Power Amplifier 45W with HEXFET

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A ideal solution for the make a good, low cost power amplifier. It’s a ideal solution for the creation a system of home cinema. The preamplifier and the driver support in a operational amplifier [IC1]. The voltage fall in resistors R5 and TR2/R6, drive the output FET's gates and is proportional with the input signal level. Transistors Q1-2 function as voltage stabilizers in the supply lines, but ensure also the essential voltage fall, because the IC1 it should not they are supplied with voltage bigger ± 18V. The consumption of all types that can be used in the place of IC1, are 2 mA, this it means voltage fall in terminal the R5 and TR2/R6 equal with the 2.4 until 2.6Volts. The current regulation it become from transistors Q3-4 in Wilson connection and the current adjustment of bias it becomes from the multiturn TR1 trimmer. The existence of trimmer TR2 in line with the R5 helps in the voltage minimization of DC offset in the amplifier output and the initially value is regulated front works the amplifier so the total resistance of TR2 and R6 is equal with the R5. TR2 can be suppressed provided that does not exist big difference in the minimization the DC offset and be replaced the R6 value with the R5 value. The Q5….8, is in Drain follower connection, in provision Push-Pull, acquiring the possibility of drive up to supply voltage, with result the refund is until 75%. For thermic stability of amplifier essential is we place the Q4 near in the Q5-6 and above in the heatsink as there is the essential thermic feedback, ensuring thus the bias stability. This happens because for small current, the thermic factor in the FET is positive Thus with increase of temperature in the Q5-6 is caused corresponding temperature increase in the Q4. Then the Vbe of Q4 is decreased, thus the current source ‘’pull ‘' less current through the R5, so that is decreased the Vgs the Q5-6. Thus the bias current comes back in his initial value and remains certain. In the bigger operation current of amplifier the thermic factor of FET, is negative. Above in amplifier PCB and near in the output stage, are found the capacitors C8-9 that they help in the benefit of current in short transient. The bias current is regulated so that it’s roughly 200mA and her regulation it becomes as follows: 1 ] In the place of each Fuse F1-2, we place resistor 10 ohms 1W. 2] Supply the amplifier. If the resistors not burned or overheating, then regulating him TR1 so that we take voltage fall, across the resistors equal with 2V. We leave the amplifier to work for half hour roughly. We again measure the voltage fall and if it needs we regulate again the TR1. If all go well then we remove the resistors and we place Fuses F1-2. 3 ] We adjust TR2 so that we take in the amplifier exit [ J2-2 ], without input signal, DC voltage ± 30mV (as smaller, so much better). In the place of IC1 they can is placed one of their LF411, LF351, AD711 opamp.

Part List

R1=47Kohm C1-2-6-7=100nF 100V MKT Q5-6=IRF9540
R2-12=1Kohm C3-4=22uF 25V Q7-8=IRF540
R3-4=3.3Kohm C5=220pF styroflex TR1=5Kohm trimmer multiturn
R5=1.2Kohm C8-9=4700uF 63V TR2=1Kohm trimmer multiturn
R6=[1.2Kohm] 820ohm *See text C10=1uF 100V MKT F1-2=fuse 3A
R7=270ohm D1-2=15V 0.5W zener J1=2pin connector 2.54mm step
R8=220ohm IC1=LF411orAD711or LF351 J2=5pin connector 5mm step
R9=27Kohm Q1=BC550C
R10-11=22Kohm Q2=BC560C All Resistors 1/4W 1% metal film
R13-14-15-16=150ohm Q3-4=BC547B

Power Supply

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The power supply circuit, is a classic circuit. Proposed power supply for amplifier 45W HEXFET is what appears in the above form. It has separated supply for the various supply stages, for power output stage, for control stage, for preamplifier supply and for protection stage. Whoever wants it can suppress department or add a power supply for each channel, separating completely the channels between them. For whoever it does not use the preamplifier circuit and protection can suppress proportional coil. In his input exist the   soft starting circuit, that combine with the delay circuit and protection loudspeakers from DC.

Supply Output Voltages
+V1=+32V -V1=-32V
+V2=+12V -V2=-12V
+V3=+32V -V3=-32V

Power Supply Part List

T1=2X22Vac 3A //2X15Vac 0.5A C5-8=1000uF 25V IC2=7912 Regulator
T2=2X22Vac 3A C6-9=100nF 100V MKT F1=FUSE 2A slow block
BR1-3=400V 35A Bridge Rectifier C7-10=10uF 25V S1=2X2 switch 10A
BR2=4 X 1N4002 C15=33nF 630V Class X2 JF1=3 pole male plug
C1...4-11...14=10000uF 63V IC1=7812 Regulator


Power [rms] 45W in 8 ohm
75W in 4 ohm
T.H.D 0.2% in 40W
Frequency Response 5-20KHZ   ±1dB
Input Impedanse 47K

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Sam Electronic Circuits 6/02