JK Audio RemoteMix C+ - Manual (Page 7)

7
To find the absolute null for any telephone line, follow this
procedure:
1. Place a call to a quiet location.
2. Send a signal down the line at moderate levels. This can
be prerecorded music or speech, or a test signal.
3. Connect the From Phone line output to a meter or scope,
or listen to the receive signal over the headphones. Be
sure to turn up the Line Out or Headphones volume
control depending on which method you are using to
monitor the receive signal.
4. On the back of RemoteMix C+, below the words Null Adj.,
there is a hole for a small flat blade screwdriver. This is a
10-turn potentiometer to allow for very sensitive
adjustments. Try to remember the position of the blade,
and the number of turns you are making as you first
adjust clockwise, and then back to the original position,
and then counterclockwise.
The goal is to adjust the null signal until the receive signal
contains a minimum amount of transmit audio. If, on the
other hand, you want a good mix of transmit and receive
signals, you must have someone on the far end speak or
send audio down the line as you adjust for the proper
balance.
5. Factory Setting: To reset to the factory setting, set the
phone line switch to On-Hook, and adjust to find the null.
We have installed a terminating resistor in the On-Hook
position that simulates the average phone line connection
and gives the right mix of transmit and receive audio.
The best tools for this task are 1) an oscilloscope connected
to the From Phone Line output, and 2) a 1 kHz sine wave
generator connected to the To Phone Line input jack.
However, the above procedure will work with a little patience.
Null Adjust (optional)