The amplifier is designed for use with stereo headphones. This audio amp is economical, can be operated from a single low-voltage power supply and at the same time has good technical characteristics. Total harmonic distortion of output signal span at 300 mV (peak to peak) at a frequency of 1 kHz is 0.079% (-62 dB with noise).
Stereo channels are input via the separating nonpolar capacitors C1 and C2. Voltage divider resistors R1-R2 and R3-R4 at the noninverting input circuits are chosen so that the outputs of op amp present voltage (1.5 V) equal to half the supply. The gain of each channel — 1.5, depending on the feedback resistors R7 and R8 (the greater the resistance, the greater the gain), but the audio amp for high gain is not necessary to choose because of the emergence of distortions. Moreover, the power supply — low voltage.
Capacitors C4 and the SOC, as dividing the DC output UMZCH constitute a load (headphones impedance 32 ohms) highpass filter (HPF) with a cutoff frequency of 5 Hz, which provides a linear response over the entire range of sound frequencies (20 .. .20000 Hz).
Do not be afraid of the fractional value of the resistor indicated in the figure. Accurate (± 1%) fixed resistors have in place is not necessary. Resistors values shown in the diagram, you can choose from a number of resistors with tolerances of ± 5 … 10% or use a combination including two resistors (parallel or sequential).
If such denominations to choose all the same is not possible, you can install dividers instead of R1-R2 and R3-R4 trimmer
100 … 200 kW (one per channel). The engines of these resistors are connected to the inputs DA1, extreme conclusions — Power and GND.
The amplifier is assembled on the circuit board of the one-sided fiberglass foil thickness of 1.0 … 1.5 mm razmerami40x35 mm. For the manufacture of the board can be used double-sided glass fiber. In this case the foil from the parts serves as a common conductor, and apertures for passage through the output board parts not connected thereto, drill Zenk 02.5 … 3 mm.
Dimensions of the board can be reduced and transfer assembly on one side of it, if you use the chip components. Most dimensional parts (oxide capacitors) can be taken from the board and place nearby. To improve reliability, it is desirable to use electrolytic capacitors with low leakage (such capacitors specially marked letters LL), or the calculated maximum operating temperature (105 ° C), as well as increased operating voltage (16 or 25 V). Instead AD822 as DA1 can use AD823.
If you check the adjustment of the amplifier assembly, short inputs to common, and immediately turn on the power control voltage at the output DA1 (must be equal to half the supply). If the board sealed trimmer, then setting their engines to center position, turn on the power and rotating axis trimmers, set at the terminals 1 and 7 DA1 voltage equal to half of the power supply (1.5 V). In the selection of offsets at the output op amp is better to include series with Trimmer restrictive constant (supply-side) resistance 2 … 5 ohms to avoid failure of the OC in the extreme positions of sliders trimmers. When serviceable parts audio amp additional configuration is required.
To get rid of bulky capacitors NW and C4 and simultaneously increase the output power of the amplifier described, you can build a bridge UMZCH. Of course, then UMZCH becomes monaural, and stereovarianta need two amplifiers. The load (stereo) BF1 have to share (four-wire connection is made, the overall lead in the headphones is excluded), and the right to phase. Each of the headphone is switched between the outputs (terminals 1 and 7) DA1 directly without coupling capacitors. At the entrance of one of the paired UMZCH to include a phase shifter 180 ° (actually inverter).
Phasing headphones can be checked at the hearing, temporarily enable both stereo inputs simultaneously and fed them a signal, for example, from the player. With proper phasing of the signal will be rich with all the nuances, if not become «dull» and even unpleasant.