Audio Amplifier Output. How to Measure !

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The output impedance of an audio amplifier is a function of several variables. These include the amplifier's topology, the amount of feedback (gain setting), the total internal resistance of the transistors (or vacuum tubes) in the power stage, and even the quality of its power supply.

It's crucial to note that the amplifier's output impedance also depends on the magnitude of the load. Generally, a heavier load (i.e., a smaller load resistance, Rload​), which means a higher current flowing through the power transistors, will increase the output impedance. This is because active components like transistors have non-linear characteristics. For example, the IDS​ vs. VDS​ curve for a MOSFET or the IC​ vs. VCE​ curve for a BJT is not perfectly linear, especially at high currents. Furthermore, the output impedance is also affected by the temperature of the power transistors, as the current gain (beta) tends to decrease as temperature rises.


Here is a practical way to measure the output impedance of an audio amplifier.


Required Tools:

  • Sine Wave Signal Generator: Must be able to precisely adjust the amplitude or level.

  • True-RMS Voltmeter or Oscilloscope: An oscilloscope is better because it allows for more accurate measurement of peak or RMS values.

  • Dummy Loads: Power resistors with appropriate values.


Measurement Procedure:

To determine the output impedance at a nominal 4-ohm load, for example, use two dummy loads with a ±5% deviation, such as 3.8 ohms and 4.2 ohms. The smaller the deviation, the more accurate your result will be.



Initial Setup:

  • Ensure the dummy loads have a sufficient power rating to handle the heat dissipation. Use a low input signal voltage (Vin​) of about 0.1 to 0.2 VRMS​. A typical audio amplifier has a voltage gain of around 30, so the output voltage (Vout​) will be between 3 and 6 VRMS​. For a 6 V output on a 4-ohm load, the power dissipated is watt. Therefore, a 10 watt or 20 watt dummy load is safe to use.

  • Aim for the lowest possible tolerance for your dummy loads (e.g., 1%). It can be challenging to find power resistors with low tolerances, but it's recommended for accuracy.

  • Ensure the input signal voltage is exactly the same for both measurements (with the 3.8-ohm and 4.2-ohm loads).


Measurement Steps:

  1. Measure the output voltage (Vout1​) when the amplifier is loaded with the 3.8 Ω resistor.

  2. Measure the output voltage (Vout2​) when the amplifier is loaded with the 4.2 Ω resistor.

  3. Calculate the output impedance (Zout​) using the following formula:

    Note: The absolute value operator (|...|) means you should ignore any negative sign in the calculation result.


Case Study:

  • In an example amplifier, with a 3.8-ohm load, Vout1​ was measured at 7.484 V. When the load was changed to 4.2 ohms, Vout2​ was measured at 7.563 V. Using the formula, the calculated Zout​ is 0.47Ω.
  • You can repeat this procedure for other nominal loads like 8 or 16 Ω . The results will show how the output impedance changes with different loads, as demonstrated in the table.


It is clear that with a change in load, a change in the Zout​ of an audio amplifier will occur (this also generally applies to any amplifier).

For transistor amplifiers that generally operate in class B, AB, or D, Zout​ will generally not be more than 0.5Ω. However, for amplifiers that operate in class A, especially vacuum tube amplifiers, Zout​ is generally higher, and can be up to around 1 ohm. (I have never built one, but from a simulation of a MosFET Zen Zen Amplifier class A (Nelson Pass), its Zout​ was measured to be about 0.9 Ω for a 4Ω load. Perhaps someone has actually performed a measurement on a real amplifier.)

Hopefully, this note can help you understand amplifier output impedance and a simple method for measuring it.

Have a good day, keep up the spirit, and always be happy. TABIK !


(This post is parallel to the status on the FaceBookGroup The Art of Electronics with the same Topic)

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