PRINCETON RECORDING AMP®™
(This is the model name for warranty claims)
7
tial line driver (U7-B). Its output signal is returned via
coax to V3-A on the Main Tube PCB (WJ33 & 30).
The line out and headphone signals arrive on the IO
PCB via PW1 (POST_B & POST_GND_B). U1-A is
a Sallen-Key filter to provide some added equaliza-
tion and rolloff for speaker emulation.
U3 - A & B are differential LINE OUT drivers. R2
acts as its output level control. S1 is the ground lift
switch for pin 1 of the XLR jack.
The headphone amp is built around U4-A. Q4 & Q5
act as output drivers. The same signal drives the tip
and ring of the headphone jack (J4) with isolation
provided through separate 33 ohm resistors.
The footswitch interfaces with the amp through a 5-
pin DIN connector (J1). R15, R16, R28, & R29 pro-
vide the Footswitch PCB’s dc bias through pins 1, 3,
4, & 5 of the connector. The switch state at the foot-
switch is static or non-momentary. Depressing the
switch causes the dc voltage change at the DIN con-
nector pins. These voltages are sensed by a quad
comparator (U5), which determines the state to set
the control signals for the analog JFET signal switch-
ing.
U5’s outputs are open drains. These 4 outputs are
effectively switching current on & off to control cir-
cuitry elsewhere through 22k resistors. Two outputs
go a current mirror on the TZ/FX subassy – P4_OD
& P1_COMP. The other 2 stay on the IO subassy –
P5_FX & P3_REV.
At the top of the footswitch interface circuitry, D20 &
21 sense whether current is being drawn by the foot-
switch, which will happen any time it is plugged in.
Q12 & Q2 acts as the switches that notify control
logic elsewhere that the footswitch is connected.
Q8 is configured with the open drain of U5-p14 to
provide a “wired OR” function allowing for the foot-
switch state. With this, the EFFECTS LOOP remains
functional when no footswitch is attached. The ef-
fects loop can only be killed with the footswitch.
Q11 is a pass transistor that provides an “AND” func-
tion. This helps allow the REVERB to function
normally when there is no footswitch. The reverb
can only be killed with the footswitch.
REV_HI_KILL is the control signal that drives the
gates of the JFET that kills the wet reverb signal on
the Tube PCB. A current sensing switch circuit is
built around Q1. This circuit is used several times on
this PCB to create JFET switching levels in an elec-
trically quiet manner. Note that R25 is in series with
R37. When the “AND” logic allows current to flow
through these 2 resistors, R26 biases Q1 into satura-
tion and REV_HI_KILL goes high. If no current flows
through R25 & R37, then Q1 is off and REV_HI_KILL
is low.
Q7 and its associated circuitry form the current sens-
ing switch that control the JFET analog switches for
the EFFECTS LOOP kill function. Q6 provides its
opposite polarity. D5 & D6 clamp these signals to
ground to protect the resistively coupled JFETs - Q9
& Q10.
FX (COMPRESSOR & OD)
The FX subcircuits are low level effects found early
in the Princeton Recording Amp’s signal chain.
These circuits are found in their entirety on page 3 of
0069364000. This circuitry is not field serviceable.
In end use, the amp’s input signal enters the Tube
Control PCB via the front panel instrument jacks,
where it then passes through its high-impedance op-
amp buffer (U1-A & B). Through a ribbon cable
(P22), it exits to the compressor & overdrive circuits
placed serially on the TZ/FX PCB. Following these
circuits, the signal couples via coax from WJ35 & 32
to the tube input amplification stage.
Both effects are footswitch controllable. The design
also provides for front panel switching when the foot-
switch is not present. The control signal
FS_ACTIVE- enables and disables the front panel
switches when the footswitch is attached to the amp.
TZ AMP
The transimpedance (TZ) amp is found in its entirety
on page 1 of the TZ/FX/IO PCB service diagram.
Most of the TZ/FX subassembly is not field service-
able.
