From: toddi@mav.com (Todd Inch)
Newsgroups: sci.electronics
Subject: Re: RCA --> phono
Date: 1 Sep 92 17:41:15 GMT
Organization: Maverick International Inc.

In article <1992Aug24.190459.21144@cbnewse.cb.att.com> parnass@cbnewse.cb.att.com (Bob Parnass, AJ9S) writes:
>In article <BtHtvt.B1t@knot.ccs.queensu.ca>,
>little@qucis.queensu.ca (Scott Little) writes:
>
>  > I want to connect the audio output from my tv to the phono jack
>  > of my amp.
>
>Those living in the USA can buy a recording-level attenuator,
>part number 274-300, from Radio Shack for $2.79 US.  I did.

If, by "audio output" you mean "earphone jack", this is probably good
advice as you MIGHT blow something by connecting a speaker-level output
to a phono-level input, especially if the TV volume isn't very low.  And
if the TV volume is very low, you'll probably hear LOTS of extra noise
such as AC hum.

tmkk@uiuc.edu (Khan) writes:

>Your friend has been leading you down the garden path, I'm afraid.

>The phono input on your amp is looking for signals in the millivolt
>range, many times lower than the line level output from your TV. You
>could match the levels easily enough, but that's only half the battle.

Which is probably good advise assuming by "audio output" you mean "line
out" jack on your TV, which is this person's assumption.

>You see, that phono input also conforms to the RIAA equalization curve
>for vinyl records. This means that the input signal is boosted in the
>low ranges and attenuated in the highs before the signal is sent along
>to the main amplification stages. Records are recorded in such a way as
>to compensate for this equalization, i.e. the lows are attenuated and
>the highs are boosted during the recording process. The output from your
>TV set, on the other hand, does not compensate for the RIAA curve. The
>result is severe distortion of the sound.

Hmmm, interesting.  Presumably this could be overcome by the tone
controls or EQ on the amp, but it might be a pain to reset them
evertime you switched between TV and something else.  It might be
negligible depending on the quality of your hearing and audio system.

>* Go down to Radio Shaft and pick up a switch box and some cables. This
>  will make your AUX (or whichever) input support more than one device,
>  yet still provide electrical isolation for safety reasons. This is
>  undoubtedly your least expensive solution.

Good advice if the "RIAA infected" audio bothers you and you are
talking "line level" TV output, not earphone.  (If you are talking
earphone, this all still applies, but add the attenuation gizmo
mentioned above as well.)  Personally, I'd try the phono input first
and see if I could hear the difference between the phone in and a line
level input such as "TAPE IN".

I don't remember any switch box they sell for line-level audio (not
that I have the catalog completely memorized) - you probably don't want
a speaker selector switch since it would probably not be sheilded,
although if it's in a metal box and you connect the metal box to the
sheild of your cables that would probably do fine.  

If you can't find a more appropriate box, a "no-brainer" solution is to
use a cable-TV A/B switch box, which I know they (and most drug stores)
sell, and is sheilded. You will have to purchase adapters to convert
your audio cables to the threaded "F" connectors on this type of
switch, however.

Hope this adds to the confusion . . . er, I mean, helps!