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Impedance Matching an audio amplifier to a transformer

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You can use a ferrite core at low frequencies, it's just that, because of its low permeability, the core will be a lot larger than one made of iron/steel.
 
As your transformer is working OK at 12V rms and 20Khz with 30 turns on the primary, you will need to increase the number of primary turns by a factor of root 1000= 32 for the transformer to support the flux at 20Hz . So your primary will need to be 32* 30= 960 turns.

As the other members have implied, what you are trying to do is impracticable. You need to start again with a transformer core suitable for audio, like a valve amplifier transformer, not a mains transformer though. By the way, a 20Hz to 20kHz, 40W audio transformer is liable to be large and expensive. The transformer will have to be wound using high frequency techniques to minimize losses at high frequencies, especially skin effects. Because of the high turns ratio, the secondary will need to be wound with litz wire. You may even be able to use a standard valve transformer and reverse the roles of the primary and secondary, as has already been implied.

You do not match the primary impedance of a transformer to the output impedance of an amplifier for maximum power transfer. Most transistor amplifiers have an output impedance of zero anyway. The VI capabilities of the amplifier must be matched to the VI characteristics of the load. The transformer must be capable of supporting the flux (turns per volt) necessary to do this.

Just a word of caution, audio transformers need to be loaded and should not be run with open circuit secondaries, or excessive voltages may damage the insulation on the windings.

spec
 
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Just a word of caution, audio transformers need to be loaded and should not be run with open circuit secondaries, or excessive voltages may damage the insulation on the windings.
That's true if the transformer is being driving by a high impedance tube circuit, where the tube AC output voltage can become very high when unloaded, but not when being driven by a low impedance transistor amp, whose AC voltage changes little from no load to full load.
 
That's true if the transformer is being driving by a high impedance tube circuit, where the tube AC output voltage can become very high when unloaded, but not when being driven by a low impedance transistor amp, whose AC voltage changes little from no load to full load.
Good point cruts. Thx

spec
 
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