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This patented amplifier technology makes practice of
voltage and current feedback in such a manner as to
accurately reflect the dynamic impedance of a loud-
speaker back into the output transformer of a tube
amp. On the Princeton Recording Amp, this tech-
nology is configured to allow a guitarist to turn down
the amplifier yet maintain all of the charm that we
love about tube amps.
The TZ amp's input (at P17) is the green lead of the
Tube Amp's output transformer (OT). In common
with this input is one lead of an 8 ohm, 50W resistor.
This resistor is mounted underneath the chassis in
the final configuration.
The TZ amp feeds back a signal to the other end of
the 8 ohm, 50W resistor using power amp U12. The
feedback signal modulates the 8 ohm resistor in
such a way that the OT's secondary appears to be
loaded with the same complex impedance as the
speaker that is being driven by power amp U11. So,
in a nutshell, the TZ amp reflects an accurate meas-
urement of the speaker's impedance back to the OT.
The signal path from the OT to power amp U11 is
termed the TZ amp's forward path. The Line Out
and Headphone functions on the IO PCB are fed via
differential buffer U15-A.
U15-B is an attenuator. The multiplying DAC at U18
(TLC7528) and the op-amp U17-A are combined into
a programmable attenuator of this signal. It is fol-
lowed by a solid-state chip power amp (TDA7293 –
U11).
Ideally, at the maximum TZ setting, the output volt-
age to the speaker equals the output voltage
generated by the secondary of the output trans-
former's white lead.
The current and voltage feedback occur at the inputs
U23-A. The 0.1 ohm current sense resistor is the
heart of the current feedback measurement.
maintain transimpedance amp operation, this resistor
cannot be shorted out! The voltage across R109 in-
dicates how much current is going through the
speaker [0.1 ohm * I(speaker)].
senses the output voltage of the speaker.
The multiplying DAC at U18 (TLC7528) and the op-
amp U17-B are combined into a programmable am-
plifier which boosts the combined current and
PRINCETON RECORDING AMP®™
The circuit built around
To
R216 & R217
(This is the model name for warranty claims)
voltage feedback output of U23-A. It is followed by a
solid-state chip power amp (TDA7293 – U12). This
power amp modulates the 8 ohm 50W resistor as
mentioned earlier.
The signal path from R109 &
U23-A to power amp U12 is termed the reverse path
of the TZ amp.
The DAC communicates with the microprocessor
through a serial-to-parallel shift register (74HC595 –
U25). It so happens that for any given TZ amp set-
ting the data input for DAC programmable attenuator
is the same as that of the DAC programmable ampli-
fier.
POWER SUPPLY
Three power transformer (PT) secondaries are con-
nected to the TX/FX PCB. Reference the lower left
hand side on page 2 of drawing 0069346000.
The power supply for the solid-state power amp's
U11 & U12 is energized using the brown bipolar sec-
ondary of the power transformer. D24, 28, 35, & 41
are the power supply rectifiers, which feed the two
4700uF filter caps C55 & C72. This yellow center
tap lead and the system's chassis connection are
placed between these caps, making this the system
ground star. Also, of important note, this power sup-
ply is noted as +/-42V on the schematic, it really
measures +/- 32V.
The violet – violet secondary drives the low voltage
solid-state supplies (regulated +/- 15.6V & +5V).
D23, 26, 29, & 30 feed 1000uF filter caps C63 &
C64.
Its positive supply feeds a 7815 (U9 - +15V
regulator) and a 7805 (U16 - +5V regulator). The
negative supply feeds a 7915 (U14 - -15V regulator).
The +/15V supplies feed all the op amps, JFET sig-
nal and relay switching controls (including the
footswitch interface), plus the Tube PCB's dc fila-
ment bias.
Note that the yellow lead mentioned
above is the center tap for the violet winding, also.
The +5V supply powers the microprocessor (U24),
its support registers (U25 & U29), the DAC, and the
power on indicator LED.
The 6V6s on the Main Tube PCB need a negative
bias to their grids. It is set by an adjustable negative
8
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