| 摘要 |
<p>An audio amplifier 50 comprises at least one thermionic valve (vacuum tube) 52, 54 having a control grid, which receives an audio signal, and a cathode. The audio amplifier 50 further comprises a biasing arrangement 108 in series with the cathode and a current sensor 110 operative to measure a bias current flowing though the biasing arrangement. The audio amplifier 50 is operative in a first mode in which no audio signal is received at the control grid, the current sensor 110 is operative to measure the bias current, the measured bias current is compared with a predetermined bias current, and a voltage drop across the biasing arrangement 108 is adjusted in dependence on the comparison to bring the bias current towards the predetermined bias current. The audio amplifier 50 is operative in a second mode in which an audio signal is received at the control grid and an adjusted voltage drop across the biasing arrangement 108 is maintained substantially constant.</p> |
