So my first question is how many condenser mics do you need to power? If it is just a couple, I would 100% just get an external phantom supply. Don’t hack up your M-320. You could totally do it. I think you can get the M-300B manual supplement pdf online somewhere, and the info for the phantom circuit with it. It’s simple enough…but it would be a decent amount of work to incorporate on a non-B model. I don’t think the non-B main transformer has the secondary taps to power a 48V regulated supply. So it’d be easiest to just get a used open-frame modular supply, like a Power One or Lambda supply…and then you’d need a bunch of DPST switches, the output of the phantom supply daisy chains through one side of the switches, and the two poles of the other side of the switches each connect to pins 2 & 3 of an XLR jack, each leg through a bias resistor…Teac spec’ed 6.8K for those. A lot of times you see a small value film cap in there too I think to deal with any DC spike when switching, but it’s not necessary.
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