The circuit is a microphone amplifier for use with low impedance (~200 ohm) microphones. It will work with stabilized voltages between 9-30VDC. If you don’t build the impedance adapter part with T1, you get a micamp for higher impedance microphones. [...]

This amplifier design really excels in simplicity and robustness, thanks to the use of the TDA2030 or TDA2040 “power op amps”, The power supply of the amplifier is built with bridge rectifier and elcap capacitor. Further stabilization is not necessary because of the high supply voltage ripple rejection (50dB). Here are some features : Short circuit proof outputs. Built in thermal protection. High supply voltage ripple rejection: no supply stabilization necessary [...]

The speakers are from the high-end series of Tonsil , good drivers built with conventional technique.The passive filter is second order (12dB/oct) and has a cross-over frequency of about 2.5 kHz.A pcb is not necessary, the filter components can be “hard wired”, all parts can be secured in the case with silicone. [...]

The Chu Moy Pocket Headphone Amplifier is a very simple, yet, quite good sounding amplifier. As the name says, it is intended for use with headphones. This little amp can be very effective in combination with a discman for example. You could put the discman volume at a very low level, thus using less battery power. The amplifier only needs a minimum number of parts and can be build as small as the 9v battery it needs for power. However, it’s not possible to convert that layout into a PCB [...]

The PPA is a high-end headphone amplifier that works for both stationary and “transportable” amplifiers. There is a high-capacity fast-charging NiMH battery supply available, allowing excellent sonics without sacrificing run time despite the PPA amp’s hunger for power. While only the most dedicated audiophile would take it jogging with their iPod, this combination makes for a fine travel amplifier. [...]