Fender BXR 200 Service Manual - Circuit Description

BXR 200

CIRCUIT DESCRIPTION

PREAMP

J1 is a high sensitivity input which can be driven with as much as 2.5 volts RMS.  J2 is a low sensitivity

input which is recommended for Bass Guitars with active pickups, and can handle as much as 5 volts

RMS.

U1A is a “cheater” Sallen Key filter with gain.  R79 and C82 create a gradual boost in the high frequencies.

This sets up the initial response of the amplifier.  C43 and C44 are used for local power supply decoupling

for the first two op-amp stages.  This minimizes noise and hum in the front end of the preamp.

U1B is a variable gain stage.  The Gain Control is used as the feedback resistor.  This allows a gain

adjustment from less than 1 to about 10.

At the input of U2A, the DEEP enhancement switch boosts and extends the low-frequency response.

Within the negative feedback loop, the BRIGHT enhancement switch adds high frequency “sparkle”.  The

output of U2A drives the Notch Filter circuit.  When engaged, this filter provides a narrow band notch of

about –20db at 650 Hz.  The signal then feeds a unity gain amp (U2B) and couples to the first tone control

stage.

From the active tone controls the signal is routed to the second Gain stage (U8B).  If the DeltaComp is off,

switch S5B places Q1 & Q2 into the feedback loop, where they are biased asymetrically to provide non-

symetrical soft clipping, which simulates a tube sound.  The diode junctions Q1,Q2, CR1 and CR2 drop a

constant knee-voltage independent of current.  As the signal increases above a certain level, the constant

voltage alters the feedback current to the op-amp in a non-linear fashion.  This non-linear transfer

characteristic is what rounds off the resulting signal into a tube-type, soft-clipping waveshape.  With the

DeltaComp engaged, the soft-clipping circuit is disabled.

CHORUS

The input signal to the Chorus circuit is capacitivly (C61) coupled to U10A, which drives the Bucket Brigade

Device (BBD) U13.  Trimpot R105 sets the DC bias for U10A and U12A.  When properly adjusted, the

output from U10A & U12A will display symmetrical clipping.  (Refer to the adjustment procedure on the

service diagram).  The BBD is clocked by U14 at 180kHz.  The 180kHz carrier clock is Frequency

Modulated by the Low Frequency Oscillator U11, which deviates the carrier at maximum DEPTH from

60kHz to 300kHz.  The RATE of deviation runs from about 0.3Hz to 8.7Hz.  The FM deviation on top of the

BBD delay results in a pleasing choral effect.

C62 and R107 (@U10A) provide pre-emphasis above 7kHz.  At the output of U12A, R121 and C74 provide

the de-emphasis.  This helps to reduce noise.  C64, C65 and C67 filter out clock noise.  The Dry and

Chorus signals are summed at U12B and then fed to the power amplifier.  Via the Chorus select switch or

the footswitch, the Chorus effect can be switched on or off.  The collector of Q4 provides the control

voltage which enables U11, and determines the state of JFET Q5.  A –15Vdc from Q4 enables U11 to

oscillate, and pinches off JFET Q5, which removes the ground from the node between R123 & R124.

DELTACOMP

 The signal from the preamp feeds from P1B pin 4, through a ribbon cable, to P1A on the power amplifier

PCB.  The signal couples (via C1) to the unity gain amplifier U1B.  U1B contains a closed loop (gain of 1)

negative feedback path, along with positive feedback through the inverting Operational Transconductance

Amplifier (OTA) U2.  U1B and U2 make up the gain reduction circuit for the DeltaComp clip protection

feature.