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Mains hum mystery...
Old 1 week ago
  #1
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🎧 10 years
Mains hum mystery...

Hi all. I hope this is the right place for this post but apologies if I've got that wrong. I am encountering a very specific mains hum issue and wondered if someone here might be able to help with attempting to fix it.

I recently bought a pair of Ex Machina Pulsar monitors and it seems like there is some gear they really don't like. Until recently I was using an Audient ASP2802 mixer but when plugged into the Pulsars they produce an audible 50Hz hum. There was no hum coming from this desk through my previous speakers (Focal Solo 6Be with Sub 6).

I changed my studio configuration and bought an Audient Nero monitor controller but this is producing identical hum through the Pulsars. If I plug my audio interface (UAD Apollo x8p) directly into the speakers there is no hum at all. I also have an ancient Mackie 1202 VLZ which does not generate any hum.

Everything I'm using is running through balanced cables and I have tried different ones and tested all the cables. I have also just moved my studio so the mains wiring has all changed but the hum issue remains the same.

My electrical knowledge is pretty much zero so if anyone has any suggestions at all I would be incredibly grateful!

Thanks in advance,
Dave
Old 1 week ago
  #2
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Thomas W. Bethe's Avatar
 
1 Review written
🎧 15 years
Quote:
Originally Posted by ragman ➑️
Hi all. I hope this is the right place for this post but apologies if I've got that wrong. I am encountering a very specific mains hum issue and wondered if someone here might be able to help with attempting to fix it.

I recently bought a pair of Ex Machina Pulsar monitors and it seems like there is some gear they really don't like. Until recently I was using an Audient ASP2802 mixer but when plugged into the Pulsars they produce an audible 50Hz hum. There was no hum coming from this desk through my previous speakers (Focal Solo 6Be with Sub 6).

I changed my studio configuration and bought an Audient Nero monitor controller but this is producing identical hum through the Pulsars. If I plug my audio interface (UAD Apollo x8p) directly into the speakers there is no hum at all. I also have an ancient Mackie 1202 VLZ which does not generate any hum.

Everything I'm using is running through balanced cables and I have tried different ones and tested all the cables. I have also just moved my studio so the mains wiring has all changed but the hum issue remains the same.

My electrical knowledge is pretty much zero so if anyone has any suggestions at all I would be incredibly grateful!

Thanks in advance,
Dave

You have a ground loop...Unplug everything and plug in one item at a time and you will "probably" find the problem. FWIW
Old 1 week ago
  #3
^^ as above. Do you have wall wart power supplies? If so start with just removing those. I had a feint earth hum that was caused by wall warts on effects units but only when they were connected via the console send/returns. Got rid of them and bought a normal mains powered effects unit in place of them and no more noise.
Old 1 week ago
  #4
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🎧 10 years
Thank you both very much for the suggestions. Forgive my ignorance but if it's a ground loop problem why would it only occur with the Nero and the ASP 2802 and not with the Apollo or the Mackie? They have all been plugged into the same 4 way power supply.

As a side note there is a DC adapter on the Nero but the ASP2802 runs off a standard kettle lead and both cause hum. The Apollo runs from a DC adapter and the Mackie from a standard kettle lead and neither cause hum. It seems most peculiar to me.
Old 1 week ago | Show parent
  #5
Quote:
Originally Posted by ragman ➑️
Thank you both very much for the suggestions. Forgive my ignorance but if it's a ground loop problem why would it only occur with the Nero and the ASP 2802 and not with the Apollo or the Mackie? They have all been plugged into the same 4 way power supply.

As a side note there is a DC adapter on the Nero but the ASP2802 runs off a standard kettle lead and both cause hum. The Apollo runs from a DC adapter and the Mackie from a standard kettle lead and neither cause hum. It seems most peculiar to me.
I'm not familiar with any of those devices but does sound strange. Different devices and manufacturers have their own way of grounding so could be something like that. Try the disconnecting tests and let us know.
Old 1 week ago
  #6
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🎧 5 years
Quote:
Originally Posted by ragman ➑️
Hi all. I hope this is the right place for this post but apologies if I've got that wrong. I am encountering a very specific mains hum issue and wondered if someone here might be able to help with attempting to fix it.

I recently bought a pair of Ex Machina Pulsar monitors and it seems like there is some gear they really don't like. Until recently I was using an Audient ASP2802 mixer but when plugged into the Pulsars they produce an audible 50Hz hum. There was no hum coming from this desk through my previous speakers (Focal Solo 6Be with Sub 6).

