Have a passive magnetic pickup Installed in your autoharp. This is with a 1/4" jack through the treble edge of the autoharp. Use any 1/4" jack guitar cord to plug into your PA system or your amplifier, where you will have the volume and tone controls. 
You can even plug in the effects pedals of your choice.
This is for an autoharp in our shop. Bring us yours, or have one installed in an autoharp you are buying.