Lighthouse Electric Power Amplifier Project

Build This 100 Watts Solid State Power Amp

        Some Application Examples:

100W Power Amp Schematic

This amplifier circuit is a stable, proven design, implementing a quasi-complementary output stage topology. Over the years it was steady improved. This version features short-circuit protection and a Baxandall diode (D7) for improved linearity. The input of the amplifier consists of an differential voltage amplifier (Q1, Q2) with a constant current source (Q3). The BIAS transistor (Q5) is mounted on a common heat sink with the output transistors for effective temperature tracking. Short circuit and overload protection is implemented by limiting the output transistors current (Q5, Q6).

Frequency Response: 5Hz - 60kHz (-3dB)
Low Distortion THD 0.1% (1kHz at 60 Watt)
Signal to Noise Ratio S/N 90dB (shorted input)
Input Sensitivity: 775mV
Supply Voltage: +/- 34VDC 2.5A
Input Impedance: 20 kOhm

Parts List:
Resistors:							Capacitors:
R1, R10= 1k		R12= 150k				C1= 1uF *note1		C6= 100pF/50V cer		
R2= 47k			R13, R17= 330R			C2= 220uF/50V 		C7= 220pF/50V cer
R3= 4.7k			R16= 510R  			C3, C11= 22pF/50V cer	C8= 0.1uF/100V film
R4= 5.6k			R18, R21= 180R	 	 	C4, C5= 100uF/50V		C9, C10= 0.1uF/100V cer 
R5, R9=680R		R19, R20= 1.2k
R6, R15= not used		R22, R23, R28= 100R	
R7= 22k			R24, R25= 0R33R/5W	
R8, R14= 2.2k		R26= 15R/0.5W  
R11= 2.7k			TR1= 1k trim-pot

Q1, Q2, Q3, Q7= BC556 PNP Transistor		Q10, Q11: 2N3773 or TIP35C
Q4: BC639 or C2383 NPN			D1: 1N5242B, 12V 0.5W Zener
Q5, Q6: BC546 NPN Transistor			D2, D3, D4, D5: 1N4148 Diode
Q8: TIP31C NPN Transistor			D6: 1N5402, 200V 3A Diode
Q9: TIP32C PNP Transistor 			D7: UF4002, 1A fast Diode

Start PCB assembly with the smallest parts like the jumper wires, resistors and diodes. Do not install more than 4-6 resistors at a time, as it will be difficult to solder them. Cut off the ends and install the next batch. Use caution when soldering diodes or other solid state components as not to overheat them. Next install the plastic capacitors, then the electrolytic capacitors. Make sure the polarity of the electrolytic capacitors is correct. Finally install the rest of the components. Q5 transistor will be later mounted on a heat-sink, together with the output transistors. However, do not install it on the heat-sink yet. Use the supplied three lead flat cable to make the connection between the PCB and Q5. Temporary solder in a jumper into the input terminals. Carefully inspect the assembled PCB for any errors. Supplied solder contains organic flux, which can be cleaned with water. To give a nice shiny look brush the solder side under hot water stream. Finally rinse off both PCB sides under hot water. Shake off excess water and set aside to dry. To speed-up drying time, use a hair dryer.

Set-Up and Initial Testing
After the board is completely dry, make a visual inspection. Look for missed solder joints, possible solder shorts, wrong polarity of diodes, electrolytic capacitors etc. The output transistors Q10, Q11 must not be connected to the PCB yet. Next, a power supply, with minimum +/- 15 Volts and 1 Amp current will be needed. If is not available, then the designated amplifier-supply must be build first. Connect the plus and minus supply leads, in series with a 1 Amp AGC (quick blow) fuse, to 'B+' and 'B-' on the PCB respectively. Connect the zero (GND) lead to 'GND' on the PCB. Switch your multimeter to 50 or 100 VDC. Connect the COM lead to 'GND'. The trimm-pot 'TR1' must be set fully clockwise (when delivered, is already set to this position).
NOTE: TR1 trim pot does not have end positions in order to avoid damage.
Make sure the board is somewhat secured, so it wont move when tested. Now turn the power on and measure B+ and B- supply voltages, directly on the board. If one or both voltages are missing, turn off power immediately. There is a wiring error present and one or both fuses are blown. Otherwise measure the output voltage at the 'OUT' point (where R24 and R25 join). There will be some voltage between + 5V and -5V present. Now turn the trim pot slowly counter-clockwise. After a few turns, the voltage at 'OUT' should change towards zero. As soon as zero volts is reached, stop turning. The board is now pre-set. At this point, the final assembly can be made. Remove the shorting jumper from the input. Install the power-transistors on a heat-sink, large enough to dissipate the rated output power. Make the connections with 18GA or larger wire. Do not make the wires longer than nesessary to prevent any feedback or oscillations. Install Q5 transistor on the same heat-sink, between the two power-transistors. Use the supplied small clamp, and silicon paste to make a good heat transfer.

Final Calibration
This final calibration is to be made after all amplifier component's are installed in a cabinet and the project is nearly finished. Again, make a visual inspection. Make sure all power supply connections are correct, the fuse is still 1A AGC. Check the power-transistor connections. Short the input terminals. Connect your multimeter to the B+ fuse-holder terminals, positive (red) probe on the power-supply side. Switch the meter to 10 Amps DC. Turn on the power. Now, remove the fuse from the fuse holder. The multimeter should indicate less than 1A (if any) current flow, otherwise turn off power immediately. Look for wiring errors. If the test was successful, switch you meter to 0.5 or 1 Amp DC. Adjust BIAS current with the TR1 trim-pot to 0.1A (100mA). Turn CCW to increase, opposite to decrease. After 5 minutes re-adjust. Let it warm-up for 30 minutes and re-adjust again. This finalises the calibration procedure. Turn off the power and replace the fuses with correct values. The amp is ready for operation.

Power Supply
Power supply is strait forward.

Power Supply Schematic

The output power depends on the rail DC supply voltages. +/- 27 Volts will deliver 60 Watts RMS and +/- 34 Volts 100 Watts. C1, C2, C4 and C5 are the filter capacitors and C3 and C6 are the noise by-pass capacitors. R1 and R2 are bleeding resistors.

Parts List:

Up to 60 Watts
Transformer AC Voltage2 x 22VAC 2A
Rectifier Bridge BR1100V 3A
C1, C2, C4, C53300uF/35V
R1, R23.3k/0.5W
C3, C60.1uF/100V
C1, C2 (omit C4, C5)6800uF/35V
Line Fuse 3/4 A MDLRail Fuses 3 A MDL
Up to 100 Watts
Transformer AC Voltage2 x 28VAC 2.5A
Rectifier Bridge BR1200V 3A
C1, C2, C4, C54700uF/50V
R1, R24.7k/0.5W
C3, C60.1uF/100V
C1, C2 (omit C4, C5)8200uF/50V
Line Fuse 1A MDLRail Fuses 4 A MDL
NOTE: Bridge rectifier and capacitor specifications are minimum ratings. These components may be substituted with higher ratings.

Download the whole project as a PDF file here


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