For reasons which would take too long to explain, I'm planning to show
a film to some friends in a large tent on a camp site.  The image will
be projected onto the wall of a tent, but I have to be very careful
about noise levels on this camp site.  I decided to offer each guest a
stereo headphone jack that they can plug headphones into.

The challange is therefore to know how to wire up a system that can
potentially drive 20 sets of headphones.

Can I simply wire them all up in parallel?  Or possibly in serial (the
sockets in question have suitable wiring to pass the signal through if
there is nothing plugged in)?  I know the basic principles of
electronics, but audio stuff has always eluded me, as it always seems
to work in a different world than the simple calculations of volts,
amps and ohms that I learnt at school.

I'm sure you're likely to want to know what will be producing the
sound output.  At the moment, I don't know.  I will be playing a DVD
and I'll be on a camp site, so the ideal options are portable DVD
playing devices or laptops.  If I look for something with more power,
I can probbly find an amplifier of some variety that can run from a
car battery.

How about going wireless. One transmitter and as many headsets as you want.

Determine the impedance of your headphones and of the audio amplifier
that will be driving them. If you hook all the headphones in parallel,
their combined impedance will be 1/20 of the impedance of each one.
For example, if the headphones have a 300 Ohm impedance, in parallel
they will represent a load of 15 Ohm's. If your amplifier is rated for
15 Ohm's or less, you will be OK. If not, you could always use a
series/parallel arrangement and hope that by turning the volume up,
you'll get enough amplitude. For this example, if you build two
parallel banks of 10 headphones and put them in series, the impedance
would be (300/10)*2 = 60 Ohms (4 times the parallel arrangement). If
you do this, you'll need to be careful to get everything balanced. I
would avoid series/parallel if you don't have 20 identical headphones,
with all of them plugged in.

If you don't have enough power to drive all 20 to sufficient volume,
you'll need to add one or more additional amplifiers. Hook some
headphones in parallel with the output of each amplifier and hook the
inputs of the amplifiers in parallel with the output of your music
source.

I hope this helps.

If you want to plug headphones in and out, series connection won't be possible. 

There's a 'workd example' using a 20 watt PA amplifier (which would be
a good choice for you as they are available using 12V power) and 6
headphones at

http://groups.google.com/group/alt.audio.pro.live-sound/msg/9619046e1b64f972?dmode=source

or search google groups for alt.audio.pro.live-sound  "driving
multiple headphones" for the thread.

Owain

For a ready solution, there are 3- and 5- output headphone amplifiers.