This is a headphone amplifier similiar to the one designed by Chu Moy. For reference, the original Chu Moy article is here
while a great tutorial on building it is here
. I have used a different dual operational amplifier, the RC4560, manufactured by Texas Instruments, in the TSSOP package, and chip resistors in order to make an extremely small printed circuit board assembly.
Step 1: The op Amp
The active device is the RC4560, which Texas Instruments claim to be suitable for, among others, Headphone amplifiers. It can work down to +/- 2 volts, and has low noise and a wide enough bandwidth.
It is available in a TSSOP (Thin Shrink Small Outline Package) which means it is small indeed, small enough for me to demonstrate my one-off microprototyping skills.
Step 2: The circuit
The circuit is the standard non-inverting amplifier with a gain of 11, with a capacitively coupled input. A small valued resistor at the output of the amplifier isolates the load and minimises any tendency of oscillation with long headphone leads.
The large size of the input capacitors make them too bulky to be included on board, so they shall be outside, between the input jack and volume control, and the board.
The power supply is provided by a nine volt battery, converted to dual +/- 4.5 volts by 4.7K resistors and decoupled by 220 microfarad capacitors.
A high efficiency Light Emitting Diode shall provide indication that the circuit is ON, it is fed with about 0.7 milliamperes by a 10K resistor.
Step 3: The components
The picture shows the integrated circuit and resistors. The chip is a free sample from Texas Instruments, and the resistors are all scrounged from old hard drives. The four digit marking means they are all of 1% tolerance, and they might even be MFR (metal film resistor). There is absolutely no way to confirm this, however.
The small size of these components make it very difficult for conventional point to point wiring techniques to be used. In fact the only viable means of connecting them up is to use a printed circuit board.