Class D Amplifier with 555 chip

This is not a new, neither an original project, but still it can be quite useful for two purposes:

Illustration of how Class D amplifiers work

Practical – the scheme is so simple, that it can be used for a cheap substitute of a low power amplifier. It operates with +4V to +10V supply and you can make it for minutes, even without a Printed Circuit Board. It works with speakers with 4, 8 and 16 Ohm
No matter its simplicity, the sound quality is surprisingly good and depends greatly on the speakers you use.

The circuit below is for a stereo variant.

555_amplifier scheme

The input signal is coupled to the base of the transistor U$2 with C14. Resistors R5 and R6 determine the transistor bias and gain. For normal operation of the input stage the voltage at the collector of U$2 should be between 1,5 and 3V.
Via C8 the amplified signal is fed to pin 5 of NE555 chip. The capacitor C11 filters higher frequencies, which when combined with the clock frequency of the chip, sound very annoying.
The elements C4, R1 and R2 determine the clock frequency of the chip - in this case about 25 kHz.
At the output - pin 3 - there is a square wave signal with an amplitude from 0 to +Vcc, frequency 25 kHz and pulse width modulated, according to the input signal.

At the diagram below, the input signal is in blue, the output one - in green.


The output signal should be fed to a speaker with a capacitor bigger than 20 µF. Its value affects the bass frequencies, so the bigger the capacitance - the better.
The choice of the clock frequency depends on the compromise between quality and consumption. Note that 555 is not a very fast chip, so low clock frequency leads to a lower consumption. On other hand, clock frequency should be higher than the frequency response of the speaker.

This is a suggested PCB original. When you open classD_555b.brd file, you may not see the ground plate as shown at the picture. In this case, hit the Ratsnest button, usually at the bottom left on the screen.
The elements are soldered in a SMD way - from the copper side of the plate.


The elements used are not critical. Transistors are Russian general purpose NPN type with beta (current gain) 100.

Note: If you have an used 555 chips, be sure they are operational, before you solder them to the PCB. Some chips look ok - they oscillate - but the waveforms are distorted. The reason in most cases is a defected output transistor within the chip.

A homemade design for the speakers :-))


Here you can download Eagle Cad file with the PCB

Here you can download LTspice simulation circuit

For stereo variant you need: 2 NE555 chips, 8 resistors, 10 capacitors, 2 NPN all purpose transistors, 2 speakers, a USB cable, a 3,5mm audio jack.