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Old 8th December 2008, 03:13 PM   #1
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Default Ground loops and containment

This thread is the beginning of a discussion on what ground loops are and how to avoid them. The three areas to be discussed will be the external environment, the internal, and later, how to force ground currents as well as measure them.. While centered on grounding and hum/noise, the concepts are fully applicable to the actual wiring within the components.

I hope everybody chips in, it should be a good learning experience for all.

I've posted a pic of three different system configurations. The assumption is 3 prong NEC type units. (two prong applies as well, but the freq response profile will be different, dependent on the impedance to chassis..)

A. This is the typical home setup. The user buys a source and an amp...connects em together with one or two IC,s then runs with it. Notice the ground loop. Any magnetic flux that goes through that loop will create a voltage (current) around that loop. If it's the romex in the wall, oh well.. If an appliance, motor, dimmer circuit goes near that loop, it will make current flow within the ground loop.

It is not obvious how to make either component impervious to that ground current. But nonetheless, current will flow through them. If they are not designed to pass that current without creating magnetic fields inside the chassis, then it will come out in the wash as non-music..

B. This is the same setup, but the pre cord has been moved to wrap around first the IC's, then the amp line cord. Notice that the big loop "antennae" is now missing. If the cords are twisted well, there is nothing an external device can do to generate ground currents in this setup.

C. This is a small varient of B, one I used for a 450 seat venue. I ran a 100 foot extension cord from the stage to the pre/mixer, and wrapped the single ended stereo pair around that cord. Both amp and pre were using the same outlet strip. This prevented the house lights dimmers, AC unit, everything external, from putting any noise into the system.

Note that both B and C are basically limited to 15 or 20 amps worth of equipment. If one requires more, a dedicated group of outlets with one common ground from the outlets back to the breaker panel is needed...one ground only. More than one ground will setup a huge loop to the service panel.

Option B or C will reduce significantly, most hum issues in the home. (not cable, that is a different nut...)

But neither will address well, currents formed by the guts of the equipment. This includes 60 hz, odd harmonics from the line side of the bridge rectifier, even harmonics from the load side of the bridge, nor actual musical content magfield generation on the supply rails back to the line. That's stuff to play with farther down the thread.

Cheers, John
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Old 8th December 2008, 08:14 PM   #2
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Here's the test setup.

I have the toroid for the current sense pickup, it's a secondary core from an old ATX supply, so it'll definitely have the bandwidth. Center hole is .550 inches, so I figure it'll take most rca's.

Gonna have to wind some copper through that puppy, see what I need to view 20 to 20K down to about 10 microamps. It'll probably require several different windings to do that sensitive at such a range..and I'll have to pickup some kinda preamp circuit also.

Don't know what to use for the drive core yet, I'm scrounging up. I was thinking of cutting one of my plitron 225 va toroids in half, I have eight more..

My intent is to use the drive core also as a test for ground loop susceptibility over the entire audio range....I'll have the amp on, the pre on but not playing anything, and then bring the drive toroid around the line cord slowly...gotta be careful of course, in case the amp is really susceptible..

Cheers, John

ps....btw, this setup is what is needed to verify that an amp design has been made to reject ground currents..it'd be silly to just post pics and verbage...gotta have some data, and heaven forbid.....graphs...
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Old 9th December 2008, 01:29 AM   #3
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I think ur in the wrong section?
Maybe... thought this was mostly about parts and electronic parts and their application.

Although your post pertains to all types of circuits, I think it is better placed in the solid state section... fwiw, imho, etc...

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Old 9th December 2008, 12:44 PM   #4
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Quote:
Originally posted by bear
I think ur in the wrong section?
Maybe... thought this was mostly about parts and electronic parts and their application.

Although your post pertains to all types of circuits, I think it is better placed in the solid state section... fwiw, imho, etc...

_-_-bear
I wasn't sure where to post it. Several forums seemed reasonable.

Tube circuits also suffer ground loops. I figured that for the external loop issues, here was most logical...Once I get into the internally generated magfields, then of course solid state low impedance circuits are the most susceptible.

For a discussion that uses "black boxes" without regard to the inner workings as my diagram has, I thought this place was best.

I wonder if perhaps a new forum under other stuff...grounding and noise perhaps...

Cheers, John
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Old 9th December 2008, 12:54 PM   #5
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Quote:
Option B or C will reduce significantly, most hum issues in the home.
I doubt it, because hum in home stereo is not related to hum pickup, but to calvanic coupling and currents flowing because of potential differences, i.e. leakage currents etc.
Cable wrapping doesn't help with this.
regards
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Old 9th December 2008, 01:46 PM   #6
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Quote:
Originally posted by Juergen Knoop
I doubt it, because hum in home stereo is not related to hum pickup, but to calvanic coupling and currents flowing because of potential differences, i.e. leakage currents etc.
Cable wrapping doesn't help with this.
regards
As I have stated, this technique works excellently when hum is induced as a result of the enclosure of time varying magnetic fields. I have used this technique with success to kill hum in magnetic cartridge phono systems, 100 foot long runs in a building with 25 kilowatts of solid state dimmers, AC power, and even a 250 kilovolt van degraf generator next to the electronics, as well as a 100 microvolt magfield measurement coil system whre the wires traverse 75 feet and past 2 to 5 tesla magnets and 600 amp 480 volt power supplies. (a rather hostile enviro, as 12 pole scr supplies are notorious generators of hash.)

