Design of practical amplifier circuit for Class D headphone driver based on 555 timer

　　The popular 555 timer can be used as a PWM/Class D amplifier for musical instruments or other applications. It can operate in a supply voltage range of 4.5V to 16V and can output a drive current of 200mA. The audio signal is sent to the CV pin of the 555 timer.

　　This Design Idea provides two simple, inexpensive drivers for headphones and audio lines, as shown in the figure. These two drivers are designed for electric guitar and violin, but can also be used in many other applications. For simple applications like this, noise and total harmonic distortion are not important considerations, so these two values ​​are not measured.

　　Figure 1: Headphone and audio line driver with op amp and NE555 timer. CMOS versions such as the LMC555 can also be used, but with lower output current. The advantage is higher operating frequency.

　　The following are some design considerations:

　　● The input resistance of the CV pin is approximately 3kΩ, and in most audio applications some kind of audio preamplifier/buffer is required.

　　● CV requires a very large amplitude input audio signal. The required amplitude depends on the power supply of the 555 timer and the required output audio power.

　　● The 555 timer is used as an oscillator, modulated by a lower frequency audio signal applied to CV. The oscillation frequency should preferably be at least 10 times the maximum desired audio frequency. For audio applications, the frequency should be between 60kHz and 200kHz. This simplifies filtering of the high frequency noise generated by the 555 timer and maintains high switching efficiency.

　　● Be careful about RF emissions. At least a first-order low-pass filter should be set between the output of the 555 timer and the speaker or headphones. If the cable is long, the parasitic capacitance of the cable should be considered (preferably twisted pair).

　　The first stage gain is set to approximately 11 by R6 and R12 using the formula Av1=1+R6/R12.

　　The frequency of the timer when there is no analog input signal on CV depends on the values ​​of R7, R8 and C5. The standard calculation formula is as follows:

　　f=1.44/((R7+2R8)×C5)(Hz)

　　The output signal of NE555 is transmitted to connectors OUT1, OUT2 and OUT3. R9, C7 and load act as a low-pass filter to filter out the high-frequency components generated by the timer. If not filtered, these components will radiate and cause problems for sensitive electronic equipment around the amplifier. The cut-off frequency of the filter should be as low as possible and headphones with higher resistance should be selected.