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How to Make a Hi-Fi 100 Watt Amplifier Circuit Using 2N3055 Transistors

The amplifier circuit explained here was built and tested by me and am extremely pleased by its performance and also ruggedness as far as maintenance and handling is concerned.
Basically, the entire configuration is a symmetrical class A amplifier incorporating an input filter stage, an intermediate driver stage and a powerful symmetrical output stage consisting of the versatile 2N3055 power transistors. The circuit efficiently drives a 100 watt 4 Ohms speaker with inputs derived from any audio source like a cell phone or DVD player etc.

Before you learn how to build this interesting and useful 100 watt amplifier circuit using 2N3055 transistors, a prior understanding of the involved circuit configuration would be very handy, let’s begin the explanation with the following points:

Circuit Description
A quick glance at the given circuit diagram makes us conclude that the output configuration is not symmetrical, since the transistors T15 and T16 are both NPN types.
The input stage of the circuit begins or initiates with a symmetrical differential preamplifier stage consisting of the transistors T1, T2 and T3, T4.
T5 and T6 are positioned as the current sources which are further extended as the driver stage consisting of the transistors T7 and T8.
However a closer inspection tells us that of course the wiring is symmetrical, having the transistors T11, T13, T15 at the upper section acting like special booster transistor package.
Similarly the lower section also employs identical super booster stage consisting of the transistors T12, T14 and T16.
The above two sections are perfectly complementary to each other, with reference to the diagram which indicates their emitters being terminated to a common point through the resistors R25 to R27 and via R28 to R30, this effectively that the wiring is exclusively symmetrical by nature.
The output stage is able to produce a massive 200,000 times amplification factor with comparatively very low quiescent current drain.
The quiescent can be set by the adjusting the preset P1.
Due to a non critical nature of the circuit, the entire project can be easily built over a general purpose PCB, however the layout of the components or rather the placement and the ratio of the distance of the components must be kept as identical as possible to the layout of the circuit diagram.
Though a common heatsink may be used for the entire set of the output devices, I personally used separate individual heatsinks for each of the transistors. This saved me from the headache of using the cumbersome and low efficient mica isolation kit between the transistors.
The inductor is kept for improving the dynamic nature of the circuit. It is built by winding 20 turns of super enameled copper wire over the 1 Ohms resistor itself. The wire is selected to be close to 1mm in thickness.
Though not absolutely necessary, for better thermal stability the transistors T9 and T11 and also T10 and T12 should be glued together, preferably by attaching the respective pairs face to face.
The quiescent current should be ideally set to 50 mA through the following initial procedure:


1) Remove the speakers, and short the input terminals (across R1),
2) Connect a DMM set at current range in series with the positive of the power supply to the circuit,
3) Next adjust the preset such that the meter reads an input of 50mA, that’s all, the amplifier’s quiescent current is set and now the connections may be restored for the normal operations of the system.
The power supply circuit is also shown along side and as can be seen there’s nothing special about it and may be built using the shown ordinary sets of components.

Parts List of 100 watt amplifier circuit

R1 = 430 K,
R2 = 47K,
R3 = 330 Ohms,
R4, R5 = 12 K ,
R6, R7, R20, R21, R22, R23, R24 = 1 Ohm, 3 Watt, Wire Wound Type
R8, R17 = 68 Ohms,
R9 = 100 K,
R10, R11, R12, R13 = 5K6,
R14, R15 = 12 K,
R16, R19 = 100 Ohms,
R25 = 10 Ohms / 2 Watts,
P1 = 100 Ohms Preset, Linear
C1 = 1 µF / 25V,
C2 = 1 n, CERAMIC,
C3, C4 = 100Pf
C5 = 100 n,
C6, C7 = 1000 uF / 35 V,
L1 = 20 turns of enameled 1mm copper wire over R24,
D1, D2 = RED LED 5mm,
All other diodes are = 1N4148,
T1 = Matching BC546 pair,
T2 = Matching BC556 pair,
T3 = BC 557B,
T4, T7, T8 = BC 547B,
T5, T12 = BC 556B,
T6, T9 = BC 546B,
T10 = BD 140  
T13 = BD 139  
T11, T14 = 2N 3055  
General Purpose PCB,
All the transistors T10, T13, T11 and T14 ae mounted on suitable heatsinks

The original design, (courtesy – elektor electronics)


196 Comments

  1. I know this circuit. Its crescendo… the best ever i had in my life. Mine is working fine…12 years now. no error at all. The driver BD139/140 can be replaced by C2073 and A940.

