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Section 2: About the Phono Preamp Upgrade
Effects of the Modification
The phono preamp will be at least 4 dB quieter than the stock preamp. Internally
generated hum will be reduced by much more than that.
The phono equalization will be much more accurate, providing truer sound.
You will change the wiring to provide 5 high level inputs and 1 phono input:
o
The PHONO LOW input will be active, accepting a moving magnet
cartridge.
o
Tape Head and Special inputs will be converted to accept high level inputs
The preamp won’t clunk when you switch it in and out of the phono position.
Your cartridge will see the correct loading impedance. The original PAT-4 design
used a form of feedback biasing that made its input impedance look inductive,
falling to 10K ohms or less at low frequencies. This kit presents an input
impedance that looks very much like 47 K Ohms in parallel with 220 pF across
the whole audio band.
Prerequisites
The PAT4PWR supply must be installed along with the phono preamp upgrade. The
reason is that the PAT4PWR supply generates a -17.5 Volt supply that is used to bias the
phono preamp in the PAT4PPR.
Building the PAT4PPR Circuit Boards
This section details the process of building the phono preamp circuit board. We start with
an overview on this page. The specifics you need to start building begin on the next page.
The bare PCB is shown in Figure 1.
Figure 1-Component side of the phono preamp PCB before loading