IC1
is an "Operational Amplifer." Essentially an op-amp is a mathematical amplifier. The purpose of all amplifiers are to take a very weak signal and make it stronger, able to "drive" more energy into something, like a speaker. When we say "impedance matching" we mean just this: using something like an amplifier to match an input signal to whatever it is being fed to.
If an op-amp's -input
were grounded, and a signal applied to the +input
, a larger but identical +output
signal would be generated, just like a traditional amplifier. So that is called a non-inverting configuration, since the same signal comes out.
Op-amps have a -input
though, which does the reverse: if the +input
was grounded, and the signal applied to -input
, the opposite signal comes out. So that would be an inverting configuration. This is very useful for many things.
In this schematic, IC1a
takes the instrument input (+input
) and outputs essentially the same thing to Out1
. That is a non-inverting configuration. Out1
is also coupled to the -input
of IC1b, which is the inverting configuration, so the opposite signal comes out of Out2
.
So Out1
and Out2
are differential outputs, because opposite signals are coming out of them. That's exactly what an XLR cable wants, and due to the physics behind sending signals along cables this way, it is very low-noise.
All of the other components are just for setting the voltage levels right (search for 'resistor divider'), blocking DC voltages (capacitors), and other signal-conditioning needs.