This versatile amplifier circuit is designed and submitted by Mr Seetharaman Subramanian from Chennai . The full credit of this article goes to him and we are very proud to publish this fabulous circuit here.

Seetharaman’s description about the circuit.

This concept has appeared long back in Practical Electronics a UK based Magazine. Based on this concept I designed this circuit during 1981 to 1986 with lots of field trials and modifications, the design was frozen in 1986. I have assembled so many amplifiers for me and for my friends based on this design with various power levels. They are still kicking in so many houses. This concept can be applied to any existing amplifier also. You must listen to believe the crystal clear thumping bass response. crystal clear mid and hi frequencies. Good transient response with very low distortion. Hope you guys  will enjoy the reproduction of this amplifier.

In the art of audio sound reproduction it is well-known that the dynamic loudspeaker is more nonlinear and generates more distortion than all the other system components combined. This is particularly true at low frequencies which require large cone excursions where the stiffness of both the inner spider and the outer surround increases rapidly as the cone approaches its peak displacement, resulting in a nonlinear suspension compliance generating high distortion.
For example, in a typical high fidelity sound system at a frequency of about 35 Hz the total harmonic distortion of the amplifier might be of the order of 0.01%, whereas the distortion of the loudspeaker might range from about 3.0% to about 50.0%, depending upon the loudness. If this cone motion can be sensed and given as a feed back to the earlier stage of the amplifier, this distortion can be reduced dramatically.

Motional Feedback (MFB) was a speaker system developed in the early 1970s by Philips Holland. It introduced a feedback system to the woofers of HiFi loudspeakers, enabling them to achieve a more extended low frequency response in a relatively small enclosure. The key benefits are a very controlled bass response. Any distortion induced by the enclosure or the woofer itself is immediately corrected by the feedback. These hand-built speakers were sounding very good and were quite expensive.

As a different approach, instead of using the cone movement, the current flow through the voice can be sensed (the current is proportional to cone movement) and can be used as a cone movement feedback. This novel idea is used in this amplifier design (I don’t claim any originality; this idea has appeared in Practical Electronics UK Magazine – long back – They might have even patented it).

The amplifier used here is a standard Philips audio application circuit, with a specification of 40 Watt RMS @ < 0.06% Total Harmonic Distortion into 8 ohms impedance speaker and having a frequency response from 20Hz to 100 KHz, suitably modified for our application.

The amplifier is a conventional class B directly coupled quasi complimentary out put stage, operating with single 56Volt supply (no need for a regulated or dual power supply). BC157 is the pre-driver and half supply stabilizer. BD 139 is the driver, a BD139 and a BD140 complimentary pair out put driver stage with 2N3055 as final out put stage. The speaker voice coil current is sensed through 0.47 ohms resistance connected from speaker one end to ground. This signal is given as negative fed back to previous stage through 470 ohms. The half supply at speaker coupling capacitor can be adjusted by varying the 39K resistance (if required you may fix a 100K pre set in the place of 39K and adjust for half supply with no in put signal at junction of both 0.47 ohms of out put transistors and speaker coupling capacitor). The quiescent current through output transistors can be adjusted with 22 ohms in series with two bias diodes 1N4007. The value for 50mA quiescent current will lie between 15 to 33 ohms for a supply of 56volts. The amplifier can be protected with a simple 1.5Amp fast acting fuse in the positive power supply.

The amplifier can be assembled on 40watt Philips amplifier application board or on any standard plain straight line board. All the three driver transistors require cooling clips. (Standard TO220 casing cooling fins). Out put transistors require a good quality extruded alloy heat sink with insulating mounting kit and with a smear of silicon conductive grease, for good conductance of heat.

The recently appeared passive Bass/Treble tone control with 12volt supply in “Circuits Today” as Baxendall tone control circuit is the most suitable preamplifier and tone control for this amplifier.

