The receive side of RemoteMix C+ has a hidden fine tuning
control which is adjusted by inserting a screwdriver into the
backoftheunitjustbelowthewordsNullAdj.andturningleft
or right. This is a ten-turn potentiometer. This adjustment
controls the amount of transmit audio that appears on the
receive audio jacks. If you would like to adjust the mix of
transmit and receive audio on these jacks, you can fine tune
using this control. The null circuit matches the
characteristics of RemoteMix C+ with the characteristics of
thetelephoneline. Althoughmuchofthetransmit audiocan
beremoved,thisis not the equivalentof amix-minus output.
The amount of transmit/receive isolation will depend on the
qualityofthetelephoneline. Thiscircuitisfactorysettogive
an equal mix on an average telephone line, and is therefore
perfectforrecordingbothsidesofatelephoneinterview.
To find the absolute null for any telephone line, follow this
procedure:
1. Placeacalltoaquietlocation.
2. Send a signal down the line at moderate levels, this
can be a prerecorded tape of music or speech, or a
testsignal.
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
methodyouareusingtomonitorthereceivesignal.
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.
HybridNullAdjustment (optional) Hybrid Null Adjustment (continued)
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. FactorySetting:
To reset to the factory setting, set the phone line
switch to On-Hook, and then 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
receiveaudio.
The best tools for this task are 1) and oscilloscope connected
to the receive line output, and 2) a 1 kHz sine wave generator
connected to the send line input. However, the above
procedure will work with a little patience.
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