In practice, there are cases when the sound reproduction devices may not provide sufficient headphone sound volume level, connected to their output.
This may be due to insufficient sensitivity headphone applied and insufficient gain level (power) of the device. In such a situation may help to use an additional amplifier stage.
For example, this article is considered the case of an additional, intermediate amplifier to the detector, assembled by a simple circuit by only two low-power transistors.
Transistors may be used, for example, type KT315, KT3102, KT342, KT361, KT3107 or similar import. The gain can be from 100 and above. amplifier supply voltage may be 3 volts.
No special setup this amplifier does not require, but you can choose the denomination of the resistor R3 of the maximum volume and the best sound quality. From the capacitance value C2 of the capacitor depends on the volume and level of reproduction of low frequencies.
By selecting the nominal resistor R4 can achieve a minimum level of current consumption at an acceptable level, the sound volume of the headphones.
Earphones can be any, resistance to 100 ohms (possibly increasing the resistance value, but in practice this has not been tested). The current consumption of the amplifier, depending on the headphones impedance is of the order of 5... 20 mA.
The sound quality of the amplifier, of course, is not the highest. But it is acceptable for use with devices such as, for example, metal or radio.
But there are cases when, on the contrary, it is necessary to lower the level of the signal to a value sufficient for headphones. For example, to connect them to the output of a power amplifier power. You can use the scheme, similar to that shown in the following figure.
Such a scheme is used in power amplifiers to 100 watts apparatus «Technics». On resistors of voltage dividers are collected for each channel, and capacitors exclude possible excitation amplifier.
In the case of such a scheme, the headphone sound quality is determined only by their characteristics and parameters of the power amplifier.