Small Signal Audio Design, 3rd Edition

CHAPTER 23

Level Indication and Metering

Signal-Present Indication

Some amplifiers and the more sophisticated mixers are fitted with a “signal-present” indicator that illuminates to give reassurance that a channel is receiving a signal and doing something with it. The level at which it triggers must be well above the noise floor but also well below the peak indication or clipping levels. Signal-present indicators are usually provided for each channel and are commonly set up to illuminate when the channel output level exceeds a threshold something like 20 or 30 dB below the nominal signal level, though there is a wide variation in this.

A simple signal-present detector is shown in Figure 23.1, based on an opamp rather than a comparator. The ...