"BHAKit" the Basic Headphone Amplifier Kit is an analog educational kit about op amp applications based on New Japan Radio NJM4556A dual high current operational amplifier capable of delivering 50mW power into 32 ohms load (enough to produce high volume levels through typical headphones); the kit is intended for breadboarding but can be built on the supplied prototyping board. NJM4556A is configured as a single supply AC-coupled non-inverting amplifier with a gain of 3 demonstrating one of the most popular consumer electronics applications of op amp, the headphone amplifier. As an analog educational kit you breadboard the circuit to have the convenience to hook up an ammeter to monitor current flowing into each component or solder the components to the prototyping board then enclose in a plastic project case to have your 1st DIY headphone amp on the go. For very detailed headphone amplifier design & testing there are many excellent technical materials at NwAvGuy http://nwavguy.blogspot.sg/
To maximize analog learning, modifying the circuit with a power transistor & trimmer resistor a basic motor speed controller can be designed, an example of DC application of the operational amplifier even though NJM4556A is not optimized for DC amplification (Analog Devices OP07CPZ is recommended to improve offset voltage). The kit is all set with 3V hobby motor and circuit operation is discussed by Andy Collinson of Circuit Exchange International.
It is suggested to check out Analog Devices’ Hardware and Housekeeping Techniques at http://www.analog.com/library/analogDialogue/archives/39-05/Web_Ch7_final_J.pdf
to avoid the pitfall of poor circuit assembly.