400 Watt Hi-fi Mono Amplifier Board - Description, installation and Specs

This High power amplifier is liked by amateurs as well as professionals.

We revised the pcb 4 times so far based on customer feedback and our own testing.

Smaller in size and with metal tab connectors to support high currents.

New version PCB having space for Fitting Heatsinks Easily.

• High Power -  400 watt Dynamic output power .
• Very Low Distortion - 0.01% Total Harmonic Distortion at 120 watt output 1KHz sine wave .
• Speaker usability - 4 & 8 ohms, 100 watt to 600 watt Speakers can be used.
• Supply voltage -  ±18 volts to ±45 volts DC supply working range.
• Protection - Fuses on each supply rail for preventing burnout due to fault or short circuit. 
• Standard Components - All components used are available online or at your local electronics store.
•  No SMD components used - only through hole components used, making soldering and replacement hassle free. 
• Low cost  - 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 amplify any instrument like Guitar, Keyboard, Banjo.
• Amplified speaker box - Compact size board that can be fitted inside the speaker enclosure.
• Subwoofer Amplifier - High power amplification of low frequency sound.
•  PA System - Public address system sufficiently audible at medium distance or over small area like family functions, Puja or Wedding Parties.
• Stereo Amplifier - 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 as differential amplifier. 

Input signal received at the base of Q1 is amplified and sent to VAS from collector. Negative feedback (NFB) received at the base of Q5 from output determines overall gain and balance of the amplifier.

Constant current is supplied to the LTP by Constant Current Source (CCS) made of Q2.

Collector currents of LTP transistors is balanced by Current Mirror (CM) made of Q3 & Q4.

In simple words input signal is amplified by the input pair Q1 & Q5 and sent to VAS for further amplification.

Voltage Amplification Stage (VAS)

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

The VA stage is Darlington (Beta Enhanced) which has better performance then single transistor VA stage.

Signal received from input stage at the base of Q6 is amplified via Darlington pair Q6 & Q9. Voltage amplified signal sent to the base of driver transistor Q11 from the Collector of Q9.

Constant current is supplied to VAS from CCS made of transistor Q7.

Biasing circuit is made of transistor Q8, which works as voltage divider. With preset RV1, base current of output transistors can be precisely adjusted. 

Important to note that the biasing transistor Q8 must be fitted on the same heatsink along with output transistors to ensure thermal tracking and feedback. 

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 buffers to prevent excessive loading of VAS and also provide input to the output transistor pairs of Q12, Q14 and Q13, Q15 respectively.

Emitter follower topology is used for output transistors which ensures minimum Crossover distortion. 

Thermal feedback via transistor Q8 is necessary to prevent output transistors from destruction by thermal runaway. 

Zobel Network - made of R30 & 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 load.