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Author Topic: Grounding/Shielding  (Read 5135 times)

j.hall

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Grounding/Shielding
« on: May 18, 2004, 12:09:25 pm »

i realize this is more of a tech topic, but.....

i tend to work in some home studios, and for all intents and purposes, i'm building a home studio right now.

in conjuction with the cell phone thread, and the basement renovation thread, and emails i've been getting.  i think it would be a good idea to dive into to "pin 1" issues.

many problems are created, and already exist, in "make shift" studios due to grounding and shielding issues.

i've invited Josh to host this thread.  i'm happy to explain what and why i'm using an isolation transformer in my new mix room.  i'll answer any interfacing questions, and grounding scheme questions that pertain to my room.  

Josh will explain some basics of grounding, shielding, and how it all relates to audio.

this is a great chance to explore all the hums and buzzes in your gear, and get some easy to implement solutions.

take it away Josh.....
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josh

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Re: Grounding/Shielding
« Reply #1 on: May 18, 2004, 12:27:01 pm »

Man, what an intro!!

Grounding and shielding are misunderstood by many engineers of equipment and designers of equipment mostly because you can achieve the desired result often times by doing the wrong thing.

I'll break up my discussion into a couple of pieces for ease of reading the thread.

josh

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Re: Grounding/Shielding
« Reply #2 on: May 18, 2004, 12:44:54 pm »

The first part of the discussion is background.

The type of electrical interference noise you hear in a recording studio is caused by one of two phenomena:  magnetic fields and electric fields.  

Magnetic fields result from high currents flowing through a conductor loop which has some significant inductance.  Similarly these fields are picked up by conductor loops with sigificant inductance and low impedance, such as a "ground loop" or a guitar pickup.  Magnetic fields' intensity decays at a rate inverse to the cube of the distance, so doubling the distance from the emitter to the reciever results in 1/8 of the field intensity.  You can think of this as an "inductive" transfer of energy, since magnetic fields couple from one "inductor" to another (such as the intended operation of a transformer).  Transformers, inductors, magnetic transducers (such as a dynamic microphone, tape head or guitar pickup) utilize magnetic field coupling as their intended mode of operation.

Electric fields result from high voltages with little or no current flowing in typically unterminated lines.  Similarly they are picked up by "antenna" that are generally high-impedance, unterminated lines like a guitar cable that is plugged into a tube amp on one end but not plugged in on the other end, or the tiny wires between a condenser mic capsule and the preamplifier.  Electric fields' intensity decays at a rate directly inverse to the distance, so doubling the distance from the emitter to the reciever only cuts the intensity by half.  You can think of this as a "capacitive" coupling where air is the dielectric of the capacitor through which the current flows.  Radio transmitters, cell phones, TV, radio, you name it, anything that uses radio energy all use electric fields as their intended mode of operation.

Now a quick touch on Ohm's Law.  Ohm's Law states that R = E/I where

  "E" is "Electromotive Force", measured in Volts, aka "voltage"

  "I" is "Intensity", measured in Amperes, aka "current"

  "R" is "Resistance", measured in "ohms", and you could substitute "Z" (impedance) in this application

Various algebra can be performed, but basically you can see that impedance can be defined as the ratio of voltage to current.  This is important in determining whether we are dealing with a magnetic field problem (low impedance), or an electric field problem (high impedance).

josh

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Re: Grounding/Shielding
« Reply #3 on: May 18, 2004, 01:02:56 pm »

So is it magnetic or electric field that's giving you grief?

Do the math.

First a constant.  The impedance of air is 277 ohms.  For electromagnetic energy purposes, below this threshold is "low impedance", and above this is "high impedance".  A tuned antenna will operate at nearly exactly 277 ohms.

The grandaddy of problems is 60-cycle hum.  Let's add up the currents and the voltage that may be causing this problem, then we can determine the impedance.  We know this is power supply noise from the AC power operating at 120V.  So E = 120.  To find the impedance, we need to know what "I" is, that's current.  In your studio, let's say you have 240W worth of lighting, a 400W console, a 300W computer, and 500W of misc other gear operating.  That's a total of 1440 watts of power, dividing by 120V you get 12 amps of current flowing in the room.

