An Octave Up guitar pedal is a fuzz-like pedal that raises your notes up an octave. This is not a general purpose pedal that you would want to use for rhythm guitar, but one you would want to engage when you are going to shred a mean solo. This pedal sounds a bit harsh and whiny, but can be very effective when used well. This is an easy pedal to build, and definitely a fun weekend project (even if you don't get a ton of use out of it).
Step 1: Materials
|2||1N34A||D3, D4||Digikey||1N34A BK-ND|
|1||9V battery connector||B1||Digikey||36-232-ND|
|1||Hammond BB Enclosure||N/A||Small Bear||0301|
|1||Adhesive Velcro Squares||N/A||Small Bear||B000TGSPV6|
Step 2: About the Circuit
This circuit is based on Gus Smalley's Simple Octave Up pedal and Scott Swartz's Octave Screamer, which in turn is based in-part on the classic Tube Screamer pedal. In my version, I took elements of all three pedals and created something relatively new-ish.
The input to the circuit has a stereo jack which acts as a switch to toggle on and off the power. To learn more about that, check out my DIY Guitar Pedal instructable. The signal from the input then goes to a DPDT switch which serves as a true bypass switch. This means the clean audio signal will bypass the circuit altogether when the switch is toggled.
Assuming that the circuit is not bypassed, the signal then passes through a 0.01uF (C2) capacitor which functions as a standard input buffer. The audio then passes to the non-inverting input of the op amp.
Also connected to the non-inverting input of the op amp is a virtual split rail supply. Put another way, the 10K resistors (R1 and R2) form a simple voltage divider and create a virtual ground at the center connection of the voltage divider. To explain the presence of this requires more information about op amps than I want to provide at this juncture, but trust me that it is fairly standard.
The 100uF (C1) and 0.1uF (C3) capacitors in parallel with this voltage divider are simply voltage filters intended to smooth out the power supply voltages. The center of the voltage divider then passes through a 100K (R3) resistor on it's way to non-inverting input. I found that the value of this resistor is not remarkably important to the sound (as far as I can tell). Frankly I am not 100% certain what it is doing, but I am certain that it needs a resistor there (as the circuit was unhappy when I removed it).
The Op Amp stage is configured as a variable gain non-inverting high pass amplifier. The 4.7K (R4) and 22uF (C4) connected to the op amp's inverting input create a high pass filter. This filter only allows frequencies over a particular threshold to pass and get boosted. By adjusting the values of R4 and C4, you can change the cutoff threshold.
The 47K (R5) resistor and 1M (R6) potentiometer connected between the non-inverting input and the output adjust the gain of the signal. Also connected between the inverting input pin and output pin are two 1N4001 diodes (D1 and D2) arranged front to back. These serve as soft clipping diodes which means they help keep the gain of the signal constrained to a hard limit and round off the top. The values of these are not remarkably important so long as they are standard silicon diodes. You can read more about the op amp circuit under "clipping stage" on the Technology of the Tube Screamer.
After the op amp stage, the signal passes through a 22uF (C5) output buffer and then a 1K (R7) resistor. This resistor serves to simply lower the level of the signal a little bit.
The transformer (T1) and 1N34A germanium diodes (D3, D4) comprise a full wave rectifier. This rectifier is where the octave shift happens. The reason a full wave rectifier doubles the octave is because it take all of the negative AC audio signal and flips it over the center rail effectively doubling it as a positive DC signal. In other words, the waveform of the note occurs twice as frequently. Thus, since there is twice as much of the signal, the frequency of the signal goes up a single octave. It should be noted that no matter what you do in the rest of the circuit, because of how the full wave rectifier works, it will only ever increase the signal a single octave.
Finally, the signal goes through a 100K (R8) volume pot, back through the switch and to the output jack.
Step 3: Build the Circuit
The attached gerber files can be used to manufacture the circuit board for this pedal. To learn more about designing and manufacturing PCBs, check out the Circuit Board Class. Should you not want to have the board manufactured from the files, you can just build it on perf-board as specified in the schematic.
Anyhow, simply solder all of the appropriate components to the circuit board as specified in the schematic. Don't worry at this moment about the jacks, potentiometer, and switch.
Step 4: Drill Guides
Cut out the attached drill guides and tape the to the enclosure.
Step 5: Mounting Holes
Use a center to punch to mark the center of the crosshairs for each of the holes you are going to drill.
Drill 1/8" pilot holes in the center of each hole.
Widen the two potentiometer holes on the front face of the enclosure to 9/32" in diameter.
Expand the push-button's hole in the front of the enclosure to be 1/2" wide.
Drill the holes on each of the sides of the enclosure to be 3/8" wide to fit the jacks.
Step 6: Potentiometer Tab Holes
We need to create holes for the potentiometer alignment tabs. To do this, insert the potentiometers into their front mounting holes backwards and upside down.
Wiggle, them back and forth, and notice you have scratched a line on the surface that corresponds to its mounting tab.
Create an indent along this line with a center punch just to the left of the larger potentiometer hole. Drill a hole where you marked using a 1/8" drill bit.
Step 7: Dial Plates
Now is time to apply the dial plates to the enclosure with contact cement.
To do this, trace the outline of the dial onto a piece of tape, and then cut it out to create a stencil.
Apply the stencil to the enclosure.
Finally, brush contact cement onto the enclosure and the back of the dial. When both dry to a tacky consistency, stick them together.
For more complete instructions on how to do this, check out the DIY Guitar Pedal instructable.
Step 8: Wire the Potentiometers
Solder two 4" green wires to the 1M potentiometer and connect this to the appropriate resistor terminals on the circuit board.
Solder two 4" green wires to the center and one of the outer pins of the potentiometer and one 4" black wire to the other outside pin. Connect the black wire to the audio out ground terminal and the outer green wire to the audio out positive signal terminal.
Step 9: Wire the Jacks
Attach 4" green wires to the signal terminals that connect to the plug's tip on both the mono and stereo jacks.
Attach a 4" black wire to the smaller signal terminal on the stereo jack and the black wire coming from the 9V battery clip to the barrel connection on the stereo jack.
A ground wire for the mono jack is not needed because it gets grounded to the circuit through the conductive metal enclosure.
Step 10: Wire the Switch
Wire two of the outer terminals on the switch together.
Connect the signal wire from the mono jack to one of the center terminals, and the signal wire from the stereo jack to the other center terminal.
Next, connect a wire between the audio-in connection on the board to the remaining outer terminal on the switch that is in line with the stereo jack.
Finally, wire the center terminal from the volume pot to the remaining outer terminal on the switch.
Step 11: Connect the Power
Now it is time to wire the 9V wires to the appropriate connections on the board.
Solder the red wire from the 9V battery connector to the 9V input.
Solder the black wire from the stereo switch to the ground input on the board.
Step 12: Install the Components
Install the external components into the appropriate holes in the enclosure using their mounting hardware.
Step 13: Attach With Velcro
Attach adhesive velcro squares to the underside of the circuit board and then affix it the inside of the enclosures lid. This will both serve to prevent the board from shorting on the bottom of the enclosure, and hold is securely in place to prevent it from bumping into other parts and shorting on them as well.
Step 14: Finishing Touches
Plug in the battery and insert it inside the enclosure.
Fasten the enclosure lid with its mounting bolts.
Lastly, attach the knobs to the potentiometer.
Step 15: Rock Out
Plug in your guitar and amp and rock out.