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This is the test circuit I have been using.
The DRIVE signal is a 6600 Hz pulse from the processor. The transistor, diodes, and current sensing resistor R form the current source. I am using R = 100 ohms to source 10 mA which gives 3.5 V p-p across the transmit coil. I have also used 10K and 33K for R to test with very small signals. I will probably set the current around 15 mA in the final version. White's uses 30 mA in the 6000 Di, but they have a higher supply voltage and have more headroom for the voltage swing on the coil.
Robert Hoolko