Circuit: 555 Class-D Audio Amplifier

Originally designed as a timer, the iconic 555 IC has proven to be far more versatile than its creators ever intended, showing up in an astonishing range of circuits over the decades. In this project, it takes on a distinctly modern job: driving a compact Class-D stereo audio amplifier using the chip’s oscillator and modulation functions. This 2014 circuit is refreshingly simple, making it an approachable DIY build — and a fun way to hear what this classic chip can do beyond pulses and delays.

The Design

Refer to the nearby schematic for the full stereo version of the class-D amplifier based on the 555 IC. A simple switch-on delay circuit is included.
  “In a Class-D audio amplifier, the analog audio signal is converted into a pulse-width modulated signal that drives the output transistors,” noted the designer, Frederik Crevits. “Here this is implemented by employing the 555 as an astable multivibrator and using the analog audio signal to modulate the voltage for the charge/discharge capacitor (C2) … Here we charge the capacitor with a constant current instead of a constant voltage. This is handled by a current source built around T2 and a voltage to current converter built around T3 and T4. This results in a fairly linear triangular voltage waveform on C3, and the ratio between the PWM output signal of the 555 and the input signal is close to linear. The input signal on connector K1 affects the charge/discharge time of the capacitor via T1, and in this way modulates the output signal. The switching frequency is approximately 250 kHz.”

The PCB layout is spacious and uncluttered, with enough room to mount an aluminum plate heat sink for the MOSFETs, although power dissipation is minimal thanks to the efficient Class-D architecture.
  Crevits also explained: “You should use the best possible components for the frequency-determining capacitors C2 and C16. Polypropylene and silver-mica are both good choices. To keep the temperature coefficients of the current sources T2 and T11 as low as possible, the (rectangular) LEDs D9 and D10 should be fitted in close contact with the corresponding transistors (T2 and T11). They are therefore located close together on the PCB. Murata pot cores and poly-propylene capacitors are recommended for the output filters (L1/C12 and L2/C26).”

For the prototype, the MOSFETs were mounted on the aluminum plate heatsink. The designer used screws with 3-mm insulating washers and ceramic insulators.
 

The 555 Class-D Audio Amp Project

The original article, “555 Class-D Audio Amplifier,” appeared in Elektor January/February 2014. Check out the article.

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Editor's Note: This article first appeared in a 2014 edition of Elektor. Given the project’s age, some components might not be available. Nevertheless, we think the design will inspire you to tackle a project of your own.