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powering 9v preamp circuit from XLR, AAA, wall wart w/o hum

Discussion in 'Electronic Basics' started by Mad Scientist Jr, Jun 19, 2007.

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  1. I built a simple preamp from this schematic -

    http://www.electronicsteacher.com/circuits-and-diagrams/audio-preamp-circuits/preamp.gif

    For the most part it works great - it makes guitar pickups and
    microphones sound much louder.

    However I do have a few issues I was hoping I could find some help
    on.

    The sound is pretty clean if you use a battery. However, I tried a 9v
    battery eliminator wall wart and got lots of loud background hum. I
    think I need to filter the power supply to clean up the sound. Does
    anyone know how to do this?

    Also I would like to get it to run off a AAA battery. I have a
    condenser mic that uses such a battery so I am hoping the circuit can
    run off of 1.5v. I think you would just have to change the two 10k
    resistors to something lower. Is this correct? Can someone explain how
    to calculate the values needed?

    Finally, how would you wire it to be phantom powered by the XLR
    connector on a mixer?

    Much appreciated...
     
  2. BD

    BD Guest

    The hum you're hearing is AC hum. There's a chance that the wall wart are
    outputing AC instead of DC. You would have to put diodes in to rectify it to
    DC. In that case you'd have to put in a filter (electrolyte capacitor).

    Get a wall wart that fits. Be sure that the outputs are DC.
    The specs are usually molded in the plastic case or on a label.
     
  3. Jim

    Jim Guest

    I suspect it's a DC supply and he's hearing excessive ripple, 120 Hz,
    from an unregulated improperly filtered supply. This is basic stuff.
    I'd measure the voltage with the circuit on. If it's in the right
    range, I'd add an appropriate electrolytic filter capacitor. Use on
    rate at at least 15 to 20V. You could try 100 uF to start, but you
    don't need a huge capacitors with small current supplies.

    If that DC supply is putting out too much voltage (not uncommon when you
    use a wallwart on a circuit that doesn't draw much current), either buy
    a regulated supply, or put a voltage regulator on that one.
     
  4. sycochkn

    sycochkn Guest

    The circuit will run on anything from 3 to 9 volts.

    Bob
     
  5. Gerard Bok

    Gerard Bok Guest

    That's because this (very basic) amplifier has a flaw.

    Any noise present on the powerline gets fed into the input.
    Replace R1 by 2 4K7 resisors in series. And connect a capacitor,
    say 10, 25 or 47 uF between the junction of those 2 resistors and
    ground. (plus to the junction, minus to ground,)

    You may also want to connect a capacitor, say 100 uF across the
    powersupply connection points.
     
  6. Les Cargill

    Les Cargill Guest

    Try a different wall wart. Of all places, I have had extraordinary
    luck with wall warts from Target in the past. If Radio Shack's
    stock of warts is like in the early '90s, those are pretty clean, too.
    You really need to get the data books out for this one. Somebody
    might know, though - and the schem says "3 to 9v"...

    Resistor network based on the fact that you get 48VDC between
    3 & 1 and 2 & 1.


    From http://en.wikipedia.org/wiki/Phantom_power :


    "The signal conductors are positive, both fed through resistors of equal
    value (for 48 volt phantom powering the standard value is 6.81 kΩ), and
    the shield is ground."
     
  7. Marra

    Marra Guest

    I noticed you have no power supply decoupling. maybe add 10uf and
    0.1uf across battery.
    Needs to be in shielded box too.
     
  8. Thanks everyone for your replies.

    Would it help to put it in this aluminum box and soldering the box to
    ground?

    http://www.geocities.com/usenet_daughter/preamp_shielding_box.jpg

    Thanks...
     
  9. Lord Garth

    Lord Garth Guest

  10. Thanks for your reply.
    I thought that too but I pulled that box out of an old mixer, and it
    already has a solder glob on it that I could solder to.

    BTW what kind of soldering iron do you need to solder to aluminum? How
    many watts? Or can you get a good electrical connection to ground by
    drilling a hole in the box & sanding the surface with steel wool and
    screwing the ground wire to it?
     
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