555 Class D Power Amplifier [130144-I]
With this project, I like to build a small class D amplifier based on a 555 timer IC. I also like to use only “common” components, so this project has great educational value and is easy to build.
My goal is to produce about 5W RMS power into 4ohm. Distortion level should be less than 1%.
Circuit description VERSION1 (see circuit diagram below)
In order to build a class D amplifier, we must convert the analog signal to a digital signal. In fact we like to use a pulse with modulated signal that follows the analog audio signal. This way, we can switch 2 power MOSFETs on and off in a very efficient way.
I use the NE555 as the heart of the PWM encoder. At the input, we have a standard NPN transistor input (Q4) biased by R10 and R9. C1 blocks all DC at the input. The idea is to modulate the way the charging capacitor C3 is loading with the analog audio signal.
In most standard 555 timing configurations, the timing capacitor is charged with a constant voltage, resulting in severe non linearities, especially at high output levels. To improve linearity, I charge the timing capacitor with the constant current. Therefore I use a current source at the input (Q3, R2 and R7) and a voltage to current convertor in the feedback loop of the 555. (Q1, Q2, R1 and R3) This way we get a real triangle wave on C3. (see image below “signal on C3 no signal.jpg”)
Oscillation frequency of the 555 is around 300 kHz.
Output signal is 0V – 18V PWM signal that switches on and off M1 and M2.
D1, D2, R4 and R5 are dealing with “on” and “off” timings and avoid that M1 and M2 are “on” at the same time (and avoiding to short circuit the power supply). R12, R14 and Q5 form a feedback circuit that improves linearity and distortion levels even more. L1 and C4 is a LPF around 25 kHz, removing all high frequency switching components.
I use a SMPS laptop power supply of 18V to test with (cheap). Initially I had switching noise in the speaker. Therefore I add a coil of 100µH to filter this out. Working fine!
I built this circuit on a standard testing board, and I’m quite happy with the result. Audio sound “direct” and “clear” to me, with tight basses! I get about 6 W RMS power, what is fine for my first attempt J I do not have any distortion measurement equipment, so I’m not sure if my goal of <1% is reached.
About efficiency, the heat sink is cool at all levels, so this should be fine.
I attached some images and of course the schematic and also some scope prints of the output square wave and charging capacitor wave. I have also added a short video of the amp @ work. Don’t bother the bad sound quality of the movie; it’s the poor mic quality of my camera J
Possible improvements (VERSION2 tbc)
- Power MOSFETs must be controlled with 18V to be fully saturated (and “on”), if I raise the supply voltage on the MOSFETs and control with 18V from 555, MOSFETs are running hot as they don’t fully switch on. A voltage level shifter circuit is recommended here.
- I will to do some tests with IR2110 IC to achieve this, maybe there's a discrete solution? Maybe with a "charge pump"?
- Now we use “half bridge” 2 MOSFET configuration, therefore needing a big output capacitor, I did some tests with full bridge configuration (4 MOSFETs) in SPICE but I get a lot of cross over distortion. Need more investigation on this….
Any help, suggestions, or hints are welcome to improve this project!
Here’s our first schematic and PCB design. Now that the design from Frederik is finalized we’ve decided to design a PCB and made some changes in component choices we think will improve the design. The PCB design is a bit more compact (131.22 x 99.47 mm). > Read more
some images of the final result!
I did some more tests and measurements to define output power and efficiency.
Please shoot if i make any mistakes :-)
I connected 4ohm 50W resistor to output and adjusted input signal to maximum output without visual clipping on scope. I got 15V peak to peak sinus on output, input signal is 1.6V peak to peak.
To calculate power: > Read more
I built a stereo version of the and i must say i'm impressed of the sound quality! I like amp without any feedback circuits, it's sounds very direct and tight....
I don't use the 555 buffer any longer, beause I had the same timing problem as mentioned before. I changed it to BD139/BD140 totempole buffer. This is working fine on the PCB. > Read more
update with the missing link added !
update with the missing link added !
Attached the final schematic ( I think) and a PCB for a stereo version.
I made some small modifications; the current source around T2 is changed now. This should be more temparature stable.
I also add a speaker protection, speaker turn on 2 secs after power on and switches off directly after power off.
I'm planning to draw a pcb of a stereo version with protection on it.
Schematic attached! > Read more
Attached you'll find the new version, this time again with the extra 555 as buffer.
I increased the timing c to 330pF what brings the oscillation frequency back to 250kHz. (It was 360 kHz with 220pF)
This way i don't push the 555 to its freq limits.
All is working fine now, no oscillations and a good sound again. > Read more
I did some more tests; and made some changes.
The 555 buffer uses the reset pin as input. This gives some timing problems, input and output are not the same....
I changed this by using BD139/140 pair as totem pole buffer. Solved.
By doing this, the amp gives a little oscillation around 15kHz, music sounds good, but i get sometimes oscillation effect. > Read more
Version4 is ready and.... working!
Great sound, no heat at all !
I add an extra 555 used as high speed buffer to drive the charge pump and mosfets. The pwm 555 was acting strange if I use it for driver too. Probably the load was affecting proper working of it.... > Read more
Hi Frederik !
You are making at least 2 fundamental mistakes:
You are Steering the MOSFet Output-Stage with +-24Volts although the Gate-Voltage of theese Devices are only specified at +-20Volt maximun Value. You can be lucky, that the MOSFets are still working, because the +-4Volt Overvoltage hasn`t destroyed the MOSFets yet!!!! > Read more
I use 2x N chanel mosfet now.
High side fet is switched with a "charge pump" as this one needs higher gate voltage than Vcc to conduct completely.
This way, we get 24V square wave on output with 15V pwm on the gates. NE555 is no longer pushed to its Vcc limits.
I spice, it looks promising, but again I need to build it and test. > Read more
I played around in SPICE with a full bridge version.
I used 2 extra NE555's to invert the PWM signal (bottom one) and buffer (top one) This way I get enough power to drive 4 mosfets.
With 2 extra 555's timings between buffered and inverted signals are ok.
I skipped the feedback loop, to make things easier at this design stage. > Read more