Austin AUSTINDRIVE, Quick Start Manual

Page: 5 / 38

AUSTINDRIVE Assembly Tutorial Rev 3.02 December, 2020
5
THEORY OF OPERATION
(Numbered on the next page)
1. The 9V power circuit is completed (powered-on) when you insert a guitar cable into the input
2. The signal from the guitar goes through two poles of the 3-Pole, Double-Throw (3PDT) switch
3. The switch either jumpers the signal directly to the output ( “ True Bypass ” ), or to the drive circuit
4. Through the drive circuit, the signal passes through a DC-blocking capacitor, C1
5. If a drive knob is installed, signal is reduced by a C250k, reverse-logarithmic potentiometer
6. If a drive knob is not installed, a jumper is installed instead.
7. The signal passes through the GAIN 1 resistor, which defines the gain for Op-Amp #1 (previous page)
8. It enters Stage 1 of the op-Amp, U1 on Pin 6.
o
This is the “inverting input” of this Op -Amp, and it flips the phase 180°
9. The feedback loop contains the feedback resistor R1, and clipping diodes which clip the peaks of the signal
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If there is high gain in the circuit, more clipping happens = more distortion in Gain Stage 1
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Also, different diodes clip the signal differently. (“Hard” or “Soft” clipping here.)
10. The distorted signal comes out of the first Op-Amp on Pin 7
11. The signal goes to the GAIN 2 resistor, which defines the gain for Op-Amp #2 (previous page)
12. Before going into the next Op-Amp, the signal passes through another DC-blocking capacitor, C2
13. Like Gain Stage 1, the signal enters Gain Stage 2 of Op-Amp, U1 on Pin 2
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This is also an “inverting input” - so the phase flips 180° again, restoring the correct phase
14. The Gain Stage 2 feedback loop contains a feedback Resistor R2, and clipping diodes
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More gain here distorts the signal more, especially if it was heavily distorted by Gain Stage 1
o
Also, different diodes clip the signal differently. (“Hard” or “Soft” clipping here.)
15. The second gain stage socket also has room for a filter capacitor , which will reduce high-frequencies
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With no capacitor here, all frequencies are allowed to pass through to the output
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As this capacitor value gets larger, more high-frequencies are removed.
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A capacitor tends to “smooth” the sound of the distortion, making it less “harsh”
16. Out of the second op-amp on Pin 1, the signal passes through a final DC-blocking capacitor, C3
17. The signal then enters a B250k linear potentiometer
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When turned “up,” the signal passes through without any resistance, for the loudest volume.
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When turned “down,” more and more of the signal is shorted to ground, reducing the output
volume of the pedal.
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This knob helps match the distorted volume to the clean (“True - Bypass”) volume.
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The volume knob does not increase or decrease the distortion… only th e output level.
18. The red LED is turned on/off with the pedal by passing voltage through the 3
rd
pole of the 3PDT switch
Notes to create a unique tone:
•
Try a high-gain setup, then use a filter capacitor to reduce harsh high frequencies for a smoother tone
•
Try different-value GAIN resistors, diodes, and capacitors with each of the 4 different Op-Amps
•
You can mix/match diodes, or point 2 in one direction and one in the other, in parallel or in series.
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Two diodes in series clip the signal less than a single diode, also known as “ u neven clipping”
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Some vintage pedals use these tricks. Here are some typical diode setups:
1N914
1N400x
Standard, “Even” clipping with
matching diodes
“Uneven” clipping with 2 pointing in one
direction, and one pointing the other
“Uneven” clipping with 2
different diodes