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Number 11 on your Feedback c. rd
Four In/Five Out
Independently adjustable mixed audio outputs.
by John R. Flint KA0 LDB
Y
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u ' re d riv ing down Ihe
hig hway , a n hou r from
home. Ir's late, and no o ne is on
the local repeater. A front is moving
in from the west, and it looks
bad . You pick up the mike, hit a
few buttons, and instantly you're
linked to another repeater a hundred
miles to the west. Someone
can fill you in on what to expect in
a few hours.
Remote bases are not new, but
an easy way to adjust, mix, and
d istribute aud io hasn 't bee n covered
in the literature. In the above
system, audio from both the UHF
repeater receiver and the 220 remole
base recei ver are fed
through a switching ci rcuit and
into the mixer. The output s o f the
mixer are connected to the UHF
repeater transmitter, the 220 remote
base transmitter, and to the
control decoder. See Figure I .
The Problem
I first worked a repeater at the University
of Missouri with the Rolla Amateur Radio
Club (W0 EEE). Funds were limi ted , but we
had a repeater on the air in four months. We'd
worked out most of the bugs, except for two
that were particularly aggravating: muffled
transmitter audio (the main problem) and an
inability to mix additional aud io signals not
o rigi nally planned for. Several local hams
were building repeaters and had similar problems.
The Answer
Mahlon Haunschild N4PSD and I re ­
searched the available anicles for a solution.
The common circuit at the time was a potentiometer
network feeding into a single transistor
amplifier. The drawback ofthis configuration
was that the input impedance , and
therefore the audio level, changes on all inputs,
when any input is adjusted. We sought a
better solution.
The solution turned out to be a simple two
input/one ou tp ut op amp mixer which
Mahlon had bui lt during a school holiday. It
performed as expected, and it' s still operating
more than five years later.
I built two more audio mixers, a four
Photo A. Four In/Five Our audio mixer board.
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