I want to know what kind of Y-adapter I need to combine the audio signals from one clip-on guitar mic, and one headset microphone, to hook-up into a PC sound card.
Reply: A small mixer is better than a Y-adapter, because you can control the volume balance between the guitar and the voice with a mixer. You can't do that with a Y-adapter except by changing how loud you play or sing, or by changing the distance of the headset microphone to your mouth.
So I would connect the guitar mic and headset mic to the mixer, and connect the mixer output to the sound card line input.
If you use a Y-adapter to combine the signals of two microphones, the mics will load down each other, and you'll get a low level signal, possibly with distortion and weak bass. A better solution is to use a combiner circuit, which has isolation resistors. Please see the diagrams below.
• The circuit below combines two low-impedanced balanced mics with XLR connectors into a single XLR mic input.
• The circuit below combines two low-impedance balanced mics with XLR connectors into a single stereo phone plug for use with a sound card.
• The circuit below combines two medium-impedance unbalanced mics (with audio, shield, and power leads in the mic cables or connectors) into a single stereo phone plug for use with a sound card.