So to find the source impedance of your problem 60 cycle field, you divide E by I, or divide 120 by 12 and you get 10 ohms.  Since this is way below 277 ohms, you know for sure that your problem is magnetic coupling.  In fact, if you look at 60-cycle hum, for it to be an electric problem, then you would have to have an impedance greater than 277 ohms.  For this to be the case, the total power load on your AC power line would have to be less than about 52 watts.  Just turning on a table lamp in most studios will exceed this and ensure that your 60 cycle hum problem is indeed magnetic and not electric.

One other way to determine if your problem is electric or magnetic is by the rate of decay.  Recall that a magnetic field drops proportionately to the cube of the distance, but electric fields are linear.  What this means is that a small change in distance from the source of the problem to the thing receiving the noise will make a relatively big change in the noise if it's a magnetic field, and will make a small change if it's an electric field.  Now, this is true for just about everything except power system hum, since most of your equipment is within the magnetic field of AC mains hum, and not some distance away from it.

So for example, a radio station tower may be miles from your studio, and you are picking up the noise from it in a condenser mic.  Moving the mic six feet across the room has no noticeable effect.  This is an electric field.  When you sit your headphone amp on top of the computer monitor it hums really loudly.  If you move it to the desk beside the monitor, the noise stops.  This is a magnetic field.  Make sense?

Generalizing, most electric field problems are going to manifest as "static", "hash", or will be immediately recognizable as a radio or tv station or other type of intelligible reception (cordless phone, baby monitor, etc.).  Most magnetic field problems you will encounter are going to manifest has hum and are due to power supply, or as broadband noise eminating from a computer or local to a device that gets hot, draws a lot of current, and has either big transformers in it or a switching power supply.  You could almost always be safe in assuming that if it's high frequency, it's an electric field, and if it's low frequency, it's a magnetic field.

j.hall

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Re: Grounding/Shielding
« Reply #4 on: May 18, 2004, 01:29:52 pm »

that's a great start....

i'm sure many people at this point want to relate that to solutions and wiring schemes

pin 1 on XLR connectors is the shield, this has a few functions, but one is the ground reference of the line.

josh, keep it simple, and give us a quick run down of what the shield is doing in audio cable.....how it helps us in the real world, and how it can hurt us.
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josh

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Re: Grounding/Shielding
« Reply #5 on: May 18, 2004, 02:25:53 pm »

So who cares if it's magnetic or if it's electric?  Aha.

SHELDING DOES NOT AFFECT MAGNETIC FIELDS

OK...  this is the most controversial thing I will say.  It's not entirely true, but for most purposes it is true.  It's true in the sense that the shielding materials in frequent use (copper, steel, aluminum, brass and alloys made from these) is not effective as a shield against a magnetic field.  There are materials such as mu metal that is used to encase magnets in shielded woofers that can shield against magnetic fields, but these materials are expensive and rare and pretty much never used except for the aforementioned shielded loudspeakers.

Shielding, on the other hand, is extremely effective at controlling electric fields.

Some other myths:

Faraday cages:  A Faraday Cage is a complete enclosure surrounding either a radiator or an emitter of electric fields that utilizes "eddy currents" to cancel fields.  You do not have a Faraday cage if it must be grounded.  A Faraday cage has to be 100% complete coverage (ideal model is a sphere), and does not have to be grounded, and therefore it is relatively impractical.

Shielded cables reduce hum:  They don't do so as a result of shielding.  Shielded cables reduce hum because the shield conductor itself is a lower impedance conductor than whatever conductor might be used for ground wire in an unshielded cable.

Shielding of equipment reduces hum:  no it doesn't, but the technique of "shielding" equipment will improve grounding, and improved grounding (lower ground impedance, smaller ground loop area) reduces hum.

Shielding a guitar control cavity reduces hum:  This works simply because you are creating a low-impedance common ground plane that reduces ground loop area and thereby makes the guitar a lower-inductance pickup.

