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From: bill@videovax.Tek.COM (William K. McFadden)
Newsgroups: sci.electronics
Subject: Re: need advice on TV sound amplifier
Message-ID: <4365@videovax.Tek.COM>
Date: Thu, 30-Apr-87 12:19:19 EDT
Article-I.D.: videovax.4365
Posted: Thu Apr 30 12:19:19 1987
Date-Received: Sat, 2-May-87 16:56:12 EDT
References: <578@stracs.cs.strath.ac.uk>
Reply-To: bill@videovax.Tek.COM (William K. McFadden)
Distribution: world
Organization: Tektronix Television Systems, Beaverton, Oregon
Lines: 41

In article <578@stracs.cs.strath.ac.uk> mungo@cs.strath.ac.uk (Mungo Henning) writes:

>A relative is slightly deaf and gets annoyed when he tries to listen
>to the TV because his hearing-aid picks up all sounds in the room.
>I bought a small general-purpose amplifier from Tandy (Radio Shack)
>and connected it up to the headphone socket so that he could adjust
>the volume to his own requirements without adjusting the TV volume.
>The trouble is that at a reasonable TV volume the little 9v amplifier
>clips when its volume is turned up too much: giving an annoying buzz
>in the headphones. The tone is not too hot on the little (cheapo)
>amplifier.
>Any ideas out there would be welcomed. I thought that one possible
>solution is to attenuate the `headphone' signal which comes from the
>TV: I think it is too strong for the little amplifier.

You are correct in that attenuation is probably needed.  I assume from your
article that the TV does not disconnect its internal speaker when something
is plugged into the headphone jack, otherwise no one else in the room would
hear the TV very well.  Anyway, if the amplifier is the kind I think it is,
then it expects a very small input level, like you'd get from a microphone.
If you put this circuit between the TV and the amplifier, it should work a lot
better:

         >-----  R1  -------+--------<  Center Conductor
                            |
FROM                        R              TO
 TV                         2           AMPLIFIER
                            |
         >------------------+--------<  Shield


For starters try 100,000 OHMS for resistor R1 and 1000 OHMS for R2.
This will attenuate the signal 100 times.  If this is too much attenuation
(e.g., not enough output from amplifier), lower the value of R1.  If the
amplifier still clips, lower the value of R2.  (I don't think this will happen,
though.  100 times is quite a bit of attenuation.)  If you want to get fancy,
you could make one of the resistors variable.  Hope this helps.
-- 
Bill McFadden    Tektronix, Inc.  P.O. Box 500  MS 58-639  Beaverton, OR  97077
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GTE: (503) 627-6920        "How can I prove I am not crazy to people who are?"