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555-based class-D headphone driver as practice amp

04 Aug 2014  | Petre Petrov

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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.

About the author
Petre Petrov worked as a researcher and assistant professor in Technical University (Sofia, Bulgaria) and as a lecturer and expert lecturer in Kingdom of Morocco. Now he is working as electronics engineer in private sector in Bulgaria.

To download the PDF version of this article, click here.

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