Circuit diagram

motional feed back amplifier circuit

Powersupply circuit.

power supply for motional feed back amplifier



  1. Hi All,

    I deeply sad to say that Mr.Seetharaman Sir, who guided me in all aspects of Electronics and helped me to built a MFB Amplifier is no more with us. He was unwell and was undergoing treatment for past one year.

    I really miss my guru and deeply regret for his depart.

    So please do not expect the reply from sir in this forum.

    “May his heart and soul find peace and comfort”


    • Hi Vinod ,its very shocking news to all,personally I don’t no him but I was regularly watching his circts,May his soul rest in peace,,,

    • if i want to give more power to my tda2050 based amp …2 nos of tip3055 can be used at o/p stage?
      if yes then how would be the connection…?

    • Who is the editor now for .Is there a contact. Thanks

      • You can contact us at – info[at the rate of]

  2. Hi, sir.. i’ve just built the amplifier… it sounds very good.. thank you for posting such a use circuit…:-)

  3. I already build this circuit in the sound is very clear. But when i turn the volume to max. without input the 470 omhs resistor burn and the emitter collector of 2N3055 got shorted. What happen?

    • Mr Sanjay S. Phodkar

      Hi ,

      I already build this circuit in the sound is very clear but when i turn the volume to max near the 470 omhs resistor emitter collector of 2N3055 got shorted. What happen when I loud the valume ? Please let us known as early as possible.

  4. I already build this its sound very clear and the bass super good. But i want more power.Sir if i use TIP35C and increase the supply to 70V what will be the output Power?

  5. Michael Dickson

    Good day to you,

    Re- Phillips Motional Feedback Amplifier Speakers [22 AH 587]

    Please can you help, I am in need of professional help in identifying the loss of amplification and renewing the failed component. I can supply more information of the fault, a PDF of my original instruction book and photos of the pc board etc.

    In return I am sure I can offer to help with my profession being a Heating and Electrical Engineer with over 30 years in this, although I did serve my time as a Joiner and could advise on this.

    If this, like me is outside of this knowledge and you know someone more knowledgeable, would you kindly pass this message over.

  6. sir what will be the output power if i add two output transistor 2N3055?

  7. sir it is ok to use bd136 and bd138 replacement for bd140?

  8. hi sir, my mfb amp output cracks and d sound is not loud.instead of 220uf I used 50v/330uf and in place of 2200uf used 50v/3300 due to unavailability is dat y d output is decrease n cracked? am using a 35 vdc and I wasn’t getting half of d supply voltage @ d output capacitor +ve terminal I had to use a variable resistor in place of 39k, after tunning d variable resistor n getting around 17v @ +ve terminal of capacitor d sound still remain low and cracked. Need ur Assistance.

  9. sir, if possible please send me the PCBs designs.

  10. Sir, may i use 2sc5200 instead of 2n3055?
    will these transistors increase sound quality?

  11. sir pls can u tell me how to know d size of a speaker(subwoofer) i.e. D inches ?do I measure with a ruler from end to end of d cone?

    • Seetharaman

      End to end of the cone paper up to the metal frame. For Sub woofer the frequency response at the lower frequencies qre important. That is 20 Hz and bellow up to 200Hz. Resonance when fitted in enclosure can be just 20Hz. 12″ is ideal 18″ is best suited for proffetional use. Modern 5 to 6″ subwoofers also give best response in the recommended tuned reflex enclosures for domestic use..

  12. thank u sir I will do d modification n tell u d result

  13. good day sir, I made made d 150watt amp as subwoofer. my problem is I sometimes have disturbance @ lower volume but @ higher volume it sounds clear .sir how can I solve dis problem?

  14. wat will b d output power of dis amp if i use tip3055..

    • Seetharaman

      With TIP3055 also it will give the same output of 40 watts RMS into 8 ohms speaker with a distortion level of <0.06%. Ensure that you use a good heat sink.

  15. Hi seetharaman, i have a 12V-0/ 15A transformer. Is this value will work on this circuit(motional feedback amp)? if yes, how much output power will be?
    and if no, how much is the minimum and maximum power supply that can be drive in the circuit, and its respective output power ratings?

