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5 Simple Preamplifier Circuits Explained

5 Simple Preamplifier Circuits Explained

The post explains 5 preamplifier circuits which can be quickly made using a couple of transistors (BJTs) and a few resistors. The first idea is based on the request presented by Mr. Raveesh.

Circuit Objectives and Requirements

  1. Electronics is my hobby since so many years. Often I will be browsing your website and found many useful projects. I require a favor from you.
  2. I have a FM transmitter module which works on 5 volts DC with provision to connect from Computer through USB or from audio out from any other device through 3.5 mm audio jack.
  3. The module works great in computer USB mode with great signal strength, quality and coverage. But when I connect the same through audio input jack from DTH set top box the signal strength becomes weak even with full volume in both set top box and FM module. I think the audio signal level from set top box is not sufficient for the FM module.
  4. Please suggest me a good quality stereo audio small signal pre-amplifier circuit which can work from 5 or 6 volts single supply, that would not load the set top box, preferably using good low noise op-amp with detailed circuit and parts label.

1) Preamplifier using two Transistors

A simple pre-amplifier circuit can be very easily built by assembling a couple of transistors and some resistors as shown in the following figure:

simple preamplifier circuit can be very easily to build by assembling a couple of transistors

The circuit is a simple two transistor pre-amplifier using a feedback loop for enhancing the amplification.

Any music as we know is in the form of a consistently varying frequency, therefore when such a varying input is applied across the indicated C1 end terminals, the same is delivered across the base T1 and ground.

The higher amplitudes are processed normally and is reproduced with a potential that's approximately equal to the supply voltage, however for the lower misc amplitudes T2 is allowed to conduct at the higher ratio which is allowed to pass to its emitter.

At this time when the actual enhancement of the music is implemented by transferring this accumulated higher potential back to the base of T1 which correspondingly saturates at a much optimal rate.

This push pull action ultimately results in an overall amplification of an insignificantly small music or data input into a significantly larger output.

This simple circuit enables boosting extremely small or minimal frequencies to an appreciably bigger outputs which can be then used for feeding lager amplifiers.

The discussed circuit was actually popularly used in old cassette type playback recorders in their preamp stages for boosting the minute signals from the tape head so that the output from this small amplifier became compatible for the attached high power amplifier.

Parts List

  • R1 = 22K
  • R2 = 220 ohms
  • R3 =100k
  • R4 = 4K7
  • R5 = 1K
  • C1 = 1uF/25V
  • C2 = 10uF/25V
  • T1/T2 = BC547

2) Using an FET

The second preamplifier design looks even simpler as it works using a single low cost JFET. The circuit diagram can be seen below.
The circuit is self explanatory, and can be integrated with any standard power amp for further amplification.

Guitar Preamplifier

guitar preamplifier circuit

It usually becomes necessary to hook up an electric guitar with a mixing panel, a audio deck or a portable studio.

As much as wiring is concerned, that may be not an issue, however matching the high impedance of the guitar component with the low impedance of the line input of the mixing panel does becomes an issue.

Even the unsuspecting high impedance inputs of those units aren't well suited for the guitar output. As soon as the guitar is plugged into this kind of input, you hardly see a signal feasible for the panel or deck to process.

It might be likely to attach the guitar to the (high-impedance) mic input, however that is commonly way too sensitive for the function, which leads to clipping of the guitar signal too easily.

The matching amplifier introduced in this article answers these difficulties: it features a high- impedance (1M) input that will stand up to voltages of over 200 V. The output impedance is fairly small. Amplication is X2 (6 dB).

Dual tone control, presence control and volume control are offered. The circuit is designed for input levels of up to 3 V. Over this level distortion rises, but that may be, naturally, a decent outcome having guitar music.

True clipping of the input signal is not going to take place until eventually significantly bigger levels above the minimum guitar specs are utilized. The circuit is powered by a 9-V (PP3) battery through which the circuit pulls a current just around 3 mA.

3) Stereo Preamplifier Using IC LM382

Here;s another nice little preamp circuit using a dual opamp IC LM382. Since the IC provides a dual opamp package two preamps could be created for stereo application. The output from this preamp can be expected to be very good.

Parts List

R1, R2 = see the below given table.
R3, R4 = 100K 1/2 watt 5%
C1, C2 =100nF polyester
C3 to C10 = see table
C11 to C13 = 10uF/25V
IC1 = LM382

4) Balanced Preamp

If you are looking for something more sophisticated, you may want to try this balanced preamplifier design. The circuit is elaborately explained in this article which you can refer for your reading pleasure.

