Here the schematic diagram of headphone amplifier which built based single MOSFET as the main amplification component. This circuit is quite simple and easy to built with minimum cost.

Circuit notes:

• The MOSFET type is IRF610.
• The above diagram design is for one channel only, just build the same circuit in one PCB to make it stereo channel.
• All resistor wattage is 0.25W except the resistor around LM317 which is 0.5W
• Connect signal ground to the power supply ground. If there is a hum, try connecting with a resistor parallel with capacitor.
• Use regulated power supply 10-12VDC

The amplifier concept is simple and follows a typical single-ended class A circuit utilizing an active constant current source (CCS) in place of a passive resistor. A CCS doubles the efficiency of the circuit over that where a passive load resistor is used, bringing it to a maximum of 25%.

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The circuit is intentionally designed using old type transistor, it is to get the harmonic distortion and to avoid the difficulty of finding good components. This 2 watt Amplifier (s) can be easily supplied by plug-in wall transformer 12V. Closing SW1 bass boost is provided but, at the same time, volume control must be increased to compensate for the loss of power at higher frequencies.

In use, R9 should be carefully adjusted to provide minimum signal for audible distortion consistent cross-over with minimal quiescent current consumption measurement, a good compromise is to set the quiescent current at about 10-15 mA.

2 Watt amplifier circuit Parts list :

P1 = 10K    Log.    Potentiometer
R1, R2 = 33K        Resistors 1/4W
R3 = 33R               Resistor 1/4W
R4 = 15K               Resistor 1/4W
R5, R6 = 1K          Resistors 1/4W
R7 = 680R            Resistor 1/4W
R8 = 120R            Resistor 1/2W
R9 = 100R            1/2W Trimmer Cermet

C1, C2 = 10μF/63V     electrolytic capacitors
C3 = 100UF                  Electrolytic Capacitor 25V
C4, C7 = CAP 470UF/25V     electrolytic capacitors
C5 = 47pF                      Ceramic Capacitor 63V
C6 = 220nF/63V         Capacitor Polyester
C8 = CAP 1000UF      Electrolytic Capacitor 25V

D1 = 1N4148                 Diode 75V 150mA

Q1 = BC560C    45V 100mA PNP Transistor low-noise, high-gain
Q2 = BC337       45V 800mA NPN Transistor
Q3 = TIP31A     Transistor NPN 60V 4A
Q4 = TIP32A    Transistor PNP 60V 4A

SW1    SPST switch

SPKR    05/03 Watt speaker, 8, 4 or 2 ohm impedance

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Power is single, despite the presence of op amp, with the polarization to VCC / 2 of their non-inverting input (virtual ground composed of R104 / R105 and R204 / R205). A small supply decoupling consists of couples R107 / R207 and C106 / C206 reduces the internal resistance of the power supply and get a better response to transistions fast. The separation of the two left and right channels is also improved slightly (less crosstalk)

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This 32 ohm Headphones Amplifier circuit built with an IC which consists of four op amps (TL074, TL084 and LM324). This amplifier circuit produces power 30 mW x 2 (1 stereo output for headphones 32 ohms) with a supply voltage + / -4.5 V (two 4.5 V batteries).

For each of the two channels, two op amp are connected in bridge amplifier, to provide headphones a voltage double that which would have been obtained with a single op amp. And in this type of wiring, which said dual voltage power quad also said, for the same load impedance. The same principle adopted by some manufacturers of car radios or amplifiers for high power while making use of reasonable voltages (more bridge circuit).

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The Stereo Headphone amplifier schematic above you can see that simplicity is really up, since it shows all the components necessary for both left and right channels. Do not expect miracles from this type of assembly, the distortion can be quite important if you push too much volume … If you are looking for a headphone amp that sounds “loud and clear” for listening to your classical music pieces in your couch, I’m sorry, this circuit will probably not meet your expectations. But ‘normal’ use, it is a good small amp with extra nice.

