Will connecting an electronic instrument's output to a computer's microphone input overload the computer?

Question

I would like to record the demo music on our Yamaha CVP 201 electronic piano, preferably by connecting it directly to a recording device's input.

Right now, I have a 3.5mm splitter to separate the L and R channels, sort of like this:

as well as a 6.5mm to 3.5mm converter:

I also have a long dual RCA cable:

My stereo requires a L and R input so I use the 3.5mm splitter and the RCA cable to connect an MP3 player or computer (or other device with just one 3.5mm output) to the stereo.

The piano has a 6.5mm output so the converter is needed to plug in headphones, etc. I am thinking I could plug the 3.5mm splitter into the converter, connect the dual RCA cable to that, and somehow connect the other end of the dual RCA cable to the computer's microphone input (no line in).

According to this website, this is possible and is known as reverse wired. I would need something like this I think: https://www.amazon.com/USB-Video-Audio-Grabber-Recorder/dp/B01GSJS0V2

First, I would like to confirm that such a setup would even work. Secondly, this question mentioned that connecting line out to line in from the same device could damage the hardware. Since this is from one device to another, would this be safe? Or would the voltage levels coming out of the piano overwhelm the computer's microphone input? I am assuming that since the piano has a 6.5mm output, like an electronic drum kit, the voltages are pretty low since they are meant for headphones, not speakers without amps.

Answer 1

In general, you won't cause any damage. A wise precaution is to turn down the volume on the instrument before plugging in and then turning the volume up until the level is adequate.

The problem is noise.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. The typical setup.

The amplifier output of Figure 1 will send out some noise irrespective of where the volume control is set.

Now imagine that the instrument is designed to give out a 1 V signal but the PC input is 50 mV mic in. You can turn down the instrument volume control to limit the signal to 50 mV but the noise level will be unaffected. The result is that the signal to noise ratio is much worse by a factor of 20.

If the noise level is unacceptable the solution is to attenuate outside the instrument.

^{simulate this circuit}

Figure 2. Fixed resistor attenuators or variable potentiometer attenuators.

If using the potentiometer option then buy a stereo log (audio taper) pot for best results.