Hook up an AC adapter across the primary windings, hook a load across a secondary tap.
Say you have 24VAC across the primary and measure 1VAC across the secondary into an 8 ohm load. Your voltage ratio is 24:1.
Impedance translates as the square of the voltage ratio, so the impedance is (24 x 24) to 1, or 576:1.
Now if you connect an 8 ohm speaker to the secondary, you multiply both sides of that ratio by 8, so you have a 4,600 ohm to 8 ohm transformer. Hook up a 16 ohm speaker to that tap and you have a 9K primary, etc..
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