Single Supply Octave Shifter: A Frequency Doubler
Effect For Electric Guitar
Octave shifting or frequency doubling effect for
electric guitar is done by rectifying the original signal, just like AC to DC
conversion inside your AC-DC power supply adapter. The circuit for this audio
effect is not new, I’ve seen it on many publications. Here I present the
circuit’s schematic diagram, but I’ve redesigned the circuit to use single
supply instead of symmetric
power supply. Thanks to
the LM324 low voltage design because we can design the circuit for standard
guitar effect’s 9V power supply.
Look at the above schematic diagram, you can see the
rectifying is done by four 1N4148 silicon diodes, configured as full-wave
rectifier bridge. Because the bridge is inserted inside the negative feedback
of the operational amplifier (op-amp) U1B, the nonlinear characteristic of the
diodes around the turn-on point (the forward bias voltage) is compensated by
the op-amp’s feedback mechanism. As the result, the output of the rectifier
looks like coming from ideal diode with no bias voltage needed.
You can variate the pre-amp gain by adjusting R3
potentiometer between clean and slightly overdriven, hear the effect, and set
as you want. The R4 pot is provided to adjust the processed signal so the
output level after the frequency doubling is equal to the level before entering
this analog effect processor. This important as we provide the circuit with
bypass switch to give an option for the player to change the mode between clean
and doubled on the fly, and normally we don’t want the effect make any volume
jump after toggling the clean-effect modes.
Make R7, R8, and C3 layout as close as possible to the
pin 10 of the LM324 IC (U1C) with shortest possible wiring to minimize
capturing any noise. This voltage (at pin 10 U1) is the reference for internal
“virtual ground” coming out from U1C output (pin 8). Make sure that the PCB
tracks for this “virtual ground” (pin 8 U1) are wider than other signal tracks
to give consistent reference for all op-amps. Make sure C4 and C5 have the
shortest possible connection to the power pins (pin 4, pin 11) and the “virtual
ground” line (pin 8).
Here is the parts list:
Item
|
Quantity
|
Reference
|
Part
|
1
|
2
|
C2,C1
|
470n
|
2
|
1
|
C3
|
22u/16V
|
3
|
2
|
C4,C5
|
100n
|
4
|
4
|
D1,D2,D3,D4
|
1N4148
|
5
|
1
|
J1
|
INPUT PLUG
|
6
|
1
|
J2
|
OUTPUT PLUG
|
7
|
1
|
R1
|
1M
|
8
|
1
|
R2
|
2k2
|
9
|
1
|
R3
|
25k Pot
|
10
|
1
|
R4
|
100k pot
|
11
|
2
|
R5,R6
|
470k
|
12
|
2
|
R8,R7
|
100k
|
13
|
1
|
SW1
|
SW KEY-SPDT
|
14
|
1
|
U1
|
LM324
|
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