ABSTRACT
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 Peak indicator is driven by fast-attack, slow-decay circuitry so that even brief peak excursions give a positive display. It is important that the circuitry should be bipolar, i.e. it will react to both positive and negative peaks. The peak-detect opamp U4 must be a FET-input type to avoid errors due to bias currents flowing in the relatively high value resistors R1–R6, and a cheap TL072 works very nicely; in fact the resistor values could probably be raised significantly without any problems. One apparently ingenious way to gain a few more square millimetres of panel space is to combine the signal-present and peak indicators into one by using a bi-colour LED. VU meters are a relatively slow-response method of indicating an audio level in ‘volume units’.
