Newsgroups: rec.audio.tech Subject: Re: TV Sound Signal Blocked by Soundcard Signal in Stereo System! References: <[email protected]> [email protected] writes: > Hi everyone, > > I have a Sony GX450 mini hi-fi system with a single auxiliary RCA audio > input and I wanted to connect both my Toshiba 27A44 TV and computer > with a Hercules Gamesurround Muse 5.1 DVD card to it. Both sources > sound fine when connected alone to the auxiliary input. I then plugged > a RCA splitter to the aux, and to it plugged the cables from both the > TV and soundcard. As long as the RCA from the soundcard is connected to > the splitter (with computer power on), the audio signal from the TV is > totally blocked. Again, both sources sound fine when connected alone to > the splitter. Is there way to circumvent the plug/unplug routine > everytime I want to change the audio source? All help is appreciated. > Sami Tuomivaara Sometimes there is need to combine the output of two line level audio sources to be fed to one input. A standard way in professional audio is to use a mixing desk, but on home audio system you might want to look for a simpler and less expensive alternative. Some people have suggested of combining two audio outputs with an "Y-adapter" which is ment to split one output. Those adapters are just one male connector and two feamle connector directly wired to each other. This kind of circuit is just a correct construction for situation where you have one line level output (connected to circuit input) which you want to split to two inputs (connected to circuit outputs). Hardwiring two inputs in parallel and ot one signal source is perfectly OK in line leve audio. But if you want to do the opposite this arrangement is not adequate. If you use this kind of "Y-adapter" to combine two line level audio signal you are effectively shorting two equipment output to each other. Audio equipments are not designed for this. When two outputs are directly wired in parallel, you are effectively fighting with each other. When they are sending out different audio signal, then most of the time one of them is trying to push the output to different voltage than other output. This effectivley causes stress and overload to outputs. In best case you get uneven mix of the signals, lowered output voltage and increased distortion in the sound. In some cases you even risk on damaging the equipments because you are constatly overloading the outputs. The reason for this to happen is that line-level outputs are low-impedance voltage sources and they just aren't going to be very happy if you connect them to each other. Anytime the output differs between the two (as it will almost all the time in stereo) each will be trying to ram current through the other -- more current than they're designed to source, and way more current than they're designed to accept (they're designed to accept practicly nothing). So "Y-adapter" is NOT the right way mix two audio sources. Depending on the equipment you connected and their output impedances the Y-adapter might work on some cases, and many cases does not give good results. Solutions: Build the circuit from http://www.epanorama.net/circuits/linesum.html Use a commercially made audio mixer to combine those sound sources Use a commercially made switch to switch between those two audio sources -- Tomi Engdahl (http://www.iki.fi/then/) Take a look at my electronics web links and documents at http://www.epanorama.net/
