davy (02/14/83)
#N:pur-ee:12000013:000:8746
pur-ee!davy Feb 13 12:26:00 1983
Due to popular request, I am posting the replies I received
regarding hooking up the audio of a television to a stereo.
I've edited out the headers, etc.
--Dave Curry
pur-ee!davy
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I have done this with great results. My tv set is a GE 19" color set.
The audio circuit consists of a VCA (voltage controlled amp), a
transistor, a transformer and speaker. The volume pot is a 15K job
from a pin on the amp IC to ground, which does not pass the signal but
provides a voltage to the VCA. Because of the output transformer I had
to provide a load replacement (4.7 ohm power resistor) for the
speaker.
What I did is add a DPDT switch to the front of the TV. One pole is
for switching between the TV's speaker and the dummy load resistor.
The other pole selects either the front panel volume pot or an
internal pot that I can use to set a fixed output level when I want to
use the stereo for sound. With this set up, one position of the switch
makes it the orignal circuit (with an audio output that I can also
tap, and by adjusting the stereo volume I can have all speakers going,
but I seldom do this). The other position cuts off the tv speaker and
fixes the output level at a constant value, regardless of the tv's
volume control. Then I adjust volume on the stereo. I find that much
of network TV has rather bad sound, but it is the source, not the
network. Commercials typically have the worst sound (overly
compressed), but some movies sound extremely good (whether HBO or
broadcast).
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You may get away with connecting a TV to your stereo but be prepared
for grief...
One problem is that TVs have (primitive) EMI filters in series with
the line cord. Therefore, the chassis will never be at ground
potential no matter which way you turn the plug around. Of course one
way will be much lower voltage than the other way and you should check
this to avoid a lethal shock hazzard.
Essentially all TVs have the chassis tied to the line in this manner.
Once you have correctly "polarized" the set for the lowest voltage,
you must make sure the plug is replaced with a 3 wire type so nobody
can plug it in wrong. The third wire doesn't get connected, it's just
to force you to stick it in the outlet correctly.
Another problem: Your stereo is almost guaranteed to consider the
building ground to be signal ground, OR it may float the signal by
using a transformer in the power supply along with various shielding
techniques. This may cause severe hum because of ground loops. One
solution is to get a small audio transformer (like 600:600 ohms) and
use it to break the ground loop. (I have done this so I know it
works...)
Finally, while todays signals are going to the station over high
quality links, remember that the detector/de-emphasis circuits in the
TV are not very good. You may have some pretty severe hiss once you
get past the other hurdles. The solution here was to use a graphic
equalizer and chop off starting around 5 kHz (not very high fidelity!)
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I just wired the TV speaker connection into an RCA tee connector on
the aux input of my stereo, and adjusted the TV volume to minimize
distortion. But for HBO, etc., our local cable service provides cable
radio which has stereo signals for all the premium channels (an extra
buck and a half a month). I just tune the FM to the appropriate
frequency and get much better sound.
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watch out for the grounding system on tvs... most are connected to one
side of the power line. even if that is "ground" it isnt; e.g. it is
noisy, and occassionally has nasty spikes.
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A few years ago, I tried to get good sound from a TV's audio circuit
(tapping the signal right before it got to the volume control) -- it
didn't work very well. There was just too much interference coming
from the video circuitry.
Along with the audio signal, there was a humming noise at 60/120 hz
and a 30 (or 60) hz modulated "clicking noise". To top it off, the TV
had such a horrible audio section, the signal that I ran into my
stereo still sounded "tinny". Only then did I realize that the problem
wasn't in the amp and speakers, but in the TV audio circuit AND in the
sound coming from the TV production itself. (NOTE: This was just a
local station that got it's audio & video signals from phone lines
rather than a satellite dish. The sound from a satellite dish is bound
to be much better).
Here are some options you might want to consider:
There are some tuners available that receive the audio portion of the
VHF TV channels and have outputs to your stereo system, just like a
regular FM tuner. (Radio Shack had one on sale not too long ago for
$50. I think they were dropping it from the catalog -- it wasn't too
popular. I don't know anything about the quality of the unit.)
Probably the best bet would be to check & see if your local cable
company has a cable FM option. Our cable company has this option
available for just a few dollars a month. They usually set aside some
positions on the FM dial for the audio portions of HBO, Cinemax, The
Movie Channel, etc.. I believe they even have the capability of
broadcasting in stereo.
If you have your heart set on getting the audio signal out of your TV,
buy a WELL SHIELDED audio transformer (the kind they use in audio
mixing boards) with a flat frequency response. I can only guess at
some impedance values -- maybe 10k ohms : 1k ohm (ball-park-ish). DO
NOT connect any wires from your stereo directly to the TV circuitry.
Sometimes (especially in older TVs) the chassis is connected directly
to one side of the AC line. A transformer should isolate the TV
circuitry from the stereo.
Connect the transformer like this:
1k side 10k side -
hot -------------O || O-------------o--| |
O || O o--| |===(knob)
gnd -------------O || O-------------o--| |
-
To Stereo Transformer Volume control
in TV
Locate the audio transformer as far away from the flyback transformer
(in the TV) as possible. If you want to get fancy, you could run the
signal through an equalizer before it goes to the stereo.
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If you can't get at an earphone plug, the best way is usually to tap
it off the volume control. The way to do it is to connect across the
two opposite ends of the control. (I.e. the control is a pot with
input on one side, ground on the other, and the tap to the amp in the
middle. Go across the two ends.) This will (1) be unaffected by volume
setting; (2) give you reasonably good sound (since you are avoiding
most of the TV's amplifier).
The input should be the AUX input (high-level, high-impedance) of your
stereo. You can either split the output and go to both L & R channels,
or go into just, say, the L and put a sorting plug on the R (to cut
down on noise).
You will probably want to use shielded cable for the connections.
Make sure that you connect the ground side of the volume control to
the shield and then to the ground side of the input, or you could be
in for a rather unpleasant surprise. (You might want to check with a
voltmeter to make sure there is no potential between the TV ground and
the amp ground. This isn't likely to be a problem...)
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Dave, if you have a solid state tv you can just locate the tv speaker
and attach two wires, one on each connect tab (clips will work but
once you find it works, solder is better). Attach the other end of the
wires to two jacks that will fit your aux inputs. (humm this is hard
to state in written form). Polarity is not important other than you
want to wire both jacks the same. I guess if you drew this out, the tv
speaker and the jacks would all be wired together in parallel. you do
not disconnect the tv speaker from the tv set in this method, you just
tap the line and the signal level is just about right (adjustable by
the tv volume control) to work very nicely. Set your Tv volume to a
moderate level and then set your stereo level to where ever you like
it and it sounds great (its not stereo of course but its nice mono and
it does really make a difference). I've done this to the last 3 tv sets
I've owned and never had any problems.
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my set has a remote control so leaving the volume control in the loop
allows me to adjust the sound as I listen. also I dont know what the
signal level would be at the volume control, I would assume you would
turn the volume all the way down, thus max signal level which may be
too high for your stereo input thus distort ot have excesive hum.
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