Simple Contact MIC Circuit

Special microphones called contact mics can pick up weird sounds if you stick them on different things.

They can even make sounds themselves if you give them a little electric boost.

If you have an acoustic guitar that needs to be louder, you can use a contact mic with a simple circuit to make it electric.

These mics use special things called piezo discs that make electricity when you squeeze them.

This makes them good at feeling tiny vibrations like a tap or a knock.

They can also be used in little speakers that make beeping noises.

Circuit Working:

Parts List:

The key lies in the preamp a fundamental circuit utilized to match the piezos signal.

The resultant piezo and preamp combination can be utilized to electrify an acoustic guitar.

Circuit operations:

The battery supplies +9V connected to the source of the JFET device MPF 102.

This voltage is directed to the source through a 1.5k source resistor.

One terminal of this amplifier is shared between both the input and output signals referred to as the JFET drain terminal.

Consequently, this amplifier circuit is sometimes labeled a ‘common drain circuit’.

The 220k drain resistor is connected to the source linked to the battery ground terminal.

Using MPF 120 the primary element in the circuit is the MPF102 transistor.

Under no signal conditions bias voltage prompts the JFET source to draw a minimal current setting the source voltage midway between the supply and ground.

This bias setting is recommended for most small signal or analog audio amplifiers allowing maximum signal before distortion.

The signal enters the amplifier through a 3.3M gate resistor with the voltage drop across it constituting the input signal at the JFET gate.

This signal is an AC voltage.

How JFET Works:

The signal enters the JFET, an amplifying device.

The difference between the source and the gate establishes the voltage drop across the 560Ω resistor.

Typically, the bias voltage across the 560Ω resistor maintains the JFET channel at a medium resistance value.

The bias voltage, a DC voltage, varies when we apply a signal altering the negative bias voltage across the 560Ω resistor.

The varying gate signal causes the JFET to fluctuate resulting in more or less current passing through it.

The 1.5k source resistor converts these current variations into voltage variations.

As the input signal controls the channel width i.e. a small signal governs a large signal the JFET gate voltage regulates the JFET source current yielding amplification.

The output signal emerges between the source and ground.

The 4.7uF capacitor blocks DC voltages in the circuit but allows the amplified AC signal to pass.

The gate is more negative than the ground terminal and the output emerges across the source and ground.

Although the source is connected to the supply making it more positive than the ground terminal the output signal exits the amplifier through the 4.7uF capacitor and appears across the 220k resistor.

This capacitor blocks DC and permits only the AC signal to pass.

Formula:

The voltage gain (Av) of a particular kind of amplifier circuit known as a common source CS amplifier using a Field Effect Transistor FET is expressed by the below formula .

Below is an explanation of the formula and its elements:

Av = gmRS / (1 + gmRS)

where,

• Av: This is an indication of the amplifiers voltage gain.
• It is a dimensionless quantity that may be computed by dividing the input voltage the voltage delivered to the FETs gate by the output voltage, or the voltage across the load.
• An amplifier that enhances the input signal has a voltage gain larger than 1.
• gm: This phrase denotes the FETs transconductance.
• It gauges how much a change in the gate voltage (the voltage that controls the channel) affects the drain current, or the current flowing through the FETs main channel.
• Typically, millisiemens (mS) are used to specify it.
• Rs:This is an illustration of the resistance from the source.
• This is the resistance connected in series with the FETs source terminal, which receives the input signal.
• It may originate from the biasing resistors in the amplifier circuit or from the signal source itself.
• Ohms Ω are used to measure it.

How the formula operates:

gm: Because minor changes in the input voltage (at the gate) result in bigger variations in the output current (at the drain), a higher transconductance (gm) often corresponds to a higher voltage gain.

Rs: The source resistance, or Rs, is an important factor, this is the reason why:

The voltage gain decreases with increasing source resistance (Rs).

This is due to the fact that less voltage is available for the FET to amplify when a portion of the input voltage is lowered across Rs.

One might think of the word (1 + gmRS) as a voltage divider.

The term approaches 1 when Rs is modest in relation to gm (gm >> Rs), and gm primarily determines the voltage gain (Av).

However, the term (1 + gmRS) grows bigger and reduces the effective voltage gain when Rs increases and approaches gm (gm ≈ Rs).

Important things to keep in mind:

Specifically, common source FET amplifiers are covered by this formula.

For alternative amplifier combinations, there are several formula.

Based on the properties of the FET and source resistance, the formula aids in the prediction of the amplifiers theoretical voltage gain.

The actual gain in real world circuits can be influenced by additional elements such as finite output resistance and transistor non linearities.

How to Build:

Building a simple contact MIC circuit for electrifying an acoustic guitar involves assembling the necessary components and following the schematic provided.

Assemble the Components on a PCB:

• Place the MPF 102 JFET transistor on the PCB.
• Connect the drain of the JFET to a 220K resistor and connect the other end of the resistor to the positive terminal of the battery +9V.
• Connect the source of the JFET to the ground terminal of the battery.
• Connect a 1.5k resistor from the source to the JFETs source terminal.
• Connect the gate of the JFET through a 3.3M resistor to the guitar cable.

Connect the Piezoelectric Transducer:

• Connect the piezoelectric transducer piezo disk to the JFET circuit.
• One side of the piezo is typically connected to the source and the other side is connected to the ground.

Add the Capacitor:

• Connect a 4.7uF capacitor between the source of the JFET and the ground.
• This capacitor blocks DC voltages and allows only the AC signal to pass.

Double-Check Connections:

• Ensure all connections are secure and follow the schematic.
• Check for any loose wires or incorrect connections.

Power the Circuit:

• Connect the positive and negative terminals of the battery to the appropriate points on the breadboard to power the circuit.

Test the Circuit:

• Connect the guitar cable to the output of the circuit and the other end to an amplifier.
• Tap or knock on the piezoelectric transducer and observe the output on the amplifier.
• Adjust the gain or volume as needed.

Fine Tune and Adjust:

• Experiment with resistor values or component placement to achieve the desired amplification and tone.
• You may need to adjust the values based on your specific requirements and the characteristics of your guitar.

Enclosure (Optional):

• If desired, you can enclose the circuit in a project box for a more polished and protected finish.

Conclusion:

Remember to take precautions when working with electronics, and ensure that you have a good understanding of the circuit before attempting to build it.

If you are unfamiliar with electronics, it might be helpful to consult with someone experienced or seek guidance online.

References:

What is a good circuit for recording a piezo contact microphone or an electric guitar pickup?

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