KXR 200
CIRCUIT DESCRIPTION
PREAMPLIFIER
Input channels 1 thru 3 will accept a balanced or unbalanced signal via a ¼” phone jack. Resistors R1 &
R2 are precision 1% parts which maximize the Common Mode Rejection of the first opamp stage U1A.U1A
provides a gain of 1, and rolls off high frequencies above 87kHz. The signal then couples to the 3 band
equalizer circuit. Gyrator U2A is tuned to 130 Hz (LOW), and Gyrator U2B to 540Hz (MID). The HIGH
control uses C10 and R14 to provide a 6dB per octave shelving filter above 4.2kHz. The boost/cut function
is handled by U3A. The signal couples to the Volume control stage U1B. The Volume control (R17) varies
the gain from less than 1 to about 23. The signal is then routed through a ribbon cable to the Channel
Insert Jack J2. This 3 conductor Phone jack sends the signal out on the “Tip” contact, and accepts the
return signal on the “Ring” contact. The signal then couples through C14 to the FX SEND bus, REVERB
SEND bus, and MAIN bus.
Although similar to channels 1 thru 3, Input channel 4 features the addition of an XLR jack which will accept
a balanced low impedance microphone input. Channel 4 also features a 4-band equalizer circuit. Gyrator
U11A is tuned to 125Hz (LOW), gyrator U11B is tuned to 320 Hz (LOW MID), and gyrator U12A to 1kHz
(HIGH MID). The HIGH control uses C56 and R92 to provide a 6dB per octave shelving filter above 3.2
kHz.
The signals from the FX SEND controls are summed at U13A, which provides a gain of 2 and drives the FX
SEND jack (J10). To minimize noise, the FX RETURN jack (J11) contains a switch contact that grounds
the signal line when the return jack is not being used. U13B provides a gain of less than 1 to a maximum
gain of 5. The FX return signal sums with the main signal at U14A.
The signals from the REVERB SEND controls are summed at U6A, which provides a gain of 2 and feeds
the reverb pan drive circuit at U6B. U15A makes up the reverb recovery circuit. U15b provides variable
gain and drive U14A. U14A is the summing amplifier for the Main, Effects Return, and Reverb return
signals. It provides a maximum gain of about 2, feeds U14B, and drives the Line Output jacks. The Line
Ouputs provide a pseudo-balanced signal. U14B feeds signal to the power amplifier.
The signal form 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.
The attack and release circuit for the DeltaComp is made up by the Diode, Resistor, and Capacitor network
which drives the Base of Q19. Comparator U1A senses the voltage from the Collectors of Q20 & Q21.
When the power amplifier approaches clipping on a positive swing, the collector of Q5 pulls up R56, which
turns on Q20, transferring –16vdc to the collector, pulling down R60. This causes the output (pin 1) of U1A
to switch from +15vdc to –15vdc, lighting the DeltaComp/Clip LEd. Note when the DeltaComp is disabled,
the Led indicates clipping of the output stage. With the DeltaComp enabled, the Led indicates an active
DeltaComp circuit. The negative control voltage from R60 also feeds through P1A to P1B on the preamp
PCB. The voltage is routed to the DeltaComp switch (S5A), located on the front panel. With the switch
engaged, the control voltage is sent back through the ribbon cable (via P1 pin 6) to the attack/release
circuit.
The negative voltage drives through Diodes CR31-CR34, and charges Capacitors C16-C19 all at once in
parallel as a one pole filter through a single time constant of R60 x (c16+17+18+19) or 2.2k x
8.8Uf=19.4mSec. As they charge, Q19 turns on and provides current to pin 5 of the QTA (U2). The
