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Amplifier circuitry

  1. May 11, 2013 #1
    I am making an amplifier with 2 lm4780 ic
    The gain is set by a resistor between the output and negative,I want to use a potenciometer to be able to adjust this, so my question is:
    will lowering the ohms lower the output or will increase the output or what?
  2. jcsd
  3. May 11, 2013 #2


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    Forum clairvoyant mode is disabled, where's the circuit diagram?
  4. May 11, 2013 #3
    In other words the feedback resister is to be adjustable? Just think for a minute. What if the F.B. resiser where zero ohms, just wire. You'd have a unity gain "amplifier". and the other extream what if you removed the F.B.? You have high-way gain.

    But the normal way to control audio volume is to set the gain to some fixed level. And adust the volume with a voltage divider potentiometer in the input. Reason is that for stabilty you'll need a capasitor in parallel with the F.B. resister to control the high freq. gain
  5. May 11, 2013 #4
    The circuit is so simple he has almost completely described it. These are only two resisters that matter and he wants to make one of them variable. He will find that a variable gain feedback amplifier is almost impossible to compensate.
  6. May 11, 2013 #5


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    I would agree if he hadn't said he was using a pot (implies more complexity than a simple connection but maybe not) instead of a variable resistor.
  7. May 11, 2013 #6


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    the volume (level) control should be on the input of the amplifier NOT between the amp output and the speaker

    Last edited: May 12, 2013
  8. May 12, 2013 #7
    I just want to be able to adjust the gain so I have full control over the system in case I need to make any changes. What if I was to use a 10k resistor in series with a 10k pot so then I have 10-20k on my feed back.
    Or would I need a 20k resistor and 10k pot for 20-30k? I am still ignorant as to weather raising or lowering the ohms would increase or lower gain because i have never used an audio chip with this resistor as an external component.
    all the circuits I'v seen used 20k and were said to be full gain, so to adjust gain which direction do i go?
    +ohms or -ohms, and by how much as to not screw something up?
  9. May 12, 2013 #8
    For my main volume i am useing a pot on the input
  10. May 12, 2013 #9


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    and and I said earlier, that's the only place you should have a vol control pot

  11. May 12, 2013 #10


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    Yes, you can do that. Why not experiment, and find out for yourself? If you do include a pot, it has a terminal at each end of the track, so as you rotate it CW you can have R increasing, or you can have R decreasing, so all bases are covered! :approve:

    The value of pot and its series resistor can be judged by the value of R shown on the schematic you are using.
    I reckon a google search would turn up the required info. Probably no one here knows offhand.
  12. May 12, 2013 #11
    Ok thanks guys I will google and experiment and everything, theres just a couple more things I am wondering though.

    My whole circuit board is going to 2x lm4780. The chip is a sterio chip and I am going to have one chip run left ear and one chip run right.

    So 1 chip has 2 outputs and in the datasheet you can run left and right speakers, put both outputs on the leads of 1 speaker to double the output, or put the outputs in parallel and ground the speaker.( I am useing one of the last 2)


    I have already planned to use the outputs in parallel circuit but does anyone have an idea weather one or the other would work better?
  13. May 12, 2013 #12


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    It depends how much power output you want. The parallel output will produce twice the power of each single amp. The "bridge" amp with the speaker connected between the two outputs will produce 4 times as much, because the voltages on the speaker terminals are equal and oppoite, instead of one of them being at zero.

    (That is assuming the speakers have the same impedance for each circuit).

    Bridge amps are useful for getting the maximum poiwer output from a low power supply voltage (e.g. a 12V car battery). If you can choose any supply voltage you want, it comes down to which design is cheapest in terms of the total cost of the components, the size of the circuiit board required, etc.
  14. May 12, 2013 #13
    wow that helps alot! yes I am looking for high power output. I already made my pcb for parallel so I will do that.

    I just ran into a BIG PROBLEM

    so for a long time I have been trying to whip together a 45v supply and after HOURS of searching online I found a 45v transformer.
    But i just relized it can only put out 1 amp and the whole project will draw alot more.

    can I instead of useing a transformer use the equation commonly used to find resistor for an led?

    like a 3.5v 20ma led with a 12v supply needs 425Ω

    except with a 120v outlet and a audio system?
    Last edited: May 12, 2013
  15. May 12, 2013 #14


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    No! It is very apparent that you do not have enough knowledge and experience yet to be dealing with AC Mains voltages in your projects. You must use a pre-packaged power supply that already has safety agency approvals.

    Please explain *exactly* what you are tying to build. What audio output power are you looking to supply into what kind of speakers?
  16. May 12, 2013 #15
    I am making a audio amplifier useing two lm4780 chips set in parallel to have 240 watts, 120 left ear and 120 right.
    It is a simple schematic I have already put into pcb form.
    I need to have at least 35v to power it sufficiently and actually I just went to the datasheet on a different website then the first time I looked for current draw and it is saying that the standard current draw is 110ma.
    if someone could check that just to be sure i am not being a D&*@!#$ that would be great.

    also, in the schematic there are filter capacitors on the V+ and V-,
    I am confused on why they are so "precise" there are 1mf, 10uf, and 100nf in parallel.
    Why must it be 1010.1uf? or can I change it to just 1000uf to save room on my board?
    Last edited: May 12, 2013
  17. May 13, 2013 #16


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    And how many seconds do you expect you hearing to last at that power level ?

    Im sure you really meant to say, left and right speakers

    No, you need a 70 V DC power supply +35V rail 0V rail and -35V rail
    and ~ 8 to 10 Amps on each of those two 35V rails
    The "standard current draw of 110mA" you mention is probably the quiescent current drain

    So you are looking at a centre tapped transformer of 70V and a current rating of ~ 18 - 20Amps
    to have a little bit of overhead
    This is a major power supply and as berkeman has already commented probably well out of your level of understanding and building ability

    No you cant and actually the main electrolytic capacitors are going to be huge !! four x 10,000 to 15,000 uF. Two for each rail, they are very big capacitors ~ 2 inches in diameter and ~ 4 inches high
    You will still need four 100nF (0.1uF) capacitors, one in parallel with each of the large electrolytics

    you shouldnt be going any further on this unless you can find a qualified electronics technician that lives near you that can help you building the power supply

    Last edited: May 13, 2013
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