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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.	Noise-free audio performance		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.

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

of		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.
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.
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.

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”.

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