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Resistors and?

Discussion in 'General Electronics Chat' started by Electronman, Apr 3, 2009.

  1. Roff

    Roff Well-Known Member

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    Are you sure your bridge is connected like that? If it is, D6 is toast.
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You have two of the rectifier diodes connected wrong:
     

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  3. Electronman

    Electronman New Member

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    Sorry I mis drew it.
    I showed the bridged by 4 diodes In relaity I have used a pole diode (those that 4 diodes are in a pack).]

    Ron,
    I noticed that when I reduce the R11 which is a 500K t to near 5k and even lower till zero, I do not have that hum problem nearly at all?!
    The R10 is 6.8K resistor and the C2 is 2.2uf cap.
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The gain of the opamp is 1+ (R11/R10) so all you did was turn the gain way down.

    The input must connect to the microphone with shielded audio cable to prevent mains hum pickup. The opamp circuit must be wired compactly and maybe in a metal box connected to 0V. It will probably pickup hum if the circuit is made on a breadboard.
     
  6. Electronman

    Electronman New Member

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    yea when I reduce the gain the hum is reduced till I put the R11 to zero and at this situstion I do not have any hum.

    audioguru,
    I do not have a such problem when use a comersial regulated power suooly, the problem araises hen I connect the circuit to a common regulated transformer.
    I have the circuit on a PCB but I have the problem yet.
    I am suspect if when I increase the opamp's gain it pick up any hum through the transformer? if so, how to solve that? I have used a 7815 so it seems it is not able o solve my problem!
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A 7815 regulator has excellent hum rejection when its 0V trace is thick and correct.
     
  8. Roff

    Roff Well-Known Member

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    Having a bridge rectifier in a package does not prevent one from connecting it incorrectly. If it is connected the way you drew it, you will have a half wave rectifier (and possibly a burned out diode), which could be the source of your hum.
    Is the transformer rated at 18V RMS?
    Are you able to measure the DC voltage at the input to the regulator?
    Do you have an oscilloscope?
    If so, observe the input and the output of the regulator, and tell us, or post a picture, of what you see.
     
  9. Electronman

    Electronman New Member

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    No, I am SURE that I have put the bridge rectifier correctely. It ha signs that show th pins for AC input and signs that show th positive, negative output.
    I will measure the rates and try it with a 24V transformer too.

    I am wondering why maybe the hums due to the sensivity of the opamps at high gain rates? Why no one tels of it exist or not.
     
  10. Electronman

    Electronman New Member

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    I tried a 24V transformer, now the hum is reduced but not removed completely, but I have a bit of it.

    The 2200uF cap at the input of the 7815 gets hot, the rectifier package gets hot too. Is this natural?
    Now the circuit draws more than 1A at some situations. I fear to lose my 7815? The 7815 gets too hot too while it has a 40mm x 20mm radiator. Now after 15minuts of working the circuit turns of by itself. I think the reason is due to the regulator. What’s the solution? Is a higher radiator helpful while the circuit draws more than 1100mA at some situations?
     
  11. Electronman

    Electronman New Member

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    Somewhere I read that it is necessary to put the C2 (see the bellow pic) which is a 0.1uf at the input of the regulator to prevent oscillation and help the regulator chip to not get so hot.
    I am wondering while the capacitance of C2 and C1 (which is 2200uF for my circuit) is added (they are in parallel) how it does help to prevent oscillations?

    Can I use another 8718 or maybe 7820 before my regulator chip to feed it just with 18V to 20V rather than 24v to prevent overheating?
    Can I achieve more than 1A at this situation?
     

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    Last edited: May 10, 2009
  12. Electronman

    Electronman New Member

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    Besides is there any simple regulator like 78xx to give more than 2A at its output? If so please let me know the part number.
     
  13. Roff

    Roff Well-Known Member

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    What circuit are you powering with the regulator other than the modulator, and possibly a preamp? Are you also powering the power amplifier?
     
  14. Electronman

    Electronman New Member

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    yea I have a power amplifier connected to the output of the modulator.
     

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