I changed my studio configuration and bought an Audient Nero monitor controller but this is producing identical hum through the Pulsars. If I plug my audio interface (UAD Apollo x8p) directly into the speakers there is no hum at all. I also have an ancient Mackie 1202 VLZ which does not generate any hum.

Everything I'm using is running through balanced cables and I have tried different ones and tested all the cables. I have also just moved my studio so the mains wiring has all changed but the hum issue remains the same.

My electrical knowledge is pretty much zero so if anyone has any suggestions at all I would be incredibly grateful!

Thanks in advance,
Dave
ground loop monster is easy to get out of this system.

The thing you need to learn is balanced outputs on non transformer coupled gear must be at some times forced into balanced operation because of the unbalanced circuits in between the balanced connections in different units will add their power supply noise onto the balanced shield ground.

So just like in PA world, you interrupt these grounds so they don't ground loop between output processing devices.

So lift the sleeve connection from the USB interface, as well as remove the sleeve connection/pin 1 xlr to the speakers to remove the equipment to equipment ground looping.
Old 1 week ago | Show parent
  #7
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🎧 10 years
Quote:
Originally Posted by mrfantastic ➑️
I'm not familiar with any of those devices but does sound strange. Different devices and manufacturers have their own way of grounding so could be something like that. Try the disconnecting tests and let us know.
Thanks mrfantastic. The problem is definitely with the Audient Nero Monitor controller. It's the only thing inducing hum when plugged in. I have written to Audient to see if they can shed any light on it...
Old 1 week ago | Show parent
  #8
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🎧 10 years
Quote:
Originally Posted by audiospecific ➑️
ground loop monster is easy to get out of this system.

The thing you need to learn is balanced outputs on non transformer coupled gear must be at some times forced into balanced operation because of the unbalanced circuits in between the balanced connections in different units will add their power supply noise onto the balanced shield ground.

So just like in PA world, you interrupt these grounds so they don't ground loop between output processing devices.

So lift the sleeve connection from the USB interface, as well as remove the sleeve connection/pin 1 xlr to the speakers to remove the equipment to equipment ground looping.
Thank you audiospecific. I partially understand your reply although there is no USB interface involved. The Nero is a monitor controller. When you say "lift the sleeve connection" - how would I go about doing this?
Old 1 week ago | Show parent
  #9
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🎧 5 years
Quote:
Originally Posted by ragman ➑️
Thank you audiospecific. I partially understand your reply although there is no USB interface involved. The Nero is a monitor controller. When you say "lift the sleeve connection" - how would I go about doing this?
easiest way would be to re-solder the trs but don't strip back the cable shield and crimp the insulation and not solder the shield.

Otherwise, you have to cut where the shield is solder, then un-crimp the end, and tape up cord shield end (with black electrical tape) and re-crimp the cable.

You should interrupt the shield on electrical balanced connections into the monitor controller, like interfaces, tape machines, etc as well as any outputs and this applies to most monitor controllers that are currently made.
Old 1 week ago
  #10
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🎧 10 years
The 2012 seminar paper:
An Overview of Audio System Grounding and Interfacing
by
Bill Whitlock, President
Jensen Transformers, Inc.

Has a section on troubleshooting, starting at page 99.
https://centralindianaaes.files.word...notes-v1-0.pdf
Old 1 week ago | Show parent
  #11
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Thomas W. Bethe's Avatar
 
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🎧 15 years
Quote:
Originally Posted by Speedskater ➑️
The 2012 seminar paper:
An Overview of Audio System Grounding and Interfacing
by
Bill Whitlock, President
Jensen Transformers, Inc.

Has a section on troubleshooting, starting at page 99.
https://centralindianaaes.files.word...notes-v1-0.pdf

An excellent guide...
Old 1 week ago | Show parent
  #12
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🎧 5 years
Quote:
Originally Posted by Thomas W. Bethe ➑️
An excellent guide...
Its incomplete in some aspects when it comes to signal, other stuff is general knowledge. But I understand its for a general crowd and not people who build the circuitry either.


Well, there are only two connections you can do balanced out to unbalanced in:


Pin 2+ to tip
Pin 1 to sleeve


This is called dc referenced common balanced to unbalanced. This means the DC grounds are connected between the two pieces of equipment and the signal common is referenced to DC ground in both units. The drawback of this connection is a -6db attenuation, and all power supply noise is applied to the signal. Some people like this connection because it attenuates it, but in certain situations, causes a ground loop (hum in the audio).

