[AdrianH]

As my thread has not been shut it gives me a chance to ask a few more questions and just check on things. I am about to wind another couple of transformers to finish this little project off for myself.

I have 'butchered' three EI-48 sized transformers, these from what application they were in were typically 6 to 7VA rated and I plan to wind as a 1 watt audio transformer, so sticking to the previous impedances 57 Volts primary and 2 Volts secondary, 3250 and 4 ohms respectively.

The actual amp can only do 200mW clean, so that gives me loads of head room and a chance to use the transformers again later.

I have made a couple of bobbins, as I seem to struggle finding anywhere that sells then privately.

When I measured outputs of my small amps using my scope, one with the hammond transformer and one with my first self wound tfmr, the one with the Hammond seemed to distort the output sooner. I think this was down to two things, the see-saw push-pull output, is simple but not the best design and the fact that one side of the primary had a higher DC resistance.

Looking on the scope one output pair had a larger peak to peak voltage then the other output pair, and it seemed to follow the transformer if I swapped the primaries around.

The one I wound was done with equal windings as both primaries were wound together so at the end the same DC resistance in both sides. The scope showed the peak to peak voltages on both legs to be very close, within a volt or two.

I did a few calculations for turns per volt.

For the 1 watt idea:-
1/(4.44 x 40Hz x 0.00032m^2 x 0.7)

is 25.14 turns per Volt. primary would be:-
57 x 25 = 1433 turns per side so 2866 total primary,
My window is 22mm by 6.2mm and I have 122 turns along the 22mm with my wire so height of primaries is 4.2 mm so then I would need 50 turns of secondary so if I am very careful I might just squeeze all that in with some insulation.

If I worked it out for the 200mW I could go down to 20Hz
TPV=1/(4.44 x 20Hz x 0.00032m^2 x 0.7)

is 50.2 turns per volt, but at 200mW the voltage would be 25.5 Volts rms so a total of 1280, both primaries 2560, same wire a little bit more space.

But, a couple of things I am not sure of, is the formula anywhere near correct?

Will I be just making up a lot of capacitance between each push pull pair winding the two primaries at the same time?

All this lot is basically me putting all my thoughts down to see if I am on the correct train of thought, and a way my brain seems to work. The more I read the more confused I can get and there is a lot of information out there.

Adrian