Constant current source for an alternating magnetic field of varying frequency and constant amplitude from a solenoid
Look at the IRS2092 app note
Infineon sells their IRAUDAMP9 evaluation board, but you may get also a fake clone from china for low price. By fake, I mean it is really fake - you might need to change transistors and driver IC.



Now, you could change the C3 and Rin, to compensate the the impedance of your load. If the input cap is relatively low value it would attenuate low frequencies, while leaving unattenuated high frequencies. This won't give you a current source, but it could make a circuit working quite well.
Next, if you can't change the solenoid, you could use a step up transformer/autotransformer. The audio amp is designed to drive low impedance, while your load is high impedance, so a transformer is needed for matching.

Original  IRAUDAMP9 500€: 



Fake  IRAUDAMP9 / L30D  30€: