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Class D Amplifier Circuit Using IC 555

A Class D amplifier also termed as a digital amplifier uses pulse width modulation technology for amplifying the fed small amplitude analogue music signal.

Why a Class D Amplifier

The main benefits of this type of amplifier are high efficiency, low cost, with the only drawback being the association of distortion if not cleaned with correctly calculated filters at the output.

Normally all amplifiers are analogue based where the input music or frequency is amplified in accordance with the same pattern that's being fed at the input.

Since a music may largely have exponentially rising and falling contents and also frequencies accompanied with all sorts of amplitudes causes heating up of the devices.

This happens because BJTs and mosfets do not "like" transitional inputs where the signal do not have sudden rise and fall rather gradually transits across the points where the devices are neither fully ON or OFF, this causes a lot of heat generation and power loss

In a class D type of amplifier, the music input is compared with high frequency triangle waves and converted into a PWM "language" at the output. The PWM content stores all the information of the music and translates it back into the connected loudspeaker in an amplified manner.

However since the PWMs will consist of non-exponential pulses where the pulses are in the form rectangular pillars switching ON/OFF suddenly without transitions can result in significant distortions at the output.

In order to smooth out the above issue, a low pass filter is generally incorporated wherein the spikes are smoothed to generate a reasonably good and clear amplified replication.

The proposed design of a class D digital amplifier circuit utilizes the famous 555 IC for the intended comparisons.

Instead of the PWM method here we use an alternative mode called the PPM or pulse position modulation which may be considered as good as a PWM.

Using Pulse Position Modulation

PPM is also known as pulse density modulation due to the specific nature of its functioning.

Here the modulation input is compared with high frequency triangle waves and the output is optimized by varying the position or the density of the generated/compared pulse output.

As can be seen in the below class D amplifier circuit design, the IC 555 is configured as a standard astable MV mode, where the resistors Ra, Rb and C determine the frequency of the triangle waves generated at pin6/7 of the IC.

The above high frequency triangle waves are compared with the music input applied at the control input pin5 of the IC.

Here the low voltage music signal is first amplified to some optimal voltage level and then applied at the control input pin#5 of the IC555.

This results in the discussed PPM output at pin#3 of the IC. This is amplified by T1 to a high current output and is fed to a loudspeaker for the required class D type amplification.

The audio trafo does a couple of interesting functions, it amplifies the output for the LS and also to an extent smooths out the harmonics which are normally a part of all class D type amplifier circuits.

A filter capacitor (non-polar) may be tried across the LS for obtaining cleaner sound outputs.


IC 555 Pinout


IC LM386 Pinouts


Need Help? Please leave a comment, I'll get back soon with a reply!


  1. Ra = 10K, Rb = 22k, C = 0.01uF, L1 could be an inductor wound on T13 torroidal ferrite core with 25 turns of 25swg copper wire.

  2. you can try this circuit:

  3. I do not have an opamp circuit, I can 555 IC circuit though.

  4. What type of diode and Mosfet used here ??

  5. diode could be any fast recovery type, mosfet could be any type matching the output power requirement specifications.

  6. I am going to build audio amps. My power requirement is 3w 3w stereo. Should i go for transistor or ic?. For better result with less cost. Suggest one ckt.

  7. go for IC amplifier, preferably by using a TDA series IC, or you can also try a couple of LM386 ICs for the same.

  8. Thanx. How? Do u publish any article for above written requirement? Or sugest a ckt.

  9. I have one article explaining LM386 IC, you can find it here:

  10. could ouu check this amplifier

    Question do you think I could add atleast 2 more pairs of output transistors to this circuit only adding and extra 10ohm and 0.1ohm to base and emmiter to get more power?

    Would anything need to be changed or added?

  11. each transistor would require a massive 1amp for for 4 transistors it would be 4amps....I don't think the MJE340/350 would be capable for delivering this much make sure these are upgraded with more powerful devices along with their bases resistances which will need to be reduced....only then it would be possible

  12. So If i was to add (4) 2sc2922 and (4) 2sa1216 I could replace MJE340/350 with tip TIP122 and TIP127. what about the MJE340 at the input, would I need to replace that one?

    Also what would the supply voltage be maximum, would it be about 50v +/-

  13. TIP122/127 are Darlington devices not sure how they would respond here? you can try nevertheless.

    50V would be a good value to start with.

  14. Ok thanks so just the way the circuit is now would 50v be too high to test it at I was looking at the A1266 in the circuit seeing that collector emitter voltage is 50v I was thinking it should be a bit lower.

  15. the MJE are rated to handle much higher than 50V so it would be OK to use 50V for these, not sure about the power can probably think about using some other variant such as 2N3055 etc or TIP35/36 which are rated to handle 100V

  16. Swagatam,

    is the 450 watt amp by Ainsworth working well now? He asked you to check the circuit, is it ok? If it is can you please share with me the power supply secondary winding center tap voltage.thanks.

  17. Hi swagatam,can i use transtorized power amp(5w8ohm) in place of LM386,if so should i change output transistor across audio transformer.And also what is power output of this circuit.

  18. Hi Charan, yes any amplifier which can work with the same voltage as the 555 will do.

    the power output will depend on capacity of the output transformer wattage and the TIP127 capacity, these two could be upgraded for higher power outputs

  19. i can change TIP127 for TIP142 ?

  20. you can try this circuit:

  21. you can change it with TI147...


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