Simple single transistor audio amp

Discussion in 'The Projects Forum' started by CTRL+X, Aug 16, 2009.

  1. CTRL+X

    Thread Starter Member

    Jul 1, 2009
    26
    0
    I'm looking for a schematic for a single transister NPN audio amp. It is will be used to amplify the beeping signal from a radar detector. Input signal will be coming from a headphone jack. I would like it to power off of a 12vdc supply, so that I can hardwire it into the vehicle, but if it's simpler to use a smaller power source, batteries are fine.

    This is not for music, so I don't care about frequencies/clarity, I just need to amplify the beeping alerts from the RD. Looking for the simplest possible.

    I tried building this below on a breadboard, but it didn't work. Of course it was probably a builder error :rolleyes:
     
  2. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    For a simple beep amplification, the following ckt is enough at 0.5W

    [​IMG]
     
    Last edited: Aug 16, 2009
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I'd put a cap in the input myself. Something around 10µF.
     
  4. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    Ya, a cap is better than a power wasting resistor.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I was thinking both actually. The resistor can be small, but you never know what kind of DC component the output of the previous device has. A cap is cheap insurance. A lot like the resistor actually.
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    I can't see any big mistake in the original circuit.

    Can you tell us where do you think the big mistake is?

    On the contrary, your proposed circuit have the possibilities of burning up the loudspeaker and/or the darlington. :(
     
  7. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    How could it be that the battery and speaker all in one line??

    Pardon me if im wrong but I havent seen any circuit like that with the supply going to collector through the speaker. The circuit I drew up is the most commonly used types( I think !!, Im not good wit transistors) . Could you explain what makes the mistakes??

    Ive removed my ckt if you think its wrong.
     
    Last edited: Aug 16, 2009
  8. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    All of the single transistor circuits I've designed never seem to have enough amplification to drive a speaker. I usually use one of those ceramic earphones you find in crystal radio sets. I would recommend that you use a transformer on the output to match the impedance. I'm not sure, but Is there a formula out there so I can calculate the impedance?
     
  9. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    Haven't you tried the above ckt i have posted in any project??

    It provides a minimum amplification. I tried replacing them with several other transistors and found that types like BEL provided very much better amplification. I can say that 1 transistor configurations can provide enough amplification to drive a speaker, a small one, ok.

    And how would you like to calculate the impedance? A transformers formula shpuld work right??
     
  10. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    Just open your mind a bit ;)
    It's just how it's drawn that's confusing you. Pull the +9V point to the top and rearrange the rest and you'll see that it's a totally standard circuit.
     
  11. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Like this:

    [​IMG]
     
  12. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The darlington transistor is almost saturated all the time and therefore is a poor amplifier. The speaker with DC in it might not work either.
     
  13. CTRL+X

    Thread Starter Member

    Jul 1, 2009
    26
    0
    Can I just substitute the darlington transistor w/ two seperate transistors? With the emiter of the first T going into the base of the second one?

    Also, what resistor values would I need to change to run this off of 12vdc instead of 9v?

    In the below crt diagram, what would I change to run it off of 12vdc? And it was suggested that I should put a cap across the imput? What value?

    And a stupid question. Input is only one wire. Where does the other input wire connect to?

    Thanks,
     
  14. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    Yes.


    Ground.
     
  15. CTRL+X

    Thread Starter Member

    Jul 1, 2009
    26
    0
    OK, went to Radio Shack to get some stuff, picked up an LM386 audio amp IC, and some connectors.

    However I also grabbed a 102db 12V piezo siren. So, slight change in direction...
    I would like to use the output from the headphone jack to trigger the 12v crt to the siren. The siren is self contained, and just needs a constant 12v supply to operate.

    I believe that headphone outputs are in the 1-2v range. So I'm basicaly looking for a crt where a 1-2v input signal will trigger a 12v crt.
     
    Last edited: Aug 16, 2009
  16. CTRL+X

    Thread Starter Member

    Jul 1, 2009
    26
    0
    Will this work? So I attach one side of the headphone output to R1, but where does the other wire from the headphone output go. Do I just Grd one side? Would that place the headphone crt on the radar detector into the 12v crt and damage it?
     
  17. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    If you add a cap of 10..20 nF across R2 it should work.
     
  18. CTRL+X

    Thread Starter Member

    Jul 1, 2009
    26
    0
    Thanks, where do I connect the other wire from the headphone output?
     
  19. CTRL+X

    Thread Starter Member

    Jul 1, 2009
    26
    0
    The crt below works as I want it too. When the radar detector beeps, the piezo siren goes off with equivalent intervals, but much louder. :D

    With just the transistor, the intervals were too short to actually activate the siren, however adding a 220uf cap across the load fixed that problem.

    Any additional resistors or diodes that I should add to protect the radar detector headphone jack crt, or does this crt not pose any risk to the RD?

    Thanks,
     
  20. CTRL+X

    Thread Starter Member

    Jul 1, 2009
    26
    0
    Not sure if I should make a new thread since this no longer involves an amp, but....

    In the crt in the post above, it works fine when the power is a 9v batt, however as soon as I connect to 12vdc auto power, it just goes off constantly as soon as the input wire is connected, whether there is a beep coming thru or not. With 9vdc it only goes off at intervals corresponding to the beeps from the imput.

    The transistor is rated to 25v, so the increased current shouldn't just push thru, should it? Do I need to put a resistor in here somewhere to limit the current?

    Thanks,

    Edit: Nevermind, put a 330k resistor in at the grd connection. Works fine, a little too quite though so just need to find the right resistor value.
     
    Last edited: Aug 22, 2009
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