Free workshop tool
Output Transformer Impedance Ratio
Estimate an output transformer’s turns ratio and reflected primary impedance from a low-voltage AC test and a known speaker-load impedance.
Use a safe low-voltage test source. Do not apply mains voltage to an unknown transformer for this test. Ensure the transformer is isolated from the amplifier circuit and that you understand the test method before taking measurements.
Formula
How transformer impedance is calculated
Turns ratio
Turns ratio = primary AC voltage ÷ secondary AC voltage. If 100 V is measured on the primary and 4 V on the secondary, the turns ratio is 25:1.
Impedance ratio
Impedance ratio is the square of the turns ratio. A 25:1 turns ratio gives an impedance ratio of 625:1.
Reflected primary impedance
Primary impedance = speaker load × impedance ratio. An 8 Ω load reflected through a 625:1 impedance ratio gives 5000 Ω, or 5.0 kΩ.