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Audio Output Transformer Load Resistance


How to Calculate Audio Output Transformer Load Resistance

Use the following instructions to determine the load resistance of an unknown audio output transformer.

  1. Get a low voltage AC transformer (about 10V or less) and place it across the speaker taps of the output transformer (OT).
  2. Next, measure the voltage across the speaker taps with a multimeter.
  3. Now, measure the voltage across the primary side of the OT (plate-to-plate).
  4. Divide the measured primary (plate) voltage by the secondary (speaker) voltage. This number is the turns ratio.

To determine the load resistance for the desired given speaker load, square the turns ratio you calculated and multiply that number by the desired speaker load (i.e. 4, 8, 16 ohms).