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Basic transistor Guitar Amp

Discussion in 'General Electronics Discussion' started by calx, May 15, 2012.

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  1. calx

    calx

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    May 15, 2012
    Hi all.

    I am new to this forum and I have what I guess must be a pretty simple problem to start with.

    I would like to make a guitar amplifier for my A2 level Product design project and so I thought I would start with a simple transistor amplifier to get things going. Basically, I have tried this a few times and watched videos, read books and seen schematics and I cannot get this circuit to work (see attachment for schematic).
    I can hear the interference when I hook up the battery so it all must be connected up, right? But when I plug the guitar in, I get no sound out of the speaker whatsoever. I have tried to follow a schematic I saw on youtube for a transistor preamp where his input was a mic and his output a PA speaker, he ran it through an IC power amp stage though. Do I need to do this to get any sound?
    If I need a power amp stage then I would prefer to make it myself out of transistors instead of an IC as I would like understand how it all works, so does anyone have a schematic for a basic power amp stage?

    Any help would be appreciated as I don't really know where I am going wrong.
    Thanks in advance
    Callum

    PS, the transistor is a 2N3904 if this matters
     

    Attached Files:

  2. BobK

    BobK

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    That amplifier will produce a gain of 10, not nearly enough to take the input from a guitar to drive a speaker. You would need a gain of about 100 to hear anything and 1000 to make it reasonably loud.

    A reasonable low-power discrete transistor amp, would have two gain stages, like the one here and a push-pull output stage.

    Edit: I take this back. The output from an electic guitar is much higher than I thought. If you just add an output stage to your gain 10 amp you should hear something. The problem with the circuit you have is that it cannot drive a speaker.

    Bob
     
    Last edited: May 15, 2012
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Your circuit is a small-signal amplifier. It can't drive a loudspeaker. Its output impedance is too high (the collector resistor is 1k). You could scale down all the resistors and replace the transistor with a big one running at a much higher current, then you would have a Class A amplifier and these are notoriously inefficient (i.e. the transistor will need a BIG heatsink).
    There should be lots of designs for simple transistor-based amplifiers available on the net. Here are a few documents I found with a quick search.
    http://users.ece.gatech.edu/mleach/lowtim/ - the Leach amplifier. Actually this one is relatively complicated; it's designed for high performance. But there is a very detailed circuit description, which might help you understand a simpler design too.
    http://sound.westhost.com/project12a.htm - Information on a relatively simple amplifier (but an old design).
    http://www.transkommunikation.ch/da...udio_circuits/60 W Simple Power Amplifier.pdf - a newer version of a similar amplifier rated for 60W output, with details of how it can be improved. These designs have been simulated but never actually built!
    There are sure to be lots of simple designs available on the net for Class B audio amplifiers but I can't find any of the really simple ones. Perhaps you should look at electronics hobby magazines; they used to publish these sorts of things.
     
  4. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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  5. calx

    calx

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    May 15, 2012
    Thanks for that guys, I will look into it. Those links look like they have some good info in, I will give them a read. Would anyone recommend the use of a specific kind of transistor for this?
    Cheers again, Callum
     
  6. BobK

    BobK

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    The output stage will depend on how much power you want to produce. If you are looking at <1W something like 2n2222 / 2n2907 pair would work fine. For more power you will be looking for power darlington compilmetary pairs.

    Bob
     
  7. calx

    calx

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    May 15, 2012
    Thanks.

    One last question, The output stage, what does that consist of? Could I create a Darlington pair from two 2n3904s or 2n2222s?

    Thanks again for your help
    Callum
     
  8. BobK

    BobK

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    You could, but you would not want to. If you are using those transistors a darlington is not needed. The darlington configuration is needed with more high power transistors which have lower gain.

    After work, I can post a circuit using 2n3904 and 2n3906 that I have built, but its output is only about 1/4 Watt, which is a guitar amp that only your neighbors could love.