    • Seetharaman

      Hi Michael minimum is 36 volts with which you will get 20 RMS into 4 ohms load @ 0.1% distortion and maximum is 60 volts DC, it will deliver 40 watts RMS into 8 ohms load @ 0.1% distortion

  16. hi. I have a transformer rated for 5 amps so I changed d secondary windings to 23-0-23 & 40-0 which I want to use to power my mfb & my 100 watt subwoofer amps .my question is will d transformer b sufficient to power d two amps? N Hw do I power my subwoofer filter using d transformer????

    • Seetharaman

      Motional feed back amp requires around 1.6 amps @ 60 volts DC and 100 watts sub woofer amplifier requires +35-0- -35 volts DC @ 2.5 amps with 23 0 23 transformer will give you +32.5 -0- -32.5 volts @ 2.2 amps will give an output of 100 watts into 4 ohms load.

  17. in the absence of 470pf can I use a 330pf & 140pf in series ?

  18. chandima fm Colombo

    Seetharaman,could I use TIP3055 transistors instead of 2N3055. Also pls advise me the current consumption at full output. I intend to buld stereo version, so I need this info to purchase the power transformer. Your early reply is appreciated

    • Seetharaman

      Yes you can use TIP3055 use a good extruded alloy heat sink. Current consumption at rated output power is approximately 1.2 Amps. For a stereo version you require 60 volt D.C at 2.5 Amps. Use a 42 volt transformer capable of 3 amps current with bridge rectifier and a 10,000uF filter capacitor and a 0.1 100volt ceramic disc capacitor in parallel.

  19. pls wat are d volts rating of all d polarized capacitors and d type of speaker 2 b used(subwoofer or ordinary speaker)?

    • Seetharaman

      All capacitors can be 63 volts Working Voltage. Speaker can be full range speaker like PHILIPS Hi Q dual cone 20 watts 4 ohms 2nos connected in series in an air tight sealed enclosure of 35 litres internal volume. (Say 50cms X 30cms X 25cms) or with 8″ 8 ohms 40watts woofer(rubber textile surround), a 5″ 8 ohms squawker and an 8 ohms 1″ dome tweeter with suitable crossover network in air tight enclosure of 25 to 35 litres internal volume.

    • Seetharaman

      It is a power darlington pair transistors use them with our 150 watts amplifier.

  20. Hi

    sir my Motional feed back amplifier doing sound like motorbike please help me

    • Seetharaman

      Dear Sandeep The motor boating sound is due to poor LF decoupling. Check the layout and ensure supply leads are not greater than 2 inches, power supply should be as close to the power amplifier as possible. have you used bread board or proper pcb with good track thickness and width. kindly send the photo graph of your finished amplifier to analyse your problem for a solution. hope you have not deleted any component especially 1K 470pf and 47pf.

      • Thanks sir I recognise my problems and solve it now it is working smoothly
        and one suggestion sir why you are not in forum please replay soon

        • Seetharaman

          Hi Sandeep hope you are fully satisfied with the quality of reproduction of the amplifier.
          yes once in a while I visit this site, but not replying for want of time.

  21. Quote : the current is proportional to cone movement.

    Sorry, not at all.

    Firstly, it is the cone movement which has something to do with the current in the voice-coil, not the reverse.

    The current controls the force on the cone with the famous equation
    F = B * l * i

    Currently, this project is not an MFB system.

    You can extract the motion of the cone with a Voigt bridge which senses the back-EMF. Back-EMF is proportional to the velocity of the cone (it is quite difficult to remove the inductive value of the voice coil from it) and you can use it in a velocity feedback project. The effect is to lower the Qes (and then Qtc) of the driver, and it then needs some bass boost to restore the low frequencies. As it is a feedback process, it lowers the distorsion.

    • Seetharaman

      Dear forr please read it as cone paper movement in a speaker is proportional to the current flow through its voice coil. sorry for the mistake.

