Mixer and Power Amplifiers

Discussion in 'General Electronics Chat' started by creaver31, Feb 18, 2012.

  1. creaver31

    Thread Starter Member

    Feb 2, 2012
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    hello,

    this is my design for pre -amp and tone control, and i need to make a mixer.
    by the way, our prof told us that we need to make 2 pre-amps for mic and 2 line-in. is my design enough?

    what should be the design of my mixer? please help, the DC voltage input is 12volts.

    thanks in advance :D

    i also need power amplifiers after the mixer, ranging from 1 watt to 5 watts
     
  2. bertus

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  3. Audioguru

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    Dec 20, 2007
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    In your preamp, why do the resistors have very low values and why do the capacitors have extremely high values?

    Why is the first transistor in your preamp biased so that its emitter is as high as it can go and it will not pass a signal? The first transistor is not needed anyway.

    Why is the second transistor in your preamp biased wrong so it has a low signal swing?

    Why does your tone controls circuit have an extremely old opamp that is missing a compensation capacitor, it cannot pass high audio frequencies and it is not powered?

    I can hardly wait to see your power amplifier schematic.
     
  4. creaver31

    Thread Starter Member

    Feb 2, 2012
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    ahm, i biased the first transistor so that i can have high input impedance, as our prof told us, about the value of resistances and capacitors, i got it from the Multisim CE BJT Amplifier Wizard, and the tone control, what opamp should i use?

    sorry about the mistakes, that's why i'm asking for suggestions :D
     
  5. Audioguru

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    Dec 20, 2007
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    Your first transistor has its base at almost +12V so its emitter is also at +12V then it cannot pass a signal. Its input impedance is less than the 20k base resistor which is not a high input impedance.

    Bias its base with a high resistance voltage divider so that the emitter will be at about +5.8V and it will be able to swing up and down with the signal. Use a fairly high emitter resistor value. My input capacitor value is 1/7000th the value of yours and its cutoff frequency is very low at 6.8Hz.
    But then the resistors in your voltage amplifier transistor circuit must be much higher.

    I simulated the circuit with a 2N3904 transistor that is similar to a 2N2222A transistor. It shows a nice high symmetrical output when its input signal is high. Yours did not have an output. Mine has an input impedance of 500k ohms. Yours had an input impedance of less than 20k ohms.

    You should use an audio opamp in audio circuits. A TL071 single, a TL072 dual (for stereo) or a TL074 quad are good inexpensive audio opamps with low noise, low distortion and a full output bandwidth of 100kHz.
     
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  6. creaver31

    Thread Starter Member

    Feb 2, 2012
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    hello, please check this, is this right? is there any way i can make the output voltage at least 50 time of input?
     
  7. creaver31

    Thread Starter Member

    Feb 2, 2012
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    is this right? i followed what you gave me...
    it's distorted..
     
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  8. Audioguru

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    Dec 20, 2007
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    The output signal level is low because you did not calculate the value of the emitter bypass capacitor Ce properly. You are using a value of only 0.1uF which will allow your circuit to have lots of voltage gain at frequencies higher than 8800Hz! Low level mid-range and no bass sounds.

    Calculate the emitter bypass capacitor value so that frequencies as low as 10Hz will have gain, then the capacitor value will be 88uF which is not a standard value. Using 100uF then the output is clipping like mad because the gain is too high. The gain with a reduced input level is 190 and the distortion at high levels is very high.

    The calculation of a capacitor value is "1 divided by (2 x pi x f x R)". When the reactance of the capacitor equals the resistor value then the gain is reduced 3dB (which is times 0.707). If the capacitor is calculated so that 10Hz is reduced 3dB then the amplifier will have a flat response down to 50Hz.
     
    Last edited: Feb 22, 2012
  9. Audioguru

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    Dec 20, 2007
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    Of course the new circuit is distorted, it is clipping like mad because its gain is 100 and your input is much too high at 1V p-p. It was designed to be a microphone preamp. A microphone has an output level of about 5mV when you talk to 40mV when you scream, not 1000mV.
     
  10. creaver31

    Thread Starter Member

    Feb 2, 2012
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    ah see, so that's it thanks!
     
  11. Audioguru

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    Do you know how to add some negative feedback so that the gain of 100 is reduced to a gain of 50 that you want and which will also reduce the distortion?
     
    Last edited: Feb 24, 2012
  12. creaver31

    Thread Starter Member

    Feb 2, 2012
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    no, how can i do that??
     
  13. Audioguru

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    Didn't your teacher or professor teach you anything about simple electronic circuits?
    Don't you have a text book?

    Which circuit do you want to add negative feedback to reduce the voltage gain and to reduce the distortion?
     
  14. creaver31

    Thread Starter Member

    Feb 2, 2012
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    i think i wanna work out with this one.
     
  15. Audioguru

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    I attach the circuit without your Multisim chicken-pox dots all over it.

    1) What is the voltage gain of the emitter follower transistor Q2?
    2) What determines the voltage gain of the common-emitter transistor Q1?
    3) How can you simply reduce the voltage gain of Q1?
     
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  16. creaver31

    Thread Starter Member

    Feb 2, 2012
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    what do you think about this mixer and PA? the 8-ohm load in the PA is the speaker, with power rating of 5 watts.
     
  17. Audioguru

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    Dec 20, 2007
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    Geez, don't you have a teacher and a text book??

    Your opamp is biased wrong so it will be a signal rectifier instead of a mixer amplifier.
    NO opamp is an amplifier when its pin3 input is at the same DC voltage as its pin4 negative supply voltage.

    The power amp has no negative feedback from the output so its distortion will be high.
    Its output will be only about 1.5W, not 5W.
     
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  18. creaver31

    Thread Starter Member

    Feb 2, 2012
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    here, pre amp then mixer.
     
  19. Audioguru

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    Dec 20, 2007
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    Good, the transistors in your preamp are biased correctly.

    But the values for the coupling capacitors are much too high:
    1) The input impedance of Q1 is its hfe (about 180) times the 1k emitter resistor, in parallel with the two 330k divider resistors. The total is 86k ohms.
    For a cutoff frequency of 10Hz the value of C2 is 0.18uF, not 22uF.

    2) The input impedance of Q2 is its hfe (about 180) times the 230 ohms unbypassed emitter resistance, in parallel with the 47k and 22k divider resistors. The total is 11k ohms. It is in series with the 22k collector resistor of Q1. The new total is 33k ohms.
    For a cutoff frequency of 10Hz the value of C2 is 0.49uF (use 0.47uF), not 10uF.

    Your labels for the volume control and the gain controls are backwards. Your "gain controls" are actually the volume controls. Your "volume control" is actually a gain control.

    The values for R10, R11 and R15 are too low, use 10k each.

    If you add a few capacitors and a voltage divider then the opamp does not need the negative supply voltage.
     
  20. creaver31

    Thread Starter Member

    Feb 2, 2012
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    how can i do that?

    also, if i replace R11 with a 10k pot, will it act as the master volume control?
     
    Last edited: Mar 9, 2012
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