  2. Also please can you show me how to connect the transistor on the PCB. please help i am new to this transistor.. i have a to-3 package which has only 2 legs. the base and the emitter…furthe the collector part is the rest of the metal body.. plz help

  3. thanks for sharing this interesting looking cirucit. I look forward to maybe trying this out with some 2n3055s. By the way the power supply schematic shows a "dual" or center tap output. But your circuit shcematic has just 25V -GND. What is the correct voltage … +25 GND -25 or +25 – GND ?

  4. supply. 32-0-32 5A. that is the best, T10 T11 T13 and T13 are directely exposed to 5A. The rest of the circuit, supply can be given through two diodes IN 4007. It will deliver only 1A max. That good for small transistors so as to keep distortion minimum as possible. U can give voltage from 27-0-27 to 34-0-34 max.

  5. In actual crescendo circuit, The instead of LED D1, there is 5.6V zener and 47uf capacitor connected in parallel. Same as in the other side D2 also. Why there is a LED here ? Is it for thermal stability ?

  6. @ power supply circuit…. what should be the rating of the transformer? And what type those diodes connected in full bridge in the power supply circuit?

    • I have a few questions and I hope someone can help me with these. I am building this circuit and in the Circuit Description Column it mentions the R25 to R27 and via R28 to R30. I see R25 but not R26 to R30 or am I missing something?
      The resistor between T8 and T7, what number resistor is it and the value?
      Then I see R18 in the drawing but not on the parts list, what is the value to R18.

      I appreciate the help,
      Thank,
      Tony

    • Thank you Swagatam. Now with the original do I use the components values from that or do I have to redo the whole circuit to the original schematic.
      And thank you for your quick response.

  7. Thats it … thats the crescendo baby. This circuit was famous in 90s. I still have the picture of printed PCB softcopy with me. This PCB was last produced by HITECH company, now they stopped producing electronics goods. Thats made the end of this PCB board. No one can reprint the board without permission. Then people are forced to relay on other circuits. The beauty is, this is a marvelous design and can handle low frequency (low bass) without any problem. Almost no other modern circuits can beat the flat frequency response delivered by this amplifier. The only thing is You should give 28-32V dual supply, 4A-5A current for superb low bass effect.
    The very first crescendo circuit is published in elektor magazine on December 1982 🙂

  8. Sir,
    I want to made this audio amplifier circuit. But in the amplifier circuit you show the 25V supply and in power supply circuit there is 30V.
    What is co-relation between them. secondly, I want to known the rating of center tap transformer you have used.

    Sudesh

  9. I had posted the amp PCB diagram ( with both sides) dude. Use photoshop, make more darker circuit and print in on copper clad glass epoxy board 🙂
    Anyway this amplifier got a special powers supply board with an input 27-0-27 and 18-0-18 AC and the output is 27-0-27, 18-0-18, +-24 and +-12v DC supply (This is for tape deck, transistor pre-amp, tone control etc.. I have the same amp + powersupply board on my old deck and powersupply board contains transistors like BD139, BD140 etc.. i dont want to reverse engineerer that stuff…. The above power supply circuit is ok for the main amplifier as told by 'Swagatam Majumdar' but actually 4700ufx4Nos capacitor is there in actual board….
    Finally my name is Lancy Felix 🙂

    • it's a big explanation, cannot provide it here….these grounds are intended for providing discrete paths for analogue and digital signals, they should not be joined.

  10. Sir,
    I have built this circuit and its working nice . Thanx for the circuit .
    I am connecting this amp to woofer and tweeter so how I should connect them by creating passive crossover . What values of capacitor and inductor should be used ?? What kind of Inductor to be used ??

  11. Sorry for the delay yar. I have forwarded the PCB diagrams . Anyway am using passive filter only for the above amplifier. Mine is BOLTON, model no 305 BN2. Its some what old but very simple construction and awesome filter. Its good choice to wind coils rather than buying. The modern multiple coil crossover is worth useless. Just open any old 10" or 12" audio box, (70's to 90's is good… Old is Gold) measure the coil gauge, ID, OD, and height of that wound core, no:of coil in a single layer. Perimeter =Pi*D. You could easily calculate the total coil length. The only important thing is coil thickness.. the more the better. You could recalculate with simple equation also.
    Otherwise measure with wire gauge, measure the inductance… done 🙂
    Lancy Felix.

  12. there are 2 types of 3055 transistor. first one is 2N3055 and other one is TIP3055. TIP3055 is a big transistor. 2N3055 is like a normal transistor. which one is good for this amplifier. I cant understand the L1 . How can i make it in home. I'm stuck in this plese reply me soon.

