2 * 40Watt Audio amplifier

Audio Amplifier, STEREO 40+40 watt Amplifier.

My recent project did in june 2022, wanted to share some pictures of it.

Final output.


Low Noise Audio Amplifier

This circuit is built with TI sound plus op-amp OPA2134 and THD of the amplifier is less than 0.3% at 12watts RMS into 4ohms driver, very low noise we can achieve using this circuit and this circuit is powered by a single power supply which is very commonly available, Circuit built with  dual op-amp(OPA2134) which is U1. 


U1B is configured as Vref the output of this op-amp is exactly mid voltage of 20V as 20V/2 is 10V of supply to ensure to have a symmetrical and dynamic range of both input and output signals and it is configured by R8, R9 resistors and C1 buffer capacitor, value of C1 can be calculated as  C1 = 1/(2π×fc×47000/2) where fc cutoff frequency should be <= 1/10th of input frequency(fin)  signal, consider fin = 10Hz; fc=1Hz so C1= 1/(2π×1×23500) = 6.77µF, 6.8µF is rarely available, if available we can use, the commonly available value is 10uF, and the output of this opamp U1B  is fed to U1A non-inverting terminal. C4 is the input capacitor to allow  input signal, R11 is the gain setting input resistor  and R12 serves as feedback resistor so the gain of the amplifier is calculated as 20log(82k/4.7K) = 24.83dB 

The following is the scope screenshot1,  probing both input and output signals of the amplifier. 

F0004TEK - Copysine wave in blue is the input signal(740mV) and yellow the output (14.2V) inverted output

Gain = 20log(14.2/0.740) = 25.66dB which is almost equal to the calculated value of gain with resistors. 

Input capacitor C4 value  can be calculated = 1/(2π×fin×R11)  = 1/(2π×10×4700) = 3.386µF The next higher value is 4.7µF. 

C2 and C3 connected across op-amp power pins U1C are electrolytic and ceramic combinations to reduce low frequency and high frequency noise in the power supply lines and must be placed close to the op-amp power pins. Main filter capacitors are C5 and C6 to reduce noise in the supply lines; the ESR(Equivalent Series Resistance) of these capacitors must be low, for better noise reduction. R4 protects the op-amp output pin by not over driving the current it is less than 6mA and drives the Q2 transistor of Voltage amplification stage. 

 R13 is the current source,R1 , RV1 and  Q4  forms  Vbe multiplier also called as adjustable diode and its purpose  to reduce the cross-over distortion by providing the bias voltage for the Voltage Amplification Stage (VAS)formed by  complementary transistors Q1,  Q2 and drop across R3 , the voltage Vce of Q4 is Vadj, this can be adjusted by varying RV1 setting it to 472

Voltage acrossR1 = 1.3mA×1000 = 1.3V and the 

Voltage drop across RV1 = 1.3mA×472=0.6136

Vadj can be  calculated by  VRV1×((R1+RV1)/RV1)) = 0.6136×((1000+472)/472)=  1.9136volts

The Vbe multiplier also acts as thermal feedback unit as Q3 and Q5 get hot during operation, Q4 senses the heat and controls the collector current  by reducing base of Q1 and Q2.  Q4 must be placed/slabbed to the heatsink or  closely to the Q3 and Q5, Vbe multiplier plays a critical role in an amplifier to make the circuit thermally stable. Emitter current of Q1 is limited by R1 and Collector current of Q2 limited by R5,  Q3 and Q5 are the output transistors using both NPN transistors; this configuration is also called the Quasi Complementary output stage, R6 and R7 are emitter resistors which are 0.22 to prevent the transistors from thermal runaway. C7 is the output coupling capacitor which is 2200uF, 3300uF would do better if you need deep bass. From the loudspeaker, R10 and C8 form a low impedance path for bypassing the high frequency noise. The impedance of the network R10 and C8  is lower at higher  frequency and higher at lower frequency components

Prototype of amplifier:




Assembly instructions: 

  1. Q3, Q4 and Q5 must be attached on the same heatsink, thermal insulators(mica) should be used for Q3 and Q5 to avoid short circuit of Power transistor Collector terminals of each,  as center terminal is attached to main tab/body inside of transistor 
  2. Adjust RV1 to 472


Output Impedance:  0.039

output :  12watts at 0.28% into 4 driver(loud speaker)

Capable of driving loudspeakers of impedance range 2-16

F0002TEKpngFFT response of the amplifier  we see only fundamental and delivering  11watts RMS into 4.7E power resistor

Note: If you want STEREO for the other channel just duplicate the same, I have configured mine in mono channel by adding two 100E resistors at the input. Schematic posted here is a single channel.



