Making an Audio Power Amplifier (Drive a Sub Woofer)


Designing an audio power amplifier is much simpler than you think:



The audio amplifier I designed here is to drive the sub woofer speaker I showed, which I used in my other experiment in “The Secret to Levitation (Experiment with Water)

The schematic of the amplifier is repeated here below:

Power Amplifier Circuit

Power Amplifier Circuit

The parts I used are listed here:

  • Q1: D44H11TU from Fairchild, NPN 80V 10A TO-220
  • Q2: KSB596YTU from Fairchild, PNP 80V 4A TO-220
  • U1: TS912IN from STM, Rail to Rail Dual OpAmp, 2.7 to 16V, 8-DIP
  • R2 = 5.6K, R1 = 1K for a gain of -R2/R1 = -5.6 V/V

They call this circuit a class AB amplifier, where there are two output power transistors, one on the high side (Q1) and one on the low side (Q2). Q1 supplies the positive output voltages while Q2 supplies the negative output voltages. So they each share half of the output power if the signal is symmetric around zero.

If we supply the input signal directly to the bases of the transistors, the output signal will be highly distorter due to two reasons:

  • Each transistor needs around 0.7V base emitter voltage to turn on. Therefore for any signal smaller than 0.7V amplitude the output remains unchanged and transistors will clip the signal.
  • The transistors show non-linear large signal behavior under load and will distort the output voltage.

Therefore I added an Operational Amplifier (OpAmp) to the circuit. What the OpAmp does is to compare the output voltage to the input voltage and generate a special signal at the OpAmp output or the transistor input, which would cause the transistor output to be almost equal to the input signal. Below is the hand drawn waveforms, if you can’t tell!

Voltage Waveforms

Voltage Waveforms

You can see that the OpAmp output voltage (VOpAmp) jumps when the voltage gets close to where the transistors turn off. Doing this, the OpAmp makes sure that the moment when transistors are both off is very short and this will cause a very negligible glitch at the output that is not noticeable.

Feedback resistors R1 and R2 are used to adjust the circuit gain. The gain here is equal to -R2/R1. I adjusted the gain for my circuit to around 6 V/V because I measured the output amplitude of my cellphone to be around 1V and I wanted a swing that could go as high as the circuit would allow.

The output of the phone is design to drive headphones with around 40 ohm impedance. I usually choose the feedback resistors above 1K Ohm. So the phone will have no problem driving such load.

The transistor outputs can still be slightly distorted, but it will not show at low frequencies, and depending on the speed and drive capability of the OpAmp it might only show at higher frequencies that can be outside the human hearing range anyways.

The circuit is design to work with a positive and negative supply with a common ground. The output voltage will be limited by the supply, and the limiting factor is the output of the OpAmp. So to get the maximum voltage swing, it is better to use a rail-to-rail OpAmp. Also you can always increase the supply voltage.

But beware that if you increase the output level, you are putting more power drop across your transistors. So make sure you do your power calculations and provide enough heat sinking on those transistors to keep them cool enough and avoid damage.

Isn’t it awesome that drill batteries have this extra center contact?! You can use one battery as a dual supply.

You also see in the video how the speaker vibrating at 31Hz seems to be moving very slow. This is the exact same effect as in my previous post, “The Secret to Levitation (Experiment with Water)“. The slow movement is something that is captured by the camera and is not visible to human eye in normal cases.

My next video will continue on a new experiment using the same components, but with very interesting and different results. So stay tuned!

55 thoughts on “Making an Audio Power Amplifier (Drive a Sub Woofer)

  1. hey i am actually designing a 2.1 home theater system. Can you please tell me what are the major differences in the design of subwoofer for 2.1 and 5.1 channels. I surfed a lot to get this answer. May be yop can help me.

  2. For a change fruit juices are quite a healthy substitution over
    whole fruits, how many calories in butter and sweet
    treat and carbonated beverage. In fact, it is said that there is more potassium in a mushroom than
    a banana.

  3. I have been browsing online more than three hours today, yet
    I never found any interesting article like yours.
    It’s pretty worth enough for me. Personally, if all website owners
    and bloggers made good content as you did, the web will be much more useful than ever
    before.

  4. Hi Mehdi, I am wondering.
    1. Can I use this power supply (http://www.ebay.com/itm/EU-12V-1A-AC-DC-Power-Supply-Adapter-for-CCTV-Camera-Free-Shipping-/350939519397?pt=US_Surveillance_Cables_Adapters_Connectors&hash=item51b5a019a5 ) connect this to connector and through LM7806 ( 6V 1A Positive Voltage Regulator) to circuit instead of use drill battery? Because I wanted to do another board with 2 regulators to supply this circuit and strobelight from one power source. If I can please tell me on what I should pay special attention? If I am wrong please tell me how to supply this from EU electrical socket in wall (230V 50Hz)?

