You can build a high-quality portable amplifier to boost the output of Sony's MiniDisc portables.
This little headphone amp from SDS Labs uses a push-pull Class A MOSFET output stage, being driven by an op-amp. The output stage is included in the op-amp feedback loop, so the distortion figures are amazingly low.
This simple amplifier is ideal for adding a headphone jack to equipment that lacks this feature. The Headphone Buffer circuit board is small enough (1.2" X 1.4") to squeeze into even the smallest spaces and power requirements are so low that existing supplies in retrofitted gear can be used.
drives up to 4 headphones from a single amplifier, has volume control and mono switch.
To realize this simple project you will need just a few parts; two resistors, two 9V batteries, one famous and popular operational amplifier IC (Integrated Circuit a.k.a. chip), LM741, a breadboard and some wire to connect the parts on the breadboard.
Chu Moy designed a very popular headphone amplifier that's easy to build, and it can be built small enough to fit in a pocket, power supply and all. It's powerful enough to drive very inefficient headphones to thunderous volumes from even weak sources, and it sounds excellent considering that you can build for just US$20.
IF amplifier filters are LC resonator stages separated by individual stages of amplification. The principle is called synchronously tuned filters stages. In if amplifier filters there can be any number of resonators per individual stage.
National Semiconductor Application Notes.