Electric Motor Synth/Drone

This is a homemade synth/drone is based on 4 tiny electric motors, each with 0-5v CV. An internal piezo amplifies the vibrations from the motors so the sound can be plugged into a mixer etc.

Here are some more build pictures on instagram: Link

Each motor is driven by an NPN-transistor in this case: 2N2222. An opamp is controlling the voltage aplied to the motor.
I used a TL074 op-amp for my project, but any op-amp should do.
potentiometers can be put between CV or 5v to attenuate the speed of the motors.
For the amplifiing the piezo i used a LM386.
I am not an expert in electronics so these scehamtics can problerbly be improved