So the summary here is, while shielding may appear to fix hum problems, it's not "shielding" that's actually fixing it, but grounding.  When you know this, then you can do a better job of grounding and forget about most of the shielding, and improve hum even more.

josh

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Re: Grounding/Shielding
« Reply #6 on: May 18, 2004, 02:44:45 pm »

OK, so shielding in an audio cable...

pin 1 of an XLR is, as j. hall stated, connected to the shield at least at one end, and perhaps at most at one end.  This is classic shielding function.  Let's look at it in two scenarios:

1.  Microphone cable:  In a microphone cable the shield is connected to ground and serves as the return for the phantom power supply.  If there is no phantom power, the shield is typically electrically floating, or attached to the microphone body.  This is in fact shielding the way it's intended to work.  It will reduce RF interference of the electric field variety (you can hear WKRP coming in on the mic).

There is no real opportunity to create a ground loop and pick up magnetic interference with this type of shield.  Balanced connections operate by cancelling hum, so only hum actually picked up in a microphone voice coil that shows up as a differential voltage, or otherwise in a microphone assembly, will be present depending on how closely balanced the circuit is.

2.  XLR interconnect at +4dBu:  In this case, the shield may be connected to ground at one or both ends and does not (intentionally) have current flowing.  Ideally, in classic shielding design, the shield is connected only at the source end and is left hanging at the load.  So inputs would float the shield, outputs ground it.  This is a ground-loop prevention scheme.

The opportunity to create a ground loop here is really great.  Let's say, for example, we have a mixing console, a mic preamp, and a DAW with converters having XLR inputs.  We attach a couple of outputs from the mixing console to the converters, some outputs from the converters to the console, the output of the console to a power amp (over XLR), and the mic pre (via XLR) to the converters.  If everything (including the converters) are connected to common (earth) ground on pin 1, then we have a huge bundle of ground loops.

In this scenario, current can flow from the console out of each of the output connectors, to the converter, and then to ground at the AC plug, or down the input connectors, to the converter, and then to ground, or down one XLR cable from the console, to the converter, back through the mic pre, then to ground at the AC plug, you name it.  There are dozens of ground paths from any point in the circuit.  Since the impedance of each of these paths is not zero, we have a ground loop and it will pick up a LOT of hum.

To correct this, the most effective way is to simply open the ground connection at the end of the shield on each XLR cable at the male end of the cable.  Every cable except ones actually used for microphones could benefit from this treatment, and it will almost eliminate the possibility of ground loops.

The real trouble with ground loops is when ground connections are made between pieces of gear.  Each piece of equipment has a ground current "return path" through the neutral line in the AC plug.  Introducing another ground conductor gives current on that equipment another ground path, and creates a ground loop.  In isolation, each piece of equipment has an "incomplete" ground loop...  ground conductor goes to the piece of gear and nowhere else.  It's like a ground "stub".  If you have to pieces of gear each attached to an AC plug, you now have two "ground stubs", and to interconnect them turns a "V" into a triangle, thus completing a ground loop.

The most effective way to reduce hum in the studio is as follows:

1.  avoid ground loops by making sure there are not continuous ground connections between different pieces of gear

2.  control ground currents and loop area especially on high-powered devices.  This will lower the amount of magnetic field intensity you have to isolate (treating the problem at its source).  Reducing the length of a power cable going to a big power amp will reduce the circuit loop area and reduce the intensity of the magnetic field (hum) it produces.

3.  proximity.  Locate your power supply circuits and high-power devices in one area, and locate your susceptible circuits in another area.  Power distribution design can really affect how much "hum" you pick up, simply by locating wiring and circuits that share return currents in a way that isolates them physically from susceptible circuitry.  For example, the way NOT to do this would be to route your XLR cables and power cables in the same cable raceway.

ebeam

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Re: Grounding/Shielding
« Reply #7 on: May 21, 2004, 04:20:13 pm »

I appreciate the info you are posting here.  I actually finished rewiring my studio a few weeks ago and have been dealing with some ground loops since.  I think I have tackled most of them but I have a question.

So, I have the output of my DAW (Lynx Two) connected to my homemade volume control/switching box (basically a pot and some switches...) using XLR to TS with the shield floating on the TS side, pin 2 to tip and pin 3 to sleeve.  So then for my outputs I decided to use the same twisted pair cable (actually star quad) to run out to my amp, TS to TS.  When I first wired this I must have been half asleep because I left the shield floating on both ends and just used the twisted pair.  This caused major hum and when I touched the cable ends or switches it went away indicating a grounding problem.  

