Circuit Notebook 97 - Earth Loops

Dealing with earth loop problems covers a huge range of probable causes, frequency effects and possible solutions. Here we look at mains frequency earth loops and its effects on audio and video equipment in the amateur's shack.

Most electrical/electronic equipment is protectively earthed by the fitting of a three core mains lead and a three pin plug, the earth pin of which connects to the mains supply earthing system.

If two pieces of equipment, for example, a tape or disc player and an amplifier are both plugged into the mains and thus earthed, then the action of connecting a screened signal coax lead, from one to the other, will result in the screen and the earth wiring forming a continuous loop - an earth loop, as shown in Fig.1.

This earth loop is not a problem in itself, but if any of the wiring is subjected to a mains a.c. magnetic field from, for example, a nearby mains transformer then the resulting induced voltage will cause an alternating current at mains frequency to circulate around the loop. In addition, a circulating current may be the result of magnetic coupling taking place elsewhere in the mains earthing system.

This circulating current will pass through equipment, possibly from the input connector, through the PC board to the mains earthing point inside the equipment, as shown in Fig.1. Depending on the resistance of the copper track on the PC board, this current will cause a small a.c. 'hum' voltage to develop within the circuit which may then impose itself on the normal signal being handled.

Similarly, if the screened connecting lead has a poor braid connection in the coax plug or the plug body has a poor connection to its other half, as shown in Fig.2 (sometimes a problem with 3.5mm jack plugs and cheap BNC connectors) then the a.c. signal developed across the poor earthing connection will be in series with the required signal, again resulting in 'hum' being added to the audio or video signal.

The result on audio systems (microphone amplifiers etc.) is a background 50Hz or 60Hz hum and in video systems the presence of light or dark horizontal bars drifting vertically over the screen. Unfortunately, interconnection of equipment in the ATV shack provides an ideal situation for developing earth loop problems. The discussion here relates to mains frequency currents circulating in the earth loop.

Resolving Earth Loop problems

  1. Initially, everything should be done in the arrangement of wiring and equipment to minimise any earth loops by keeping cables tidy and close together but not coiled and away from any magnetic fields, to minimise inductive coupling.
  2. Where possible feed all equipment from the same distribution point, this will reduce the earth loop length between equipment.
  3. Avoid connecting some equipment to one mains socket and associated equipment to a different mains socket in another part of the shack. Keeping earth loop length to a minimum will reduce any possible coupling to circulating currents in the shack (house) wiring.
  4. Even when reasonable care has been taken, it is still possible that due to unavoidable poor layout, long cables etc. earth loop problems may still persist. The option then is to break the earth loop in some way so that signals can be carried but loop currents can no longer circulate. In the case of cables carrying audio, using a suitable 1:1 audio isolating transformer will usually suffice, as shown in Fig.3. A suitable transformer is shown on the left in Fig.4. Where video signals are concerned, then again, a suitable video isolating transformer may be used. The special video transformer, shown on the right in Fig.4, uses a high frequency core material capable of providing a bandwidth of 20kHz to about 6MHz. when terminated in 75R.

Warning

Hazard warning signUnder no circumstances must the earth wire in the three pin mains plug be disconnected in order to break the earth loop. Doing so could result in lethal voltages being present on the case of the instrument when the signal cable is unplugged.

Earth loops are not only prone to couple to mains frequency magnetic fields. Those fields produced by television scan coils and radio frequency fields produced by transmitters may also cause problems. Reading through 'Digital Modes for all Occasions' [1], 'The Radio Amateur's Guide to EMC' [2] and 'Personal Computers in the Ham Shack' [3], all provide specific information for avoiding earth loops when interconnecting amateur radio equipment.

In professional audio systems twin screened cables are used almost exclusively and amplifiers have balanced input circuits with good common mode signal rejection. Video is dealt with in a similar way with video amplifiers also having balanced inputs. See reference [4].

References

[1] Digital Modes for all Occasions, Murray Greenman ZL1PPU, Published by RSGB

[2] Personal Computer in the Ham Shack, Paul Danzier N1II and Richard Roznoy K1OF, Published by ARRL

[3] The Radio Amateurs Guide to EMC, Robin Page-Jones G3GWI, Published by RSGB

[4] Video and Audio over CAT5, Mike Cox, CQ-TV 220 pages 9-12

Web:

www.jaycar.com.au/images_uploaded/humloop.pdf

www.epanorama.net/documents/groundloop/index.html

Figures

Fig.1. Current Loop

Fig.2. Effect of faulty Connector

Fig.3. Use of Isolating Transformer

Fig.4. Audio and Video Isolating Transformers