Low impedance input preamplifier | microphone speaker circuit

I am going to show you a low impedance input transistor preamplifier circuit. Why should we make this circuit? Read before

In an old amplifier circuit, such as in an intercom system Use 2 or more speakers instead of a microphone.

Low impedance input preamplifier circuit using transistor


Note: This circuit is one of 4 transistor preamplifier circuits. All circuits have PCB so easy to learn and having fun.

  1. Low impedance input Preamplifier
  2. The medium Impedance Preamplifier circuit
  3. High Impedance Preamplifier circuit
  4. Simple Preamplifier using FET

You should read all 4 articles for more understanding.

In this case, the voice coil of that speaker has a very low impedance, not more than 20 ohms only Voltage from the voice coil is very low, only less than 0.01V. And older AM radios also use low impedance speakers, too.

Recommended: Simple Transistor intercom project

We can increase impedance by using matching transformers. To convert both the higher impedance and voltage. But using transformers will lose high frequency. Therefore use transistor better.

Matching transformer

The Common base amplifier or the circuit that connects the base to the ground. (Ground Bases). As in the example in the circuit diagram below is a low impedance amplifier of approximately 50 ohms.

Read first for beginners: How do transistor circuits work

How it works

When considering circuits We can distribute the details as follows

Low impedance input Preamplifier circuit using transistor
  • Transistor-Q1 is an NPN transistor. So, its power supply circuit is + VCC power, by using the + 9V to + 12V power supply through R5 before. And, there is C4 maintain stability as a decoupling to prevent any spike signals.
  • R1 and R2 act as the input and output of the transistor.
  • The transistor conducts current or amplifies the signal. There must be a bias between the lead base and emitter. The voltage of the base must be higher than the lead emitter.

    Therefore use voltage divider circuit with R3, R4 to act as biased of the transistor.
  • Capacitor C1 is a signal coupling on the input and prevents DC voltage or noise-free pulses from the front circuit to this circuit.
    Recommended: How to use capacitor coupling
  • The capacitor C2 is signal-coupling to output and prevents the DC voltage from disturbing the next circuit.

Read Also:

Customization and experimentation methods

This time it is a test play, supply low voltage test first. And then measure the voltage at the collector lead of Q1. Without entering the signal. We should read half the voltage of the power supply.

If you want the voltage at this point to be greater than half We can do this by changing the value R3 to more values.

If using 9 volts power supply, R3 should be at 6.8K. But If using 12V power supply we should R3 is 12K.

Hum high-frequency protection

The low impedance circuit as the circuit diagram is an easy hum. Especially the signal cables that are twisted together may cause alarms easily.

If high-frequency noise occurs, it is called RF Interference. We use the 0.1uF capacitor at the emitter (E) of transistor Q1 down to the negative voltage (or 0 volts) and in the PCB the device hole is designed to add this device already.

Also: Low noise preamplifier

How to build it

Look at the Actual-size of Single-sided Copper PCB layout below. And the component layout, too.

copper pcb layout low impedance transistor preamplifier
The Copper PCB layout low impedance transistor preamplifier
components layout low impedance transistor preamplifier
The components layout of low impedance transistor-preamplifier

Check out these related circuits, too:

Parts you will need

0.25W Resistors, tolerance: 5%

  • R1: 1K
  • R2: 10K
  • R3: 18K or See text
  • R4: 2.2K
  • R5: 100 ohms


  • C1, C4: 100uF 25V Electrolytic
  • C2: 4.7uF 25V Electrolytic
  • C3: 47uF 25V Electrolytic
  • C: 0.1uF 50V Ceramic


  • Q1: BC548 or equivalent, 45V 0.1A, NPN Transistor

A Microphone From Normal Speaker

We sometimes need a Simple microphone, But we cannot find it. This circuit can help us modify the normal small speaker to the microphone as a pre-mic circuit.

This is the generally Microphone Preamplifier circuit. When entering the power supply circuit for Q1 will work as an amplifier in a common base.

A Microphone From Normal Speaker

Microphone speaker circuit

That matching the input impedance of the Impact of Impact Speaker impedance low. When the signal sounds into the speakers. The audio is converted to electrical signals through C2. In addition, C2 to prevent voltage direct current flows into the speaker.

The signal that goes through C2 drop across R3 is a voltage signal sound. When Q1-BC109 works, will result in pressure changes pin B of Q2.This is the amplifier changes, the current implementation of Q2 change. If the speaker signal to drop across R3 is positive.

The Q2 will be very current. If the incoming signal is negative, Q2 will be less current. The output is a voltage drop across R4. The C3 protects DC, not to flow into the.

Read other circuits about transistor preamplifier

Not only this circuit we have other circuits that you should play with us.


I always try to make Electronics Learning Easy.

6 thoughts on “Low impedance input preamplifier | microphone speaker circuit”

  1. I apply a 100mV 1000Hz signal as the input signal (sine). When I connect an oscilloscope to the Output, the signal comes out clipped. I’m viewing a clipped square wave in the range of + 4.8V to -7.70.
    My aim is to amplify waves with 100-200mV amplitude and 100-3000 Hz frequencies.


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