  22. Hit here can you please send me the PCB board design
    many thanks

  23. Hi sir

    can i used this circuit for 15 to 20wts in to 8ohm load by make some changes because I have few 2n3055 transistors and i have 22-0-22 1A transformer i please help

    • Seetharaman

      HI ram you can with lots of changes in resistance values, but without any modification you can use the transformer as 44 volt, which will be ok, at higher volumes only the sound will be distorted due to current limitation of the transformer but at 15 to 20 watts level it will be quite good.

  24. Seetharaman

    Hi Sandeep From power supply the Positive and 0 volt to be connected with short thick wire directly to power transistors and to driver stages. the grounding of zero volt to be done only at the signal input socket. If these basic cares are taken then the amplifier should sound great. What about the voltage at the speaker coupling capacitors positive side. it shuold be exactly half the supply voltage(if supply is 56Volts it should be 28volts) check and let me know through my email ID for detailed layout etc. Hope you have not done any modifications or deleted some components. For stability of the amplifier all the components used are very important.

  25. HI sir
    Where are you Seetharaman sir please replay soon. I have a problem on MF amplifier i am mail you detail

  26. sir
    I assemble this amplifier it works good. But when i increase volume it start distorting,i am using 10″ Woofer and 1″ tweeter in shield encluser box is any earthing problem in my amplifier please help me

  27. hi sir
    i made this amplifier it working good but when i incress valuem it start distoring sound and few days ago mistakenly short the out put and one registar is burn please help me

  28. Seetharaman

    Hi Rahul for clarity sake the line is left as earth (that is going to common that is zero volt bus). For this amplifier only a single 56volts DC supply is used that is common and +56volts. Here the zero volt (shown as – of the 56 volt is connected to GND Bus). You have connected it correctly to 0.47 ohms. (the end which is connected -ve Bus).

  29. sir
    what’s the difference between “ground” ,”common” and the “-ve supply”? In the circuit u have written “GND” below and have drawn the ground symbol above (with the 4.7uF cap)are these different? I have connected both of them along with my other end of the input. where should I connect the negative supply in my circuit i have connected it near the 0.47ohm resistance (below the speaker).

  30. Seetharaman

    Hi Rahul you have to assemble the amplifier on an metal chassis and ground the zero of the power supply to chassis at only at one point at the input signal socket only. i think you have kept your wiring totally floating. For high power amplifiers proper dressing of cables in input, output, power supply are very important. Please take care.

  31. sir
    sorry to annoy u gain but the motor boating sound is still there.I have even used very short thick copper wires that are used in the house hold wiring but couldn’t reduce the sound . however I have noticed that if I touch the 3055(the one with +ve supply ) with my hands it reduces.
    but if I touch the one with -ve supply it produces an high pitch sound.
    couldn’t understand what’s happening.

  32. thank you sir i will make the modifications and let u know .
    sir u were about to send me an detailed lay out of home theater using 2 of MF amp one 100 watt sub mail id is

  33. Seetharaman

    Hi Rahul the length of the wire connecting positive and common of the supply to amp board should not be greater than 2 inches it should be as short in length as possible. 0.33ohms / 3 watt is OK. Unless otherwise leakage and noise levels are high the standard trasistors available in the market are good enough.

  34. sir
    i could not understand what does dc supply short to the out put transistor mean ? i have put 100uf & 0.1uf on the mf amp board not on the supply do i need to put one more pair of that?? the pcb quality is not that good .the polarities of 139 & 140 are ok.the wiring is also fine. the bias diode polarity is also good .can the quality of the transistors effect the sound quality??i have used the cheap ones .
    i forgot to mention i have used .33 ohm 3 watt resistors instead of .47 ohm 2 watt.

  35. Seetharaman

    Hi Rahul check the bias diodes polarity. If there is motor boating sound check whether your DC supply to output transistors are quite short and with copper wire not less than 1.2mmdia(18SWG). ensure decoupling capacitors 100uF and 0.1uF are not the power supply it should be on the power amplifier board on each channel. Are you using good quality PCB or on some proto clip on board(which are not suitable for high power application. Check BD139 & 140 their pins are different from others. center collector, left emitter(in other cases it is base) right is base. Check your wiring thoroughly.

