Hello. I need safely transform PC stereo audio output to mono. After signal going to one-channel self made amplifier. Now i'm listening only right channel. Show schematics please.
You should be able to just use a Y cable to combine the two outputs into a single input. The PC output drivers should be able to handle a little extraneous voltage from the other channel without damage.
If you want a little protection, include a 10Ω resister in series with each output.
The next level would be to use a hybrid arrangement which I doubt you would want to build yourself because of the need for specially wound transformers. You can find the things on the market for use in 600Ω telephone circuits and I suppose you could design an active one for 8Ω but, that is a fair amount of design work and balancing the thing, either active or passive, takes skill and a very sensitive meter to get a really good return loss.
The LINE OUT on most sound cards has a 100Ω resistor between the output amplifer and the jack, so it is ok to short the Left Channel to the Right Channel to create a mono signal.
Generally, with things like MP3 players, IPODs, etc that are designed to drive headphones, it is not OK to short the Left and Right channels without using series resistors.