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Author Topic: Proper line grounding Fred Tucker Film Handler Posts: 90 From: Sugar Land, TX Registered: Sep 2007 posted 11-06-2008 02:47 PM                     John, Having an isolated ground rod, one that is not tied to any other ground system, gives the potential for electricity to travel through the earth to another grounding point. Obviously the earth makes an excellent ground and the electricity dispiates quickly but not all soils are created equal in this respect. Normally the greatest potential is between the isolated ground rod and the electrical service's bonded ground rod. Another big no-no is bonding the ground to the neutral more than once, the neutral should ONLY be bonded to the ground at the first termination. Although many utility companies do no want you to bond inside of the meter can so the disconnect switch/main breaker panel is a commonly excepted practice. Fred  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 11-06-2008 02:52 PM                     Right, the case of the isolated ground rod is specifically not what I am asking about. --jhawk  |  IP: Logged Edward Jurich Master Film Handler Posts: 305 From: Las Vegas USA Registered: Jul 2003 posted 11-06-2008 06:45 PM                     Ditto on twisted wires...but I have had the same experience with cables that have very little twist. The magic is mostly in the balanced input though, ground one side of the line and the hum is almost as loud as the audio. This may have been mentioned but another things that helps sometime is to ground the shield through a .1mfd capacitor. Sometimes an isolation transformer on the 120VAC line helps by isolating the electrical ground (the hot ground not the ground ground).  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 11-06-2008 07:05 PM                     It is absolutely against code to not have the ground rods bonded to each other where they enter the building. Thus, the sound system ground and the building ground are indeed at the same potential, at that point and are common. However, the path and potential currents on "clean" ground are vastly different. Edward, believe what you want, it was the combination of the differintial input AND the twised wire. Don't believe me, just you two ordinary wires and keep em separated. Run one by something with a decent EM force on it...say the feed to the power panel...and listen to the lack of quiet. Do the same thing with a twisted pair and it will be much quieter. Steve  |  IP: Logged Oscar Neundorfer Master Film Handler Posts: 275 From: Senoia, GA Registered: May 2000 posted 11-07-2008 05:18 AM                        I would add one thing to the discussion of twisted wires. I am in total agreement that twisting the wires makes a difference. The thing to note is that it is not really the twist per se, but it is the fact that the 2 wires have a better chance to be exposed to the same fields at the same point in the length of the wire, so that at all points the induced currents have a chance to better cancel each other when they appear at the balanced interface. Even a little phase shift or amplitude difference in the currents in the 2 wires can have a big impact on the common mode rejection of the balanced input system. Twisting the wires helps to ensure little phase shift or amplitude differences can exist between the wires. Your example of separating the wires and laying one near a source of interference does indeed point out that a signal induced in one wire and not in the other is no longer common mode, and therefore will not properly cancel. The idea is that it is a combination of equal induced currents in the wires and the common mode rejection capability of the balanced interface that does the trick. Anything that degrades either of these will result in less than perfect cancellation of the comon mode signals. The wire twisting should be done as symetrically as possible so that the lengths of the 2 wires that are twisted are as nearly equal as they can be. In high gain circuits like microphone preamps, this can be a factor in how much common mode rejection is achievable.  |  IP: Logged Edward Jurich Master Film Handler Posts: 305 From: Las Vegas USA Registered: Jul 2003 posted 11-07-2008 10:46 AM                     Has anyone used plastic conduit between projector and audio rack? That might eliminate one extra ground path. I had an occasion while tracing a ground loop to just run a wire across the floor between projector and rack and had no hum while the cable in the conduit did.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 11-07-2008 12:12 PM                     If putting it in plastic conduit reduced the hum says something else is wrong  |  IP: Logged Fred Tucker Film Handler Posts: 90 From: Sugar Land, TX Registered: Sep 2007 posted 11-07-2008 09:16 PM                     I am not an audio tech by any stretch of the imagination, but I would think that a clean solid ground always reduce hum, is this not the case? Guess I need to give my Dad a call, he served in broadcast radio for close to 20 years.  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 11-08-2008 12:01 AM                     The problem is there is rarely such a thing as a "clean solid ground." Any current flow across any resistance (such as a wire) produces a voltage change. If any small current flows, such as through bypass capacitors, then there are voltage differences, and they can affect audio. What is apparently clean and solid for normal electrical distribution purposes is not necessarily sufficient for audio. --jhawk  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 11-08-2008 08:03 AM                     For mere electrical safety, small currents are insignificant. Get a short, it will trip the breaker. Ground, for audio at least, provides an electrostatic shield from induced noise (both from radiating out and from receiving it in). Its all in the current flow.  |  IP: Logged

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