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