Simple Contact MIC Circuit

Contact mics are specially designed microphones that can record strange noises when placed on various objects.

Giving them a small electric boost will even allow them to produce noises on their own.

A contact microphone and a basic circuit are used to turn an acoustic guitar into an electric one if it is required it to be louder.

Piezo discs are special components used in these microphones that generate energy when pressed.

This helps them to detect little vibrations such as a knock or a tap.

Small speakers that produce beeping sounds can also use them.

Circuit Working:

Parts List:

The key element is the preamp a simple circuit that matches the piezos signal.

An acoustic guitar is powered using the resulting piezo and preamp combo.

Circuit functioning:

The JFET device MPF 102s source is connected to the batteries 9V supply.

A 1.5k source resistor delivers this voltage to the source.

The JFET drain terminal is the single terminal of this amplifier that is shared by the input and output signals.

As a result this amplifier circuit is often referred to as a “common drain circuit.”

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

The MPF102 transistor serves as the circuits main component when MPF 120 is used.

Bias voltage causes the JFET source to draw a little amount of current when there is no signal putting the source voltage halfway between the supply and ground.

For the majority of tiny signal or analog audio amplifiers this bias setting is helpful since it allows for the maximum signal before distortion.

The input signal at the JFET gate is the voltage drop across a 3.3M gate resistor which allows the signal to enter the amplifier.

This is an AC voltage signal.

How JFET Works:

The JFET is an amplifier that receives the signal.

The voltage drop across the 560Ω resistor is controlled by the distance between the source and the gate.

The JFET channel is usually kept at a medium resistance value by the bias voltage across the 560Ω resistor.

When we apply a signal that changes the negative bias voltage across the 560Ω resistor the bias voltage which is a DC voltage than changes.

More or less current flows through the JFET as a result of the fluctuating gate signal.

These changes in current are transformed into changes in voltage by the 1.5k source resistor.

The JFET gate voltage controls the JFET source current which results in amplification just as the input signal affects the channel width meaning that a tiny signal controls a big signal.

Between the source and ground the output signal appears.

The amplified AC signal can flow through the circuit while DC voltages are blocked by the 4.7uF capacitor.

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

The output signal leaves the amplifier through the 4.7uF capacitor and emerges across the 220k resistor despite knowing that the source is connected to the supply making it more positive than the ground terminal.

Only the AC signal can go through this capacitor since it blocks the DC.

Formula:

The following formula represents the voltage gain (Av) of a particular kind of amplifier circuit called a common source CS amplifier using a Field Effect Transistor FET.

The formula and its components details are explained below:

Av = gmRS / (1 + gmRS)

where,

• The voltage gain of the amplifier is shown by Av.
• The voltage gain of an amplifier that amplifies the input signal is greater than 1.
• The word gm indicates the transconductance of the FET.
• Rs gives as an example of the sources resistance.

How the formula operates:

gm: A greater transconductance (gm) often results into a higher voltage gain because small changes in the input voltage (at the gate) produce larger variations in the output current (at the drain).

Rs: The source resistance or Rs is an important factor for the following reasons:

As source resistance (Rs) rises the voltage gain falls.

This is because when part of the input voltage is dropped across Rs there is less voltage available for the FET to amplify.

A voltage divider is one way to think about the term (1 + gmRS).

When gm is the main factor that affects of the voltage gain (Av) and Rs is small with respect to gm (gm >> Rs) the term approaches 1.

But when Rs rises and gets closer to gm (gm = Rs) the term (1 + gmRS) gets larger and lowers the effective voltage gain.

Important things to keep in mind:

This formula refers mainly to common source FET amplifiers.

There are other formulas for possible amplifier setups.

The formula indicates the possible voltage gain of the amplifier based on the FETs features and source resistance.

In practical circuits other factors like transistor nonlinearities and limited output resistance might affect the actual gain.

How to Build:

Building a Simple Contact MIC Circuit follow the below mentioned connection steps:

Put the Parts Together on a PCB:

• The MPF 102 JFET transistor should be placed on the PCB.
• Connect a 220K resistor to the JFETs drain and then connect the resistors other end to the batteries +9V positive supply.
• Connect the JFETs supply to the batteries ground terminal.
• A 1.5k resistor should be connected from the source to the JFETs source terminal.
• Connect the guitar cord to the JFETs gate through a 3.3M resistor.

Put the piezoelectric transducer in place:

• Connect the piezo disk of the piezoelectric transducer to the JFET circuit.
• Usually the source connects to one side of the piezo and the ground to the other.

Put the capacitor in:

• A 4.7uF capacitor should be connected between the JFETs source and ground.
• Only the AC signal can go through this capacitor since it prevents DC voltages.

Verify Connections Again:

• Make sure that every connection is safe and that the diagram is followed.
• Verify that there are no loose wires or improper connections.

Give the circuit power:

• To power the circuit connect the batteries positive and negative connections to the proper spots on the breadboard.

Inspect the circuit:

• Join one end of the guitar cable to an amplifier and the other end to the circuits output.
• Check the amplifiers output after tapping or pushing on the piezoelectric transducer.
• As necessary change the volume or gain.

Adjust and fine tune:

• To get the appropriate amplification and tone experiment with resistor values or component arrangement.
• Depending on specific requirements and the features of the instrument one might need to change the parameters.

Enclosure if required:

• For a more refined and secure finish one might choose to wrap the circuit in a project box.

Conclusion:

When working with electronics take extra care and make sure one should fully understand the circuit before beginning to build it.

It might be helpful to consult an expert or look up help online if unfamiliar with electronics.

References:

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

Piezoelectric contact mic for Audio line in Teensy 4.0 Rev D Board High Voltage issue