555-based class-D headphone driver makes great practice amp
The popular 555 timer can be used 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 analog 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.
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.
The input stage of the circuit is built around the JFET T1. Resistors R4* and R5* should have the minimal applicable values. They should produce some gain, and have low output impedance to drive the 555.
Without an input signal, the DC voltage between points A and B should be around VEE/3.
This circuit is simpler than Figure 1, but we may need to adjust R4* and R5* according to the selected transistor T1 and the required voltage gain from the first stage. The problem is that JFET parameters for a given type can differ by more than 4:1. When switch S1 is closed, the gain of T1 is set to the maximum.
Figure 3 Using two 555 circuits working at different frequencies to obtain different sound effects.
The bipolar transistor T2 improves the drive capability of the JFET. Also, it allows the use of a higher R4* value, and that will increase the voltage gain of T1.
The circuits can work over the entire power supply range of the 555 (4.5V to 16V), but higher +VEE is preferred; e.g., from 12V to16V. This will produce more output power, and most op-amps and JFETs will work better with these voltages.
The circuits can drive high impedance loudspeakers and headphones – greater than 24Ω is preferred. In all cases it is preferable to keep the peak output current of the 555 below 150mA. That will keep the power dissipation of the chip to an acceptable level. The voltage drop across the 555’s output transistors increases rapidly if the output current is much above 100mA.
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