Help with power supply for a phono preamp

Discussion in 'The Projects Forum' started by art_12345, Jul 3, 2015.

  1. art_12345

    Thread Starter New Member

    Mar 26, 2015
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    Hi everyone, I am new in this forum.

    I am working on a phono preamp for a turntable, and I am having troubles with the design of the power supply.

    Mi question is how can I get a negative voltage from a single dc source (like having only 12vdc), cause I have seen some comercial preamps that have a single dc input, also is it really necessary to make a design with negative voltage, because I found this schematic of a preamp and it does not seems to be using negative voltage at all.

    Screen Shot 2015-07-03 at 2.50.28 PM.png

    Thanks for your help
     
  2. crutschow

    Expert

    Mar 14, 2008
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    You don't need a negative voltage, you just need to bias the op amp in its active region.
    That's provided by R2 and R3, which generates a pseudo-ground bias voltage (VB+) which biases the op amp at 1/2 the supply voltage.
    That should work fine to allow the op amp to operate properly with a single supply.
    The capacitors block this DC bias and allow ground based input and output AC signals.
     
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  3. ian field

    Distinguished Member

    Oct 27, 2012
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    Some manufacturers use up a spare op-amp element as a voltage follower to buffer 1/2 Vcc provided by a pair of equal resistors in series across the single ended supply. A capacitor is used to decouple the resistor mid point - but not on the output of the voltage follower, it can cause it to oscillate.

    For a buffered divider, a couple of 100k is about right. A lot of manufacturers don't bother with the buffer, to bias a couple of op-amps 100k resistors are probably still OK, but any more than that - notch them down to 47k each.
     
  4. art_12345

    Thread Starter New Member

    Mar 26, 2015
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    Thanks, your explanation really help me
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    There's a pretty good op-amp applications manual that's free to download.

    I think it might be a TI publication, but you'll probably find others while searching for it.

    The title: "Op-amps for everyone" has stuck in my mind - but I can't remember for certain if that's the one.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Here the pseudo-ground load is only the current from the input AC voltage across R4 and R5, which is very small, so I see no advantage of using an op amp for that purpose (and the significant disadvantage of adding the noise of the op amp directly to the input signal, which of particular concern for a phono preamp).
     
  7. #12

    Expert

    Nov 30, 2010
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    That schematic seems entirely complete to me. Re: crutschow, It even comes with the biasing network included (R2 and R3).
    R14 could be adjusted to 5.6k if you're going to use a 12 volt supply. If you don't need D2 as bright as it will go, 10k will make it last longer and look pretty much the same (for an indicator light). ;)
     
  8. DickCappels

    Moderator

    Aug 21, 2008
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  9. ian field

    Distinguished Member

    Oct 27, 2012
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    I didn't say it was good practice - I just said some manufacturers sometimes do it.

    Maybe it just looks better on the schematic than having a left over op-amp strapped up to keep it out of mischief.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Yes, that is certainly what you did and did not say. No disagreement there.
    But since you didn't comment one way or the other about whether it was good practice, I thought it appropriate to mention it's deficiency when used in this application.
    Is that somehow a problem? :confused:
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    You came across as a bit adversarial, so I felt it necessary to point out what I hadn't stated. Mud can stick if you don't brush it off every once in a while.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    I truly apologize if I came across that way.
    That's the problem with the written word. Sometimes the tenor of the communication comes across different then intended.
    I perhaps have an abrupt way of writing sometimes but my intention was simply to point out the problem with using an op amp for a virtual ground in this particular application so no one would be temped to use it here.
    I certainly was not trying to make any negative implication about your post.
    In the future I'll try to word things as to sound less adversarial.
    Cheers. :)
     
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