Using the LM48824 integrated circuit , can be designed a very simple stereo headphone amplifier for portable devices .

Stereo Headphone Amplifier Circuit diagram


The LM48824’s Stereo Headphone Amplifier (Class G architecture ) increases audio (MP3, mobile TV, etc.) playback time with its adaptive power supply approach that enables very low supply rails, which doubles the power-efficiency compared to typical Class AB headphone amplifiers.

A high output impedance mode allows the LM48824's outputs to be driven by an external source without degrading the signal.

The project produces noise-canceling headphones which allow the background noise to be minimized while listening to music.

To keep the microphone signals in phase, the project consists of a phase switch that amplifies the background sound making it lend itself to several other interesting functions. There are three op-amp circuits that make up the electronics where each is built around one half of a dual op-amp NE5532. The op-amp is used by each circuit in a different configuration. A non-inverting pre-amp is done by the first circuit, a unity-gain phase-inverter by the second, and an inverting headphone amplifier by the third. The circuit is actually two circuits side-by-side since the noise-canceling headphone is a stereo device.

All inverted sounds played back through the headphones cancel out the original sounds, leaving nothing but silence since the noise-canceling headphone is a simple phase-inverting amplifier. For different situations, the amount of canceling can be adjusted. The circuit is relatively simple and can be easily assembled on a perfboard with two parts containing the construction of electronics and modifying a pair of headphones. The microphones are mounted on earpieces of the headphones with a dab of silicon sealant.

This has nearly unity gain combined with a large amount of local feedback. Unfortunately the output impedance of an emitter follower is dependent upon the source impedance. With a volume control, or even with different signal sources this will vary and could produce small but audible changes in sound quality. To prevent this, the output stage is driven by a cathode follower,based around an ECC82 valve (US equivalent: 12AU7).

This device, as opposed to a transistor configuration, enables the output stage to be driven with a constant value, low impedance. In other words, the signal from the low impedance point is used to drive the high impedance of the output stage, a situation which promotes low overall THD. At the modest output powers required of the circuit, the only sensible choice is a Class A circuit. In this case the much vaunted single-ended output stage is employed and that comprises of T3 and constant current source T1-T2.

Hybrid Headphone Amplifier Circuit Diagram:

The constant current is set by the Vbe voltage of T1 applied across R5 With its value of 22R, the current is set at 27 mA. T3 is used in the emitter follower mode with high input impedance and low output impedance. Indeed the main problem of using a valve at low voltages is that it’s fairly difficult to get any real current drain. In order to prevent distortion the output stage shouldn’t be allowed to load the valve. This is down to the choice of output device. A BC517 is used for T3 because of its high current gain, 30,000 at 2 mA! Since we have a low impedance output stage, the load may be capacitively coupled via C4. Some purists may baulk at the idea of using an electrolytic for this job but he fact remains that distortion generated by capacitive coupling is at least two orders of magnitude lower than transformer coupling.

The rest of the circuitry is used to condition the various voltages used by the circuit. In order to obtain a linear output the valve grid needs to be biased at half the supply voltage. This is the function of the voltage divider R4 and R2. Input signals are coupled into the circuit via C1 and R1. R1, connected between the voltage divider and V1’s grid defines the input impedance of the circuit. C1 has sufficiently large a value to ensure response down to 2 Hz. Although the circuit does a good job of rejecting line noise on its own due to the high impedance of V1’s anode and T3’s collector current, it needs a little help to obtain a silent background in the absence of signal.

The ‘help’ is in the form of the capacitance multiplier circuit built around T5. Another BC517 is used here to avoid loading of the filter comprising R7 and C5. In principle the capacitance of C5 is multiplied by the gain of T5. In practice the smooth dc applied to T5’s base appears at low impedance at its emitter. An important added advantage is that the supply voltage is applied slowly on powering up. This is of course due to the time taken to fully charge C5 via R7. No trace of hum or ripple can be seen here on the ‘scope. C2 is used to ensure stability at RF. The DC supply is also used to run the valve heater. The ECC82 has an advantage here in that its heater can be connected for operate from 12.6 V. To run it T4 is used as a series pass element.

Base voltage is obtained from the emitter of T5. T4 has very low output impedance, about 160 mR and this helps to prevent extraneous signals being picked up from the heater wiring. Connecting the transistor base to C5 also lets the valve heater warm up gently. A couple of volts only are lost across T4 and although the device runs warm it doesn’t require a heat-sink.

This schematic circuit diagram project is a very simple and very good quality Electronic Circuit of  Precision Headphone Amplifier. Designs for good-quality headphone amplifiers abound, but this one has a few special features that make it stand out from the crowd. We start with a reasonably conventional input stage in the form of a differential amplifier constructed from dual FET T2/T3. A particular point here is that in the drain of T3, where the amplified signal appears, we do not have a conventional current source or a simple resistor. T1 does indeed form a current source, but the signal is coupled out to the base of T5 not from the drain of T3 but from the source of T1. Notwithstanding the action of the current source this is a low impedance point for AC signals in the differential amplifier.  