Output Power Table :

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Headphone amplifier based OPA134PA Input stage
The input is on the inverting terminal of the op amp U1, through resistor R1 and a part of the gain adjustment potentiometer in RV1. Input impedance is high enough to allow the retribution at each line output level of AF without fear of interference, as long as it still remains that the output impedance of the circuit in which you will not connected to 2 KOhm or 4.7 KOhm …

Power Stage
It consists of the op amp U1 supported by a pair of transistors to enhance capacity in the output current. The op amp is polarized in its non-inverting input (terminal 3) so that the signal AF revolves around a voltage equal to half the supply voltage. This environment is achieved through the presence of the two resistors R2 and R3 of the same value, which form a voltage divider bridge, we are dealing with a virtual ground. The capacitor C1 guarantee a good stability to the reference voltage. The voltage thus generated can be found at the output, and should definitely not reach the earphones of the headset. This is the reason for the presence of the coupling capacitor C2, which passes only useful AC signal.

Headphone amplifier based OPA134PA Power supply
The circuit can draw power from a 9V battery, which of course can be replaced by a combination of a larger battery (2 x 4.5 V or 6 x 1.5 V), to allow greater autonomy. You also can use a 12V lead acid battery, or a set of rechargeable Ni-Cd or Ni-Mh provide voltage between 9V and 15V. Power supply decoupling is provided by capacitor C3, to be placed closer to the supply of integrated circuit terminals (terminals 4 and 7).

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The  100mW stereo amplifier based TL 082 circuit is based on the combination of op amp and an output stage (pseudo) two push-pull transistors, capable of delivering power to feed more than comfortable headphones, even if it at an average performance.

Input of both left and right channels will be carried out through a stereo female jack if you want to embed this amp in a box reserved for this purpose. Of course, this jack will not be necessary if the integration is made with your own mixer.

The amplification of around of 10 (20dB), is determined by the ratio of resistors R2 / R1 (left channel, top) and R10 / R9 (right channel, bottom). You can always change the report if you feel the need to. The presence of knobs for adjusting the input level, however, you should avoid changing the value of these resistors, the adjustment range to suit a wide variety of situations. The presence of the capacitor C1 in series with the resistor R1 (with R9 and C5) limits the DC gain to a value of 1. In this way, any DC voltage output (offset) will be minimal and will not risk damaging the headphone. You have indeed noted the absence of coupling capacitor output …

The diodes D1 to D4 compensate the threshold base-emitter junctions of transistors, and in combination with resistors R5, R6, R13 and R14 can set the base bias to a value that guarantees operation with minimal crossover distortion.

Circuit Notes:

• The resistors R3, R4, R11 and R12 can easily Show their value rise to 10 ohms. The power dissipated in these resistors should be 0.5 W at a minimum, I recommend the same to limit the use of models 1W.
• Many may agree transistors output: a pair of 2N1711 (NPN) + 2N2905 (PNP) will do just fine, and a pair of BC547 (NPN) + BC557 (PNP) or BC337 (NPN) BC327 + (PNP).

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Parts list :
R1 : 100K
R2 : 47 ohm
R3 : 4,7K
C1 :0.47uF
C2, C3 : 470uF
VR1 : 50K
Transistor : 2N3565 and 2N3706

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Chip operates at supply voltage range 1.8-6V. THD – 0.7%. Frequency – 40-40000Hz. Resistor R1 is used to adjust the volume.

Parts list :
R1 : 50K variable resistor
C1, C2, C3, C9 : 10uF/10V
C4, C5 : 3.3uF/10V
C6, C7, C8 : 0.1uF
C10, C11, C12 : 220uF/10V
DA1 : CXA1622P

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KR1436UN1, KR1064UN2, KR1054UN1, KF1054UN1 – most Likely of “Angstrom” 34 119 – Motorola, AN7050 – Matsushita, TDA7050T, TDA7050 – Philips.

Parts list :
C1, C2 : 47uF/ 6V
C3 : 100uF/ 6V

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