  36. Sir I m in a bit of trouble with one of my MF amp. The audio is very noisy specially for high frequency sound even the bass is not that clear. A loud baaarrrr.. sound is there when powered on with no input. One of the 3055 (da 1 with positive supply )become very hot after few min of operation. Need ur help.

  37. Seetharaman

    Hi Rahul it will a stereo amplifier with 2 channels and 2 speaker boxes(from motional feed back amplifiers of 2 channels) sub woofer amp will be seperate with its own speakers. kindly send me your email i will send a detailed layout. Motional feedback amplifier speakers can have one 8″ full range speaker each or you can have a pair of 3 way speaker systems with each containing one 8″ woofer 5″ midrange and a 1″dome tweeter with suitable cross over network.

  38. sir
    I couldn’t understand your last line can u please explain me in detail. The 8″ woofer will contain the mid range woofer in or it is a separate one and how will I connect the tweeter, with one of the MF amplifier or one one separately at with both??

  39. Seetharaman

    Hi Rahul use 100watts amp with suitable 10″ speaker and enclosure as sub woofer. use MF amplifier as 2 speaker stereo system For MF amplifier you can use one 8″ woofer with 5″ mid range and a 1″ dome tweeter. (that is 2.1 system)

  40. sir
    will increase the no of speakers increase the sound quality??

    what I am planing to do is I have built two of these and one 100 watt sub woofer and use all of these as one sound system like home theatres . will it work ??


  41. Seetharaman

    Hi Rahul you can use PHLIPS 2nos Hi-Q speakers 8inch of 4ohms impedance in series in each air tight enclosure of size 24″ X 20″ X8″ to handle the power of 40watts RMS. For lower power level operations you can use PHILIPS Hi Q speakr 1no with 8 ohms impedance fixed on air tight enclosure of 20″ X 15″ X 8″ to handle 20watts RMS continuous power. These are the most recommended speakers by PHILIPS.

  42. hello seetharaman sir
    I have built two of these and want to use as two channel speaker is there a way of doing so ???
    what should be dimensions or volume of the box??? please reply soon..

  43. Boyet0173

    Hi SEETHARAMAN, Thank you for your reply. I’ll do it again when I came home.. I will update you soon
    if it will work. Again thank you.


  44. seetharaman

    Hi Boyet already there is a DC blocking capacitor 2200uF so you should not get any DC output. if you have not connected any load then you can get it as the capacitor is trying to get charged. Have a practice of connecting a dummy load say 10 ohms 10 watts in the place of speaker and giving zero input,(no input signal) before doing DC tests. The PCBs are available in India as 40watts philips amplifier circuit costing around Rs100/- (INR) per pair of this board. I will send you the design soon.

  45. boyet0173


    I built this design but unfortunately have no luck. Theres dc volt on the output around 40 volt. i re-checked the connection is ok but the problem same. HELP ME MR SEETHARAMAN..Thank you for your reply in you have a pcb layout i really appreciate if you give me.


  46. seetharaman

    Hi Boyet they are suitable equivalents please proceed all the best.

  47. Boyet0173


    I bought the parts list of MFB amplifier, I used TIP31C,TIP32C for the driver and TIP35C for the output. What do you think, it is possible to work without affecting the sound quality? I need your comment on this. Thanks


  48. seetharaman

    Hi Herbert the motion of the speaker cone is proprtional to current flow through that. hence it is used as feed back not just a overal feed back from output. the feed back level is varied with the current flow through the speaker. hence no over loading as a open gain amplifier. you must build and klisten to appriciate the quality. the speakers you are taking about are very expensive and they require lots of mainteneance. Philips Holland introduced such speakers in the year as early as 1972. why they could not sustain in the market?

