Modern loudspeakers boxes are relatively insensitive to overdriving signals, however it is still important to limit the power driving it to avoid clipping of the audio signal. A broken sound from an overdriven loudspeaker is not only annoying to the the ears but also the loudspeaker itself can be damaged by the continous uncontrolled signal peaks.
This peak indicator circuit is very useful aid in detecting the driving limits of a loudspeaker system. It can e directly connected to the existing speaker wirings and needs no extra power. It can detect very short voltage overswings and therefore provides reliable means of determing the driving limits of a loudspeaker.
The threshold of the peak indicator can be set for speakers ranging from around 15 watts to 125 watts with 8 ohms impedance. For 4 ohms speakers, the indicator can be set for 30 watts up to 250 watts. By testing speakers, occasional blinking of the LED does not mean danger but when the LED blinks very often then it is advisable to reduce the volume of the amplifier.
The circuit operates this way: During operation the signal charges C2 through R1 and D1. In standby periods, all transistors are turned off and no current flows through the LED. A sample of the signal flows through P1 and enters T1. P1 controls the threshold level of T1. Once the signal sample exceeds the threshold lvel, T1 and T2 switch on and charges C1 as a result; T1 conducts amd switches T4 on.
When signal goes down, T1 and T2 turn off but since C1 discharges slowly T3 and T4 remain conducting for about one second longer causing the LED to also light up longer. This tehnique has the advantage of indicating short time signal overswings which are normally not detectable.
In constructing this peak indicator circuit it is advisable to use high luminance LEDs with a diameter of not less than 3mm.