5) Preamplifier with Tone Control

A tone control normally includes bass and treble features for tweaking the dynamic quality of the music. However, since a tone control also has the ability to amplify the incoming it can be effectively used like an outstanding Hi-Fi preamplfier circuit stage. This we have a system which works two ways, foe enhancing the tone quality of the music and also preamplifying the music for the subsequent power amplifier stage.

The complete circuit of this fifth preamplifier can be seen below:

preamp circuit with tone control

About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

22 thoughts on “5 Simple Preamplifier Circuits Explained”

    • TUERESUECO, it is for providing a feedback loop which enhances the amplification many folds, without R3 the amplifier will be unable to amplify the input efficiently

  1. But sir what should I do if I am not able to get a 10 amp transformer in market. And I am using 7.6Ah battery. I'm using IRF 540 MOSFETs and 2.2uf capacitor in its astable multivibrator.

    • then there's no other way of achieving the mentioned results, you can do it only by upgrading the trafo and the battery specs.

      7.6AH is quite less and will not provide more than 100 watt at the output

    • What is the ampere of Transformer with turn ratio 19:1 12v centre tapped? How to do all that calculations. And other thing I Want to ask that by adding a 8A OR 10A transformer, will the CFL stop that minor blinking problem?, because all other things I have checked correct.

    • If the problem is regarding the flickering of a CFL then increasing the trafo amp will not help, it could be due to a faulty oscillator circuit or some other fault….because any CFL under 100 watt will easily light up brightly with your existing trafo and battery.

    • Sir, it's an astable multivibrator oscillator using 2n2222a and 2.2uf capacitor, and there are MOSFETs IRF 540 on each side, now I have checked the components using multimeter and the value of two transistors just different by 2 or 4 number and MOSFET are good but could you tell me what could be the fault and factors on which I should focus? And can replacing MOSFETs IRF540 with IRFZ44N give me good results.

  2. Dear sir I want a circuit to amplify an ac signal of 220-240v ac output from my inverter to amplify current. Its a square wave Inverter I have made it by astable multivibrator and two MOSFET and a 5 amp 12-0-12 transformer. The output Is fluctuating. Please suggest a circuit which do not contain any IC, to stabilize the output or amplify it, I have transistors 2n2222a,2n3904,2n3906,bc547,bc 557,and MOSFET IRF 540 and 630.and 68ohms, 510ohms, 10k ohms resistance as unused component so suggest me to utilise them all in that circuit

    • Dear Abhishek, no need of any external circuit, to amplify you just have to upgrade the trafo from 5 amp to 10 amp or as desired.

      and thee battery AH must be also upgraded appropriately.

      The trafo voltage rating should be preferably 9-0-9V for a 12V battery

  3. Hi sir,
    I tried to run a DC motor (found in trimmer) using a power supply circuit from LM317 , 12v transformer. The diodes & IC's were over heating, please provide me a driver simple circuit.
    Thank You

    • Hi Karthik, the IC and the diodes will heat up if the load current is above the 50% of their maximum handling capacity…..so if the motor is rated at around 500mA then those parts will heat up….for the diodes you can try 1N5408 and for the LM317 use a large hetasink to control the dissipation.

  4. Pls sir what power can this amplifier produce and can i use this circuit as a preamp in my homemade lm3886 amplifier in the sense that i want to use the input of this amp as my music input and the output as the lm3886 amplifier input so the amp will sound much louder at it's final output(speaker) is that a good idea?

    • victory, yes you can use this circuit for your application, it's exactly meant for such applications.

      however if the source music is already loud enough for the LM3886 input then in that case the above circuit could be useless and an overkill….

  5. Dear sir good day to you
    I have used 12v motor It is made to put the pcb hole but it often reaches temperatures. How can I avoid it? Please Provide appropriate protection circuit
    Motor specification:
    25000 RPM no load speed at 12V
    No Load Current – 1A, Stall Current – 10A
    0.36 Kgcm torque

    Thank you

    • Dear Rajkumar…power the motor through an LM338 circuit….the LM338 has an internal high temperature protection….you can attach this IC with the motor body, so if the motor heats up, the IC will also heat up, and its internal circuitry will shut off the current preventing any possible damage to both the devices, whenever the heat crosses sat around 90 C.

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