  49. Herbert Rutgers

    Current-feebback has NOTHING te do with motional feedback!! The only thing you do is extending (a bit) the frequency response at the low and ON PENALTY OF A LOT OF DISTORTION. The only right way is to connect a accelerator detector to the cone of the woofer and build a correction network to integrate the signal obtained into an amplitude sensitive system which should be put in the feedback loop.

  50. seetharaman

    Hi EJ you have to measure between 2200uF + terminal and ground. if it is pulsating it indicates your amplifier is having low frequency instability. this is mainly due to long supply leads from power pack to amplifier PCB. If so ensure a 100uF 63 volt in parallel with 0.1 uF 63 volt disc capacitor is connected from positive to common in the PCB.

  51. sir i measured the voltage across the 2200uF and the common ground it it turns a value that is not constant. also the speaker, when no input connected, is pulsating. like a heartbeat. and when connected to an input, it hums very loud.

  52. Seetharaman

    Hi EJCAB7 kindly check the bias loop 2nos 1N4007 and 27 ohms between driver BD139 collector and pre driver output BD139 for correct polarity. without any input signal try the voltage between speaker coupling capacitor 2200uF and common negative. it should be half of supply voltage around 28volts. if not check the wiring completely ncluding transistor leads electrolytic capacitor polarity and diods polarity. Pl remember BD139 / 140 center collector right base and left collector, different from other TO220 transistors.

  53. okay sir, this is what ive observed.

    when it is properly connected, (based on the design of the pcb), when turned on, the speaker is distorted. cant barely hear the sound. and the power resitor (2n3055) heats up dramatically.

    ive tried to interchange the base and emitter of both the power transistor (2n3055) and the sound was better, (but not like the quality when it was on the breadboard), the problem is, the 56 ohms connected to the ground, for the bottom transistor,
    and the other 56ohms connected to the 0.47 and 4.7K ohms, for the upper transistor burns. the smell was awful btw.

    i dont know what seems to be the problem. please help me, been trying to fix this for 3 days. checked the connections several times, doing manual simulation, etc…

  54. hello again.

    i have another problem. after making this amp on a breadboard. i tried doing it on pcb. when everythings connected, the speaker is pulsating. ive tried to look for a shorted coppers on the pcb but theres none.


  55. Dieudonne

    hi Seetharaman i have a circuit diagram to share with, how can i upload it.

  56. @EJCAB7
    thanks a lot… it really worked…………..

  57. Seetharaman

    Hallo EJCAB7 you can use 100k log(if not available use lin) potentiometer. one end connect to ‘in’ GND slider (centre tag of pot) to ‘in’ 1k and input signal between GND and other free end of the potentiometer.

  58. hello mr Seetharaman.

    how can i put a volume control on this amplifier? i read from your another circuit, the 50watt transistor amplifier, putting a 10k POT in series with the input would do it.

    ive tried it and its not working. i checked the POT with a DMM and its properly working.


  59. @RAHUL

    the voltage rating from my capacitors is more than 60 volts. (just to be in the safe zone).

    some are rated 63v, some 100v, well basically it depends on the availability of the component. 🙂

  60. i got it working! i found the problem, the cap before the speaker on my circuit was 220uF instead of 2200uF.

    darn the sound quality is good!!! thank you very much sir Seetharaman!!!

  61. hi
    you have assembled the circuit …. can u please tell me about the voltage rating of the capacitor’s used I am having a bit of problem with the capacitors.

  62. sir Seetharaman,

    the output sound of the amp is a bit distorted. do i have to make the pre amp from the article? the baxendall tone control? or this amp would just do fine without it. thank you. 🙂

  63. hi seetharaman sir i am trying to assemble the circiut on my own .i am having a difficuilty with the capcitors .can u please tell me about the voltave rating about the capacitors….


  64. Seetharaman

    Hi EJCAB7 the other one which is cool might have got shorted. with a DC voltmeter kindly check the voltage at the junction of 0.47 ohms it should be half the supply voltage. if it is not kindly check the component connections, and also for a dry joint. the speaker one terminal is connected to the 2200uF capacitor and the other end is connected to a 0.47 ohms, which samples the current flow through the speaker and give the feedback.