What you are speaking of is something entirely different, but one which will be important to the equipment design aspect, which is making it ignore the current while still allowing it to flow (the NEC defines this as unintended current, and they provide almost ZERO assistance in working around this).. This is also what happens when one connects to the "cable", as they wreak havoc with grounding because they are required to protect against lightning..independent of the power company.

I am the first to admit that what I've stated, that of wrapping the power cables together, or even the pre cable wrapped around the IC's, seems counterintuitive..but it is not. It works, and works very well

But as you point out, it will not counter a hard leakage path, that must be addressed seperately. I started this thread with a basic description of the magnetic coupling effect, so that I could detail the method and equipment necessary to measure the equipment's ability to reject the ground current regardless of the inducement methodology. I chose magnetic induction to create the ground current BECAUSE trying to inject it via wiring means connecting to the circuit, and given the low resistances involved, I prefer the drive NOT alter the loop which we are trying to measure.

Cheers, John

Oh, btw. Within the home, if you measure the safety ground to safety ground potential between two outlets in one room, you will find a voltage. You will find this occurs even if there is no current draw in the wires or the ground...it is because the 60 hz magnetic field in your house (or 50 hz) is coupling to the loop you form when you measure from one ground to the other. Voltages in the 1 to 2 volt range are not uncommon, but for a daisy chained outlet string that would be an IR drop consistent with half a kiloamp or so...which is of course not the case. It is magnetically induced, which is what this thread is all about... my office has two outlets currently unused, on two adjoining walls, they daisy from one to the other, and they run 400 mV to 1.5 V differential depending on the time of day..again, unloaded..
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Old 9th December 2008, 04:37 PM   #7
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Quote:
Within the home, if you measure the safety ground to safety ground potential between two outlets in one room, you will find a voltage.
but it needs a current for hum. Connect a class2 thingie to a class1 amplifier and this will result in app. 0,1-0,5mA hum current flowing through the interconnects.
regards
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Old 9th December 2008, 05:16 PM   #8
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Quote:
Originally posted by Juergen Knoop
but it needs a current for hum.
If it is magnetic coupling, yes it needs the current. Electric coupling, no current needed.

The point is, faraday's law of induction essentially guarantees that if you enclose time varying magnetic field lines with a conductive loop, there will be currents.

The goal of the thread is to figure out how to reduce the currents that will be generated, and how to reduce the equipment sensitivity to those currents.
Quote:
Originally posted by Juergen Knoop
Connect a class2 thingie to a class1 amplifier and this will result in app. 0,1-0,5mA hum current flowing through the interconnects.
regards
I believe you meant to say it will result in current flowing on the shield of the interconnects.

And even that case can be dealt with once it is understood w/r to e/m theory. It's trivial to run the shield current over all the IC's with a cylindrical braid, bond that to each chassis, and remove the magnetic coupling.. But I fear I'm jumping the gun with that solution, it'll be along soon enough.

You did notice that I specifically address standard NEC grounded chassis equipment, right? I'm trying to remain specific.

Cheers, John
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Old 9th December 2008, 05:44 PM   #9
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obviously we are talking about different things. The balancing currents due to construction of power supply sections, can't be adressed by better shielding.
It is not about fields, antennas and these things.
I don't think the usual hum problems can be solved that way.

Quote:
You did notice that I specifically address standard NEC grounded chassis equipment, right? I'm trying to remain specific.
Safety grounded equipment allows residual currents of a few mA and provides an earth path for insulated equipments leakage current.
I believe this is the main cause of hum in consumer audio.
regards
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Old 9th December 2008, 07:06 PM   #10
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Quote:
Originally posted by Juergen Knoop
obviously we are talking about different things. The balancing currents due to construction of power supply sections, can't be adressed by better shielding.
You're going to have to explain that statement a little better. It is not difficult to prevent suppy rails from generating external magnetic fields, but it requires both a triaxial feedwire (for plus/minus plus ground), and it requires a net zero current through that wire.

But as you can see, I am discussing the safety ground wire and the current caused by induction within that loop. Where did "power supply sections" come from?

Quote:
Originally posted by Juergen Knoop
It is not about fields, antennas and these things.
It is entirely about fields. And the interception of them by a conductive loop. (it's a near field problem, I believe you are thinking far field with antenna..
Quote:
Originally posted by Juergen Knoop
I don't think the usual hum problems can be solved that way.
My experience certainly is not consistent with that statement. I have used the techniques many times with success.

Quote:
Originally posted by Juergen Knoop
Safety grounded equipment allows residual currents of a few mA and provides an earth path for insulated equipments leakage current.
I believe this is the main cause of hum in consumer audio.
regards
You forgot the case of two safety grounded equipment pieces, which is actually what I drew. Your scenario falls apart when both pieces are bonded to ground, but yet that is the case I am discussing.

There are certainly cases of your scenario.. What is of concern is when that leakage current is travelling along an IC shield, and what both the source and sink equipment does with it.

As I stated several times now, using test equipment to force and measure the impact of shield currents on the audio equipment without altering the paths is the best way to find and fix ground current sensitivities.

There are no good designers out there who intend to design their equipment to hum as a result of those currents..some try to eliminate the sensitivity by trial and error, some ignore it and blame the consumer.

Whitlock has the right idea...force the current into the ground, then fix the problem.

My methodology goes a tad farther, and is applicable to single ended equipment as well as balanced. And it provides insight into the mechanism of magnetic induction coupling..

Cheers, John
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