If you want to have an indicator for your amplifier that tells you that the signal is clipping then this is the one for you. This circuit can be used at the input or output of the amplifier. You just need to change some resistor values to make it work.
About the Circuit
The circuit uses comparator LM393 that have an open collector output. Basically any comparator can be used as long as the operating voltage is within the supply voltage.
figure 1 – peak/clipping detect schematic
There are 2 comparators used, 1 for comparing the positive side of the signal and 1 for the negative side of the signal. R1 and R4 should be same value as well as R2 and R3 to keep the complimentary voltage. R5 is used to set the input of the comparators to half of VCC. R6 is just a pull-up resistor of the comparators because these comparators have open collector output. Without R6, the output of the comparator won’t go high.
The current threshold of this circuit is around 1.1Vpp. If you want to increase the threshold you can increase the volue of R2 and R3, make sure they have same value.
Threshold Voltage in peak-to-peak is computed as:
Vthreshold = (VCC) * (R2/(R1+R2))
Note:
R1 = R4
R2 = R3
Simulation Result
figure 2 – input signal below threshold
figure 3 – input signal above threshold voltage
Improved Version
The problem with the first design was the led will not light up steadily and will not light up on its maximum brightness. This is because the signal will always reach in between the limits and the output will always turn on and off because the signal will always cross zero volt. Because of this behavior of AC signal, we will need a device that can be able to store energy such as capacitor to prolong the on time of the LED when it detects peak voltage.
figure 4 – improved peak detect circuit
Q1 becomes PNP transistor which is used to charge C3 when peak voltage is detected. R7 is used to limit base current of Q1. R8 is used to limit base current of Q2 and to allow C3 to charge at higher potential. Without R8, the charge voltage of C3 will only be limit to Vbe of Q2 which is around 0.7V. The discharge rate of the capacitor is around 100ms meaning the LED stay on for about 100ms when peak voltage is detected. If you want it to make longer, increase C3. LED will be connected in series in R9.
PS:(the labeled “D1-cathode” in the circuit is because there is a diode on the first version of this circuit, I made some modification and the diode was removed but I forgot to rename the node probe. 🙂 )
DOWNLOADS:
audiopeakdetect-tataylino.com.wxsch