Integrated headphone amplifier (Class A)

It is a very useful thing , to have a small headphone amplifier , capable of driving a couple of pairs phones. Fortunately, the headphone amp has a much easier job to do, in that neither the output power requirements nor the load characteristics are so severe, since headphones typically have a load, impedance , higher of 50 ohm, (typical 600 ohm), and only require 1-2V RMS. max, for normal output. Since only a low power output is required, a Class A stage, is perfectly feasible. For adequate Class A operation the output transistors Q5-6, should pass say 100mA each. With ?15V supply this would mean 1.5Watts dissipation, so a smallish Heatsink, will be needed for each. With RV1 adjust input audio signal and Tr1 adjust the output DC Offset voltage for 0V (?30mV). For stereo application , you need two unit, of this amp. by L. Hood

R1-4= 1.2Kohm R12= 4.7 ohm C8-10= 470uF 25V

R2-3= 3.9Kohm RV1= 10Kohm Log. pot. C9-11= 100nF 100V ceramic

R5-10= 10Kohm TR1= 10Kohm Trimmer Q1-3= BC184

R6=100Kohm C1= 4.7uF 63V MKT Q2-4= BC214

R7-9= 2.2Kohm C2-7= 100uF 25V Q5= BD136 or BD538

R8= 150 ohm C3-5= 100nF 100V MKT Q6= BD135 or BD537

R11-13= 6R8 ohm C4-6= 100uF 16V All resistors is 1/4W 1%

Audio amplifier 40W.

Audio amplifier BTL (Stereo)

Audio amplifier BTL (MONO)

Integrated microphone mixer

R1, R2, R3 1K 1/4W Resistor

R4, R5, R6 10K Logarithmic Pot

R7 1Meg 1/4W Resistor

R8, R10 10K 1/4W Resistor

R9, R11 100K Linear Pot

C1, C2, C5, C6 0.1uF Ceramic Disc Capacitor

C3, C4 22nF Ceramic Disc Capacitor

U1, U2 741 Op Amp

J1, J2, J3 Input Jacks Of Your Choice

Audio amplifier 400W (Mosfet)

Audio amplifier 400W (Mosfet)

Audio amplifier 300W (Bipolar)

Audio amplifier 300W (Bipolar)

Amplifier circuit 250 mW.

High temperature warning circuit.

R1= 820 ohm D1= 5.6V 0.5W Zener K1= 6V 200 ohm Relay

R2-3= 1Kohm D2-3= 1N4148 BZ1= Buzzer

C1= 220uF 16V Q1-2= BC550C S1= 1x2 Switch

TR1= 2.2Kohm Trimmer TH1= Thermistor 10Kohm at 25 องศา BATT= Battery 9V or external supply

Delay relay circuit open.

After a time delay determined by the RC time constant values of C1 and R2, this voltage rises to the threshold value of the CMOS inverter gate. The gate's output then falls toward zero volts and drives Q1 and relay RY 'ON'. The relay then remains on until power is removed from the circuit. When that occurs, capacitor C1 discharges rapidly through diode D1 and R1, completing the sequence.

The time delay can be controlled by different values for C1 and R2. The delay is approximately 0.5 seconds for every µF as value for C1. The delay can further be made variable by replacing R2 with a fixed and a variable resistor equal to that of the value of R2. Taken the value for R2 of 680K, it would be a combination of 180K for the fixed resistor in series with a 500K variable trim pot. The fixed resistor is necessary.