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Preamplifier design for power amp

Discussion in 'General Electronics' started by Inspar8r, Feb 1, 2005.

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  1. Inspar8r

    Inspar8r Guest

    Hey guys, i have another question for a project in one of my classes.

    We have to design and build a power amplifier as part o fthe final
    project. Now the rquirements are really basic, in fact, so basic, i
    want to do more, which is encouraged.

    My question is, using the 741 as an op-amp for my pre-amp circuit, can
    i just daisy chain several op-amp circuits???

    I want to have the process go as follows:

    mono rca input>>stg 1 pre-amp>>active filter (high low and
    pass, using a three way switch)>>stg 2 pre-amp>>stg3
    pre-amp>>power amplifier>>speaker.

    With the setup they require, there only needs to be one pre-amp, and
    using a single darlington tansistor, the output power is at a mere
    .5W with the pre-amp's Vcc @10V.

    When chaining them up, does the output of one pre-amp feed into the
    top (-) terminal of the op-amp, and the (+) side goes to ground on
    all proceeding op-amps???

    On the (+) side of the op-amp, we designed 2 resistors as a voltage
    divider to give us 1/2Vcc, do i need to do this for all pre-amps as
    well.

    Hopefully you guys can follow. I want a relatively usable amplifier.
     
  2. Karl Uppiano

    Karl Uppiano Guest

    If you choose something better than a 741, you might not need to chain
    op-amps. I mean, a microphone or phono preamp is frequently done in a single
    op-amp. What are your requirements: Gain? Bandwidth? Noise? Distortion?
    What's it going to be used for?
     
  3. Robert Baer

    Robert Baer Guest

    You can (almost) double the output voltage swing to give (almost) 4
    times power capability by taking driving a follower and an inverter at
    the same time.
    That way, when the ouput of the follower is going positive, the output
    of the inverter is going negative.
    Add a current amplifier (voltage follower) like the LH0002 inside the
    loop of each (output of uA741 drives the current amp and output of
    current amp goes to load and feedback).
    The load connects between the outputs of the two (composite) op-amps.
     
  4. SteveB

    SteveB Guest

    You could use the circuit at http://www.elexp.com/t_audio.htm as a pre-amp
    building block, AC coupled for no biasing problems, set the gain with R2/R1.


     
  5. Inspar8r

    Inspar8r Guest

    Karl Uppianowrote:
    If you choose something better than a 741, you might not need to
    chain
    IT's for a audio amplifier that we are building, but it's only to
    learn the aspects of the amplifier design. I'm a bit advanced in the
    class, so instead of just meeting the basics, i want to build and
    amplifier that has USABLE power, not .5W, but has a similar circuitry
    to the requirements.

    The basic requirements is a regulated Power supply that has 10V
    output, a power amplifier that utilizes a Darlington TIP120
    transistor, and a pre-amp that uses a 741 inverting op-amp...

    I found a op-amp IC that has 4 op-amps in one chip (LM148/248/348),
    and my plan was to have 3 stages of pre-amplification.

    1st stage- Pre-amp
    2nd stage- pass-through/active high pass filter/active low pass
    filter
    determined by a 3 way switch
    3rd stage- Pre-amp

    I have the circuit drawn out that i have planned, if you want me to
    scan it. The filters are first order active filters.

    Thanks for the help guys.
     
  6. Dope mcSmoke

    Dope mcSmoke Guest

    Active filters are at least 2nd order.
     
  7. Inspar8r

    Inspar8r Guest

    Dope mcSmokewrote:
    [quote:e998840fcd]
    Active filters are at least 2nd order.[/quote:e998840fcd]

    Maybe an ideal design, yes, but not in general... The order depends on
    the number of reactive elements in the circuit.
     
  8. Dope mcSmoke

    Dope mcSmoke Guest

    Yes, but If you only have 1 reactive component i would call it a passive
    filter and an amplifier..
     
  9. Dope mcSmoke

    Dope mcSmoke Guest

    To make myelf clear. With a first order filter, what you mean is a plain R-C
    link, either lowpass or highpass. IF you build a filter wih an OP-amp you
    can build either a first order Bandpass filter or a second order
    Low-/Highpass, What you describe seems to me like a RC-filter and an
    amplifier, so if it is, check out Sallen-Key and if it isn't, sorry for
    misunderstanding..

    /Johan

     
  10. Inspar8r

    Inspar8r Guest

    Well,

    I'm going to dump the filters, because I am having a hard time giving
    the filters (the lowpass in particular) initial bias without
    affecting the cutoff point.

    So I'm dumping them.

    My problem now is, my 2 stage pre-amp is not working. I have a gain of
    10 on the first pre-amp, with a 200mV input, and the second stage
    isn't giving any gain. I have the Feedback resistor on the second
    stage as a potentiometer for volume control, and the waves are all
    messed up.

    Keep in mind the op-amps are in single supply config as well.

    Isn't daising chaining pre-amps the best way to get a decent ouput on
    a power amplifier???

    My power amp is just a simple class AB push pull config with a couple
    darlingtons.
     
  11. Dope mcSmoke

    Dope mcSmoke Guest

    What kind of filter are you using? it wouldn't be too hard to give them a
    bias voltage from half V+ through a large resistor. Anyway, you can always
    use passive filters and boost the signal later (earlier?) in the chain.

    If you post a schematic someone can probably find what's wrong. What voltage
    are you driving the power transistors with?

    /Johan
     
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