  65. hi again mr. Seetharaman.

    turnso out my circuit has many incorrect connection. the sound was a bit louder than before but still not clear. and i notice that the first power transistor (2N3055), the upper one, is not getting hot unlike the one on the bottom. does this mean that its not working and may be the cause of unclear sound?


  66. sir i dont really get the speaker to be not grounded. or floating.

  67. thank you for the reply MR Seetharaman

    im sorry if im asking you this questions, im not really an experienced person yet in terms of this electronic devices. :p

    another concern is that i bought the wrong power rating for the resistor 56ohms (resistor burnt on the circuit). i read one of your reply that the power rating of 56ohms is 1/2 watt. but why is it that, that specific resistor is the only one that was burnt. (other 3 56ohm was doing fine)
    my 0.47 ohms also has a power rating of 1/2 watt, but doesnt burn also. could this be the problem of a noisy sound?

    thank you again for assisting. :)))))

  68. Seetharaman

    Hallo EJCAB7 if the transformer is of bad quality it will vibrate. if the transformer is over loaded also it will vibrate. Check all your connection, cold check all the transistors check the polarity of all bias diodes, check feed back line, check the speaker is not grounded (it should be floating)check BD139 and 140s for their lead identity-it is different from standard transistors.

  69. Hello.

    im just a student and i have a problem making this amp.

    first is that, my transformer from the power supply vibrates. (its not normal right?) can you identify the problem? 🙂

    and the output sound is very noisy. pls help me. thank you. 🙂

  70. gregtherider

    I have to say that this is one really nice amp. Been listening it now for about a month and im impressed. In my opinion the main difference to the other similar amps is that this one has way much more tight and defined bass which I like. I used 10000uF cap in power supply so the hum really almoust dissapears.
    It is really a nice amp to build! Thank you mr Seetharaman for sharing!

  71. Seetharaman

    Hi Rup when you connect a tuner signal to the amp and tune into a station you will get in back ground humming sound (even if you increase the filter capacitor value it will not vanish). when you of tune the station it will vanish. this phenamina is called tunable hum problem, to clear it you have to add 10nF as shown in the above circuit. you can practically try without the capacitor and with it by using an AM tuner you can see the difference distinctly.
    Hi Chris by sensing heat using thermister or a silicon diode can be fed back for controlling against a set level.

  72. sir i really need to know how an temperature control using feed back amplifiers work?

  73. Thanks for reply,sir can u plz tell me about the power supply how it works. Here u use two 10nf capacitor for what purpose.
    Thank u again

  74. Seetharaman

    Hi Rup 0.47 ohms are 2 watts, 56ohms are 1/2 watt and all others are 1/4 watt.

  75. Hi Seetharaman Sir,
    I am trying to make this motional feed back amplifier,plz tell me about the wattage of the component(resistor) used.

  76. Seetharaman

    Hi Greg yes it requires 60mm X 25mm 1mm thick bright aluminum sheet as heat sink (or you can use any standard TO220 heat sink,) as they get warmed up.

  77. gregtherider


    Do transistors BD 139 and 140 need cooling or not?


  78. Seetharaman

    Hi Reji the motional feed back principle is to give the movement of the speaker as a feed back so that they start moving but not infinitely to avoid large excursion of voice coil and getting damaged with sudden peak poweror to produce huge distortion of sound. the current flow through the speaker is sensed across 0.47 ohms and fedback, (as the movement is proportional to I sq T where I is current flow through voice coil and T is voice coil number of turns).
    Since these current setting pots 1K give way over a period of time and blow the output trasistors it has been changed into a simple rugged biasing system.

  79. Dear Seetharaman,

    Thanks a lot for your article. In fact the Philips 40 W amp beats most of the designs that came afterwards in quality, despite the fact that it has no differential input and uses a single rail power supply. I had made it in 1991 and it was in use until recently, when one of the output transistors blew and I didn’t get the time to repair.

