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

[synchro]

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 ).

The early version of the Harold Beveridge electrostatic speaker had an OTL (output transformerless) direct drive from two vacuum tubes operating at a ~4 kV DC plate voltage. A schematic from his patent is below. Later versions used a step-up audio output transformer to drive the speaker electrodes, as does nearly every other make of electrostatic speakers.

Back again to the discussion of ground loops and hum ...

 

[zbang]

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

Yes, my proofreading isn't what it used to be.

Take a look at this one, the "final" uses a bipolar 150v supply into a pair of triodes.
https://audioxpress.com/article/your-can-diy-a-25w-otl-tube-amplifier

 

[Fred B]

Maybe dumb question but, OP states EMT wiring method. Did any of this EMT share a box or Strut that could be connecting the IG system to the "dirty" ground system prior to the panel?
Had a trace issue with my fox and hound that the EMT acted like a large antennae and a small local radio broadcast was coming through on same frequency as the fox and hound. So thinking the EMT could be acting to induce across the system an interference onto your IG system.

 

[tom baker]

There is an entire code article on balanced power systems, it may be Art 647. When that article was introduced the reports were it eliminated him. But I don’t know the cost or if you can use it

 

[tortuga]

There is an entire code article on balanced power systems, it may be Art 647. When that article was introduced the reports were it eliminated him. But I don’t know the cost or if you can use it

The ones I worked on were not crazy expensive compared to the cost of the equipment it powered, they used all two pole breakers and then dead front GFCI's out of the panels.
The crazy expensive part was that they did everything twice, first they did all that IG nonsence that did not work, then flew out an audio engineer who designed/specified a 60/120 system, then everything had to be re-piped/cabled into that.

 

[ggunn]

FWIW, I have a garage music studio - not professional but more of a rehearsal space where I do some recording. I have a 60Hz noise issue in some situations, so years ago I decided to try to track down the source. I looked at the Usual Suspects - "dirty" power, bad grounding, ground loops, etc., but it turned out to be none of those things. There is an overhead medium voltage distribution line that runs up the side of my property about 20 feet from my studio, and the 60Hz RF from it sneaks into some unbalanced signal lines. If I am, for example, playing my Strat set to a single coil output through a high gain amp it wails like a banshee when I am facing the wall between me and the MV lines, but if I rotate 90 degrees, the noise goes almost completely away.

When I had figured out what I thought was going on, I had the POCO send someone out to verify it with their equipment. Yep.

 

[ggunn]

FWIW, I have a garage music studio - not professional but more of a rehearsal space where I do some recording. I have a 60Hz noise issue in some situations, so years ago I decided to try to track down the source. I looked at the Usual Suspects - "dirty" power, bad grounding, ground loops, etc., but it turned out to be none of those things. There is an overhead medium voltage distribution line that runs up the side of my property about 20 feet from my studio, and the 60Hz RF from it sneaks into some unbalanced signal lines. If I am, for example, playing my Strat set to a single coil output through a high gain amp it wails like a banshee when I am facing the wall between me and the MV lines, but if I rotate 90 degrees, the noise goes almost completely away.

When I had figured out what I thought was going on, I had the POCO send someone out to verify it with their equipment. Yep.

I'll just add that another observation that led me to the solution was of our old CRT TV (long gone now) in the living room that is on the same side of the house as my studio. It was on a swiveling tabletop and when it was pointed in a particular direction the upper left part of the screen looked greenish and the lower right looked purpleish.

 

[hornetd]

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 sq ft 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.

Is HCF a recognized cable type? It wasn't the last time I checked. You said that the Isolated Ground branch circuits are run in MC cable. I've run dozens of IG circuits in cable and in all cases the Cable was type AC Armored Cable with a green colored insulated Equipment Grounding Conductor (EGC) included with the other insulated conductors inside the bonded armor of the type AC cable. Since the interlocking spiral wound metal tape jacket of type MC, Metal Clad, cable is not listed as providing an Equipment Grounding Conductor through the Jacket type MC cable used for isolated ground circuits would have to have 2 insulated green colored wires to serve both the receptacle grounding ports and the Grounding Electrode conductor for the boxes and other conductive enclosures along it's path back to the panel from which the branch circuit is supplied. I've never seen type MC cable with 2 insulated Grounding Conductors but I certainly haven't seen every cable ever produced in the US. What makes me think that your actually using Type AC cable with an additional insulated green colored conductor to serve the Grounding Sleeve ports of the receptacles is that type MC cable with 2 insulated Grounding Conductors would need to be a special order and would incur all of the cost of testing by a Recognized Electrical Testing Laboratory to be listed as a new cable type. Those 2 green colored insulated wires would have to be distinguishable from each other. There are only 3 NEC compliant ways to identify a wire Equipment Grounding Conductor, Those are that the conductor is neither insulated nor covered, the insulation is green in color, or the insulation is green with one or more yellow stripes. The least expensive way to make a cable for isolated ground circuits is to place a single green colored insulated conductor in with the other insulated conductors in Type AC cable. The insulated green colored wire would connect all of the receptacle grounding port wiper sleeves back to the Isolated Grounding Conductor busbar at the service equipment were it is bonded to the Grounded Current Carrying Conductor (neutral) of the Service Conductors. If you want to break the DC pathway that provides the carrier current for the induced AC hum you are going to have to have an opto-isolator or a DC isolation transformer in every audio port on every piece of audio equipment. If the transformer approach is used care must be taken to assure that it has an adequate passband for the full range of audio signals that must pass through it to maintain the desired level of fidelity. It would also help if all the AC power supply transformers were wired as a DC current isolating transformer. That would require that the audio equipment's circuitry be double insulated to protect users from contact with current escaping from the Isolated circuitry of the audio equipment through a fault of those circuits to a conductive surface of the device.

