Audio Amp / Intercom

Discussion in 'General Electronics Chat' started by crivoli, May 4, 2006.

  1. crivoli

    Thread Starter Member

    May 4, 2006
    26
    0
    I am trying to build a simple intercom for personal use.

    I have built one from the schematic found at ( http://ourworld.compuserve.com/homepages/B...owden/opamp.htm ) ( scroll all the way to the bottom ), but when I speak into the speaker the sound coming out has way too much static and is not understandable at all.

    This circuit uses a LM741 chip to amplify the sound. Is there anything I can modify on this circuit to make the sound quality better?

    Would I be better off using a LM386 audio amplifier chip and building a completely different circuit?

    If so can anyone please give me a link to a schematic that is pretty simple to build but has a very good quality sound coming out! I appreciate it very much.

    Thanks
    -New User
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    The circuit you used looks reasonable.

    Provided you did not make any component value changes, the circuit should work.

    The distortion you hear is probably being caused by clipping. The capacitor in series with the speaker being used as the microphone could be leaky. A leaky cap would change the dc operating point of the amplifier and create conditions for clipping. If you have access to a voltmeter you can measure the voltage at the emitter of the transistor. You should measure a voltage that is equal to half the supply. With a fresh 9V battery you should measure 4.5V. If this voltage is higher or lower by a volt or more then clipping is the likely your problem.

    If you measure 4.5V then most likely you have too much gain and clipping again would be the result. The design calls for an 8 ohm speaker at the output. You may need to check the dc resistance of your output speaker and make sure that it is not a 16 ohm speaker or greater.

    You may also want to confirm that the dc voltage at the positive input of the opamp measures 4.5V with a fresh 9V battery installed.

    hgmjr
     
  3. crivoli

    Thread Starter Member

    May 4, 2006
    26
    0
    hgmjr

    Thanks for the feedback.

    The only difference between what I build and what the schematic shows are:
    1. I am using a different transistor. I am using a MPS2222A transistor
    2. The cap in series with the speaker I am using is .47uF
    3. I also replaced the 33 ohm resistor with a 10 ohm

    Would these affect the circuit enough to distort the sound terribly?

    Thanks again
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I think the use of the mps2222a should be fine. Also, the value of the capacitor of 0.47uF should be OK.

    I think that the use of the 10 ohm in place of the 33 ohm may be a problem. The use of a lower valued resistor may be causing the clipping. The lower resistor increases the gain of the speaker driver. If you can't get a 33 ohm resistor then at least use a value that is greater. Something in the range of 33 to 75 ohms should be fine.

    hgmjr
     
  5. crivoli

    Thread Starter Member

    May 4, 2006
    26
    0
    Hey man, I know you have better things to do than to keep offering suggestions, but here goes onem one more thing. :)

    I replaced the 10ohm resistor with two 100ohm's in series to create a 50ohm resistance. It greatly improved the static, but the clarity of the sound is still about a 1 on a scale of 1 to 10. I am not sure where to go from here.

    Another thought I was going to try is this ( http://www.uoguelph.ca/~antoon/circ/intercom.html ) ( the schematic at bottom with 8 ohm speakers ). Are the BC558 and BC548 really necessary or could I get by with another generic PNP and NPN transistor?

    This design looks pretty good, and I was going to use two LM386 audio amps with this. I am not exactly sure which pins to hook up where on the schematic. The datasheet can be found here ( http://cache.national.com/ds/LM/LM386.pdf ).

    I would appreciate any comments you have on whether you think this new intercom would be a better solution, and any ideas on which pins of the LM386 go where.

    I'm sorry, but I am a novice at this and I don't remember much of my Comp. Engineering classes....... I graduated in Computer Science... forgive me :)

    Thanks again
     
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