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So I need two primaries to feed a single secondary with a 2:1 turns ratio. I'm figuring 20W RMS (maybe 30W) but have been told the low frequencies of a sub make the transformer large (and I thought it was more a function of VA capacity/saturation level than freq).

I'd like to think I could ballpark this thing and buy a cheap torrid doughnut and some 22ga enameled wire and wrap a couple hundred turns to start testing and then scope it to see how bad it performs. So can anyone give me a idea where to find a cheap doughnut (or U-shaped laminate core) and an idea what gauge/turns to start with?

I'm guessing that if we start with say 60Hz like mains power will the performance suffer greatly at 30Hz, and if we start with 30Hz then the inductance would need to be managed to keep the performance in-check at 60Hz? Ahh, I have a ton of questions but will try to keep this short so any help is appreciated.