 

[Little Bill]

Is HCF a recognized cable type? It wasn't the last time I checked. You said that the Isolated Ground branch circuits are run in MC cable. I've run dozens of IG circuits in cable and in all cases the Cable was type AC Armored Cable with a green colored insulated Equipment Grounding Conductor (EGC) included with the other insulated conductors inside the bonded armor of the type AC cable. Since the interlocking spiral wound metal tape jacket of type MC, Metal Clad, cable is not listed as providing an Equipment Grounding Conductor through the Jacket type MC cable used for isolated ground circuits would have to have 2 insulated green colored wires to serve both the receptacle grounding ports and the Grounding Electrode conductor for the boxes and other conductive enclosures along it's path back to the panel from which the branch circuit is supplied. I've never seen type MC cable with 2 insulated Grounding Conductors but I certainly haven't seen every cable ever produced in the US. What makes me think that your actually using Type AC cable with an additional insulated green colored conductor to serve the Grounding Sleeve ports of the receptacles is that type MC cable with 2 insulated Grounding Conductors would need to be a special order and would incur all of the cost of testing by a Recognized Electrical Testing Laboratory to be listed as a new cable type. Those 2 green colored insulated wires would have to be distinguishable from each other. There are only 3 NEC compliant ways to identify a wire Equipment Grounding Conductor, Those are that the conductor is neither insulated nor covered, the insulation is green in color, or the insulation is green with one or more yellow stripes. The least expensive way to make a cable for isolated ground circuits is to place a single green colored insulated conductor in with the other insulated conductors in Type AC cable. The insulated green colored wire would connect all of the receptacle grounding port wiper sleeves back to the Isolated Grounding Conductor busbar at the service equipment were it is bonded to the Grounded Current Carrying Conductor (neutral) of the Service Conductors. If you want to break the DC pathway that provides the carrier current for the induced AC hum you are going to have to have an opto-isolator or a DC isolation transformer in every audio port on every piece of audio equipment. If the transformer approach is used care must be taken to assure that it has an adequate passband for the full range of audio signals that must pass through it to maintain the desired level of fidelity. It would also help if all the AC power supply transformers were wired as a DC current isolating transformer. That would require that the audio equipment's circuitry be double insulated to protect users from contact with current escaping from the Isolated circuitry of the audio equipment through a fault of those circuits to a conductive surface of the device.

How about breaking up your post into paragraphs. It's extremely hard to read a wall of jumbled lines.

 

[LarryFine]

Is HCF a recognized cable type? It wasn't the last time I checked. You said that the Isolated Ground branch circuits are run in MC cable. I've run dozens of IG circuits in cable and in all cases the Cable was type AC Armored Cable with a green colored insulated Equipment Grounding Conductor (EGC) included with the other insulated conductors inside the bonded armor of the type AC cable. Since the interlocking spiral wound metal tape jacket of type MC, Metal Clad, cable is not listed as providing an Equipment Grounding Conductor through the Jacket type MC cable used for isolated ground circuits would have to have 2 insulated green colored wires to serve both the receptacle grounding ports and the Grounding Electrode conductor for the boxes and other conductive enclosures along it's path back to the panel from which the branch circuit is supplied. I've never seen type MC cable with 2 insulated Grounding Conductors but I certainly haven't seen every cable ever produced in the US. What makes me think that your actually using Type AC cable with an additional insulated green colored conductor to serve the Grounding Sleeve ports of the receptacles is that type MC cable with 2 insulated Grounding Conductors would need to be a special order and would incur all of the cost of testing by a Recognized Electrical Testing Laboratory to be listed as a new cable type. Those 2 green colored insulated wires would have to be distinguishable from each other. There are only 3 NEC compliant ways to identify a wire Equipment Grounding Conductor, Those are that the conductor is neither insulated nor covered, the insulation is green in color, or the insulation is green with one or more yellow stripes. The least expensive way to make a cable for isolated ground circuits is to place a single green colored insulated conductor in with the other insulated conductors in Type AC cable. The insulated green colored wire would connect all of the receptacle grounding port wiper sleeves back to the Isolated Grounding Conductor busbar at the service equipment were it is bonded to the Grounded Current Carrying Conductor (neutral) of the Service Conductors. If you want to break the DC pathway that provides the carrier current for the induced AC hum you are going to have to have an opto-isolator or a DC isolation transformer in every audio port on every piece of audio equipment. If the transformer approach is used care must be taken to assure that it has an adequate passband for the full range of audio signals that must pass through it to maintain the desired level of fidelity. It would also help if all the AC power supply transformers were wired as a DC current isolating transformer. That would require that the audio equipment's circuitry be double insulated to protect users from contact with current escaping from the Isolated circuitry of the audio equipment through a fault of those circuits to a conductive surface of the device.

