The op-amp usage is pretty straight forward but there is some good stuff going on here. The first op-amp uses a virtual ground that is also used for positioning the HDD arm based on the voltage on the far side of the voice coil and also for the audio input and overall gain of the system. The second op-amp is being used to quickly overcome the dead zone of the transistors configured as a voltage follower with high current output. Position of the HDD arm is being fed back with a 10k pot hot glued to the pivot point of the HDD arm. On the right side of the voice coil I wanted a simple circuit to both source and sink current from the output of the Drive op-amp. I wanted a set voltage of about 5VDC, so came up with using a 7805 raised up by a diode voltage drop to give ~5.7V to source current back through the HDD voice coil. To sink current forward through the HDD voice coil I used a 5.1V zener diode. Needed 5.7V in order to be greater than the 5.1V zener as to sufficiently bias it enough, measured current was ~10mA and measured voltage was 4.9V with voice coil disconnected.
See the attached final circuit diagram, pictures and videos of the project in operation. Enjoy!
Notes: Circuit design could be improved but was limited by the components I had on hand. 1. The 2nd stage of the High Pass Filter by best practice should be a 22K resistor and .1uF cap to avoid loading. 2. Each transistor should have its own base resistor however after adding them in parallel it did have more drive power so I left it. 3. The 100 Ohm input resistor was intended to be higher but I did not have a bigger potentiometer for the feedback resistor, like a 100K of 1M Ohm, so I made do with what I had. 4. It was a bit challenging without a "big" potentiometer to find the proper tuning value for the feedback resistor to keep it from oscillating or being severely under damped, after several attempts I found that 7.3K Ohms in series with the 10K pot allowed it to be over damped to just under damped. It is tuned just over damped so as not to oscillate but also have descent response when the Vref pot is turned.