GROUND LOOPS
CLEANING/ MAINTENANCE
Ground loops | are characterized | by a low level hum | or buzz | in the system. Loops are caused by | |
a voltage potential difference between two points in | a ground circuit, and aggravated when multiple | ||||
paths | for a | given circuit exist. | is dependent upon all grounds being | ||
at the | same | potential, with a | single path for each | ground connection. Ground loops can exist in | |
two forms: 1) loops created in audio interconnects, | and 2) | loops created between earth grounded | |||
chassis. |
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Mounting components to a rack with metallic rails may introduce ground loops between associated equipment, because the rails can introduce a second ground path. The extent of this problem will depend on the grounding arrangements of associated equipment. Ground loops can occur in non- rackmounted equipment, though it is less common.
If ground loops occur, and any | other | component in the | system has a three wire grounded power | ||||
cord, the first step should be to | use | a | ground | adaptor (with the ground tab or wire of the adaptor | |||
not connected) on the power cord plug | of | the | preamplifier. DO NOT cut off the grounding pin on | ||||
the plug! It may | be necessary | to use additional adaptors on other grounded components if more | |||||
than two components are earth | grounded. | (In | other words, only one earth ground per system | ||||
should exist.) Another potential source of multiple earth | grounds is from coaxial antenna or cable | ||||||
service feeds for FM or video | sources, which | usually are (and should be) earth grounded. The | |||||
ground adaptor(s) | should cure | this | grounding | problem | as well. |
WARNING: The use of ground adaptors (with the ground tab or wire of the adaptor disconnected) will eliminate the safety feature of the grounded power cord. This safety feature is intended to reduce the risk of electric shock should an internal fault in the equipment result in an electrically “live” chassis. Therefore, this method of ground isolation should be employed only when absolutely necessary, rather than as a general practice. When using a ground adaptor for isolation, make sure that the power cord plug is inserted into the receptacle in the same orientation as if no adaptor were present, to maintain the same hot/neutral polarity.
SPECIAL NOTE ON VIDEO CABLE “GROUNDS”: As mentioned above, the ground conductor
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| cable | service | and | antenna | feeds should | be connected to | earth ground. Often, however, due | |
to | long feed lengths, | especially | in apartment | distribution | systems, | this ground will be of poor quality | |||
and | could | be a | source of noise. This interference | is usually | characterized by a high frequency |
noise, rather than a hum or a buzz. If such interference is experienced, a separate earth ground
connection | should | be | made | from the | cable outlet | nearest | the audio/video | equipment to | a known |
good earth ground. |
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If ground | loops | exist | due | to potential | differences | in the | interconnects in | rack mounted | systems, |
then it is advisable to isolate the preamplifier chassis from the rack rails. This may be accomplished by sandwiching a thin sheet of insulating material (i.e., plastic washers or several layers of electrical tape, etc.) between the rack and the preamplifier, and securing the unit with non-
metallic | (i.e., | nylon, | etc.) | hardware. It may be necessary to insulate other | components | so that | only | |
one component is | directly connected to the | rack rails. |
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If the faceplate should become soiled, it may | be cleaned with a slightly damp, soft cloth, and, if | |||||||
necessary, a | mild detergent. Do not use any | abrasive cleaners or solvents. | Unplug the | power | cord | |||
before | attempting | any | cleaning operations. |
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Except as specifically detailed in the owners manual, there are no user serviceable parts or adjustments inside the 915 Preamplifier, and all servicing should be referred to qualified, authorized personnel. The only user access to the inside of the chassis should be for changing the optional loading for the phono inputs (when installed). See “Phono Section”.