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Ron Natalie wrote: No, I'm saying that unless you know the characturistics of the last amplifier stage you don't have a clue how it's going to perform when you feed it into a different impedence than what it is spec'd for. But you can make a pretty good guess... A lot of headphone amps used in intercoms and audio panels use an off-the-self IC audio amp to drive the headphone(s). This this application, the amp is running "closed loop" (i.e. its gain is determined by putting some resistors around it). This makes it have low output impedance (i.e. it looks like a voltage source). Practically, the loudness you hear in your headset isn't effected by having additional headsets plugged in. Put another way; the voltage out of the amp is independant of how many headsets are plugged in (up to 4 or 6). This would make you think that it could drive an unlimited number of 150 Ohm aviation headsets or even an 8 Ohm speaker! The fly in the ointment is that the poor little audio amp is limited by how much current it can deliver without "clipping" or "distorting". The amp is selected to be able to drive up to 6 150 Ohm headsets. If you try to plug in a "low-impedance" load like a 4 to 8 Ohm home HiFi headset in parallel with your aviation headsets, then two things happen. First, the amp distorts like mad due to its inability to deliver the peak currents required by the low impedance load. Second, the power delivered to the low impedance phones would blow the eardrums of the wearer, while the aircraft headset wearers would complaining about low levels and distortion. Now, back to the original poster's problem: Hooking the headphone bus to the line input of a CamCorder. Since the audio line input impedance of a CamCorder is likely to be in excess of 1K Ohm, and since the headphone amp is already capable of driving 37 Ohms (4ea 150 Ohms headsets in parallel), then adding the CamCorder will have negligable impact. Any patch cord that has the appropriate plugs will work. MikeM Skylane '1mm Pacer '00z |
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