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-5 volts?

Discussion in 'Electronic Design' started by DaveC, Feb 10, 2005.

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  1. DaveC

    DaveC Guest

    Hello.. I've built a little Guitar effects circuit, Its basic
    configuration is as follows Input -> LM741(variable Gain) -> ADC -> PIC
    -> DAC(MAX5354) -> output. I used a voltage divider to impose a 2.5 volt
    DC offset on the analog input of the OPAMP so I could get a clear reading
    as the signal swings in either direction.

    It all works fine, except that the 2.5volt offset is still on the output.
    Due more to the DAC then the above mentioned. Any way. I wish to power
    the whole thing of a regular 9 or 12 volt DC wall wart. So I need -5
    volts to pull the signal down again (removing the DC offset.

    Are they called charge pumps.? somthing? At the moment I only have the 5v
    digital supply and ground.


  2. "At the moment I only have the 5v digital supply and ground."

    First thing....
    it sez the minimum supply rail is 10V, so select another opamp,
    preferably with good rail to rail specifications


    Serious error.
    All shortcuts have disappeared.
    Screen. Mind. Both are blank.
  3. Richard Webb

    Richard Webb Guest


    If I'm correct all you want to do is remove the DC offset from the output of
    the DAC.

    A capacitor passes AC but blocks DC.

    Putting a sensibly sized cap in series with the output of your circuit will
    only allow the AC components of your signal to pass and block the DC

    Its hard to tell from your description but it sounds like you might be
    trying to impose a 2.5v DC offset onto your input directly which could be
    feeding back to your guitar. No idea if this will affect your guitar or not.
    Use a capacitor in series with the input and after the cap place a resistor
    to ground, this will form a high pass filter. Set the break frequency nice
    and low (20Hz ish).

    As for the power, why not use a regular 9v wall wart and regulate it with
    something simple like a 7805?

    Hope this helps,
  4. SCADA

    SCADA Guest


    Have a look at the LMC7660...
  5. DaveC

    DaveC Guest

    Thanks.. The cap worked. I'm only in my first year as electronic degree
    so forgive me :)

    What is the use of positive and negative voltage on many opamp circuits
    I've seen then?

  6. Mook Johnson

    Mook Johnson Guest

    This allows for DC coupling (direct to the opamp output) without a coupling
    cap. This is useful when cascading stages so you don't have to worry about
    all those AC coupled stages rolling off you low frequency gains (bass).

    As a guitar guy, you might want to look into Tubes. I'm in a project with a
    musician friend of mine and even though he will admit that solid state get
    much better specifictions and cleaner sound, tuba amps are more desirable
    because they become part of the musical instrument. Tube distortion due to
    signal overdrive (hitting the string hard) is more pleasing to the ear and
    make the guitar sound fuller rather than raspy. I'll admit that he is
    correct in that as I have heard teh difference for myself. not saying that
    solid state cant be made to mimick the shound of a tube, but tube amps are
    pretty much the norm for musicians I know.

    This is NOT the Hi-Fi tube amp vs Solid State discussion/arguement and
    please lets not open that can of worms. :)

    good luck
  7. Fred Bloggs

    Fred Bloggs Guest

    It is not necessary to invert for your slight loading, a simple power
    supply split will suffice-use figure 10 in the MAX5354 datasheet and run
    the input and output 741's off this:

    View in a fixed-width font such as Courier.

    +-------------+---+---|IN OUT|---+-(+)5V
    | | | | | |
    | | | | | |
    | | 680 | GND | ===
    | | | +-------+ 1u
    | | | | |
    (+)---+ | | | |
    | +-------+-------+----+-->
    | | | |
    12v === 1n5231B| | ---
    1u ---/ === ///
    | // \ 1u
    (-)---+ | --- |
    | | | |
    | | | |
    | | | |
    | | | |
  8. Chris Carlen

    Chris Carlen Guest


    LM741, I love it!

    Consider a more modern general purpose OP-amp like TL081,2,4.
    Rail-to-rail opamps are really cool, but expensive. Avoid if not
    needed. Spend a lot of time reading various op-amp datasheets to
    understand the specifications, and read "The Art of Electronics" a bunch
    of times.

    You should stick with higher slew rates to provide enough full power
    bandwidth (not the same as small signal bandwidth, which is the front
    page spec given in datasheets). For an interesting extra homework
    problem, determine the slew rate r needed from an op-amp to avoid
    distortion for a desired signal output of frequency f, and peak amplitude A.

    For audio, there are some really cool op-amps that aren't expensive:

    OPA134, OPA2134, etc.

    From TI. Go to TI's web site, register, and they will send you samples
    overnight (yes, to students as well) You can get about 4 each of at
    least two part numbers per order.

    Here's a neat article:

    Good day!

    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
    -- NOTE: Remove "BOGUS" from email address to reply.
  9. Jamie

    Jamie Guest

    he said effects amp, maybe the effects he is getting from the 741 is
    what he wants! :)
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