Coupling Capacitor

Valve stages sit at very different DC voltages (a plate at +200 V, the next grid at 0 V), so they are joined by a coupling capacitor that blocks DC but passes audio. Together with the following grid-leak resistor it forms a first-order high-pass filter:

Below fc the response falls at 6 dB/octave and the phase leads by up to 90°. In a multi-stage amplifier, the corners stack up; n identical stages are −3 dB at a much higher frequency, and the total phase shift approaches n×90°, a real concern in feedback amplifiers where it can cause low-frequency instability ("motorboating").

Guitar amps deliberately use small coupling caps (1–22 nF) to tighten bass before distortion stages, whereas hi-fi designers place the corner a decade or more below 20 Hz and stagger the corners between stages.