    • Both the transistor types are almost identical with their specifications, only the package is different.

      You can use any of these as per your convenience….I have updated L1 data in the parts list.

  13. in our city there was a young man. he makes subwoofers in home and sell them. but i dont have money to buy that. he has many types of subwoofers. like 5.1 , 2.1 , 7.1. now he is not here. but i want to make one. quality bass sound subwoofer circuit diagram. thats what i need. if you have that send me. i will give my email. thankz

  14. This looks like a very nice amplifier, thanks for the detailed schematic and items list! I have just ordered the components and I'm excited to get started!

    I have one questin though – I can't seem to figure out the recommended power suppy voltage/current. I'm confused with the shchematic sayin "Tr1, 2 x 25" Do I need a dual transformer, or?

    If I only need one, will 30 Volts at 5 Ampere be suficient? Can less do it?
    Im planning on winding my own transformer, so no need for "standard values" 🙂

    • Thank you!
      The transformer needs to be a center tap transformer, that is with two sets of winding on the secondary, rated as 24-0-24V at 2amps or more.
      you can use a transformer rated at 30-0-30V/2amps also.

    • Thank you very much. If I want to make 2 of these amplifiers, one for each channel, can i just make two identical amps and ground them to the same "0" on the transformer (offcourse with twice the current), or do I need 2 seperate center-tapped windings with their own respective "ground/0"

  15. Hi again

    I have built the amplifier, and i am getting an amplified signal out of the speaker, but it is very quiet, and a bit distorted. I tested the voltage over all the tranistors, and its at approximately 35 volts, except the T7+T8 – they are only showing between 0,1 – 1V. What could I be doing wrong?

    • No, there's definitely something hugely wrong with your connections or the parts.

      Once built and set the output should provide a large amplification, even without any input music you should be able to hear a big Hiss on the speaker.

      did you adjust the quiescent current as suggested in the article??

    • there's something not correct in your circuit, check all the connections many times you should be able to locate the fault…I built this circuit some 10 years ago and succeeded at the first attempt.

  16. good day sir. I am currently making this circuit. I just have a question. At the bottom left, is there really a connection under R12? I just notice that there was an excessive line going to the left and it is not like the one on the top left. I am just confused because if they are really connected, then the non-connected lines between R14 and R19 are useless. Can you please clarify sir? I will wait for your response to be able to continue the build. Thank you.

    • good day Evo, i can't see any extra line anywhere, R12 and R11 rails seem exactly symmetrical…did you click the image to enlarge?? please click it to enlarge.

      R14 and R19 are wide apart, they have no link at all…I hope there's some confusion in reading the diagram from your side…enlarging it will clarify it

    • Good day sir. I already decided to make the original version and currently building it. My only concern is that my transformer is only around 21-0-21 and after the rectification it goes up to 28-0-28. It would be good to the new amp design but in the original it indicates around 30 to 35v input. Im a little under voltage. Will this be a problem?

    • thank you very much for your help sir.
      I would also like to inform you that my classmates had a problem with the new version of the design. the LED on the upper part wont turn on, T13 explodes and C7 is connected on the wrong terminal. The first time they turned it on, C7 exploded. I think its because the positive terminal of the capacitor is connected to the negative of the power supply.
      That's why I decided to make the original version. I'll also update as soon as I finish this project. Thank you very much sir Swagatam.

    • One last thing sir, in the original design, there are C3, C4 and C5 connected in parallel with 820nF each. We have a difficulty in resources here so I cant find an 820nF. But when the three are added, they will have a sum of 2.46uF. The nearest capacitance I can buy is 3.3uF. And I have no choice but to change the three capacitors with one 3.3uF. Im just not sure if this is still okay. I apologize sir but I'm new to this. You've been a great help.

    • Evo, the new version is also correct except C7 which needs to be reversed and R12 ground side short, except these two drawing mistakes all seems right in the diagram.

      connecting 3 in parallel will have other benefits but you can also try a 3.3uF instead it would do any harm.

    • Good day sir. I used 4 capacitors for the parallel connection with 560nF. their result will be closer to the sum of 3 capacitors of 820nF. I havent tried it yet coz I have a difficulty having the result of 4-6uH for the inductor and my potentiometer broke. thank you for your help sir.

    • do you mean i'll put the inductor on top of the resistor? or wrap the resistor? sorry sir coz im a little confused. then connect their legs in parallel. thank you sir. I still have to wait until monday to resume making the amp. thank you very much sir for your non stop help.

