[sdiy] Any idea how Behringer Eurorack volume controls work?

[Tom May]

This is sort of off topic, but if I find the answer I'll learn
something new about electronics.  I've been trying to figure out how
the volume pots on my Behringer MX802A mixer work.  See below for the
background on why I'm doing this if you're curious.  I don't have a
schematic.  Poking around with eyeballs, meter, and scope, I have
discovered the following:

1. The pot has three terminals.  So far, so good.  (For simplicity I
   was looking at a mono channel volume control, but the stereo phones
   and master volume seem to work the same way.)

2. The middle terminal is the wiper.  I say it is the wiper because
   the middle terminally usually is the wiper, and because it shorts
   to one of the other terminals when the knob is turned completely to
   one side or another.

3. The wiper is grounded.  Scope, ohm meter, and eyeball all agree on
   this.

4. There is audio signal on the bottom terminal (the one the wiper
   connects to when the volume is turned all the way down).  When the
   volume is turned all the way down, the amplitude of the signal is
   zero.  It increases as the pot is turned to unity gain (center
   detent), and increases only slightly (maybe an additional 10% or
   so) when the pot is rotated beyond unity gain.  The signal at the
   output increases about 15dB (as advertised) as the volume is
   increased from unity to maximum.

5. There is audio signal at the top terminal.  It is zero amplitude
   with the volume turned down, increases to a maximum when the volume
   is at unity, and decreases to zero again when the volume is maximum
   (and the grounded wiper has reached that side of the pot).

I just can't puzzle out how this thing is working.  Anybody have any
ideas?  About the only thing I can figure is that the behavior at the
bottom terminal is consistent with connecting it to the audio signal
through a finite impedance, but that doesn't get me very far.  Is the
audio being represented by currents?  Even if it were, I am still
clueless . . .

Background:

I got a Behringer MX802A mixer which I planned to plug my computer's
audio card into and use to send the signal either to my headphones
(using the headphone volume) or my monitors (using the main volume).
I've been doing this with a Mackie MS1202 (the original one), but I've
moved and rearranged my studio the Mackie is now being divested of
some of its many duties.  But it turns out that you only get sound
from the headphones on the 802 if the main volume is turned up as
well, which means sound comes from both headphones and monitors which
is not what I want.  It seems that the Mackie 1202VLZs may work this
way as well, although I can only wonder why.

So I was thinking I could do a little rewiring to make the phones and
main volumes on my 802 independent.  But I can't figure out how the
volume controls on it work!  

Tom.