Pin 2+ to tip
Pin 3- to sleeve

This is called virtual ground reference balanced to unbalanced connection. This means the virtual ground from the previous unit is tied to the shared dc ground and signal ground of the next unit. The advantages of this are that you eliminate the ground loop that would be present in the other connection, and the signal maintains its signal level. The drawback to this connection is that the line level might exceed the maximum input level of the unbalanced connection (i.e: +22dbu vs +18dbu).


So there you go: the both ways you can do it, and the advantages and drawbacks to both.
Old 1 week ago
  #13
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🎧 10 years
Thank you all for these very thorough answers / suggestions. Guess I've got some reading (and possibly soldering) to do!
Old 1 week ago | Show parent
  #14
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🎧 15 years
Quote:
Originally Posted by audiospecific ➑️
Its incomplete in some aspects when it comes to signal, other stuff is general knowledge. But I understand its for a general crowd and not people who build the circuitry either.
Bill Whitlocks efforts on this subject are considered comprehensive, and well respected by audio professionals.
Quote:

Well, there are only two connections you can do balanced out to unbalanced in:


Pin 2+ to tip
Pin 1 to sleeve


This is called dc referenced common balanced to unbalanced. This means the DC grounds are connected between the two pieces of equipment and the signal common is referenced to DC ground in both units. The drawback of this connection is a -6db attenuation, and all power supply noise is applied to the signal. Some people like this connection because it attenuates it, but in certain situations, causes a ground loop (hum in the audio).

Pin 2+ to tip
Pin 3- to sleeve

This is called virtual ground reference balanced to unbalanced connection. This means the virtual ground from the previous unit is tied to the shared dc ground and signal ground of the next unit. The advantages of this are that you eliminate the ground loop that would be present in the other connection, and the signal maintains its signal level. The drawback to this connection is that the line level might exceed the maximum input level of the unbalanced connection (i.e: +22dbu vs +18dbu).


So there you go: the both ways you can do it, and the advantages and drawbacks to both.
Surely there are more than just those two. RANE published an application note documenting the sundry interface connections.

I am not familiar with this terminology. This is relatively mature technology.

JR
Old 1 week ago | Show parent
  #15
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🎧 5 years
Quote:
Originally Posted by JohnRoberts ➑️
Bill Whitlocks efforts on this subject are considered comprehensive, and well respected by audio professionals.
I don't want to sit here and pick apart a paper designed for the operator, but he does assume some things. One example is the Hum-x he picked on, which is the same old chassis lift that is in thousands of tube gear. That no one has an issue with. Besides that, The diodes short when the circuit is faulted.
Quote:
Originally Posted by JohnRoberts ➑️
Surely there are more than just those two. RANE published an application note documenting the sundry interface connections.

I am not familiar with this terminology. This is relatively mature technology.

JR
No, there is only two real valid connections.
I remembered seeing the RANE publications some time ago, but If I remember correctly, they only really had the unbalanced connections to dc ground connections and not the ones that only reference zero crossing of the signal.


Its old EE terminology that is self descriptive.

The reason why the speakers are humming on the monitor controller is both grounds on the speakers are grounded to mains FG and the controller's power supply is unfortunately grounded to Mains neutral. If it was a double insulated power supply it wouldn't have the issue. Or one that is referenced to FG, like in the other devices that were tried and worked hum free.

So the OP has to lift the shield/sleeve on the trs side that goes to the monitor controller from the speaker and every trs, or replace the supply or even use the computer's power supply. Its just 12VDC

Most likely there is a mains neutral to dc ground cap in the monitor controller supply that is causing the loop. A quick test of a battery or different supply in place of the monitor controller's power supply will tell.
Old 5 days ago
  #16
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🎧 10 years
Bill Whitlock, retired president of Jensen Transformers and past co-chair of the Audio Engineering Society EMI/RFI committee. Mr. Whitlock has been authoring AES papers, magazine articles and doing live seminars for well over two decades.
When it comes to safety, Bill Whitlock does not assume anything.
Old 5 days ago
  #17
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🎧 15 years
Mr. Ragman.
in agreement with Thomas, evidence suggests a ground loop. suggest you find a local professional, familiar with the subject, who can help identify the problem.

apologies for the fan club promoting an article with zero practical suggestions.

ground lifts are widely used by professionals to identify loops. they are employed in studio sessions filled with union scale musicians, legal or not, to fix noise issues.

likewise, power isolation transformers are exceedingly helpful not only to resolve ground loops but to reduce dangerous leakage currents.

am suggesting one or more 240:240 power isolation transformers to isolate your problem.


to the fan club more interested in promoting some questionable paper on grounding,
there is not one scintilla of evidence that electrostatic shields are named incorrectly.
been called that for 80+ years, by professionals.
Old 5 days ago
  #18
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🎧 15 years
"Ground loops" are a vague catch-all term used to describe all product interface mains hum issues. In fact ground loops*** are a real thing, but typically not "the" thing causing interface hum problems.