So I pulled the soldering iron back out and tied the sleeve to the shield on one side.  The buzz is now mostly gone (except when I touch the pot there is a slight buzz) and I am happy but confused.  I thought the floating shield should go on the input side like you mention above.  However, I notice no difference in noise when I switch it either way.  Also, when I looked in the manual for my amp (Hafler P3000) they say the shield should be tied to pin 1 for both sides when using XLR to prevent 'instability'.  Should I do this or just screw the extra conductor and just use one with the shield or what?
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Ben Myers

josh

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Re: Grounding/Shielding
« Reply #8 on: May 21, 2004, 04:45:55 pm »

The source/load grounding thing only applies for active devices.  You are inserting a passive device.  The load in this case is your amp input, and the source is the output of your DAW.

If it hums when you touch it, then there is something grounded (attempting to be grounded) somewhere.  You're completing the circuit when you touch it (your body is a very big capacitive load to ground that is pretty much resonant at 60 Hz).

I would connect pin 1/sleeve throughout your switch/volume box and if the Hafler amp says to connect it on their end, then experiment with that, but you shouldn't need to.  I can't imagine why they'd say to do so for "stability"...  Try it both ways, if it stops humming, then great.  Otherwise, teach it the words!  Wink

weihfool

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Re: Grounding/Shielding
« Reply #9 on: May 26, 2004, 11:47:25 am »

Hey Josh,
Thanks for being part of this thread. Very, very informative. I have a question and if this is something you were going to tackle later on, then I'm sorry for jumping the gun. I'm experiencing the electric interference you spoke about. There is a radio tower a few miles from where I'm setup and the interference isn't constant. It comes and goes. I don't have it at all with dynamic mics. It's only there with condensers (as you mentioned earlier). Is there ANYTHING I can do to combat this, barring moving my studio's location? It seems as if the radio transmitter is cranking up the power only at certain times of the week. Thanks again.

-Sean
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josh

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Re: Grounding/Shielding
« Reply #10 on: May 26, 2004, 04:49:54 pm »

weihfool wrote on Wed, 26 May 2004 16:47

There is a radio tower a few miles from where I'm setup and the interference isn't constant. It comes and goes. I don't have it at all with dynamic mics. It's only there with condensers (as you mentioned earlier). Is there ANYTHING I can do to combat this, barring moving my studio's location? It seems as if the radio transmitter is cranking up the power only at certain times of the week. Thanks again.

-Sean


What kind of condenser mics are doing this?

Or, more specifically,

#1 do you have more than one condenser mic, and are the multiple mics different brand/type/model ?

#2 if the answer to #1 is yes, then what are the mics that have the problem?  all of them?  some worse than others?  Do you have the problem with phantom power on, but the mic unplugged?  Does turning on the pad switch on a mic pad the noise?

Also, is this an AM radio station (540-1600 kHz) or FM (88-108 MHz)?

Need more data!

Yes usually radio stations will operate a schedule of broadcast power and often are limited in broadcast power by the FCC during certain times of the week, or day, or whatever.

Phillip Graham

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Re: Grounding/Shielding
« Reply #11 on: May 26, 2004, 06:34:47 pm »

josh wrote on Tue, 18 May 2004 12:44

The first part of the discussion is background.

The type of electrical interference noise you hear in a recording studio is caused by one of two phenomena:  magnetic fields and electric fields.  

Magnetic fields result from high currents flowing through a conductor loop which has some significant inductance.


It should be mentioned that all magnetic fields result from moving electric charges.  Einstein covers how this works in his discussion of relativity.

Quote:

Similarly these fields are picked up by conductor loops with sigificant inductance and low impedance, such as a "ground loop" or a guitar pickup.


Ground loops are a function of voltage potential differences, creating a current flow.  That flowing current has a magnetic field, of course.

Quote:

 Magnetic fields' intensity decays at a rate inverse to the cube of the distance, so doubling the distance from the emitter to the reciever results in 1/8 of the field intensity.


Since electromagnetic forces are defined in terms of their fluxes, it is the increasing area, not the volume, which defines the decay rate of the field.  For a point source this follows the inverse square law, and for an infinite line source, this is proportional to 1/r.

Quote:

  You can think of this as an "inductive" transfer of energy, since magnetic fields couple from one "inductor" to another (such as the intended operation of a transformer).


