video-cam adaptor

I have the PS Engineering PM 1000 II, the specs say the headphone
impedance range is 150-1000 ohms.

Craig Prouse wrote:
Newps wrote:


wrote:


I don't know of an off-the-shelf adapter, but it needs to be an
impedance adapter. Headsets are typically 150-300 Ohm, with voltages
running upwards of 10V. Portable devices like cd players and other
line-in critters use a much lower voltage, and expect a lower source
impedance. If you truly want to go into the mic port, it'd have to accept
and modulate the bias voltage coming from the video-cam as well, I'd
expect.

If it were me, I'd build one... but need to do more research
first.

If you have a Sony with a mic in a simple cord from Radio Shack will
suffice.


Unless you have an intercom that can't drive an 8 ohm load, in which case
you'll get nothing on the tape and nothing in your headset. Or a fire in
your intercom. Your intercom sees that Radio Shack cord as almost a short
circuit.

Craig Prouse wrote:
Newps wrote:


I have the PS Engineering PM 1000 II, the specs say the headphone
impedance range is 150-1000 ohms.


That's right. When you use that Radio Shack adapter cable, you're plugging
an eight-ohm load into your headphone jack. Eight is not between 150 and
1000. Consequently you're asking your amp to drive nearly 20 times its
rated output on that jack.

I don't know what the load is. The cable isn't relavant, it's the mic
jack off the camera that matters. And looking thru the camera manual it
does not list the load for that jack. In any event I don't care. The
audio laid on the tape, both intercom and radio transmissions, are
perfect and at the proper volume. If I turn my walkman type AM/FM radio
that I have velcroed to the window and fed into the intercom that really
sounds good too.

"Newps" wrote:

I have the PS Engineering PM 1000 II, the specs say the headphone
impedance range is 150-1000 ohms.

That's right. When you use that Radio Shack adapter cable, you're plugging
an eight-ohm load into your headphone jack. Eight is not between 150 and
1000. Consequently you're asking your amp to drive nearly 20 times its
rated output on that jack.

I don't know what the load is. The cable isn't relavant, it's the mic
jack off the camera that matters. And looking thru the camera manual it
does not list the load for that jack.

The cable is totally relevant. It determines your load.
Radio Shack provides the spec for the cable:

http://support.radioshack.com/suppor...oc33/33094.htm

It places a 10 ohm resistor directly across the plug contacts, in parallel
with the voltage divider (attenuator). This is to provide the proper load
to consumer audio devices which are designed to drive eight ohm speakers and
headsets. When you put a low impedance in parallel with a much larger
impedance, the lower impedance prevails and you can essentially ignore the
large impedance; the effective impedance of the circuit will be just
slightly less than the smaller parallel impedance. The impedance of the
camera's mic input basically doesn't matter at all. All the amp sees is
that tiny little 10 ohm resistor.

Another concern would be the amount of power dissipated in that 10 ohm
resistor. The intercom can put 70 mW into a 150 ohm load. If you
extrapolate, then that 10 ohm resistor ought to be rated for about a watt or
so, and would therefore be somewhat physically large. The actual resistor
in that patch cord is probably rated no more than a quarter of a watt.


In any event I don't care.

Your personal experience notwithstanding, it's poor advice to recommend this
product to others for this application. EE101 says that something is likely
to get hot.

On Wed, 16 Jul 2003 18:59:38 GMT, Newps wrote:
The testing did involve a flight (I don't need much of an excuse to go flying,
and testing the camera stand etc. was ample excuse).

Who gives a rats ass if the compass is affected? We're talking VFR flight.

Erm, it occasionally comes in handy when the goal is to do a coast-to
coast flight across the USA using a map, clock and compass!

--
Dylan Smith, Castletown, Isle of Man
Flying: http://www.dylansmith.net
Frontier Elite Universe: http://www.alioth.net
"Maintain thine airspeed, lest the ground come up and smite thee"

"Craig Prouse" wrote in message ...

That's right. When you use that Radio Shack adapter cable, you're plugging
an eight-ohm load into your headphone jack. Eight is not between 150 and
1000. Consequently you're asking your amp to drive nearly 20 times its
rated output on that jack.


No, no, no. Power transfer is NOT directly proportional to impedence.

OK... I can't take this crap anymore. He was right saying that
power is NOT proportional to impedance, NOR is it *inversely*
proportional. If the voltage source has a zero output impedance, then it
would go with 1/Z. HOWEVER, the output stages on these doodads are
typically impedance protected so a short circuit won't blow them out. In
other words, the perfect source that drives them has a series matching
load (probably of 100 Ohms or so). That means you'll get the most power
out of it at 100 Ohms total impedance. If you hook up an 8 Ohm load, you
will get much less power output than your factor of 20. Might get a
factor or 2 or 3, but at the expense of killing everything else on the
same bus expecting high-Z. Also, a typical walkman-style headphone tends
to have a higher than 8 Ohm impedance. Often it's 16 or 32. In any
event, you don't need much power into it, so a voltage divider to cut
down the voltage while not swamping the output should work fine. Look at
my schematic from a few days ago... should work.

-Cory


Craig Prouse wrote:
: "Ron Natalie" wrote:

: That's right. When you use that Radio Shack adapter cable, you're plugging
: an eight-ohm load into your headphone jack. Eight is not between 150 and
: 1000. Consequently you're asking your amp to drive nearly 20 times its
: rated output on that jack.

: No, no, no. Power transfer is NOT directly proportional to impedence.

: Of course it's not. I said it's inversely proportional.

: Now do you want to argue the wisdom of sticking a hairpin in a light socket
: or do you want to argue my precise derivation of the number 20?


--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

"Craig Prouse" wrote in message ...
"Ron Natalie" wrote:

That's right. When you use that Radio Shack adapter cable, you're plugging
an eight-ohm load into your headphone jack. Eight is not between 150 and
1000. Consequently you're asking your amp to drive nearly 20 times its
rated output on that jack.

No, no, no. Power transfer is NOT directly proportional to impedence.

Of course it's not. I said it's inversely proportional.

Now do you want to argue the wisdom of sticking a hairpin in a light socket
or do you want to argue my precise derivation of the number 20?

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.

That's what I was driving at.

-Cory

: 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.



--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

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