Circuit Description; Audio Section - Shure U1 Service Manual

Shure U1/U1L Body-Pack UHF Transmitter

Circuit Description

Audio Section

Circuit Description
Audio enters L201, an inductor that is used as an rf choke. The
signal enters a 6 dB pad made up of R203, R204, C204, and C206.
The signal is ac-coupled through C205 into a 40 dB (30 dB for JB
frequencies) user-adjustable gain stage around U201A.
This gain stage is externally accessible to the user. C201, C202,
and C207 protect the pre-amplifier and bias circuits from rf interference.
R205 and R207 set up a half-supply bias, and R206 sets the ac input
impedance.
The amplified audio signal is then passed through a pre-emphasis
network before entering the compression stage. R208, R211, and C211
set up two corners for the pre-emphasis network. The pre-emphasis
network boosts the high frequencies before transmission and helps set
the hinge point (0 dB gain) of the compandor. The network feeds NE575
compandor U203, which utilizes an external amplifier U202B. The com-
pandor performs 2:1 logarithmic compression of the audio signal.
Transistors Q201 and Q202, along with crystal Y202, form the tone key
oscillator circuit, which provides a stable, continuous 32.768 kHz sine wave.
Transistor Q204 buffers the tone key signal before it is added to the audio
signal.
The tone key signal is used in the receiver to provide audio output only
when the tone key signal is present in the transmitted signal. Therefore, if
the tone key or the transmitter is turned off, the receiver will be muted.
Q203 acts as a switch for toggling the tone key on and off. It is controlled
via the microprocessor. R224 allows the tone key amplitude level to be set.
The tone key circuit is powered by U208B, which is a dc amplifier
that multiplies the 3 V battery voltage to 4.5 V (gain = 1.5). As the battery
voltage drops, the tone key supply voltage also drops, which decreases
oscillator amplitude. The tone key amplitude is used by the receiver
to indicate transmitter battery strength.
The tone key feature indicates battery level at the receiver. It is not
available on JB models.
The tone key signal, along with the processed audio signal, is fed to
summing amplifier U202A. Resistors R232 and R233 set up a half-supply
bias. Resistor R231, located at the output of the summing amplifier, pre-
vents spurious oscillations from the operational amplifier. After passing
through ac-coupling capacitor C232, the signal is fed to the rf module.
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25C1021 (BG)