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Making this Small Pre-Amplifier Circuit

The post explains a simple pre-amplifier circuit which can be quickly made using a couple of transistors (BJTs) and a few resistors. 

The idea was requested by Mr. Raveesh.

Technical Specifications

I am working in LIC of India and electronics is my hobby since so many years. Often I will be browsing your website and found many useful projects. I require a favour from you.

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.

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.

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.

Sincerely hope you will help

With regards


The Design

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:

Pre-Amplifier Circuit

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

Need Help? Please leave a comment, I'll get back soon with a reply!


  1. Dear sir
    I can not believe it !!! It is very pleasant, how to set volume control

  2. 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

  3. Dear Rajkumar...power the motor through an LM338 circuit....the LM338 has an internal high temperature 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.

  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?

  5. 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....

  6. you can connect a pot across the C2 for implementing the volume control feature.

  7. 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

  8. Hi Karthik, the IC and the diodes will heat up if the load current is above the 50% of their maximum handling 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.

  9. 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

  10. 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


  11. 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.

  12. 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

  13. 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.

  14. 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.

  15. 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.

  16. Abhishek, get rid of the mosfets and build it using 2N3055 BJTs which will give you confirmed results.

    Make it exactly as shown in the following article:

  17. Sir excuse my ignorance but, what is the function of R3?. Your website is amazing!!!

    Tranks you.

  18. 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

  19. Dear Sir!Can we use this Circuit instead of a Transformer of Loudspeaker?

  20. Sir can I use lm386 as amplifier for this? tnx Sir


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