For some light reading search "Pin 1 problem". SKUs that improperly sink and ignore chassis ground current flows (like from shields) that corrupt the signal are a design flaw. The industry generally became aware of the pin 1 issue years (decades?) ago, but some SKUs are still susceptible.

Safety ground lifts are by definition unsafe. Properly designed gear is happy to be ground bonded to safety ground.

The OP appears to have identified the rouge SKU. Surely the manufacturer has encountered this problem before and can help, or not.

Good luck.

JR

***Ground loops, effectively a one turn transformer winding are sometimes encountered inside high power audio amps due to strong internal magnetic fields. Not rocket science but something for skilled design engineers to manage.
Old 4 days ago
  #19
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🎧 15 years
quick ground loop fix

The iFi GND Defender fixed ground loop hum for me. About $60 on Amazon.

WW
Attached Thumbnails
Mains hum mystery...-screen-shot-2022-12-01-09.08.50-.png  
Old 4 days ago | Show parent
  #20
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🎧 5 years
Quote:
Originally Posted by JohnRoberts ➑️
"Ground loops" are a vague catch-all term used to describe all product interface mains hum issues. In fact ground loops*** are a real thing, but typically not "the" thing causing interface hum problems.

For some light reading search "Pin 1 problem". SKUs that improperly sink and ignore chassis ground current flows (like from shields) that corrupt the signal are a design flaw. The industry generally became aware of the pin 1 issue years (decades?) ago, but some SKUs are still susceptible.

Safety ground lifts are by definition unsafe. Properly designed gear is happy to be ground bonded to safety ground.

The OP appears to have identified the rouge SKU. Surely the manufacturer has encountered this problem before and can help, or not.

Good luck.

JR

***Ground loops, effectively a one turn transformer winding are sometimes encountered inside high power audio amps due to strong internal magnetic fields. Not rocket science but something for skilled design engineers to manage.
I think there is more to this, as I sampled a schematic of a unit (made by them) and found improperly designed balanced outputs. So its actally is an unblananced circuit, and a valid interconnect is going to be tip and ring to 2&3 with no shield and you just lift pin 1 of the xlr going into the speaker. If you still have a hum, then the power supply needs to be changed to something isolated dc power that its ground is not referenced to mains power in any way.

Besides it only is a single supply, and tying signals to that ground instead of the virtual ground that would be developed in a single supply circuit. This might lead to noise injection if the dc supply is grounded to FG or Neutral.


Btw John, you guys over at Peavey wouldn't make an obvious mistake with designing a balanced output.
Attached Files
File Type: pdf iD14 15002-c XMOS & PSU.pdf (201.7 KB, 3 views)
Old 3 days ago
  #21
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Seems like I accidentally threw a bit of a grenade with my OP so apologies for that!

Just to update you all, I tried running the monitor controller off a car battery with a power inverter to completely isolate it from the mains. Same problem annoyingly so I've written to Audient to see if they can shed any light on it. I'll let you guys know as it's obviously caused some head scratching ;-)
Old 3 days ago | Show parent
  #22
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Quote:
Originally Posted by ragman ➑️
Seems like I accidentally threw a bit of a grenade with my OP so apologies for that!

Just to update you all, I tried running the monitor controller off a car battery with a power inverter to completely isolate it from the mains. Same problem annoyingly so I've written to Audient to see if they can shed any light on it. I'll let you guys know as it's obviously caused some head scratching ;-)
I think I figured out the mystery, You see, The Audent's engineers are using a bad design on their balanced trs connections. The output is a balanced type, but its not commonly used for audio circuit because balanced audio is symmetric. Attached is their circuit. Now the way your speakers are behaving, their circuit is called a dc coupled differential input. The connection it wants is an electrically balanced or transformer balanced , which are symmetric balance audio. The hum is not from a ground loop, its the fact is that you are plugging a symmetrically balanced speaker into an asymmetric balanced output and its causing 50% of the power supply noise of the speaker to be injected into the speaker circuitry. 2nd attachment is an approximation of the input circuit of the speaker (by the way it is behaving)
Attached Thumbnails
Mains hum mystery...-badbalanced-stage.png   Mains hum mystery...-differential_line_driver.png  
Old 3 days ago
  #23
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🎧 5 years
The hum free instance of the Mickie mixer to speakers confirms what I am saying.

