δειτε αυτο (λιγο πριν τη μεση) απο εδω.........
http://gilmore2.chem.northwestern.edu/faqs.htm
How can I drive headphones from a power amplifier?
Small power amplifiers (20 - 40W per channel) can be configured as distribution amplifiers for driving 1 to 10 or more headphones simultaneously. The simplest setup is to place a current-limiting 100 ohm, 2W resistor at the L and R speaker outputs and then connect the headphones to the resistors (see diagram above). Add another set of 100 ohm resistors per additional headphone. Each resistor forms a voltage divider with the impedance of the headphone transducer. Thus, if the headphone has a 50 ohm impedance, the voltage across a transducer will be:
Vout = Vin(Z(phones)/(100 + Z(phones))) = 50/150(Vin) ~ .33(Vin)
Headphones will reach full volume with only a few milliwatts of input. If the volume is too high, increase the value of the resistor. When driving many headphones from one amplifier, the total impedance of the resistor array should not fall below the rated load of the amplifier. For example, two pairs of headphones with 8 ohm impedances and 100 ohm current-limiting resistors will present a load of about 50 ohms per channel to the amplifier. If the headphones are different models (e.g., they have different impedances and sensitivity specs), they will be playing at different volume levels.
The current-limiting resistors will increase the output impedance of the amplifier as seen by the headphones. Whether the higher impedance affects the sound of the headphones depends on the design of the headphones. The IEC 61938 international standard recommends that headphones should expect a 120 ohm source (5V rms max.) - regardless of the headphone's own impedance. If the manufacturer followed this standard, the 100 ohm resistor will not affect sound. See also the FAQ regarding the value of damping factor measurements for headphone amplifiers.
A slightly more elaborate scheme is to use two resistors as a voltage divider across each channel. Here, the output impedance of the amplifier is lower (essentially the resistance of the two resistors in parallel) and the output voltage is less dependent on the impedance of the headphones. Although the effective output impedance is lower, the amplifier itself sees the voltage divider as a series load (which is good). Begin with R2 = 10 ohms (1/2W) and scale R2 up or down to adjust volume. For more information about distribution amplifiers, see A Quick Guide To Headphone Accessories.
εχω κανει την version2 .