Inductors, et al. work because of Lenz's law, a principle that says the current induced in the secondary body will flow in such a way to create a magnetic field opposing the original.

Quote:

Transformers, inductors, magnetic transducers (such as a dynamic microphone, tape head or guitar pickup) utilize magnetic field coupling as their intended mode of operation.


Yup!

Quote:

Electric fields result from high voltages with little or no current flowing in typically unterminated lines.


Great point, Josh.  Many people assume that flowing current must accompany a voltage, and this is explicitly not the case.

Quote:

 Electric fields' intensity decays at a rate directly inverse to the distance, so doubling the distance from the emitter to the reciever only cuts the intensity by half.


E-fields behave the same as the H-field, inverse square or 1/r for the infinite line source.

Quote:

 You can think of this as a "capacitive" coupling where air is the dielectric of the capacitor through which the current flows.


While this is useful, it is misleading.  No current is required to flow.  This is one of the fundamental pieces derivable from Maxwell's equations, that the collapsing magnetic field drives the electric field, and vice versa (by Faraday's and Ampere's laws respectively).


Quote:

Now a quick touch on Ohm's Law.  Ohm's Law states that R = E/I where...
various algebra can be performed, but basically you can see that impedance can be defined as the ratio of voltage to current.  This is important in determining whether we are dealing with a magnetic field problem (low impedance), or an electric field problem (high impedance).


Ohms law may be thought of a transform between voltage and current (which causes the H-field).
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Phillip Graham

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Re: Grounding/Shielding
« Reply #12 on: May 26, 2004, 06:50:08 pm »

josh wrote on Tue, 18 May 2004 13:02

So is it magnetic or electric field that's giving you grief?

Do the math.

First a constant.  The impedance of air is 277 ohms.  For electromagnetic energy purposes, below this threshold is "low impedance", and above this is "high impedance".  A tuned antenna will operate at nearly exactly 277 ohms.


Impedance is not a materials constant, resistivity or conductance are.  The discussion of the definitions of characteristic impedance for transmission lines is probably outside of this discussion.  The proper materials constants are permittivity and permeability.

Quote:

So to find the source impedance of your problem 60 cycle field, you divide E by I, or divide 120 by 12 and you get 10 ohms.  Since this is way below 277 ohms, you know for sure that your problem is magnetic coupling.  In fact, if you look at 60-cycle hum, for it to be an electric problem, then you would have to have an impedance greater than 277 ohms.  For this to be the case, the total power load on your AC power line would have to be less than about 52 watts.  Just turning on a table lamp in most studios will exceed this and ensure that your 60 cycle hum problem is indeed magnetic and not electric.


The classic 60hz hum results from voltage differences in the ground plane between pieces of gear.  These voltage differences cause current to flow in the shields of the equipment.  If the shields are incorrectly terminated to the signal ground of the equipment, then the input impedance of the device will determine the voltage induced in the audio circuit from this current.

This topic is covered with authority by:

Neil A. Muncy, "Noise Susceptibility in Analog and Digital Signal Processing Systems," 97th AES Convention of Audio Engineering Society in San Francisco, CA, Nov. 1994.

Rane offers a pretty good distillation in RaneNote 151:
http://www.rane.com/note151.html

The other type of 60hz interference results from the inductive coupling of two devices.  The (unshielded) transformer of a tube amp and the (improperly shielded) transformer of a direct box.
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Phillip Graham

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Re: Grounding/Shielding
« Reply #13 on: May 26, 2004, 08:27:20 pm »

josh wrote on Tue, 18 May 2004 14:44

OK, so shielding in an audio cable...

pin 1 of an XLR is, as j. hall stated, connected to the shield at least at one end, and perhaps at most at one end.  This is classic shielding function.  Let's look at it in two scenarios:

1.  Microphone cable:  In a microphone cable the shield is connected to ground and serves as the return for the phantom power supply.  If there is no phantom power, the shield is typically electrically floating, or attached to the microphone body.  This is in fact shielding the way it's intended to work.  It will reduce RF interference of the electric field variety (you can hear WKRP coming in on the mic).


The shield of a cable, in equipment that successfully passes the "Pin 1" test, should be connected at both ends.