Attached is the mixer's output circuit, and the 2nd is the redraw of the generic design (that was also in the Neve 1272) that are electrically balanced outputs.
Attached Thumbnails
Mains hum mystery...-mackie_board_output.jpg  
Attached Images
Mains hum mystery...-balanced-out.png 
Old 3 days ago | Show parent
  #24
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🎧 15 years
Quote:
Originally Posted by audiospecific ➑️


Btw John, you guys over at Peavey wouldn't make an obvious mistake with designing a balanced output.
busted... I presume you found a mistake (i didn't open that file). I worked at Peavey for 15 years last century. I want to think I fixed more mistakes than I created but doubt I was perfect.
===
active balanced outputs are also a mature topic with numerous examples of better and less good approaches. Nowadays we can purchase off the shelf ICs that work well.
===
@ ragman ... no worries, some people just like to argue.

===

Years ago I sent a private communication to Hum-X questioning their use of small signal diodes in series with a safety ground bond path. On my test bench small signal diodes do indeed short, then vaporize. IIRC that SKU is not UL approved and would not pass ground bond testing because the small diodes ultimately fail open circuit (if you can find anything left). FWIW I did not get a response from Hum-X.

===

Quote:
This is called virtual ground reference balanced to unbalanced connection. This means the virtual ground from the previous unit is tied to the shared dc ground and signal ground of the next unit. The advantages of this are that you eliminate the ground loop that would be present in the other connection, and the signal maintains its signal level. The drawback to this connection is that the line level might exceed the maximum input level of the unbalanced connection (i.e: +22dbu vs +18dbu).
Not to get overly pedantic but that seems to be in fashion here. "Virtual ground" in circuit design refers to an input node (typically - input) of an active negative feedback circuit, where the amplifiers uses negative feedback to hold that "virtual ground" identical to the actual ground reference (typically connected to the + input). This is a common topology used in "virtual earth, active combining circuits". Since that virtual earth is an input to a gain stage, it is not suitable for use as an output ground reference.

===

My apologies to the OP this veer is not very helpful.

JR
Old 3 days ago | Show parent
  #25
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🎧 5 years
Quote:
Originally Posted by JohnRoberts ➑️
busted... I presume you found a mistake (i didn't open that file). I worked at Peavey for 15 years last century. I want to think I fixed more mistakes than I created but doubt I was perfect.
===
active balanced outputs are also a mature topic with numerous examples of better and less good approaches. Nowadays we can purchase off the shelf ICs that work well.
Yes, major improvements from hand selecting parts to go around a ne5532 to laser trimmed on die equivalents like a THAT 1606 that make making good audio circuits not time consuming.

Quote:
Originally Posted by JohnRoberts ➑️
Years ago I sent a private communication to Hum-X questioning their use of small signal diodes in series with a safety ground bond path. On my test bench small signal diodes do indeed short, then vaporize. IIRC that SKU is not UL approved and would not pass ground bond testing because the small diodes ultimately fail open circuit (if you can find anything left). FWIW I did not get a response from Hum-X.
That is crazy.
It was always stated wherever I read on this subject that the diodes are suppose to be silicone power supply type that stays shorted and people usually use a cheap rectifier bridge. and the paralleled line cap is suppose to be a Y type (and UL listed) that shorts on fault. Some people put an X type here, but I think it should always short so the fault is noticeable in the circuit when it fails.




Quote:
Originally Posted by JohnRoberts ➑️
Not to get overly pedantic but that seems to be in fashion here. "Virtual ground" in circuit design refers to an input node (typically - input) of an active negative feedback circuit, where the amplifiers uses negative feedback to hold that "virtual ground" identical to the actual ground reference (typically connected to the + input). This is a common topology used in "virtual earth, active combining circuits". Since that virtual earth is an input to a gain stage, it is not suitable for use as an output ground reference.

JR
This is parts of electronics theory that hasn't been sorted out in years. They need to make a new term for the common reference (zero crossing) of the signal and not call it a "virtual ground" because technically its a "virtual common" that has nothing to do with the circuit and anything with the words "ground" and "earth" should be applied to the circuit operational terms only.

So, when I told the OP to use pin 3, on a real symmetrically balanced output, its signal common is only the signal itself, and this virtual common is used instead of the common that is referenced at dc ground of the monitor controller. However, since the monitor controller's output is not the standard symmetric electrically balanced output, it hums, but that is because the total circuit connection between the two cause the effect at the speaker's line stage.
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