This may come as a surprise to some, but I love to sing karaoke. For several years we have owned a karaoke "machine"; a microphone with line level outputs and a reasonable selection if songs. The thing is that these songs get old fast, and it is not easy (nor cheap) to keep-up by buying new "music chips". As it turns-out, it was brought to my attention that YouTube has a huge selection of karaoke songs. In fact, I have yet to find a song that doesn't have a Karaoke version on YouTube! This was music to my ears (pun intended) as I have a streaming box that can output YouTube content to the TV (an older model Roku box; though new ones don't support YouTube. look for an AppleTV). Now the only issue I had was how to mix the microphone output with the TV output. I wanted to use the same Karaoke microphone since it is already amplified, has nice sound volume controls and outputs at line-level (RCA connectors) so doesn't require a pre-amp.... Furthermore, using the same microphone allows me to easily switch from the "old karaoke" mode to the new "streaming karaoke" with the TV remote.
My complete Karaoke setup is shown in Figure 1 (though not shown below, there are speakers connected to the amplifier; the built-in TV speakers are not used). As depicted, the solution I found was to build a "sound mixer", one of the easiest circuits I've ever described in this site.
Figure 1 - The Karaoke Setup
The mixer simply takes the output of the TV plus the Karaoke microphone output and combines them before sending the signal to the amplifier (aka "Receiver") input. The circuit is shown in Figure 2. The advantage of a fully passive circuit like this one is that it doesn't require a dedicated power supply and doesn't add much noise or distortion. On the other hand, it does attenuate the signal somewhat, though the amplifier volume control can easily compensate for this loss. The optional input switch disconnects the microphone when not in use to remove loading from the normal TV output signal path.
Figure 2 - Circuit Schematic
Figures 3 through 5 show how the I built the circuit using a simple mint tin can as an enclosure. These cans are readily available and are actually pretty good for audio applications as the metal shields the signals from unwanted noise interference. I'm happy to report the system performed flawlessly through the Christmas eve party. Have fun singing... few things in life are better to lift the spirits!
Figure 3 - Circuit Assembly
Figure 4 - Inside the enclosure
Figure 5 - Final Assembly