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Power amplifier 65W with HEXFET

power amp 65w hexfet [circuit]

  A medium power amplifier   that is characterized by a lot of good sound quality, but simultaneously is very simple in the construction. Him uses, enough time in my active loudspeakers. In his output stage exist the very good FET transistors, technology HEXFET, transistor which are controlled by voltage and no by current as the classically bipolar transistors. The circuit has symmetrical designing, resolving thus the harmonic distortion problem. All the transistors that are used in the circuit are simple and they exist in big clearings in the market. The pairs of differential amplifiers Q1-2 and Q3-4 should be matched between them and near the one in the other. Thus you can buy enough transistors of types BC550C and BC560C, and with a multimeter you match between them creating pairs with same characteristics, ensuring thus uniform behavior in the temperature changes etc. Networks RC from the R7/C3 and R12/C4 decrease the bandwidth of differential amplifiers and power amplifier in the 6.5MHZ. Resistors R8-9-10-11 function as local feedback in the differential amplifiers improving the linearity. The differential amplifiers are supplied with constant current from him current sources Q5 and Q6. The bias of current sources becomes from the combination of diodes LED D1, D2 and R20. This becomes because the combination transistor/LED ensures big thermic stability, for this reason should they are in very near distance [1]. With the TR1 trimmer we regulate the bias current of output power stage. For this reason Q8 should find itself on the heatsink so that it ensures thermic stability in the bias, so that it does not change with the temperature changes. The resistors R32-33 shape a local feedback bronchus in the output stage, because this functions as voltage amplifier. With the TR1, R3-4, C14 we regulate the amplifier output DC offset voltage, near in the zero. The transistors Q8-10-11-12-13,  [Fig.1] should are placed on heatsink, adding between the transistors and the heatsink of good quality leaves mica and ointment. Inductor L1 is constituted by 6 coils of insulated cupreous wire of diameter 1.5mm, with internal inductor diameter of 16mm [2].


  Previous to we supply the amplifier with voltage, we regulate the trimmer TR2 in the mid of his way and the trimmer TR1 in biggest resistance. We connect a multimeter in output pin J4 (measurement range 200mV DC), we give voltage in the amplifier and with the TR2 we regulate so that we take DC voltage in the output as possible near in the zero. We remove the supply and connect a multimeter (range 1A or 2A) in line with one from the supply cables. We again give supply in the amplifier and with the TR1 regulate late the current, so that we take clue roughly 330mA. We leave the amplifier to work for 10 min., without audio signal in his input. Afterwards the 10-min. usually the current stabilized in 230mA roughly. If it needs we adjust late so that we take clue near in 230mA. We again check the amplifier DC offset [J4] output, for changes DC and if it needs we regulate again with the TR2. [Elektor12/93].

Part List

R1-15-16-21-22-17-18=1Kohm R33=68ohm 5W Q1-2=BC550C [matched]
R2=47Kohm R35-36=6.8ohm 1W Q3-4=BC560C [matched]
R3-4=10Mohm TR1=1Kohm trimmer Q5=BC560C
R5-6-13-14=1.2Kohm TR2=1Mohm trimmer Q6=BC550C
R7-12=47ohm C1=2.2uF 63-100V MKT Q7-11=BD140
R8-9-10-11=22ohm C2=1nF 100V MKT Q8-9-10=BD139
R19=82ohm C3-4=2.7nF 100V MKT Q12=IRF9540
R20=22Kohm C5=330pF ceramic or mylar Q13=IRF540
R23-26=56ohm C6-7=100uF 16V F1-2=Fuse 2.5A Fast
R24=2.2Kohm C8=1uF 100V MKT L1=see text [2]
R25=560ohm C9-11-12=100nF 100V MKT J1= 2pin connector 2.54mm step
R27-30=150ohm C10-13=470uF 63V
R28-29-31-34=15ohm C14=33nF 100V MKT Q8-10-11-12-13 on Heatsink
R32=150ohm 5W D1-2=LED RED 3mm [see text] [1] All the resistors is 1/4W 1% except quote differently



POWER RATING at 1KHZ with 0.1% THD 63W/ 8 ohm, 105W /4 ohm

IMD <0.008%

THD at 60W into 8 ohm and 1 KHZ <0.005%

THD at 60W into 8 ohm and 20 HZ....20 KHZ <0.05%

FREQUENCE RESPONSE [at 35W/8ohm, +0...-3dB] 1.5 HZ.....125 KHZ



SLOW RATE with input filter 20V/uS


DAMPING FACTOR at 8ohm [20HZ...20KHZ] 160


Power Supply


The power supply circuit, is a classic circuit. Proposed power supply for amplifier 65W 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.

Output Voltages For Power Supply

+V1=+35V -V1=-35V
+V2=+12V  -V2=-12V
+V3=+35V -V3=-35V


Part List

T1=2X25Vac 160VA //2X15Vac 20VA C5-8=1000uF 25V IC2=7912 Regulator
T2=2X25Vac 160VA C6-9=100nF 100V MKT F1=FUSE 2A slow block
BR1-3=200V 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 AMP. 65W top side


power amp. 65w bottom side

Fig.3--POWER AMP. 65W HEXFET [Bottom side] PCB


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