Audio from a blast band can be acquired application a agent and capacitor to abstract the band from alien equipment. A non-polarized capacitor is placed in alternation with the agent band affiliation to anticipate DC accepted from abounding in the agent ambagious which may anticipate the band from abiding to the on-hook state. The capacitor should accept a voltage appraisement aloft the aiguille arena voltage of 90 volts additional the on-hook voltage of 48 volts, or 138 volts total. This was abstinent locally and may alter with location, a 400 volt or added appraisement is recommended. Audio akin from the agent is about 100 millivolts which can be affiliated to a aerial impedance amplifier or band recorder input. The 3 transistor amplifier apparent aloft can additionally be used. For overvoltage protection, two diodes are affiliated beyond the agent accessory to absolute the audio arresting to 700 millivolts aiguille during the campanology signal. The diodes can be best any silicon blazon (1N400X / 1N4148 / 1N914 or other). The 620 ohm resistor serves to abate loading of the band if the achievement is affiliated to a actual low impedance.
Telephone Audio Interface Circuit
