFI shielding and use that to wrap the toroid to cut down the buzz and hum without having to use a steel food can with a hole drilled into it, Do any of you think that would work or would that make it worse? I've seen from toroid's wrapped in copper foil tape when looking into sheading online.
Copper will not work because it is non-magnetic.
 

MisterBill2

Joined Jan 23, 2018
8,735
First, any copper wrapping on the toroid will act like a short circuited turn and may lead to a lot of grief. I am suggesting measuring the resistance of the different windings in each toroid and comparing them. A single short circuited turn will cause problems on occasion.
I think that I read that moving the toroid changes the hum, and so it may be that there is an unintentional loop in the one package that is not present in the hum-free one. That would be a reasonable cause, but a big challenge to locate.
One other thing to try is swapping the two power transformers.
 

Thread Starter

SEGACDX

Joined Oct 12, 2018
15
Copper will not work because it is non-magnetic.
That makes sense, maybe I can find something at the hardware store that's solid steel flat on the bottom and round. A thick steel cup might work, I may even be able to find something that can fit on top, then it would be almost closed off from any interference. Would just have to make a cutout for the wires or just go the full mile and rewire everything. I'll look around for the next while and see what I can find, if all else fails I know I can just use a can even if it's not ideal.
 
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