Inhale, man!

 

[hornetd]

Inhale, man!

WillCo.

 

[ggunn]

How about breaking up your post into paragraphs. It's extremely hard to read a wall of jumbled lines.

Yes, please do.

 

[LarryFine]

Allow me:

"Is HCF a recognized cable type? It wasn't the last time I checked. You said that the Isolated Ground branch circuits are run in MC cable. I've run dozens of IG circuits in cable and in all cases the Cable was type AC Armored Cable with a green colored insulated Equipment Grounding Conductor (EGC) included with the other insulated conductors inside the bonded armor of the type AC cable.

"Since the interlocking spiral wound metal tape jacket of type MC, Metal Clad, cable is not listed as providing an Equipment Grounding Conductor through the Jacket type MC cable used for isolated ground circuits would have to have 2 insulated green colored wires to serve both the receptacle grounding ports and the Grounding Electrode conductor for the boxes and other conductive enclosures along it's path back to the panel from which the branch circuit is supplied.

"I've never seen type MC cable with 2 insulated Grounding Conductors but I certainly haven't seen every cable ever produced in the US. What makes me think that your actually using Type AC cable with an additional insulated green colored conductor to serve the Grounding Sleeve ports of the receptacles is that type MC cable with 2 insulated Grounding Conductors would need to be a special order and would incur all of the cost of testing by a Recognized Electrical Testing Laboratory to be listed as a new cable type.

"Those 2 green colored insulated wires would have to be distinguishable from each other. There are only 3 NEC compliant ways to identify a wire Equipment Grounding Conductor, Those are that the conductor is neither insulated nor covered, the insulation is green in color, or the insulation is green with one or more yellow stripes.

"The least expensive way to make a cable for isolated ground circuits is to place a single green colored insulated conductor in with the other insulated conductors in Type AC cable. The insulated green colored wire would connect all of the receptacle grounding port wiper sleeves back to the Isolated Grounding Conductor busbar at the service equipment were it is bonded to the Grounded Current Carrying Conductor (neutral) of the Service Conductors.

"If you want to break the DC pathway that provides the carrier current for the induced AC hum you are going to have to have an opto-isolator or a DC isolation transformer in every audio port on every piece of audio equipment. If the transformer approach is used care must be taken to assure that it has an adequate passband for the full range of audio signals that must pass through it to maintain the desired level of fidelity.

"It would also help if all the AC power supply transformers were wired as a DC current isolating transformer. That would require that the audio equipment's circuitry be double insulated to protect users from contact with current escaping from the Isolated circuitry of the audio equipment through a fault of those circuits to a conductive surface of the device."

 

[rambojoe]

I read "the problem" multiple times and i swear it seems a contradiction...
Do you mean alls well in each studio until a direct line out goes to the board? Or a mic cable?!
And does it go to a patch or straight to the board?
One thing i know is effects or instrument cables within 6" of a power transformer will make noise.
Also external power supplies for effects or processors must be plugged in to a different recept than a amplifier. It can be the same circuit- just NOT the same recept.
This is where i.g.'s get a bit goofy (unnecessary) or at best, very easy to screw up...
Is the framing steel stud?

Funny how the old and awesome analog studios of old did great w/knob and tube and a rudimentry grid... Hhmmm.

Btw, i cant tell you how many times a i.g. buss was either free floating (thus open) or never makes it back to the customers bullseye..

I thought the o.p. said "emt" not mc...

 

[synchro]

Allow me:
...

"I've never seen type MC cable with 2 insulated Grounding Conductors but I certainly haven't seen every cable ever produced in the US. What makes me think that your actually using Type AC cable with an additional insulated green colored conductor to serve the Grounding Sleeve ports of the receptacles is that type MC cable with 2 insulated Grounding Conductors would need to be a special order and would incur all of the cost of testing by a Recognized Electrical Testing Laboratory to be listed as a new cable type."

https://www.southwire.com/wire-cable/metal-clad-cable/armorlite-type-mc-isolated-ground/p/MC40

...
I thought the o.p. said "emt" not mc...

Yes, he did.

 

[rambojoe]

And should be green w/yellow spiral to boot...