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The input buffer allows both unbalanced and balanced input signals to be
applied to the UAM4SYS. First we will take an unbalanced signal into the
unbalanced input at J4.
An unbalanced single is one that uses two conductors, one for the audio
signal and one for ground. This is the type of signal that comes out of an
MP3 player, CD player, tape deck, VCR, computer, or other similar device.
J4 takes a 3.5mm Stereo plug. A 3.5mm stereo plug consists of three
conductors called tip, ring, and sleeve. The tip at the very end of the plug is
normally the left audio channel. The next conductor down is called the ring
and is normally the right audio channel. The last and biggest conductor is
called the sleeve and is the ground connector that is common to both the left
and right channels.
Now that we know what a 3.5mm stereo plug is lets follow an unbalanced
signal from the connector through the Input Buffer. When a plug is inserted
into J4 the electrical connections between pins 2 & 4 and 1 & 3 are opened.
The left channel signal from the tip enters J4 pin 2 through C10 and R1 to
U1A's non-inverting input at pin 3. The left channel signal is buffered and
exits at pin1. The right channel signal from the ring enters J4 pin 1 though
C11 and R23 to U1B's non-inverting input at pin 5. The right channel signal
is buffered and exits at pin 7. Ground comes in on the sleeve and is
connected to the circuit board ground plane.
A little information about balanced signals is probably a good idea at this
time. Balanced audio signals use three conductors, positive, negative, and
ground for each channel. The actual signal is not referenced to ground but is
applied on the '+' and '-' lines. The signal on the '+' line is exactly the
opposite of the '-' line. The two signals are applied to the two amplifier inputs
and the amplifier uses both of these to produce an output. The ground is
simply there to provide some shielding of the cable and is not used in any
other way. Most noise that sneaks onto the lines will be the same level, both
in amplitude and polarity at any given time. Since it is applied to the opposite
polarity inputs the noise will be canceled and not appear at the output. This
is called common mode rejection, a big advantage of the differential inputs of
the op amps. Balanced lines are used in most professional equipment and
for long signal runs.
For separate unbalanced inputs we will again use a 3.5mm stereo plug but
now we need one for each channel. When a plug is inserted into J2 and J3
the electrical connections between pins 2 & 4 and 1 & 3 of these jacks are
opened. Let's follow the right channel and leave the left channel to you. The
positive tip signal enters J2 pin 4 goes to J4 pin 3 which is connected to pin 1
(no plug in J4). From J4 pin 1 the signal goes through C11 and R23 to U1B's
non-inverting input at pin 5. The negative ring signal enters through J2 pin 1
UAM4SYS • 5
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