DYNACO PAT-4 Assembly Manual - Page 5

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Section 2: Line Amp Distortion Reducer

Line Amp Characteristics

The line amp in a properly working PAT-4 has about 0.02% distortion when driving a

100K Ohm load at 1 Volt RMS. Typical units measure about -74 dB for the second

harmonic of a 1 kHz test signal, and perhaps -90 dB for the third harmonic. Distortion

increases for lower impedance loads and at higher levels.

By making the modification described herein, you can drop the 2

dB below the fundamental, and make the 3

about 0.002% distortion, rivaling the best preamps. Distortion reduces with reduced

output level.

How the Distortion Reduction Works

This modification changes Q4, a standard bipolar transistor, for the T0-92 Darlington

transistor and heat-sink supplied in the kit. The Darlington transistor greatly diminishes

the loading of the output stage on the first gain stage, increasing the overall open loop

gain. This leaves the closed loop gain essentially unchanged, but reduces distortion by a

factor of nearly 10.

The Modification in a Nutshell

You will be replacing Q4 with the supplied transistor and heat sink. If you're quite

familiar with the PAT-4, that's probably all you need to know. Otherwise, you may find

the information in the following sections helps you keep your place and avoid trouble.

Preparing to Remove the Circuit Boards

1. Disconnect the PAT-4 from your music system.

2. Unplug the power cord and allow the preamp to sit for one minute before moving

on.

Caution: Be sure that the preamp power is unplugged! 120 VAC can be lethal! 240 VAC

3. Remove the 4 screws that hold the cover in place, 2 on the left side and 2 on the

right side.

4. Lift the cover straight up and set it aside in a safe place.

5. Get masking tape and a pen ready. You will be labeling the wires that you are

about to remove.

6. Desolder the following wires from the left channel PC board, the one closer to the

back of the preamp. As you remove each wire, place a clearly marked masking

tape label on the wire with the number of the eyelet from which the wire was

removed. If you think it's helpful, add the letter R or L to the tag to designate

Right or Left channel. It may be easier instead to use F or B for front or back PC

Board. Don't put too much stress on the selector switch as you remove these

wires.

a. Eyelet 1

b. Eyelet 2

harmonic invisible. That corresponds to

can be lethal!

Page 5 of 20

Harmonic to about 94