This amplifier is intended to be used in conjunction with the popular Sennheiser stereo headphones (HD 414, HD 424). These headphones have a 2 k impedance. According to the factory specifications, a voltage of 1.41 V rms is needed for a sound press- ure of I 02 dB. (That is 4 Vp _ p, or I mW into 2 k). To meet the needs of the hard of hearing, an output of at least 40 V p _ p can be reaclied with this design, so that a maximum sound pressure of about 122 dB is obtainable. This is not recommended, however ....
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