It Ain't Just Dirt… Grounding to Reduce Hum and Buzz – Part 2
In this article I will discuss the role of grounding in keeping audio and other technical systems free of hum, buzz, and other unwanted noises.
In Part 1 I discussed grounding for safety. In this article I will discuss the role of grounding in keeping audio and other technical systems free of hum, buzz, and other unwanted noises.
“Ground” is not a magical pile of sacred dirt which absorbs hum and buzz and absolves us of any guilt regarding improper interconnection practice. Quiet systems are the result of all noise sources and noise entrance points being dealt with adequately. We will only be dealing with the grounding aspects today.
Audio systems should only be grounded according to local electrical codes, not by any exotic bizarre means, such as a separate ground rod, which may not provide electrical safety under fault conditions.
In addition to safety, the desirable characteristic of a good ground system for technical equipment, is to provide as nearly as possible, the same ground voltage at all technical equipment locations. This is not automatic, in fact it is seldom even possible. However, the closer we can approach this ideal, the better. Ground voltage differences can come from leakage current from motors and other heavy equipment. Inductive coupling between the current carrying conductors and the ground wire can generate voltage.
For various reasons there will often be some small voltage difference between the grounds at two locations, even if it is only a few hundredths of a volt. For analog audio with its high dynamic range, even this is too much. With an unbalanced signal connection any ground difference becomes part of the signal. This leads to the unsafe practice of lifting the AC safety ground to eliminate the ground difference. Stay safe! Keep the ground in place and fix the connection.
We strongly prefer a balanced signal connection which essentially carries its own reference with it instead of relying on the local ground reference. This greatly reduces noise pick up problems. Even so, there are benefits to keeping the ground differences small. Active balanced inputs have a limited voltage range over which they will reject common mode signals, and any ground difference voltage they must cancel out means less voltage available for the audio signal. Ground voltage differences also provide the necessary current to expose problems such as Pin-1 and SCIN. Of course these problems are best fixed at their source, the badly designed equipment and wire.
To reduce ground voltage differences use an Isolated Ground (IG) system, sometimes called Insulated Ground. The basic principle is to use a separate ground system for the technical equipment only. The IG is the only path to ground for any technical equipment. No other equipment, especially the high current stuff, may share any part of this ground path.
There are two separate ground paths. Both function as safety grounds, and both ultimately return to the ground to neutral bond in the main service entrance panel, or at the bond of a separately derived source such as an isolation transformer.
The regular ground wire connects to any conduits, junction boxes and outlet boxes. It may touch building steel. There is no need to isolate conduits and boxes from the building steel.
The Isolated Ground connects only to the third (ground) pin of the outlets and to the ground-neutral bond. Special outlet devices are used which do not have a connection between the mounting tabs and the outlet ground pin and screw. In the US, these are either orange or have an orange triangle to signify isolated ground. By the way, they may or may not be Hospital Grade, which is a separate issue and refers to the insertion force of the contacts. IG wiring must be insulated so it does not contact boxes or conduit. It should be a special color used only for IG, preferably green with a yellow or orange stripe. This prevents dangerous mistakes.
Isolated Ground wire connections in sub-panels are made on isolated bus bars. Equipment racks should be part of the IG system, with a separate AWG #6 ground wire from the ground stud in the rack to the IG system. They should be electrically isolated from building steel, and from conduits. Use plastic couplers where the conduit comes into the equipment rack.
Since there are so many places where an innocent mistake can corrupt an IG system, it’s important that it be tested after everything has been installed. The procedure is as follows.
- – Plug in and interconnect all technical equipment.
- – Turn off the power to all branch circuits
- – Remove the IG wire from it’s connection at the ground-neutral bond
- – test for continuity between the wire and the bus bar you removed it from. Any continuity means there is another ground connection out there somewhere. Use a robust DC current tester, such as a small battery and bulb tester. Your DVM may be fooled by tiny voltages picked up from other circuits.
- – find and fix any shorts to ground. These may be on signal lines as well as power wiring.
Common sources of ground contamination include CATV and antenna grounds, signal runs to other locations not on the IG system, connectors on metal wall plates screwed to boxes grounded to building steel, mounting brackets for projectors, powered speakers, and other equipment, and conduits into equipment racks. Signal isolation transformers and non-conductive mounts can fix these problems.
With proper care a well designed and verified Isolated Ground system can result in a quieter sound system, with much less noise for the normal noise reduction techniques to deal with. It is not a substitute for good wiring and connection practice. If the power wiring is already in place, it may be more economical to chase noises from the signal connection end. However, with new construction or remodeling, an IG does not add significant cost and is well worth the effort.
Your sound system can be both quiet and safe. ds
