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I get asked a lot how to use field coil speakers. To me, most of them sound great and they are available everywhere because they aren't easy to power up and use. Here's a circuit you can use to power up the field coil. Then, you connect the speaker signal from the amp to the voice coil the same as you would any speaker. Here are the steps: 1. Measure the DC resistance of the field coil. 2. If it is a small coil, in the 2" diameter range, we are going to put not more than 12 watts of power into it. If it is a big coil (e.g. Jensen F12N), we can go up to 17 watts. 3. Now multiply the power number (12 or 17) by the DC resistance you measured. 4. Now take the square root of that number. 5. You should have a number between 100 and 300. That is the DC volts you need to apply to the coil to energize it.
6. Next, build this circuit for the coil power supply. Don't show it to any anti-Ted's because they will tell you I'm an idiot for suggesting you use a power supply that has no transformer (line operated). However, this power supply is only for the field coil. It and it's load are isolated from ground. It is totally safe providing you contain it in a small box and don't have any exposed wires.  The zener diodes are 5 watt units. The rectifiers are 1N4007's or 1N5403's, your choice. The ground symbols in the schematic are for this circuit only. They do not connect to chassis ground in the amp. 7. You should remote a small pilot lamp in a box to be placed on top of your amp to remind you to turn off this power module when you turn the amp off. No need to keep the coil energized when you aren't using the amp. 8. Don't be tempted to run the coil at a higher power level to increase the sensitivity of the speaker. It will burn up. The voltages shown are for field coils from 750-1000 ohms and 3K to 5K ohms. You can recalculate and use different zener diodes to get to the voltage you need if yours is not in the ranges I have shown. |