  1. U1A, U1B and U1C  OPA2134 (Texas Instruments)
  2. Q1  BD139 (ST microelectronics) 
  3. Q2  BD140 (ST microelectronics)
  4. Q3, Q5 TIP3055G (ST microelectronics) 
  5. Q4 ZTX450 (Diodes Incorporated)


  1. R1 1K  1/4watt
  2. R3 , R5 330  1/2watt
  3. R4 100 1/4watt
  4. R6 , R7 0.22, 2watt 
  5. R8 , R9 47K 1/4watt
  6. R10 10 , 1watt
  7. R11 4.7K 1/4watt
  8. R12 82K 1/4watt
  9. R13 1.8K 1/4watt
  10.  RV1 1K, trim potentiometer
  11. C1, C2  10uF, 50V electrolytic
  12. C3, C9 1uF 50V ceramic
  13. C4 4.7uF, 50V ceramic
  14. C5, C8 100nF 50V ceramic
  15. C6 220uF, 50V electrolytic 
  16. C7 2200uF   electrolytic
  17. C10 47pF 50V ceramic


  1. Heatsink 
  2. Thermal insulators or mica  with screw fittings
  3. LS1 loudspeaker 4, 20watt rated. 

Equipment used to validate the amplifier circuit: 

  1. Function generator AFG1022
  2. Scope TBS1072
  3. 20V, 3A  adapter  
  4. Fluke 115 DMM
  5. Power resistors 3.33 12watt and 4.725watt

I will be very happy if you want to make this circuit. If you do, send me the pictures of your circuit, would like to see. If you feel any trouble in building  this, let me know. Happy building.

Acoustic Guitar Preamp

I like Guitar very much and taking classes and I got new yamaha FSX315C Electro-Acoustic Guitar, it comes with electronics and audio output of 1Kohm impedeance, So I thought I could make one small preamp with OPA2134 JFET opamp, so that we can use it with a normal power amplifier.

acoustic guitar preamp

acoustic guitar preamp pic2

I soldered 3.5mm female stereo sockets on both sides of amp as INPUT and OUTPUT so I had to use 1/4inch female to 3.5mm male connector. I have connected the preamp circuit to my PC and recorded few tracks I have played using audacity software.
audio track1:
   I hope you like it.
I will add few things to the circuit like Equalizer thing and post the schematic. Thank You


Yes, it somewhat looks a spider with 6 legs (:P) and I believe it scares the real spiders and even bugs LOL.

Update: 17-01-2018

spider pic 003

design drawing, yes it’s not perfect and please excuse for this.  spider idea

spider pic cons1

A small hexapod with copper legs and HS-311 actuators/servo’s, I have used 3mm thick copper wire for legs, I had no better option at the of making So I did go with the things I have and a sheild wire, copper wire and paper tape to attach the 3 servo’s

spider pic cons2

With bending tools I was able to mould legs like and fix them to the motors.spider cons4

spider pic cons3

Yes It may not look good.  Soon the spider hits the floor and the spider is still in progress.


  1. power Source: 3.7V,2600mAh
  2. CPU: ATmega48 clocked 1.536MHz
  3. Actuators/servos: HS311
  4. Sensors: yet to be added
  5. Programming: AVR C
  6. Environments: indoor

and many thanks to the Sir protowrxs.

Little update: 17th january of 2018

Modifed the legs style looks perfect and walking is also good.

spider pic 006

spider pic 003

spider pic 004

spider pic 005

spider walking. 😛

I will add sensors to it and will update as soon as possible. Thank you all.



Class A MOSFET amplifier

I always enjoy building audio amp’s particularly Class A topology.

PCB’s of class A MOSFET amp


pic c a .png

pic c a 2.png

pic c a 1.png

PCB’s of class A MOSFET amp.

PCB class A .jpg

assembled class A amp board.jpg


irfp250n mosfet.JPG


power amp ca .JPG


I used two MOSFET’s  (IRFP250N) one is configured as current source and the other one is to amplify the signal, for the input stage I have used JFET 2SK209 provides high input impedance and the

output impedance of the amplifier is 2.6ohm’s

output of the amp is 5watts at 3% THD and each MOSFET dissipates 9.6watts of power efficiency of the amp is 26.04%


12AX7 Amplifier

I always wanted to build vacuum tube or valve amplifier. finally I made one hybrid amp.  just combined both valve and solid state.  For the preamp section I have used 12ax7(dual triode) and for the solid state I built opamp based power amp configured in unity gain..

This is the low power version and I’m sure I am going to build medium power amp delivers atleast 5-10watts to the speaker and no solid state 😀

Here is the picture of my construction:



Audio Exciter amplifier

Audio exciter:

Exciter is also known as bodyshaker, Audio Exciter is  just basically a loud speaker without a membrane, and it consists of voice  coil , oscillating mass and mounting ring/plate.  If the audio/music signal is applied to the contact pins of the voice coil, the oscillating mass starts shaking with the frequency of the applied music, the oscillation is transmitted to the mounting ring and from there on  to the surface the exciter sticks to, thus the excited surface emits the music signal.

audio exciter

audio exciter2

For testing the audio exciter I did built a small opamp based amplifier which delivers 2Watts rms at 6ohms impedance amplifier exciter

This amplifier circuit is nothing new. and you can find a better one here  (discrete transistor amp)