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From: mlf@panda.UUCP (Matt L. Fichtenbaum)
Newsgroups: net.audio
Subject: Re: Audio Transformer (Really TV audio)
Message-ID: <520@panda.UUCP>
Date: Mon, 20-May-85 09:42:23 EDT
Article-I.D.: panda.520
Posted: Mon May 20 09:42:23 1985
Date-Received: Thu, 23-May-85 00:46:22 EDT
References: <1989@topaz.ARPA> <10785@brl-tgr.ARPA>
Reply-To: mlf@panda.UUCP (Matt L. Fichtenbaum)
Organization: GenRad, Inc., Concord, Mass.
Lines: 40

>> I am trying to connect the earphone jack of my television set to an unused
>> auxilliary connector on my NAD 7020 receiver.  When I just build a cable
>> with a mini phono plug on one end (for TV) and an RCA plug on the other and
>> connect the units, I get a LOT of hum.

>The best thing is to grab the audio signal before it gets into the TV's
>own audio stages, and also strip off any garbage like DC offsets or stray
>AC power-line noise or leakage. This is usually done by getting the signal
>at the volume-control pot, and checking it with a DC-coupled 'scope.

CAREFUL CAREFUL CAREFUL CAREFUL!  KNOW WHAT YOU'RE DOING BEFORE YOU DO THIS!

   It is common nowadays for TV receivers, especially the $300 for 19" consumer
kind, to derive their operating voltages from the AC line using a power supply
without a transformer.  In other words, "ground" within the TV is one side of
the power line, or sometimes ~100V DC away from one side of the line.

   IF YOU PLACE YOURSELF BETWEEN TV GROUND AND A REAL GROUND IT CAN KILL YOU.

   If you connect TV ground to a grounded audio system it can blow a fuse or
one or both pieces of the electronics.

   If you connect TV ground to an ungrounded audio system it can place lethal
voltages on the audio equipment's front panels.

   Sets with earphone jacks isolate the earphone jack through a (cheap) audio
transformer so as not to put dangerous voltages where people can touch them.
(UL approval requires a certain amount of isolation, too.)  Direct audio
connections are kept within the cabinet.

   If you don't KNOW that your set has a power transformer, and is thus
isolated from the line, don't attempt to find a direct audio output.

   Sorry to clutter up the whole net with this, but I think it's important.

-- 

					Matt Fichtenbaum
					"Our job is to rescue fires,
					not put out your cat."