    Bob
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    What kind of speaker are you going to be driving? The 2N2222 is used in applications as a small signal or switch. It's not intended as a power amplifier. Power transistors are most commonly found in TO3 or TO220 case styles and are much larger than a 2N2222 to facilitate dissipation of heat.
     
  10. BobK

    BobK

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    As I stated above, it would be good for up to about 1 Watt output.

    Bob
     
  11. calx

    calx

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    May 15, 2012
    Ok, thanks guys, a diagram would be great :)
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Here's a stereo power amplifier chip. You only need to use one half of it. I'm certain there are a host of mono chips available too. On the other hand, if this is something that you want to build with mostly discrete components please post so.

    http://www.datasheetcatalog.org/datasheets/166/378181_DS.pdf

    EDIT: Alternatively, you can use the other half of this chip to drive a second speaker.
     
    Last edited: May 16, 2012
  13. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Yes, I've used the 2N2222 to drive speakers =>30 Ohm operated class D but the application was mainly for producing an alert tone, where distortion wasn't a factor. I don't see them as a good choice to drive 8 or 4 Ohm speakers., where faithful signal reproduction is vital.

    The 2N2222 is spec'd @ 1.2W Max @ 25 degC (Case Temp). It's doubtful that this spec can be maintained without running it in a freezer. :D
     
  14. calx

    calx

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    May 15, 2012
    Hi, It would be nice to build this type of thing from discrete components, I kind of like to understand how things work on the inside, so building one first, even if I use a chip later on, seems like something I would like to do. Also, The more understanding that can be shown in the coursework the better.
    Cheers
    Callum
     
  15. BobK

    BobK

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    The power dissapation in the output transistors is not the same as the output power. For a 1 Watt amplifier, in a push pull stage, they would only be dissapting 1/2 Watt each of the efficiency was 50%, which would be way low. So, I still maintain that you would easily get 1W out of a 2N2222 / 2N2907 pair. I have a working 1/4W amp using 2N3904 / 2N3906 and they do not even get warm to touch at full output. And the 2N2222 is spec'ed at 4 times the current.

    Bob
     
  16. BobK

    BobK

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    Here is the amp I have been talking about. It is intended to take the output from an MP3 player and so a guitar is a bit lower than than that and it put out just barely audible sound. If you combine it with your original amp it should produce a little bit of sound from a guitar. This amp runs off a 9V battery.

    If you play with this and understand how it works, you should be able to make it operate a higher power.

    AudioAmp.JPG

    Bob
     
  17. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    You're right it isn't the same. It's more. Your logic only holds true for Class D switching. Do the math on this simple representation of a 9VDC source feeding an 8 Ohm load. Assume 500mW for RL power dissipation. Then calculate what value RS must be to deliver 500mW to RL. Finally, calculate the PD of RS. RS represents the Emitter- Collector nodes of a BJT, FET, or any other analog source you want it to be. Because Ohms Law doesn't care what it is.
     

    Attached Files:

  18. calx

    calx

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    May 15, 2012
    Thanks for the schematic, I take it R9 8 ohm represents the speaker?
    So there the first part is similar to the one I already made but with a push pull output.
    And so what you guys are saying is that it is important that the transistor can handle the power?
    Cheers again
    Callum
     
  19. BobK

    BobK

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    CDrive,

    Okay, I will do the calculation. For 1 Watt output into an 8 Ohm speaker the RMS voltage has to be 2.8V. The push-pull stage will have one side of the speaker at half the 9V so the transistor has to drop, on average, (4.5-2.8)V or 1.7V. The current for 1W output into 8Ohms is 2.8/8 = 350ma. Which comes to 1.7 * .35 = .6 W, but each transistor is handling only half the wave form and carries no current during the other half, so the average power dissapation is 0.3W in each transistor. Well within the limits of a 2N2222.

    Bob
     
  20. calx

    calx

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    May 15, 2012
    Thanks Bob, That makes sense really :)
     
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