If one end is to be unconnected, that end should generally be the transmitting (i.e. microphone) end.  The shield reduces RF radiation by two means, eddy current losses in the shield itself, and by shunting the induced current to a low impedance ground point.

Quote:

There is no real opportunity to create a ground loop and pick up magnetic interference with this type of shield.


Leaving the shield unconnected at one end prevents current, resulting from voltage differences between connected pieces of equipment, from flowing.

Quote:

2.  XLR interconnect at +4dBu:  In this case, the shield may be connected to ground at one or both ends and does not (intentionally) have current flowing.  Ideally, in classic shielding design, the shield is connected only at the source end and is left hanging at the load.  So inputs would float the shield, outputs ground it.  This is a ground-loop prevention scheme.


Here the classic scheme is to have the shield connected at both ends.  The reason that it needs to be left floating at one end to prevent ground loops is explicitly a function of improper treatment of the shield termination to the audio device!  This is the so-called "Pin 1" problem.

Quote:

If everything (including the converters) are connected to common (earth) ground on pin 1, then we have a huge bundle of ground loops.


If the ground plane is proper, and the equipment deals properly with the shield termination, all shields can be happily connected.  Equipment not dealing properly with the shield termination is a huge issue, however, so learning the workarounds is useful.

Quote:

The most effective way to reduce hum in the studio is as follows:

1.  avoid ground loops by making sure there are not continuous ground connections between different pieces of gear


The correct way to avoid ground loop issues is to have a stable, low Z ground plane for your technical power, and to modify gear that does not pass the "Pin 1" criteria.  Or, in the case of consulting where you don't want to chase this stuff down later, one only specifies pin 1 compliant gear.


Quote:

2.  control ground currents and loop area especially on high-powered devices.  This will lower the amount of magnetic field intensity you have to isolate (treating the problem at its source).  Reducing the length of a power cable going to a big power amp will reduce the circuit loop area and reduce the intensity of the magnetic field (hum) it produces.


The conducting cables inside of your equipment has a "twist" to it, specifically to make sure the open loop is as small as possible, and to minimize the magnetic field.  It also minimized the inductive behavior of curled cabling.  For large scale portable power distribution, individual untwisted feeders are allowed, here the magnetic fields generated by the cables can be appreciable, and the curling of such cable is a definite safety hazard.

Quote:

3. proximity.  Locate your power supply circuits and high-power devices in one area, and locate your susceptible circuits in another area.  Power distribution design can really affect how much "hum" you pick up, simply by locating wiring and circuits that share return currents in a way that isolates them physically from susceptible circuitry.  For example, the way NOT to do this would be to route your XLR cables and power cables in the same cable raceway.



Yup!  I would add to this that power and signal cables, if they must cross, should do so in a perpendicular fashion.
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Phillip Graham

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Re: Grounding/Shielding
« Reply #14 on: May 26, 2004, 08:41:38 pm »

ebeam wrote on Fri, 21 May 2004 16:20


So, I have the output of my DAW (Lynx Two) connected to my homemade volume control/switching box (basically a pot and some switches...) using XLR to TS with the shield floating on the TS side, pin 2 to tip and pin 3 to sleeve.


Part of your problem is that you have unbalanced your setup by the use of T/S cabling.

Quote:

So then for my outputs I decided to use the same twisted pair cable (actually star quad) to run out to my amp, TS to TS.


The extra cable capacitance of star quad is not worth it for your unbalanced interconnects.

Quote:

When I first wired this I must have been half asleep because I left the shield floating on both ends and just used the twisted pair.  This caused major hum and when I touched the cable ends or switches it went away indicating a grounding problem.


It's hard to say what this actually indicated.  Your shield does still have some effect due to eddy current losses.


Quote:

Also, when I looked in the manual for my amp (Hafler P3000) they say the shield should be tied to pin 1 for both sides when using XLR to prevent 'instability'.  Should I do this or just screw the extra conductor and just use one with the shield or what?


If the shield is left unterminated, is can be possible to create an internal amplifier feedback loop, where very high frequencies on the speaker output cables couple back into the signal inputs, resulting in ultrasonic oscillations.  It is rare, but known to exist.  Connecting the shield provides a shunt for most of this.

Start by reading Rane Note 151:
http://www.rane.com/note151.html

Then read Rane note 110:
http://www.rane.com/note110.html

Then properly balance your setup utilizing 1:1 isolation transformers and proper cabling.

