60hz hum issue - have checked usual suspects but problem persists

[FC Owens]

The conditions:

New recording studio construction, all audio circuits are IG receptacles with home runs to IG bus at main panel, which then goes / is bonded to the system ground rods..of which there are 2 per LA city code.

Single phase 200 amp service

Each studio room has 2-3 IG circuits and 1 dirty power circuit. All circuit routing are home runs via EMT from the panel to a main pull box in each room, then from there we used HCF MC to all of the IG circuit receptacles, all of the isolated grounds are separated per circuit in the pull box so there is no cross wiring between each other or the HCF and dirty power system grounds.

The approximate distances between all of the IG home runs and the main panel / IG buss are all similar, like, within 15-25 ft max difference, but across a 4000 sqft building.

The Problem:

When connecting an audio channel through 2 or more piece of gear plugged in to AC power within any single room, regardless of if they are on different circuits / phases / etc, there is no noise, however as soon as an audio signal is run between two devices which are connected to AC power in different rooms, there is significant 60hz hum.

What we have tried/tested already:

- Tested devices on different IG circuits which are on the same phase/leg but in different rooms - 60hz hum
- Tested devices on different IG circuits which are on different phases but in the same room - no hum
- Bypassed all in-wall audio cabling and ran directly between devices in different rooms - 60hz hum
- Lifted either audio or AC ground at one end when going between rooms - no hum (but neither are viable actual solutions of course).
- Connected devices to the "Dirty" power circuits in the same room - no hum
- Connected devices to dirty power in different rooms - noticeably louder 60hz hum than the IG circuits

The thing I cant wrap my head around is why there is no issue / hum when different devices are connected to different circuits/phases within the same room, but as soon as we move to a different room its a problem. Again, as all of the circuits going to each room are home runs back to the panel there isn't a huge disparity in wire length for the isolated (or system) grounds.

Any ideas or suggestions would be great. as this is a newly built commercial facility the client wants to figure out a permanent solution which wont require using any device to lift grounds either ac or audio when connecting between rooms.

 

[hillbilly1]

Does the isolated grounds go only to ground rods? If so, that is part of your problem. It is supposed to terminate at the service neutral bar or GEC.

 

[LarryFine]

Does the isolated grounds go only to ground rods? If so, that is part of your problem. It is supposed to terminate at the service neutral bar or GEC.

. . . also.

 

[FC Owens]

Does the isolated grounds go only to ground rods? If so, that is part of your problem. It is supposed to terminate at the service neutral bar or GEC.

So here is the current rounting:

IG buss at main panel -----> Bonding jumper @ service connect w GEC/Neutral
system ground buss at main panel ------> same bonding jumper as above.

 

[infinity]

I would start with disconnecting the IG at the panel and test each IG receptacle to ensure that it is not grounded.

 

[zbang]

This sort of thing is almost never an AC wiring issue but rather a signal reference ("ground") issue. Are these balanced or unbalanced lines? IMHO do not expect unbalanced line to be clean over more than 5'.

Also, it only takes one device to compromise an IG circuit

 

[ruxton.stanislaw]

Do you have an isolation transformer that you can test with? Or even portable battery powered inverter.

 

[FC Owens]

This sort of thing is almost never an AC wiring issue but rather a signal reference ("ground") issue. Are these balanced or unbalanced lines? IMHO do not expect unbalanced line to be clean over more than 5'.

Also, it only takes one device to compromise an IG circuit

Yeah, this is also something I'm well aware of...like 90% of the time i get a noise complaint its a actually a ground loop happening in the audio signal/patch bay somewhere.

However - This definitely seems to be an AC power related issue. one of the things we tried was placing a pedal which required AC power next to an the amp, then simply running an extension cord to power the pedal from another room and then using short unbalanced cable ...this = 60z hum. But if we change nothing else and use that same extension cord to any outlet in the same room, regardless of circuit/phase then its totally quiet.

 

[FC Owens]

Do you have an isolation transformer that you can test with? Or even portable battery powered inverter.

I do have a battery inverter..didn't have time to bring that today to test as they were about to start a session. as far as the iso transformer is concerned, would need a multi circuit one to properly test...and don't have off hand access to one off hand.

 

[GoldDigger]

One possibility which fits the observations is that there is a strong non-uniform 60 Hz magnetic field in the building and that magnetic field is inducing a voltage on the IG wires which are bundled by room. All of the IGs for one room share a common physical routing which is different from that of the other rooms and therefore will have an induced voltage differing in amplitude and possibly phase. Or else the different common induced voltage on each rooms set of IGs is caused by induction with the currents in wires in a single home run conduit for each room.

A much simpler explanation would be that there is some unrecognized connection among the IGs for a given room.

Have you checked whether the hum changes as breakers are turned off for various rooms in the building?
How about looking at the ground offset using the differential input on a scope? Possibly even a battery powered scope?

 

[synchro]

... Each studio room has 2-3 IG circuits and 1 dirty power circuit. All circuit routing are home runs via EMT from the panel to a main pull box in each room, then from there we used HCF MC to all of the IG circuit receptacles, all of the isolated grounds are separated per circuit in the pull box so there is no cross wiring between each other or the HCF and dirty power system grounds.

The approximate distances between all of the IG home runs and the main panel / IG buss are all similar, like, within 15-25 ft max difference, but across a 4000 sqft building.

The Problem:

When connecting an audio channel through 2 or more piece of gear plugged in to AC power within any single room, regardless of if they are on different circuits / phases / etc, there is no noise, however as soon as an audio signal is run between two devices which are connected to AC power in different rooms, there is significant 60hz hum.
...
The thing I cant wrap my head around is why there is no issue / hum when different devices are connected to different circuits/phases within the same room, but as soon as we move to a different room its a problem. Again, as all of the circuits going to each room are home runs back to the panel there isn't a huge disparity in wire length for the isolated (or system) grounds.

