BJT Audio Amp [Help]

Discussion in 'General Electronics Chat' started by Deko Chis, Nov 25, 2012.

  1. Deko Chis

    Thread Starter New Member

    Nov 25, 2012
    5
    0
    [​IMG]


    This is the Amp I'm working on, I haven't soldered everything in place yet, but everything is connected correctly on my breadboard. It's giving me a bit of noise, but not as much as I want. Also, I can't hear my voice through the microphone. I just don't know why, oh, and for my 27k resistor, I simply put a 22k and 4.7k and a 1k in series, do I even need the 1k?

    Thoughts, please.
     
  2. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    I think you do not need the 1k. I also think the 8 ohm speaker is too low impedance for the circuit.
    It would be interesting to me if you have a DVM to measure the voltages on the various elements of the two transistors. I looked at the data sheet and did some calculations and this is what a calculated for the transistor on the left. All voltages with respect to ground. C=4.5V B=.98V E=.68V The transitor on the right. B=4.5V E=4.2V
     
    Deko Chis likes this.
  3. Deko Chis

    Thread Starter New Member

    Nov 25, 2012
    5
    0

    I believe that all your voltages are right, I used NTE192A transistor, I was thinking about pulling that 1k resistor out. If I wanted to increase the volume or even hear myself, what may need to be my first steps to config? Oh, at [C] I got 6.1V when I measured using a DMM.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,987
    3,733
    One problem is that your bias transistors (two at far left) may be attenuating the small signal from the mic. Use your volt meter on AC volts and connect directly to your mic (mic not connected to your circuit). Then hold the mic up to your favorite loudspeaker playing some loud music and record the readings - (being exact is not important, if you see much lower values in the next step, there is a problem). Then add a 27k ohm resistor in parallel with the mic. Measure the voltage again. If the AC voltage is much lower, you might want to consider a different circuit design. This design may be allowing too much signal to be lost in the input stage.

    Your dynamic microphone delivers a pretty small signal to your first transistor. Two problems here. You will need more voltage gain out of this stage so google transistor calculator and play with the resistor values to see hoe you can get more range out of The first transistor. The second transistor only gives increased current to drive the speaker without adding (or dropping) voltage swing.
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,421
    3,355
    Are you certain that the microphone is a dynamic mic?
    Electret mics are very common and will not work in the circuit as shown.
     
  6. Deko Chis

    Thread Starter New Member

    Nov 25, 2012
    5
    0
    Oh wow, my microphone wasn't dynamic and I don't know where I can find one! They don't seem to sell it here int he Bahamas, anyone know a device of some sort that has one in it I can remove? I have to present this on Dec.5th! Darn, how did I not see this..!
     
  7. Deko Chis

    Thread Starter New Member

    Nov 25, 2012
    5
    0
    No one knows where i could find a dynamic microphone?
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,421
    3,355
    All is not lost.

    You can use an electret microphone in that same circuit by adding a 10kΩ resistor from the mic to +9V.
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    3 days ago I tried to reply but this website was not working correctly.

    The transistor is so old (GE, 1971 databook) that I couldn't find a detailed datasheet, just a list of a few spec's. It is silicon so its base-emitter voltage is 0.6V to 0.8V, not 0.3V.

    You are making a "Class-A Amplifier" that needs a LOT of current to make any output power. The high current causes its output transistor and resistor R5 to get VERY HOT!

    I simulated your circuit and its output power in the speaker is less than only 0.0013W.

    Then I increased the currents a lot and the output increased to 0.016W but the output was distorted.

    You need to learn Ohm's Law, Class-AB output circuits and negative feedback.
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    If you are using an electret mic then simply power its Jfet with a 10k resistor from +9V.
     
  11. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your circuit is not a power amplifier. Instead it is a preamp that can drive the high input impedance of a power amplifier.

    Make a normal complementary-symmetry power amplifier with 3 transistors and drive it from a 1-transistor preamp.

    Here is a simulation of a 3-transistors power amplifier that works well. Replace its R4 input resistor with your first transistor as a preamp:
     
  12. Deko Chis

    Thread Starter New Member

    Nov 25, 2012
    5
    0
    How does the 10k ohm resistor turn my Elect. mic into a dynamic one?
     
  13. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Look at the microphones in Google:
    1) A dynamic mic generates a signal with a moving coil surrounding a magnet.
    2) An electret mic is a "condenser mic" with a high voltage stored in its electret material (a condenser mic needs an external high voltage).
    The 10k resistor from +9V powers a Jfet transistor impedance converter inside. If the Jfet is not powered then the mic has no output signal.
     
Loading...