LDstudios Posted February 10, 2019 Report Share Posted February 10, 2019 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? Quote Link to comment Share on other sites More sharing options...
Dalton Patterson Posted February 11, 2019 Report Share Posted February 11, 2019 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. Quote Link to comment Share on other sites More sharing options...
Philip Perkins Posted February 11, 2019 Report Share Posted February 11, 2019 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. Quote Link to comment Share on other sites More sharing options...
LDstudios Posted February 11, 2019 Author Report Share Posted February 11, 2019 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... Quote Link to comment Share on other sites More sharing options...
henrimic Posted February 11, 2019 Report Share Posted February 11, 2019 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. Quote Link to comment Share on other sites More sharing options...
LDstudios Posted February 11, 2019 Author Report Share Posted February 11, 2019 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. Quote Link to comment Share on other sites More sharing options...
mikewest Posted February 24, 2019 Report Share Posted February 24, 2019 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 Quote Link to comment Share on other sites More sharing options...
mikewest Posted February 24, 2019 Report Share Posted February 24, 2019 It was a KEMO product KM50 A 0.22mFd capacitor and a 0.03mH inductor mike Quote Link to comment Share on other sites More sharing options...
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