It sounds like there is a ground loop formed between the two devices by their connection to equipment grounds, as well as their being connected together by the shield of the audio cable between them.

Some speculation on what might be happening:
Perhaps the arrangement of a separate home run in EMT for each room could be a factor in what you are observing. For example, if two receptacles in the same room are used (even if they are on different circuits) then their isolated ground conductors will be in the same conduit. And if the other wires within that conduit have a net common mode current (maybe from powerline filters or equipment with L-G capacitors) then a current could be magnetically coupled into the isolated ground wire(s) going to the two receptacles. Then a very small AC voltage would be developed across the series reactance and resistance of the isolated ground conductors, and which would be approximately the same voltage on separate isolated ground wires going to each of the two receptacles. Therefore, the voltage difference between the receptacle grounds would be quite small, and therefore contribute a negligible AC current on the ground loop between the devices.

However, if the two devices are each connected to an isolated ground conductor in a different EMT home run, then the current coupled into each IG conductor and the voltage developed on them will be different. Therefore, when these two IG conductors are connected to the two devices a larger ground loop current will flow than if the IG conductors were in the same EMT run. The ground loop could also have a significantly larger area because of the space between the EMT home runs, and this could allow more hum to be picked up from stray AC magnetic fields.

One way to break the ground loop is to connect the shield on the cable only on its input end, and not connecting the shield on its output end. For low frequencies like 60 Hz or its low order harmonics, electrostatic shielding with one end connected is all that is necessary. If you need to shield from RF interference, then a shield connection on one end may not be adequate.

A test you might try is to power each device with a L-N from a different room, but connect them to isolated grounds from the same room. Then if there's not a hum problem, that would confirm that only grounds are involved and not the power conductors.

 

[Speedskater]

Those dumb IG circuits with home runs are the problem!
Run a 120 Volt feeder to a central point near the control rooms. The a 6 breaker box.
The equipment in almost all control rooms will work on one 20 Amp circuit.
The idea is to have short Safety Ground (EGC) runs from one audio component to all other components.
To keep a the components chassis's at the same potential.

 

[Speedskater]

Everything you need to know about AC power and audio systems (part 1):

from Jim Brown
Power and Grounding For Audio and Audio/Video Systems
-- A White Paper for the Real World

http://www.audiosystemsgroup.com/SurgeXPowerGround.pdf

from Bill Whitlock
An Overview of Audio System Grounding & Interfacing

https://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

a great decade old Middle Atlantic paper that disappeared from their site when their ownership and product focus changed.
Integrating Electronic Equipment and Power into Rack Enclosures

https://www.ground1.com/pdf/WhitePaper_IntegratePowerIntoRack.pdf

 

[synchro]

... Any ideas or suggestions would be great. as this is a newly built commercial facility the client wants to figure out a permanent solution which wont require using any device to lift grounds either ac or audio when connecting between rooms.

Yes, lifting AC grounds is definitely unacceptable from safety and code compliance considerations. And lifting audio grounds can break up ground loops, but it may also allow entry of other noise sources. A high quality audio isolation transformer would avoid those issues, and therefore might be considered as one tool in minimizing the occurence of ground loop problems.

Pro Audio | Jensen Transformers

Jensen Iso-Max pro-isolators present a complete range of passive, hum-eliminating solutions for balanced and unbalanced signals where absolute audio performance is required. These feature ultra-wide frequency response, exceptional signal handling (up to +24dB) and immunity to external...

www.jensen-transformers.com

 

[tortuga]

Everything you need to know about AC power and audio systems (part 1):

from Jim Brown
Power and Grounding For Audio and Audio/Video Systems
-- A White Paper for the Real World

http://www.audiosystemsgroup.com/SurgeXPowerGround.pdf

from Bill Whitlock
An Overview of Audio System Grounding & Interfacing

https://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

a great decade old Middle Atlantic paper that disappeared from their site when their ownership and product focus changed.
Integrating Electronic Equipment and Power into Rack Enclosures

https://www.ground1.com/pdf/WhitePaper_IntegratePowerIntoRack.pdf

Great links, the systems I have installed that have worked are 60/120V described on page 21 of Jim Brows paper:

 

[zbang]

Good audio transformers aren't cheap, but neither is troubleshooting time and reworks, and transformers are not inherently evil as some audio people seem to think.

Definitely go play "ground hog" for a while- turn off the power then lift all the IG leads from the bus and see if individually any of them have continuity to the real EGC.

 

[synchro]

Good audio transformers aren't cheap, but neither is troubleshooting time and reworks, and transformers are not inherently evil as some audio people seem to think.

And all of the vacuum tube power amplifiers cherished by audiofiles have audio output transformers, because of the relatively high impedance load that tubes require. And so as you say, high performance audio transformers are available for a price.

 

[ggunn]

And all of the vacuum tube power amplifiers cherished by audiofiles ...

And guitar players

 

[zbang]

Side note-
There were audio power amps without an output transformer*, but most of those either required a high impedance speaker (500-800 ohm), had fairly low power (maybe 15-20 w), had an unconventional circuit, or paralleled multiple sets of tubes to get the current required. AFAIK in all cases, these were not commercially successful except to the high-end audiophile crowd (where 100 units might be wild success ).

*look up Julius Fetterman(?) or some of the P-P designs with power triodes

Back to the OP- balanced lines or unbalanced lines?

 

[ggunn]

Side note-
There were audio power amps without an output transformer...

I am guessing that you mean tube power amps without output transformers.