Circuit Description

The circuit diagram of audio distortion meter has been described here. Audio distortion meter is used to measure Total Harmonic Distortion (THD) on any load. This device can measure harmonic distortion at any output power. In this circuit, different load resistors of R1 (2 ohm), R2 (4 ohm), R3 (8 ohm), R4 (16 ohm) is selected by means of a switch S1. This circuit a filter called Notch filter which is composed of resistor R1, capacitors C1 (0.01 uF) and C2 C1 (0.01 uF) and inductor L1.

Feed the input of 1 KHz signal from a signal generator amplifier circuit. Connect the out put of amplifier to points P1 and P2 of the circuit. Then select the required load resistance using switch S1. Slide the switch S2 to CAL position (to left as in circuit) and note the reading on the AC meter (Vcal). Then slide switch S2 to THD position (right) and adjust L1 and R5 (250K) to get the minimum meter reading (Vmin). Calculate THD by using the following equation.

THD (%) = (Vmin / Vcal) *100

Circuit Diagram

Audio Distortion Meter Circuit

Audio Distortion Meter Circuit


  • Load resistors used here should have a rating at least twice of the power dealing with this circuit.
  • Inductor L1 is a UTC VC 15 variable inductor.
  • Meter M1 should be a general purpose AC voltmeter with range determined by the maximum possible output voltage of the amplifier under test.