I tried regular iron cores, and they are more than 50 percent lossy, so tried Ferrite. The size of the core is 2 3/16 inches square and no gap. It is wound based on RF tech used for transistor linear power amps, so not an easy job. For ribbon resistance of .03 ohm, which is what the ones I made are, the ratio is 17 to 1. just for 36 watts, that is 36 amps on secondary! I used 5 mil copper sheet that is wound so it is a tube when done. layer of foil, wind 16 turns of 2 ea #21, then another foil. Side ends are pinched together and soldered making it a flatened tube. 4 of these in series to complete it. The connection ends of both of the foils has a piece doubled over and soldered. This is because of the high current. I tested it for the amount of voltage it would take with little loss. For 3 Kc lowest frequency, it would take 35 volts RMS, which at 8 ohms is over 150 watts! The ribbon won't even handle that! This has almost 100 percent coupling, and very little loss.