  17. Good day sir. We already made the amp but without the inductor first because we didnt have the chance to buy today. We have a little problem. the sound was too low. like there was no amplification. but we used a mono jack to connect to our cellphones. maybe it was the problem. or is it something else? we are so close to finishing this project. is there something we can do? we will try to use a 3 pin stereo jack tomorow and short the left and right. will it be okay? thank you sir.

    • Good day Evo,

      do not connect any music at the input just touch it with your finger, it must produce a very loud buzz in the speaker, and a small hisss when the finger touch is removed…..if this is not happening surely there's something terribly wrong with your circuit assembly.

      I built the second design in the year 2000 when I was quite new to electronics, yet I succeeded to get the best response from it at the first attempt….i had used a general purpose board…not even a PCB

    • by the way make sure to set the quiescent current with the help of the given preset in the amp….otherwise the circuit will waste a lot of power unnecessarily.

    • it should be very loud, the vibration on the speaker cone should be very high and visible to eyes…something may be not correctly done or the components may be faulty

      check and set quiescent current as explained in the article.

    • presets do not have polarities, just use the center lead and any of the outer leads of the preset for the connections….. anyway round…the other outer lead can be left open or unconnected

  18. Can I build this without any printed circuit board? Or is printed circuitry required?
    It is suggested to stick to diagram layout yet two pairs of transistors are suggested to be glued together. They are in widely different parts of the diagram. How to do this? 'Fold' the circuitry up? Or have long leads to the transistors, putting them above the rest of the circuitry.
    This will be a first project ever for me.
    🙂

    • yes you can do it, because I myself built it on a dot matrix PCB and succeeded in the first attempt…refer to the diagram that's given in the bottom of the article, I think the transistor glueing info may not be correct so don't do it, I'll check and update the correct data soon.

  19. Hi thanks for the info,actually my aim is to build a HI-FI 5.1 home theater system (100 watts in each channel if possible) with tweeter + mid range speakers in 4 of the channels and the rest would be center and sub.So pls advise me what circuit would be the best for this….thanks in advance.

    • Thank you Swagatam Sir, for your quick reply. Please give your email id for share some valuable information with picture about this project. I will expect support like that from you in future also.

    • Abinash, commenting in the my website is the best way to get in touch with me without fail….for image presentation you can submit it to:

      admin @ homemade-circuits.com

    • Good Evening Swagatam Sir,
      If possible, can you please upload a video in youtube of 2N3055 amplifier you have already made in the year 2000. So we can able to hear sound quality of this Class A amplifier.

  20. I have made this apm on a protoboard…and for the love of God i cant make it work…the input just gose to the speaker with no amplification and the amp draws 2A, i have used the undmodified circut…i have used bc557c and bc447c insted of the b version, is that a problem?Suplly is +-25V

    • BC447 is not the problem unles it's faulty…the circuit is huge and the fault could anywhere in the design.

      the quiescent current consumption of the circuit without load should be first set to 50mA

      this is done by shorting R1/C1 junction of the amp to ground, speaker connections open, ammeter connected in series with the ground line, and P1 adjusted until the meter reads 50mA

  21. This amplifier is the modified version of the original 40+40w amplifier circuit by MD Hull a pioneer designer for Philips nv Netherlands. The circuit has very good bass and treble at very low volumes. I built this amplifier completely as per instructions ( book was published by Norman Brothers in India) in 1990 and is working perfectly to date. I had used only with modification of bd182 with 2n3055. The bass and treble control and the preamp were build and used as were given in book. The bass is far better than the now days used driver driven subwoofers. I had used 0-54v transformer 3A (guru make), dome tweeters*4 of Philips, mid range of 80w and woofers of 10 inch Bolton make with cross over. This whole audio system cost me around 8230rs in 1990's. But believe me even Bose or marantaz or sonos does not compete with it any way in any frequency levels. The bass and treble is very good.
    n
    By

  22. I was Constructed a 30W+30W Cassette Deck very long ago, using N055 & P055 Transistor in Golden PCB (Gemini Radio, Chadni Chawk). I get the Circuit diagram from a book of Debashis Bandhopadhyay (ERC publication). Recently all Transistors are burnt out but, I want to repair the Amplifier. I have lost the book. I did not find the Transistor or Circuit Diagram in Gemini Radio. Please give me the circuit diagram of the amplifier of N055-P055. And suggest me where can I find the Transistor or it’s substitute.
    Thanks in advance.

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