Impedance bridging

[1][2][3] The load measures the source's voltage while minimally drawing current or affecting it.

When the output of a device (consisting of the voltage source VS and output impedance ZS in illustration) is connected to the input of another device (the load impedance ZL in the illustration), these two impedances form a voltage divider: One can maximize the signal level VL by using a voltage source whose output impedance ZS is as small as possible and by using a receiving device whose input impedance ZL is as large as possible.

(typically by at least ten times), this is called a bridging connection and has a number of effects[4] including: Impedance bridging is typically used to avoid unnecessary voltage attenuation and current draw in line or mic level connections where the source device has an unchangeable output impedance ZS.

Fortunately, the input impedance ZL of modern op-amp circuits (and many old vacuum tube circuits) is often naturally much higher than the output impedance of these signal sources and thus are naturally-suited for impedance bridging when receiving and amplifying these voltage signals.

The DI box is placed close to the source device, so any long cables can be attached to the output of the DI box (which usually also converts unbalanced signals to balanced signals to further increase noise immunity).

The red curve is the power in the load, normalized relative to its maximum possible (which occurs when R L == R S ). The dark blue curve is the power transfer efficiency η , which asymptotically approaches the maximum of 100% as the ratio increases.