Phantom-powered inline preamplifiers capable of variable impedance loading are disclosed with unique adjustable interfaces. By enabling a user to adjust impedance loading from an actively-powered audio preamplifier which takes a microphone electrical signal or another sound source signal as an input, this unique audio preamplifier design with various adjustable impedance loading interface configurations can change sound characteristics according to the user's preference in a recording, production, or live concert environment. In addition, a high pass filter incorporated in a preamplifier with the variable impedance loading feature allows the user to further customize sound characteristics in the recording environment. This novel inline preamplifier, which may be standalone or integrated into a microphone casing, is powered via a microphone cable from a component (e.g. another preamplifier) providing the phantom power. This inline preamplifier may be connected to a conventional microphone and receive phantom-power into the inline preamplifier from a conventional preamplifier.