Then go to AES.org and download Preprint 5747, "Common-Mode to Differential-Mode Conversion in Shielded Twisted-pair Cables (Shield-Current-Induced-Noise)"

Finally, build the device in the posted figure and test all of you equipment to see if it is Pin 1 compliant.
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Phillip Graham

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Re: Grounding/Shielding
« Reply #15 on: May 26, 2004, 08:45:24 pm »

weihfool wrote on Wed, 26 May 2004 11:47

Hey Josh,
Thanks for being part of this thread. Very, very informative. I have a question and if this is something you were going to tackle later on, then I'm sorry for jumping the gun. I'm experiencing the electric interference you spoke about. There is a radio tower a few miles from where I'm setup and the interference isn't constant. It comes and goes. I don't have it at all with dynamic mics. It's only there with condensers (as you mentioned earlier). Is there ANYTHING I can do to combat this, barring moving my studio's location? It seems as if the radio transmitter is cranking up the power only at certain times of the week. Thanks again.

-Sean


Sean,

This is covered nicely in a Synergetic Audio Concepts Tech Topic.  There is an online version of that tech topic hosted on Prosoundweb, as a part of the excellent "Study Hall:"

http://www.prosoundweb.com/install/synaudcon/tt25_2.php

And, yes, radio stations are constrained to different power outputs at different times (generally lower at night).
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Phillip Graham

weihfool

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Re: Grounding/Shielding
« Reply #16 on: June 08, 2004, 03:46:10 pm »

Hey Josh and Phil,
   THANKS!!!  I very much appreciate the assistance.

Josh, to answer your request for more info, I've done a bit more investigating.  I get the noise from all of my condenser mics (qty:2, BPM CR-73 LDC, qty:2, AKG C451E).  

I've noticed the RF gets worse when I use 4 conductor twisted pair wire over standard 2 conductor+shield.  

I've also noticed that the noise is present in the console when the 48V is turned on with NOTHING connected to the board.  

However it is CONSIDERABLY louder when the mics are connected to those inputs.  

With nothing connected, I have to pretty much pin the mic gains to hear anything, which I figured might be something that can't be eradicated.

I've tried placing the mics in different parts of the building (the place is about 3000sqft.), varying proximities from the console, etc.

Let me know if you need any other info.

Phil, I'm reading that article you linked me to as soon as I post this.

Thanks to the both of you again!!!

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ebeam

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Re: Grounding/Shielding
« Reply #17 on: June 10, 2004, 04:45:31 pm »

gtphill wrote on Thu, 27 May 2004 01:41



Part of your problem is that you have unbalanced your setup by the use of T/S cabling.


Yes, I realize this and it was intentional.  The Lynx outputs can be switched +4/-10 and connected using the cabling I described.  The reason I did this is that to maintain balanced signal through my passive volume control would require a 4 gang pot and I'm using short cable runs anyhow.

Quote:


The extra cable capacitance of star quad is not worth it for your unbalanced interconnects.


OK, but its what I already have from making mic cables and such and its really not much more expensive.  There's not really any harm is there?

Quote:


If the shield is left unterminated, is can be possible to create an internal amplifier feedback loop, where very high frequencies on the speaker output cables couple back into the signal inputs, resulting in ultrasonic oscillations.  It is rare, but known to exist.  Connecting the shield provides a shunt for most of this.


Interesting...I'm planning on wiring it with the shield connected.

Quote:


Start by reading Rane Note 151:
http://www.rane.com/note151.html

Then read Rane note 110:
http://www.rane.com/note110.html

Then properly balance your setup utilizing 1:1 isolation transformers and proper cabling.

Then go to AES.org and download Preprint 5747, "Common-Mode to Differential-Mode Conversion in Shielded Twisted-pair Cables (Shield-Current-Induced-Noise)"

Finally, build the device in the posted figure and test all of you equipment to see if it is Pin 1 compliant.



Good reading, I already read 110 and I'm printing 151.  I think I'm going to stay unbalanced for the time being until I have a lot of time and more money to invest.  I still have quite a bit of unbalanced gear and my patchbay is unbalanced...

Thanks for your help.
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Ben Myers
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