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Mike. Watterson · 30th Jul 2019, 5:00 pm

The DC resistance can be from 200 Ohms to 600 Ohms for an audio out transformer depending on size. The the AC load resistance is the voltage ratio squared x speaker ohms (The DC resistance of that can be close to AC impedance).

You can't tell from DC resistance. One way is to drive transformer primary from 115V shaver transformer and measure AC on secondary

Vin/Vout = turns ratio.
Z primary = (turns ratio) squared x Speaker impedance.

The actual DC resistance depends on the size of the transformer, not the turns ratio. A 100V line speaker transformer, one for 16 Ohms and one for 3 Ohms could all be the same DC resistance on the primary.

A 20W 240V to 6V mains transformer is about 40:1. With 3 Ohm load that looks like 4,800 Ohms. That will work to test. DC bias current is an issue with single ended transformers. That cancels on Push Pull, however a larger transformer than audio power suggests mitigates it. Never test single ended with a toroid transformer as they will saturate at a much lower DC current.

30th Jul 2019, 5:00 pm	#23
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Unknown Home Built TRF radio The DC resistance can be from 200 Ohms to 600 Ohms for an audio out transformer depending on size. The the AC load resistance is the voltage ratio squared x speaker ohms (The DC resistance of that can be close to AC impedance). You can't tell from DC resistance. One way is to drive transformer primary from 115V shaver transformer and measure AC on secondary Vin/Vout = turns ratio. Z primary = (turns ratio) squared x Speaker impedance. The actual DC resistance depends on the size of the transformer, not the turns ratio. A 100V line speaker transformer, one for 16 Ohms and one for 3 Ohms could all be the same DC resistance on the primary. A 20W 240V to 6V mains transformer is about 40:1. With 3 Ohm load that looks like 4,800 Ohms. That will work to test. DC bias current is an issue with single ended transformers. That cancels on Push Pull, however a larger transformer than audio power suggests mitigates it. Never test single ended with a toroid transformer as they will saturate at a much lower DC current.