Section 2: About The Phono Preamp Upgrade; Effects Of The Modification; Before You Begin; Prerequisites - DYNACO PAT-4 Assembly Manual

Section 2: About the Phono Preamp Upgrade

Effects of the Modification

 The phono preamp will be 4 to 8 dB quieter than the stock preamp.
 Phono High and Phono Cer inputs will no longer be active.
 You'll have two more phono inputs because:
o The special input will be equalized for phono.
o The Tape Head input will be equalized for phono.
 Alternatively, you can change the wiring to provide 5 high level inputs and 1
phono input:
o The phono input will be active
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 phono positions.
 Your cartridge will actually 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. The upgrade modifies
the topology such that the input impedance looks very close to 47K across the
whole audio band.

Before you Begin

This modification gives you a wonderful opportunity to tweak the gain of your phono
preamp. That lets you trade off sensitivity with phono preamp overload. There is no one
right answer. The best answer depends upon your equipment. Here's the procedure:
Listen to a record with your preamp, cartridge, and turntable in its stock form. Note the
"clock position" of the volume control when you're listening about as loud as you'd
every reasonably expect to.
 Write down the clock position of the loudest sane setting here____________.
 If the volume "clock position" (for any input) is never above 12 O'clock, then you
might want to drop the gain of the phono preamp by 6 dB. You'll do that by
installing R2 equal to 243 Ohms. That makes the preamp more resistant to
overload, and will make it cleaner on the most highly modulated records.
 If you often run your preamp with the volume clock position higher than 12
o'clock, then you should maintain the stock gain, and you'll install R2 equal to
121 Ohms (very near the original stock value of 120 Ohms).
Think about how you'd like to proceed, and circle your preference here:
 Drop gain by 6 dB to increase overload margin, R2=243 Ohms
 Keep gain at stock level. R2=121 Ohms.

Prerequisites

The PAT4PWR supply must be installed in order to install 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 upgraded design.
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