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From: wmartin@brl-tgr.ARPA (Will Martin )
Newsgroups: net.audio
Subject: Re: Audio Transformer (Really TV audio)
Message-ID: <10785@brl-tgr.ARPA>
Date: Fri, 17-May-85 12:03:43 EDT
Article-I.D.: brl-tgr.10785
Posted: Fri May 17 12:03:43 1985
Date-Received: Sun, 19-May-85 08:12:57 EDT
References: <1989@topaz.ARPA>
Organization: USAMC ALMSA
Lines: 64

> 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.

There are several problems with this set-up. I don't think that a "matching
transformer" is the cure, though it might function to solve some of the
problems. It would function more as an "ISOLATION transformer" here, to isolate
the TV circuitry from the hi-fi audio stages.

You are probably getting lots of power-supply leakage into that earphone
jack from the TV circuits, which, given the poor bass response of most TV 
audio stages or speakers or small earphones, is never apparent until you 
feed it into a higher-quality audio system. 

You are probably also getting some DC offset and maybe 60 Hz AC from the TV
into the heart of your audio system's preamp, which can wreak havoc with
various elements of the circuitry. (I believe that it can damage volume
and other pots, for example -- a friend with a Mac amp was told this at one
of the old Mac free clinics when they replaced his volume pot; they said DC
from a malfunctioning tape deck had caused the pot to become noisy. Since
they were replacing the pot free, with no sales pitch or income involved,
they had no reason to lie to him about that...)

Also, you are amplifying the signal AFTER it has passed through all the 
(usually) poor-quality audio stages inside the TV. The signal you are then
feeding to your audio system has already been distorted and band-limited,
so you are not getting the best possible sound that your TV's 
detector/demodulator circuits can give you (often surprisingly good).

> Both the TV and the receiver are plugged into the same outlet to try to avoid
> ground loops.

I fear you might be *causing* a ground loop, especially if both are equipped
with three-prong grounding plugs. The shield of the cable would cause a ground 
loop. Maybe try a quick experiment, and disconnect and isolate the shield
at one end (and then reconnect it and disconnect the other end). One or
both of those options might reduce hum.

> Do companies make products that do this?  

A company called "Rhodes" (or maybe "Rhoades"?) has been living off this
need for decades -- they make a doohickey called a "Teledapter" to do just
this. They have (at least had) several models, and also did market a TV-audio
tuner for a while. Look in the little ads in the back of any of the slick
hi-fi mags for their ad.

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.

Some fancy TV's make it hard to do this -- I have a Zenith System III set
(a real dog!) where the volume pot has only DC on it -- it just varies
a control voltage and the real audio signal is buried down inside one
of their overpriced modules! But if you can get clean line-level audio
at your TV's volume pot, you can tap it off to a line-level-output jack
which you can install anywhere convenient. (Suggestion: don't put it on
the back panel were you'll have to disconnect it to take off the rear for
servicing!)

Will Martin

USENET: seismo!brl-bmd!wmartin     or   ARPA/MILNET: wmartin@almsa-1.ARPA