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555-based class-D headphone driver as practice amp

04 Aug 2014  | Petre Petrov

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The popular 555 timer can be employed as a PWM/Class-D amplifier for musical instruments or other applications. It can use a wide supply range of 4.5-16V and produce 200mA of drive. The audio is fed to the 555's CV (control-voltage) pin.

This design idea presents two simple, cheap drivers for headphones and audio lines. The drivers were designed for electric guitars and violins, but have many more applications. For such simple applications, noise and THD are not a primary concern and they were not measured.

Here are some design considerations:

1/ The input resistance of the CV pin is around 3kΩ and in most audio applications we need some kind of audio preamplifier/buffer.

2/ CV requires significant amplitude of the input audio signal. The required amplitude depends on the power supply of the 555 and the required output audio power.

3/ The 555 works as an oscillator modulated by the lower frequency audio signal applied to CV.

The frequency of oscillation should preferably be at least ten times higher than the maximum audio frequency of interest. For audio applications, the frequency should be between 60kHz and 200kHz. That simplifies the filtration of the high frequency noise produced by the 555, and maintains high switching efficiency.

4/ Care should be taken re RF emissions. We should have at least a 1st-order low pass filter between the output of the 555 and the loudspeaker or headphones. If we have long cables, the parasitic capacity of them should be taken into consideration (twisted pairs are preferred).

Figure 1: Headphones and audio line driver with op-amp and NE555. The CMOS version (e.g., LMC555) also will work, but the output currents are lower. The advantage is the higher working frequency.

The gain of the first stage, Av1, is set by R6 and R12 to around 11, given by: Av1 = 1 + R6/R12.

The frequency of the timer without input analogue signal on CV depends on R7, R8, and C5, and is calculated with the standard formula below:

f = 1.44/((R7+2*R8)*C5) (Hz)

The output signal of the NE555 is available on the connectors OUT1, OUT2, and OUT3. R9, C7, and the load work as a low pass filter for the high frequency components produced by the timer. If not filtered these components can be radiated, and can create problems with sensitive electronic equipment around the amplifier. The cut-off frequency of the filter should be kept as low as possible. Headphones with higher resistance are preferred.

JFET version
We may also use FETs or bipolar transistors to obtain high input impedance and to amplify the audio signal before the NE555.

Figure 2: Driver for headphones and lines with JFET input and NE555.

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