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Sound Advice

Star Grounds, Loop Areas, & Electrical Safety In Project Studios, Edit Suits, & Other Compact Audio Installations Part III by Neil A. Muncy

Still have noise left? If you have reworked your power as described in previous issues, you have done everything you need to do to make your power and grounding system safe and legal.

The Pin-1 problem is a term coined to describe the almost universal practice employed by most audio equipment manufacturers, in which the old-fashioned (pre-1970) method of connecting cable shield terminals (Pin-1s) on I/O connectors directly to the chassis at the point of entry has given way to connecting Pin-1s to some convenient nearby ground circuit trace on the motherboard. The consequence of this practice is that the moment you connect a cable, you have just attached an antenna to the most sensitive inner workings of your equipment! See the AES publication cited below [1] for how to do a Pin-1 test, and suggestions on how to deal with the consequences.

Once you uncover Pin-1 problems, send the manufacturer a letter/e-mail outlining your observations. Surveys conducted by the author suggest that only about 10 per cent of all the equipment presently in use in the audio industry is demonstrably free of Pin-1 problems. If the manufacturer in question doesn’t respond, or implies that you’ve gone bonkers, tell them that you are going to sell off the offending equipment and buy an equivalent unit from another manufacturer who has seen the light. That should get their attention. If not, you now know whom you’re dealing with.

If you still have RF Interference (RFI) problems, start looking for equipment with less than major Pin-1 problems. Just because a piece of equipment doesn’t exhibit a significant Pin-1 problem at powerline frequencies doesn’t guarantee that it will not be susceptible to RFI. A piece of ground wire a couple of inches long inside a piece of equipment, which is employed to internally chassis ground Pin-1(s) can be a very effective re-radiator from well below 100 MHz to the upper limit of the RF spectrum. An RF signal generator can be utilized for this type of Pin-1 test.

The scenario described above will make your system virtually immune to farfield magnetically coupled interference. Wall warts, line lumps, and power transformers in your gear are all sources of strong extreme nearfield magnetic field energy, which will also cause hum problems if you aren’t careful. Locate wall warts, line lumps, and anything else that has big power supply as far away from your low level equipment as practical. Make use of the Inverse Square Law, which dictates that as you increase the distance between a source of interference and the “victim” equipment and cables, the strength of the interference decreases as the square of the distance. In other words, in this case an inch is (almost) as good as a mile.

Pick up the December issue of PS for Muncy’s conclusion and his invaluable tips on MOV surge suppressors.

References:
[1] The June 1995 issue of the AES Journal, Shields & Grounds reprinted as a Special Publication by the Audio Engineering Society. On the web at: www.aes.org.

Neil Muncy has been around since the days when recorded sound was analog mono and vacuum tubes ruled the audio landscape. He has been a consultant in the audio field for many years, and can be contacted by e-mail at: nmuncy@allstream.net.

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