Loudspeaker Impedance Meter

A simple Impedance Meter can be useful to measure the actual impedance a loudspeaker or headphone is presenting @ 1kHz standard frequency. The circuit, designed on request, relies on an earlier design (Spot-frequency Sine wave Generator) to obtain a stable, low distortion 1kHz sine wave avoiding the use of thermistors, bulbs or any special amplitude-limiting device. The sine wave output, after some amplitude setting obtained by means of P1, is sent to the device under measurement through a resistor.

A regulated supply is necessary to obtain a stable output waveform. D1 and D2 force IC1 to deliver 6.2V output instead of the nominal 5V. The measurement is done in two stages: as a constant current supply of the device under test is necessary, this can be set at first by adjusting P1 and measured across the series resistor (R7 or R8, depending on the impedance value to be measured); then, the meter is switched across the device under test and the actual impedance will be read directly on the meter display.

Loudspeaker Impedance Meter Circuit diagram: 

Parts:

P1_______________4K7  Linear Potentiometer
R1______________12K  1/4W Resistor
R2_______________2K2 1/4W Resistor
R3_______________1K  1/2W Trimmer (Cermet)
R4_______________1K5 1/4W Resistor
R5_______________4K7 1/4W Resistor
R6_______________3K3 1/4W Resistor
R7_____________100R  1/4W Resistor (See Notes)
R8_______________1K  1/4W Resistor (See Notes)
R9_______________1K  1/4W Resistor (Optional)
C1______________22nF  63V Polyester Capacitor
C2_____________330nF  63V Polyester Capacitor
C3______________22µF  25V Electrolytic Capacitor
D1,D2_________1N4148  75V 150mA Diodes
D3_______________3mm  Red LED (Optional)
Q1,Q2,Q3_______BC550C 45V 100mA Low noise High gain NPN Transistors
IC1____________78L05   5V 100mA Regulator IC
SW1,SW2_________SPDT Toggle or Slider Switches
SW3_____________SPST Toggle or Slider Switch
B1________________9V  PP3 Battery

Clip for PP3 Battery

Circuit set-up using an oscilloscope:

Connect the oscilloscope in place of the DVM and rotate P1 fully clockwise.

Short the speaker output and adjust R3 to obtain a sine wave of about 2.2V peak-to-peak amplitude.

"By ear" circuit set-up:
Connect a small loudspeaker or one of the two earpieces forming a pair of headphones to the circuit output and rotate P1 to obtain a moderate output sound level.

Carefully adjust R3 until the output sound will stop; then turn back the trimmer very slowly and stop adjusting immediately when the sound will start again.


Measurement:

• Connect a Digital Voltage Meter set to 200mV ac range to the DVM output terminals
• Connect the device under test to the Speaker terminals.
• Switch SW1 in the position towards R7 if the impedance value to be measured is below 100 Ohm or towards R8 if above.
• With SW2 in the "Set" position power-on the circuit by means of SW3
• Adjust P1 in order to read exactly 100.0mV on the DVM display
• Switch SW2 in the "Measure" position and read directly the loudspeaker or headphones impedance value on the DVM display, e.g. 8.5mV = 8.5 Ohm.
• Please note that when measuring devices with impedance values above 100 Ohm (SW1 set towards R8), the decimal point in the DVM reading must be ignored. E.g. if the display shows 70.5mV, the impedance will be 705 Ohm

Notes:

• For very precise measurements use 1% or 2% tolerance resistors for R7 and R8.
• D3 LED pilot light and its current limiting resistor R9 are optional.