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Okay, I'm not going to go into how it works and all, suffice to say, that the coil is the most important component here. It should be rated at 5A. Typically you can use a smaller coil with a higher frequency, but certain problems arise with driving the MOSFET and increased EMI, so i suggest you keep it as it is. I do recommend though, to increase the 220nF cap, to something between 1uF and 47uF for supreme stability and shield the whole thing in a grounded metal case to eliminate EMI leakage, which i hate to say is a lot, even up in the FM radio range, despite all the measures taken. The good thing about this power supply is that virtually no hum can possibly ever enter into the amplifier, nor the preamp. Typically the preamp stage uses RC filtering, that at these frequencies will give out around -120dB ripple rejection and the power stage eliminates hum by using the primary of the output transformer as a filter, that will also give out exceptional filtering. Even if the impossible happens, the hum's frequency will be outside the audible range, so it won't be reproduced by the speaker. This is absolutely supreme to the normal techniques in regards to low hum, high stability power supply. For best results use DC for the heater, or an artificial center tapped ground for an AC source with a low value potentiometer. The output shouldn't wobble a lot, because there's a negative feedback that compensates for load variations and such. The efficiency is not great at 66%, and I guess a little more can be juiced out of it, but it's acceptable. Real life tests can best show how to tweak for best efficiency. Use a good heatsink for the MOSFET as it'll dissipate around 15W. And last recommendation, separate the 12V source with a Ge diode from this circuit and add a large capacitor after the diode (470uF-4700uF), followed by a small capacitor (100nF) close to the coil. All diodes are fast recovery schottky, and the MOSFET is IRF740, all resistors are 1/4W and all capacitors before the coil are 16V and all to the right of the coil are at 450V. ⚠ Enjoy, and be cautious, this thing more than has the full capacity to kill!!! ⚠
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