Circuit Description - Fender BULLET 15 DSP Service Manual

This section provides an overview of any new or
unusual circuitry incorporated into this amplifier
model. The purpose is to aid the service technician
by providing insight into the design areas most
likely to present a challenge in troubleshooting.
This overview is focused for the efficient use and
security of Fender® proprietary information.
PRE-AMPLIFIER
The guitar input signal is fed into the high pass filter
at U1–B before splitting into two signal paths. The
Normal Channel signal path (U1–A to U3–B) is ac-
tive until the Drive Select switch is pushed "IN" to
change the signal path from the Normal Channel to
the Drive Channel (U2–B to U3–B).
amplifier U4–A and follower U4–B adjust the signal
level and provides +1.65VDC bias voltage to the
uDSP effects card. The preamp is in series with the
digital effects card, therefore, disconnecting the
card breaks the overall signal path. The signal from
the uDSP card is recovered at U6–A. It is neces-
sary to bring the signal back to its level prior to
entering the uDSP card. This is accomplished by
increasing the signal gain at U6–B. The signal is
fed through the Passive Tone Control circuitry to the
summing amplifier at U3–A, where it is mixed with
the CD/Tape Input. The signal is fed through the
Headphone buffer amplifier (U8–A and U8–B) and
on to the Power Amplifier. FET Q2 is used to mute
U6–A at power shutdown, preventing any uDSP
card signal transients from getting through to the
Power amplifier.
.

CIRCUIT DESCRIPTION

Differential
BULLET® 15 DSP
(This is the model name for warranty claims)
POWER AMPLIFIER
Current Feedback topology is used for the Power
amplifier. The Power amplifier IC (U5) is configured
as a non-inverting amplifier. Diodes D7 and D8 are
used for common-mode protection of the amplifier.
Current feedback is achieved by sampling current
at the negative speaker terminal and feeding the
signal back to the inverting node of the Power am-
plifier through R41 and series capacitor C25. The
negative speaker terminal is referenced to ground
through resistors R42 and R43. FET Q1 provides a
mute function at the Power amplifier input.
POWER SUPPLY
Standard bridge rectifiers for high voltage (+/-22V),
with zener regulation (D14–15) for the +/–15V sup-
plies. +15V supplies U7, 7805 regulator for +5V
supply for the uDSP card and digital user–interface.
DIGITAL USER INTERFACE
The uDSP Effects control (R65) is a linear pot that
sets a DC level (0–5V), which is read by an A/D
converter located on the uDSP card. These DC
levels determine which desired Effect is selected.
DIGITAL NOISE
The uDSP effects card generates some high fre-
quency noise (>50kHz), which is a normal product
of DSP processing. This does not affect amplifier
performance, but may be visible when observing
the output of the amplifier on an oscilloscope.
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