    2. How I have to connect cables from jack (Vin) in this circuit because i am not 100% sure? If I have stereo jack I have to connect right and left channel to Vin and rest to ground?

    Sorry for my English and please explaine like for silly guy because I am beginer.

    • Answer 1. http://lh3.ggpht.com/_gnU30vMbtYg/Tcsp3cNq3oI/AAAAAAAAAlY/zKc93faxPoE/virtual%20ground_thumb%5B1%5D.png?imgmax=800

      Resistors 100Ohm.

      Answer 2. Blue and red (right and left channel) to Vin and both gold/orange to ground.

      I post answers on my questions in case somebody had same problem ;P.

      Already did it, but I have one more question:
      Maybe you know how to get bigger current in this virtual ground solution because i got -/+6V +/- 60mA if I have right I need 400mA ?

      My DC have 12V 1.2A.

    • Well, with one supply like this it is tough to make the circuit. The issue is that for the circuit the way I made it there will be also high current through your ground, so you can’t just make a virtual ground with resistors. Also you can’t use a capacitor to decouple your load easily as the frequency is very low for my application. If you want regular audio though, a 1000uF capacitor could work well. Then you can make a virtual ground as a reference for your OpAmp, have the big capacitor connected between the circuit output and your load (speaker) and then connect the speaker to supply negative.

  5. Hi again ;p,
    I wanted to ask, what if I have a 8 ohm 35W speaker? Please tell me what I have to change in circuit and what will happen if i leave like this?

      • I would like to use almost full power of speaker. -R2/R1 should be a little more than -8V/V to get about 7,6V – 7,8V output? If I am wrong, please correct me.

        sorry for my english and silly questions ;p

          • In order to have 35W of power across 8V speaker with a single tone means a 24V amplitude signal, so you would need over 50 volt supply for that. with a 12V battery and this circuit you won’t be able to provide that much power, maybe close to 3 W. You would need something else…

    • and i am wondering what cables i can use to connect resistors with opamp and etc. I thought about LAN UTP but i dont know maybe that will be to thin wire ? can you say something about this because i am beginner and i am afraid that it will be warm or something

    • These transistors seem powerful, but the VCE saturation of them is pretty high, like 3V. It means when the current is high, there will be a large voltage drop across the transistors and will distort the output too much, which in return limits the output swing.

  6. Hay, nice video, and good instructions. I’m building a portable phone amp/radio thing which will have monster bass for its size. but I’m building on the cheap and wanted to know if your amp sircus will not only work for a woofer, but for regular audio. Will the sound be high quality or am I going to need to build something more complex?

  7. Hi Mehdi, i m very love your stuff :D
    can i ask something, how you make your circuit to connect to your phones in your videos?
    Circuit that you touch and very hot in your videos above (that’s funny :D)

    • The amp is necessary if you want to drive the speaker with a smart phone like I did. But if what you use to play the tone already has a power amplifier in it, then you won’t need to make a separate one.

  8. HI mehdi, Sorry for the trouble…. :), Now i am making the amplifier you have show I can not get the Transistors you specified I found the ones available to be here are 2N3055N and 2N3055P ?? other than that got everything the Op-Amp I am using is Lm393 which I checked have the same features as the one you specified, if I am making any error please guide! Thanks

    • make sure your transistors can drive a couple of amps, and with hef of 50. Any OpAMp with output current capability over 20mA should work fine. If your OpAmp is lower current, make sure hef of your transistor is larger. Good luck!

  9. i have a scool project where im supposed to use 100 watt bass and a cople of speakers and i need to make a curcuit so i can play music on it and the have to play thogether. its supposed to fit in a backpack so i can bring it to my school and i ned help with the curcuit. i dont know where i’ll get the parts and i dont know how to get them playing thogether.

    plz help!

  10. hello mehdi.
    Im a computer science student so my knowledge in circuits is quite low. ive been wondering if you know any good book to read as an intro to circuits and from there to a more complex reading on this topic?
    thank you.

    • I wish I could remember, I started my bachelor 18 years ago. And I don’t recall using many text books, just the usual classroom data given out by professors. I guess internet is your best friend!

  11. Mehdi,

    You are the man. You make electronics easy and fun; I always laugh when I watch your videos. I am trying to build the circuit above in order to create a Chladni Plate (http://www.youtube.com/watch?v=GtiSCBXbHAg) – using water, though, which is more suited to a sub.

    I may or may not be able to get a drill battery, so could you tell me if it would also be possible to use a virtual ground built off another op-amp? Maybe the other amp in the dual package? See this resource: http://www.swarthmore.edu/NatSci/echeeve1/Ref/SingleSupply/SingleSupply.html … I would go with the inverting flavor.

    Thanks again!