Precision Headphone Amplifier Circuit Diagram:

Measurements show that this trick by itself results in a reduction in harmonic distortion to considerably less than –80 dB (much less than 0.01 %) at 1 kHz. T5 is connected as an emitter follower and provides a low impedance drive to the gate of T6: the gate capacitance of HEXFETs is far from negligible. IC1, a volt-age regulator configured as a current sink, is in the load of T6. The quiescent current of 62 mA (determined by R11) is suitable for  an output power of 60 mWeff into an impedance of 32 Ω, a value typical of high-quality headphones, which provides plenty of volume.

Using higher-impedance headphones, say of 300 Ω, considerably more than 100 mW can be achieved. The gain is set to a useful 21 dB (a factor of 11) by the negative feedback circuit involving R10 and R8. It is not straightforward to change the gain because of the single-sided supply: this voltage divider also affects the operating point of the amplifier. The advantage is that excellent audio quality can be achieved even using a simple unregulated mains supply.  Given the relatively low power output the power supply is considerably overspecified. Noise and hum thus remain more than 90 dB below the signal (less than 0.003 %), and the supply can also power two amplifiers for stereo operation.

The bandwidth achievable with this design is from 5 Hz to 300 kHz into 300 Ω, with an output voltage of 10 Vpp. The damping factor is greater than 800 between 100 Hz and 10 kHz. A couple of further things to note: some-what better DC stability can be achieved by replacing D1 and D2 by low-current red LEDs (connected with the right polarity!). R12 prevents a click from the discharge of C6 when headphones are plugged in after power is applied. T6 and IC1 dissipate about 1.2 W of power each as heat, and so cooling is needed. For low impedance headphones the current through IC1 should be increased. To deliver 100 mW into 8 Ω, around 160 mA is required, and R11 will need to be 7.8 Ω (use two 15 Ω resistors in parallel).

To keep heat dissipation to a reasonable level, it is recommended to reduce the power supply volt-age to around 18 V (using a transformer with two 6 V secondaries). This also means an adjustment to the operating point of the amplifier: we will need about 9V between the positive end of C6 and ground. R4 should be changed to 100 Ω, and R8 to 680 Ω. The gain will now be approximately 6 (15 dB). The final dot on the ‘i’ is to increase C7 by connecting another 4700 µF electrolytic in parallel with it, since an 8 Ω load will draw higher currents.

This is Vice Control Music Outlet circuit using SL517A, this electronic circuit project build a very easy. The circuit is shown in Figure, and it is composed of acoustic sensor, voice control IC, relay control circuit, song voice circuit and AC buck rectifier circuit.

Vice Control Music Outlet Circuit using SL517A:





Voice control IC uses SL517A which contains high-gain amplifier, bistable flip-flop and buffer output level, and it has two packages of dual in-line and black ointment. Its internal functional block diagram is shown as below.

2x300W or 600W Power audio Amplifier Circuit Diagram:




The circuit includes thermal shutdown circuitry that activates when the die temperature exceeds 150°c. CIRCUIT’s mute function, when activated, mutes the input drive signal and forces the amplifier output to a quiescent state. Maximum Output power @ 8ohms : 300watt. Absolute max power supply voltage :±38V to ±40V. Recommended power supply voltage :±30V to ±35V.

It is a very simple build a simple Audio-oscillator Circuit Diagram  Project. The circuit`s frequency of oscillation is/= 2.8/ [C1 x (R1 + R2)]. Using the values shown, the output frequency can be varied from 60 Hz to 20 kHz by rotating potentiometer R2. A portion of IC1`s output voltage is fed to its noninverting input at pin 3.

Audio Oscillator Circuit Diagram:



The voltage serves as a reference for capacitor Cl, which is connected to the noninverting input at pin 2 of the IC. That capacitor continually charges and discharges around the reference voltage, and the result is a squarewave output. Capacitor C2 decouples the output. 

Here is a very simple and easy to use audio amplifier using I.C BEL(Bharat electronics limited)1895 , a very common IC. This circuit can run on 3V to 6v , making it easy to use in pocket amplifier. Cost is under 25/-

Sooper Amplifier Using BEL1895 I.C:



 
Parts list:

BEL1895 I.C (DIP8),
C1 = 470uF/10V,
C2 = 1000uF/16V,
C3 = 220uF/10V,
C4 = 100uF/10V,
C5 = 4.7uF/10V,
C6 = 47pF,
C7,C8 = 1uF,
R1 = 47Ohm,
R2 = 470Ohm,
R3 = 100K,
R4 = 1Ohm,
R5 = 10K V/C,
speaker, etc…
Total cost is around 20-30 rupeess(INR) or 0.6USD.

Monolithic Voice Record-replay Intergarted Circuit QX-R42 can constitute a monolithic solid recorder and its sentence is input by users and played repeatedly.Please press button SA3 when you record and at the time,27 feet of the IC is in low PWL and voice signals enter storage unit via Microphnone,MIC.

Monolithic Voice Record-replay Intergarted Circuit Diagram:




Plesae loosen the button and it can play the voice after finishing recording.There are two palyback buttons.SA1 is low PWL triggering playback.SA2 caues tirggering playback by pulse falling.SA1 can be chosen and SA2 is not used when we choose playback button.When we choose to use other circuit'spulsetriggering but button and the pulse will be input IC's 24 feet and it constitute automatic playback mode.

LED is recording indicating light and it sparks during the recording process.R5 and R6 constitute automatic gain control net.R2 and C3 constitute analog signals which iuput and output coupling loop.Thus the 14th and 15th feet output playback signals and the speaker is drived to playback directly.