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Bag harnesses and proper grounding techniques

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Hey everyone, 

 

Can anyone shed some light on proper grounding techniques when wiring up a sound bag? Is the usual one-path-to-ground rule still the best approach to constructing a bag that is as quiet as possible? Perhaps centred around the mixer/recorder with the shields floated at the receiver end? 

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You do have to be careful of the ground or "cold" lines of the system you use to distribute power your gear.  In the past that has been a source if RF and TC issues between the components of a bag system.

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3 hours ago, Dalton Patterson said:

Buy a portable mixer that has decades of intellectual property dedicated to this issue. 

 

I'm not 100% but I think the bag is entirely DC. There is nothing to ground. 

 

From what I understand there is always something to ground, even in DC systems. It all has to do with ground reference - the common point which voltages in the system are measured from. It is why you can have a +9v or -9v or +/-4.5 volt circuit powered off a simple 9v battery. 

 

I wasn’t particularly succinct in my first post. The power aspects of the system seem pretty straight forward. But what about the audio interconnects? In the studio, the usual convention is to leave the shield of your audio interconnects floating at one end to prevent multiple paths to the ground reference. That is why studios often run a single “star” grounding system. I think it has less to do with a physical ground connection, and more to do with how each device references that common ground reference point. 

 

When you have something in your bag or cart like a Lectrosonics quadpack or octopack, suddenly there is the possibility of 9 paths to ground when connecting what essentially is one device, to another. At least in theory...

 

 

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If I am not wrong, the reason why the ground shield is lifted on one side of the cables between analog studio equipment is related to different power sources and ground references in these devices.

If the ground reference is not common between equipments, especially with floating ground or different AC circuits, there will be some current travelling in the shield and thus audible hum in the audio.

I just made some measurement on my SD633, the Wisycom MCR42 and Sony wireless transmitters.

On all these, the minus of the DC source and the audio ground are linked.  

So, as all the grounds are common, no current can travel in the shielding of the cables. 

Also, in a bag, the cables are also really short, whereas in a studio they can be quite long. 

The only problem I experienced a few times did appear when I cable my mixer to a camera linked to a monitor powered by the mains. 

If I plug a return cable from the camera monitor, then some hum could appear. 

But this is a very exceptional case.

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2 hours ago, henrimic said:

If I am not wrong, the reason why the ground shield is lifted on one side of the cables between analog studio equipment is related to different power sources and ground references in these devices.

If the ground reference is not common between equipments, especially with floating ground or different AC circuits, there will be some current travelling in the shield and thus audible hum in the audio.

I just made some measurement on my SD633, the Wisycom MCR42 and Sony wireless transmitters.

On all these, the minus of the DC source and the audio ground are linked.  

So, as all the grounds are common, no current can travel in the shielding of the cables. 

Also, in a bag, the cables are also really short, whereas in a studio they can be quite long. 

The only problem I experienced a few times did appear when I cable my mixer to a camera linked to a monitor powered by the mains. 

If I plug a return cable from the camera monitor, then some hum could appear. 

But this is a very exceptional case.

 

Most studios also operate with common ground and power. It’s impedance to ground that really matters. All it takes is a less than average IEC cable

or cheap and nasty power board to throw spanner in the works. By lifting the shield at one end, you avoid multiple paths to ground, and no closed loops for stray ground current to congregate.

 

While there is no 50 or 60hz AC buzzing around a bag, does a common ground necessarily mean all devices in the bag see it at the same impedance? Sound bags are exposed to the elements quite a bit. I might do some measurements in the coming days, and try and find out if ground wiring configurations do have any impact on the audio noise floor of a sound bag. 

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I have experienced the need for isolation between some battery equipment.

Most things have dc/dc converters in them that can impose noise on the

common lines from the battery.

 

Using a Zaxcom stereo transmitter I found that I needed to isolate it and discovered

that a package car generator filter did the trick

I cannot find the details but it consisted of a series inductor and a parallel capacitor.

 

mike

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