    • You can do it two ways:

      - Use two batteries powerful enough to drive the output, put them in series and use the center as ground.
      - Use a single high enough voltage battery, like you said, use an OpAmp or just a good resistor divider to create a center voltage. But you can’t drive the output off of that voltage. So connect you load to battery negative, and use a capacitor in series with the output to DC isolate the load from output. If you are using very low frequencies, your capacitor needs to be large in value.

      Good luck!!

      • Thanks for your quick reply Mehdi.

        I definitely understand the first strategy – it’s like your original setup with the drill battery. To check my understanding, you’d wire everything up the same way, except your supplies would look like this: http://draw.to/DFp31e.

        As for the second strategy, what you’re saying is that the center “ground” created by the divider can’t independently source current without affecting the other supplies. So instead of connecting the load to Vout and ground, you’d connect it to -Vcc, which can source current, and Vout which will provide the amplified AC signal that will be transduced into sound. The DC offset that would appear at Vout due to -Vcc is filtered by the big cap. So the wiring is the same, except for the source of ground (top of drawing) and the connections to Vout and the load (bottom of drawing). Is that right? http://draw.to/Dn0pCo.

        Thanks so much!

        ET

        • Both drawings are correct. Regarding the second one, it’s not the supplies that are affected, it is the divided voltage. Because if you assume that as ground and connect your load negative side to it, the load current will need to go through the divider. Your load current is large and an OpAmp or resistor divider can’t hold the level.

          Using a capacitor, assuming your speaker is 6 Ohm, you would need the capacitor to have an impedance lower than 0.6 Ohm at least at 10Hz to get a good output level. So your capacitor must be around 30mF or larger.

          • Hi Medhi,

            Right. Your explanation on the current through “ground” makes more sense (:

            Would you please explain your calculation for the cap impedance? I tried to do the math with an RC series circuit but feel like I’m missing one piece of information/assumption. I need to build this tonight so will continue to look at it, but want to make sure I don’t blow anything up (one thing I have definitely learned from your videos is that this can happen)! I have a 4 Ohm speaker.

            For my picture, I used Gravatar!

            Thanks,

            ET

          • I just assumed a capacitive impedance much smaller than the speaker resistance of 6 ohm. For much smaller I went with 0.6 ohm. And for low frequency I assumed 10Hz. You can change the assumptions. Capacitor impedance = 1/(2*Pi*f*C) so 0.6 = 1/(2*3.14*10*C) and so C=26mF. The larger value, the lower impedance, so I rounded it to 30mF.

  12. Pingback: The Strobe Light Effect (Levitating Water) | ElectroBoom

    • The transistors are rated for 30W and above. So if you increase the voltage rails as much as the circuit can tolerate, you might be able to achieve close to 30W, although I doubt it if you can go that high. You should do some heavy heat sinking on the transistors too.

  13. Hi Mehdi,

    One basic question about the amplifier. You mentioned that the gain of the op-amp is around -6. In your awesomely hand-drawn waveform, why isn’t the output of op-amp 180 degrees out of phase with the input? (i.e. shouldn’t the output of the op-amp be negative when Vin is positive?) Consequently, shouldn’t Vout (transistor output) also be opposite of Vin?

    Thanks in advance for the explanation!

  14. Oh how I wish you made this just a couple months earlier! My friend and I were in an engineering class in high school and we decided to build an guitar amplifier. Where we got stuck on was how to start designing a schematic for a 40W amp. Your explanation surely clears up some problems we had!

    Thank you for doing this experiment!

  15. Pingback: Shocking truth about making an audio amplifier | BRYAN LENETT OFFICIAL WEBSITE - BryanLenett.com

  16. Thank you, Mehdi! I hate to ask, but could you provide a (rudimentary) bill of materials for that circuit? (i.e. what part# did you use for R1/R2, Q1/Q2, the opamp, etc.?) I’ve got perfboard, I’ve got hookup wire, I’ve got solder and an iron and I’m not afraid to use ‘em – but I’m hopeless at figuring out the BOM from a schematic, and I bet I’m not the only one among your fans.

    Thanks again for the videos and blog!

    • it the gains is 6 V/V and the feedback resistor (R2) is 1Kohm then, because gain = -R2/R1, R1 should be around 165 ohm…opamp must be a LM741 (check DC voltage)…
      Q1 should be a npn BC547, Q2 pnq BC547

      • Thanks Chuck for the parts. There are some issues with them though: LM741 is not a rail to rail OpAmp and will limit your output. Also the transistor you provided (same part number for both) can only handle 100mA which is not enough for a 6 ohm load. I’ll provide my BOM soon.

    • Hi Marc, I updated my post and added my BOM there. You might want to put heat sink on the transistors if you are taking high power from them. Good luck!

Leave a Reply

Your email address will not be published.

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>