200 Watt Hi-fi Amplifier - 200 Watts of Pure Sound - No Distortion, No Noise

• High Output Power -  200 watt  Dynamic output power .
• Very Low Distortion - 0.01% Total Harmonic Distortion at 80 watt power output using 8 ohms speakers and 1K Hz sine wave.
• Speaker usability - 4 & 8 ohms, 100 watt to 300 watt speakers can be used.
• Supply voltage -  ±18 volts to ±45 volts DC supply working range. 
• Standard components - All components used are available online or at your local electronics store. 
• No SMD components - Only through hole components used, making soldering and replacement hassle free.  
• Cost effective - This amplifier can be made in a fraction of cost compared to similar amplifiers available in market. 

• Musical instrument amplifier - Frequency response 20 Hz to 50 KHz, you can amplifiy any instrument like Guitar, Keyboard, Benjo and many more.
• Amplified speaker box - Compact size amplifier board that can be fitted inside the speaker enclosure with minimum effort.
• Subwoofer Amplifier - High power amplification of low frequency sound, stomp the dance floor with your boombox.
• PA System - Public address system sufficiently audible at medium distance or over small area like family functions, Puja or Wedding Parties.
• Stereo Amplifier - Use two 400 watt hifi amplifier boards to make stereo amplifier and listen to your favorite songs in clarity never experienced before. 

Input Stage

 Input stage works as a  transconductance amplifier ( voltage in - current out ).

This stage consist of Long Tailed Pair (LTP) made of Q1 & Q5, working in differential mode. constant current flows from Constant Current Source (CCS) made of Q2, collector current of LTP is balanced by Current Mirror (CM) made of Q3 & Q4.

In simple words input signal is amplified by the input pair Q1 & Q5, current amplified input signal is sent to VA Stage for voltage amplification.  

Voltage Amplification Stage ( VAS )

 As the name suggests all the voltage GAIN of the amplifier is achieved in VA Stage.  

VAS of this circuit is Darlington ( Beta Enhanced ) so low GAIN transistors can be used.

Signal received from input stage is amplified via Darlington pair Q6 & Q9. Constant current flows from CCS made of Q7. 

Biasing circuit is made of Q8, preset RV1 is to adjust output stage biasing voltage. 

it is important to note that the biasing transistor Q8 must be mounted on the same heatsink along with Output transistors to ensure thermal tracking. 

In simple words signal received from input stage is amplified by Q6 & Q9. voltage amplified signal is sent to output stage.

Output Stage ( OPS )

 This circuit has Class AB output stage, so each half of the signal is amplified by each pair of NPN and PNP output transistors.

Driver Transistors Q10 & Q11 work as buffer to prevent excessive loading of VAS as well as to provide input to the output transistor Q12 and Q13 respectively.

Emitter follower topology is used for output transistors. Thermal feedback via transistor Q8 and correct bias voltage adjustment via preset RV1 is necessary to prevent output transistors from destruction by thermal runaway. 

Negative feedback via R11 to the base of Q5 ensures overall amplifier performance and stability.

Zobel Network - made of R23 & C10, provides protection against high frequency oscillations.

Inductor L1 is used to reduce harm from Capacitive load of Speakers Cables.

In simple words signal received from VAS is given to the output transistors via driver transistors. output transistors conduct high current to drive the speakers in accordance with signal.