Introduction: Simple Single-Frequency Guitar Amp Circuit
This circuit will take input from an electric guitar. This input will then be amplified and filtered such that the output will be the fundamental pitch of one of the strings. This specific circuit is built to isolate the fundamental pitch of the top E string on the guitar, but you can use this format to isolate any of the 6 strings.
Step 1: What You Will Need
These are the parts you will need to build the circuit for the top E string.
1. 8x 47k ohm resistors
1x 333 M ohm resistor
1x 220 ohm resistor
1x 5k potentiometer
2. 2x 10.3 nF film capacitors
3. 2x OP27 or OP37 Op-amp ICs
4. a power supply capable of producing positive and negative 5 volts
5. an 8 ohm speaker
6. a breadboard and plenty of wires
Step 2: Preparing the Parts.
Using either alligator clips or a soldering iron, connect two wires to either terminal of the speaker.
Make sure you pick your ground. This can be either a small to medium sized piece of metal or the earth plane for the building you are in. All connections to the ground symbol must be connected to this plane.
Using a multimeter, measure the value of all the parts you are using to make sure they are the right kind.
It is not necessary, but it may help to plan out the breadboard layout before building the circuit.
Step 3: Build the Filter
On the breadboard, build the following circuit. It is incredibly benificial to either write down the tie points that you use, or label your parts as you connect them. This will help prevent a "rat's nest" style circut. Note that R1 and R4 are made of 4 47k resistors in series
R1 = R4 = 188k ohms
R2 = 10k ohms
R3 = 3.3 M ohms
C1 = C2 = 10.3 nF
Step 4: Make the Filter Active
As the filter is constructed right now, it doesn't have any way to power the speaker. To fix these, we will make it active. complete the circuit shown using the op-amp pinout guide.
Step 5: Build the Pre-amp
Build the circuit shown somewhere else on the breadboard. This circuit will account for the fact that the output of the guitar is incredibly low (70 - 100 mV). Without the pre-amp, the signal will not be strong enough to drive the active filter. When this is done, connect the output of the pre-amp to the Vin of the filter. The guitar should be connected to the input of the pre-amp, and the speaker should be connected to the Vout of the filter.
Step 6: Test Your Circuit
Your final circuit should look like this. You should now be able to test it out. You will need to adjust the potentiometer to best suit the guitar you are using. It is most likely that it will not work the first time. If this is the case, double check all of your connections against this diagram are correct. If the circuit still doesn't work, make sure you have an adequate ground plane, and that all ground connections are on the same ground plane. If this does not work, try picking a different ground plane.
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