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Electronic Amplifier Circuits-Power amplifier, audio amplifier designs and circuits

May 13th, 2010

Audio Distortion Meter Circuit

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.

April 21st, 2008

Low cost Audio level meter circuit.

About the circuit.
An audio level meter is a must in the workbench of an amplifier specialist and if it this audio level meter circuit then it will be more fine.You will like this because of its simplicity and of course effectiveness.
The circuit depicted here is designed to get a steady frequency response over the audio frequency range.The circuit has an input sensitivity of 100mV to get a full scale deflection on the meter.Two common emitter amplifiers form the heart of this circuit.The preset R3 in the first stage can be adjusted to get a full scale deflection on the meter.For getting maximum AC voltage deflection the second stage is kept biased at half the supply voltage.Capacitors C1,C2 and C3 act as filters.The diode bridge will covert the signal at the collector of Q1 to a varying DC voltage and this voltage will be proportional to the input signal level.The meter will show this voltage as reading.That’s all about the circuit.

Circuit diagram & Parts list.



  1. This circuit will give only the instantaneous reading and peak readings will not be available.
  2. The calibration can be done in one step.Feed the input with a 1Khz 100mV peak to peak sine wave and adjust R3 to get a FSD on the meter.
  3. The circuit can be powered fromĀ  a 12 V DC supply.I recommend a 12V battery for a noise free operation.