I plan to connect my guitar and digital piano to RC 500 loop station. I will connect guitar via "INST IN" jack. Am I able to connect my digital piano via "MIC IN" XLR jack? Since I won't be using that jack for the mic.
No, you can't generally connect a digital piano directly to a mic input. Ideally the piano would connect to a line input, but the RC 500 doesn't appear to have one. But there are devices/converters that make it possible.
For a bit of background, there are a few different types of electric signal involved:
- line level: fairly strong signal, from say a CD player or digital piano
- mic level: very faint signal from a mic, 1/10th to 1/1000th of the strength (voltage) of line level
- instrument level: typically from a guitar/bass pickup, somewhere between mic and line level. Passive pickups in particular need a high impedance input to sound as intended, whereas line inputs are low impedance and mic inputs are very low.
Connecting either line or instrument level directly to a mic input will probably distort the signal beyond recognition, it's a lot stronger than a mic input is designed to handle.
Connecting instrument to a line input will normally work, but you lose the higher frequencies (at least with passive pickups). Audio example in the DI box link below.
Connecting line level to instrument input (low impedance into high impedance) will often be fine if you can dial down the strength of the signal a bit a the source. But it varies with the device you plug into, some devices are very sensitive to these mismatches, for others it matters little.
There are devices that can translate between the different types of signal:
- A DI box converts an instrument signal to mic level.
- A reamp box converts a line signal to high-impedance instrument level.
So a DI box will let you do what you want (piano -> DI box -> mic in). Whether you will benefit from a reamp box too (piano -> reamp -> DI -> mic in) depends on the DI box.
Alternatively, a small analog mixer with mic/instrument/line in, plug both the guitar and the piano into the mixer and connect the output either to "INST IN", or to "MIC IN" through a DI box. This lets you add more instruments in the future.
(Example mixer: Note that the mic input on this mixer is an XLR/jack combo input. The XLR goes through the mic preamp, the jack doesn't, and the button labeled "HI-Z" lets you switch between line and instrument/guitar input for the jack. So this type of input can handle all three types of signal.)
Or get a looper with a line in in addition to the instrument in.