audio preamplifier circuit

The Real Audio Preamplifier- DIY

I made many amplifiers and tone controlling boards in past. But most of them either work on dual mode power supply or need a lot of external components. Today, I have an idea of single power supply transistor based stereo type preamplifier. This preamplifier circuit is used to control volume, bass and treble of any audio circuit. I used two small signal amplifier transistor in this circuit to make it stereo type. Gain of the amplifier is controlled via these transistor and then there are low pass and high pass filter to pass the frequency of one type.


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  • Single power supply 7-12volt
  • Stereo operation
  • Bass, treble control
  • Less noisy than other tone controllers

Components required:


1) C1815 NPN transistor

2) 100nf, 470pf, 10nf, 1nf and 100pf polyester film capacitor

3) 1M, 15k, 10k, 6.8k, 4.7k, 100R resistor

4) 47k, 100k potentiometers

5) 12v power supply or battery

6) Custom PCB from JLCPCB

Circuit diagram:


There are many circuit based on single transistor method and most of them are useless because of wrong choice of preamp transistor. I modified my circuit according the C1815 audio transistor and then bass, treble filters are designed separately. After that both the channels are embedded on a single piece of printed circuit board.

PCB designs and Gerber files:

I created these Gerber files in EasyEDA and you can download the same files from here. I used white solder masked FR4 based 1.6mm dual layer PCB. Every component value is printed on the silkscreen which make the project easy to repeat. Sign-up now to JLCPCB and get your first order done just $2 for 5pcs of 2-layer PCB.


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Components placement and soldering:


I will always suggest first to place small components like resistor and diodes, then go with the capacitors, transistors and integrated circuits. After that place big filtering capacitors and potentiometers. Solder all the component properly because in audio circuit any missed connection or dry solder may generate noise and hum.

Connection With Amplifier:


You can use any stereo type amplifier, there are 3 pin headers given on the top. Two for the input/output and one for the power. It can be directly supplied from transformer linear power supply. Input of the preamp is connected to the Bluetooth module, output is connected to amplifier input stage. And then speakers are connected to the amplifier TDA2030.

Testing with Stereo amplifier:


I have tested this amplifier in different condition first purely with bass and subwoofer then only with treble at full volume and third at every pre-set on 50%.

This preamplifier is controlling the bass and treble very well, and just taking a 100mA of operating current. Bass control and treble control is depended on the component’s values of RC filter. You can watch the video and listen the quality of audio produced after using this board. But the circuit has best frequency response in the treble range.

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