    One doubt I have is the following: in the original Philips circuit motional feedback is taken from the ungrounded speaker terminal, whereas in your modified circuit it is taken from the ungrounded end of an added series resistor (0.47 Ohms). Is this the major difference between the two circuits? The other changes I noted are removal of bias servo TR7 and addition of base resistors to the output transistors, but these may not affect the signal quality. Or am I missing some other point here?


  80. Hi Seetharaman,

    Thanks very much for all your kind advises and recommended preamp circuits. I will proceed with the construction of the MFB amp using the simple transistor pre-amplifier with single supply and passive tone control as suggested for best results.
    Will write to you again for the results of performance.


  81. Seetharaman

    Hi Suresh no it will over compensate hence use on PC location itself as it has to compensate for internal ambient temperature.

  82. Hi Seetharaman,

    I have another question about the circuit. Do we need to mount the two biasing 1N4007 diodes (in series with 27 ohm resistor) onto the heatsink?


  83. Hi Vinoth,

    Thanks very much for your reply concerning my questions on the Motional feedback power amp. I’ll stick to the original components as suggested in the design. I understand that if we replace the original components with something else would also change the quality of sound.
    I have intentions of building this amp. More power is not important to me as long as the quality of sound remain good.

    Once again thanks very much for reply.


  84. Hi Suresh
    BC 157 is equal to BC557,higher version u need to goin for MOSFET which will cost u high, but i doubt whether it will give MFB clarity
    As i made a comparison of TIP 142,147(100w) & MOSFET IRF250(300w), compare to both, this 2n3055(40w)in clarity is the best… my view……

    Even the clarity depends on preamp also.

    I sugget get advise from Mr Seetharman on Preamp also power amplifier


  85. Seetharaman

    The pre-driver can be any general purpose PNP transistor with a Vce of 60 volts and Ic of 100mA and hfe around 100 at 10mA. like BC556, BC557, 2N3906, 2N2907A, BC327 even MPSA92 or 93. You can use any amplifier suitably modified for this function. I prefer you to use a simple transistor pre-amplifier with a single supply and a passive tone control will give you the best results. you can use the following circuit
    if you want an IC based try this the best one.

  86. Hi Seetharaman,

    The pre-driver transistor BC157 is not easily available here for the motional feedback power amplifier of your article. Any recommended equivalents?
    Also, do you have a higher power version circuit of about 100w or more? And do you have a nice preamplifier with tone control circuit for me to built for other power amps?

    Appreciate your reply.

  87. Hi Seetharaman Sir,

    At presemt i am developing this amp(2n3055)for 5.1 home theater, post doing reserch of other mostfet &TIP amp, i found this phlips 2n3055 is “better than the best”, also i understood that only universal preamp suits for this amplifeir,
    i need some advice from you for develooping this amplifier on power voltage and preamp. can you send me your contact number to my mail ID so that i can call you for clarification,


  88. Seetharaman

    Hi Mahajan the kit form is not available. vinoth is correct you can get all the components in Ritchi st. you must listen to beleive the quality. application book 40watt PCB you can use as we fix power transistor on chassis with heat sink, the protection circuit is not required motional feed back will take care. I can send you a nice most tried and suited preamplifier with LM387 / TL072 and passive tone control. send your email id.

  89. Hi
    This is wonderful amplifier and we need to use the universal peramp for better output ,
    BD182 were used in70s, at present v need to alter with 2n3055,PCB is verymuch available in the market ritch st,
    For further details mail me


  90. A Mahajan

    Kind Attn Mr. Seetharaman,

    Dear sir,

    Is this circuit using the same PCB as the one published Philips Application Book Audio Amplifier Systems?
    The circuit in this is built using BD182 output transisters.

    Also is it possible to get this circuit in a kit form or as a completed PCB?

    Thanks and Regards,

    A Mahajan

  91. Narayan Singh

    Can we make such transister as amplifier working only with one npn or pnp transister. If yes what will be the circuit diagram. please mention how speaker and micro phone can be connected.