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I need a low-noise guitar pick-up preamp...

Discussion in 'Electronic Design' started by powerampfreak, Mar 10, 2007.

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  1. Hello,
    Anyone having good suggestions what kind of preamp is best for use in
    guitar pick-up applications.
    These pick-ups has very high impedance.
    Would a I.C based circuit or a discrete provide the best results?
    Your ideas are most appreciated,
    Regards
    PAF
     
  2. Eeyore

    Eeyore Guest

    Best ? Depends what you call best. An IC design is going to be very distortion
    free which may not be what you want !

    What exactly are your criteria ?

    Graham
     
  3. Low noise.
    Low distortion.
    Just a preamp to buffer the high impedance pick-ups, no coloring of
    sound.
    All tone shaping is done later.
     
  4. Guest

    It would help to specify the pick-up. Low noise is generally
    associated with high current. Is this a battery powered project?
     
  5. Ken Smith

    Ken Smith Guest

    The LSK170 + a LT1128 may be what you are looking for.



    Like this:
    Vcc (about 12V)
    !
    Vcc \
    ! / 1K
    \ \
    /100K ! 47u
    \ +------!!----------
    2.2u ! !- d !
    ---!!----+-+--->! LSK170 !
    ! !- s !
    / ! LT1128 !
    100K\ +--------!+\ !
    / ! ! >---+--+---+--- Output
    ! \ ---!-/ ! !
    GND / ! ! !
    3.3K\ ---------- !
    ! !
    +-------!!-------------
    ! 47u
    \3.3K
    /
    \
    !
    GND
     
  6. Yes, it will be battery powered.
     
  7. Phil Allison

    Phil Allison Guest

    "powerampfreak"

    ** A FET input pre-amp.

    Do a Google search.



    ....... Phil
     
  8. tempus fugit

    tempus fugit Guest

    Check this one out:

    http://www.till.com/articles/GuitarPreamp/
     
  9. Phil Allison

    Phil Allison Guest

    "tempus fugit"


    ** The 2.2k in the source can be bypassed with an electro of about 10uF for
    increased gain.

    10 times gain or 20 dB is easily possible.



    ........ Phil
     
  10. Guest

    Hmmmh, doesn't like the sound of op amps, but big ass polarized caps
    in the audio path are just fine. Uh, right.

    I have this general distaste for circuits that use 9V batteries unless
    the current draw is on the order of self-discharge. Always design with
    AA cells if possible.
     
  11. Eeyore

    Eeyore Guest

    In which case a FET input op-amp (e.g TL07x family) is ideal.

    Graham
     
  12. Eeyore

    Eeyore Guest

    Slightly overkill for a guitar pickup though.

    Your Zin is way too low btw.

    Graham
     
  13. Phil Allison

    Phil Allison Guest

    <

    ** Piss off - you pathetic audiophool wanker.





    ........ Phil
     
  14. Eeyore

    Eeyore Guest

    Adequate but not very low distortion and won't tolerate as ageing battery very
    well. End of life for a 9V alkaline is actually about 6 volts !

    Graham
     
  15. Eeyore

    Eeyore Guest

    Actually I thought he was mocking the apparent audiophoolery in the article.

    Graham
     
  16. What's the problem with 9V (6F22) batteries? Why use AA cells? Tell
    me.
    I will probably need two 9V batteries since I don't like using single
    ended supplies,
    and I will need higher supply to have a decent output level and
    headroom.

    Also, how could anyone say "don't like to sound of op-amps". It all is
    up to how to op-amp is wired up.

    For the moment, I think a TL072/TL074 will do just fine, I'm a little
    afraid of it's noiselevel though.
    Besides, going for a NE5532 will make the batteries last much less.

    Regards
    PAF
     
  17. Eeyore

    Eeyore Guest

    I don't know what he had in mind there. I gather that they are sometimes less
    reliable since they consist of a stack of cells and the inter-cell connections
    can be unreliable.

    Ah ! I just drew a scheamtic using a single 9V battery since that's the norm.
    Never mind. It's easily adapted.

    That's always good practice for sure. The twin supplies also mean you can use an
    op-amp intended for split supply operation and these tend to be lower noise than
    those intended for single supply (and lower voltage) e.g. TL071 vs TLC271

    Well... the problem is that good op-amps don't really have 'a sound'. Many
    guitarists do actually like intentionally added (low order) distortion.

    Not a problem. They're perfect for this application. From practical experience I
    can tell you that a guitar signal is a decent ~ 10mV (when not played hard - you
    can get over a volt from some pickups when played hard).

    The noise of a TL07x op-amp at 12nV/sqrt Hz is 1.7 uV in the audio band. That'll
    give you a 75dB signal to noise ratio.

    Only about 1.4 times less actually but the 5534/2 wasn't intended for high
    impedance inputs. You're better off with a fet input op-amp here.

    Graham
     
  18. Phil Allison

    Phil Allison Guest

    "Eeysore = the never check a fact plagiarist "



    ** TI spec says 18 nV/rtHz, typical

    Noticeably noisier than a most single FETs.




    ........ Phil
     
  19. Eeyore

    Eeyore Guest

    You're quite right. I was thinking of an LF357.

    Perfectly true but easier to use.

    Graham
     
  20. Hi PowerAmpFreak, I have been thinking, and it is so many years ago I had
    a guitar with electric pickup coils, and those were inductive.

    So if the 'sensor' is like this:


    -R-L--------
    |
    ------------

    where R is the resistance of the windings, why not drive into a LOW impedance:
    +9V
    |
    [ ] 10k
    470k |-----------------0
    --===---| c
    | b |/
    ---||-----------|
    |\/ e
    -- |
    | |
    /// ///

    The idea is, that as freq goes higher, the impedance of the picup coil _also_
    increases, so the Ib is actually set by the R in the coil in series with the Zi
    of the transistor circuit.

    There are several advantages in using a low impedance circuit, no effect of cable
    capacitance, low noise if input open, linear freq ??? response?
    (I am not sure if lower frequency snares make the same EMF..., but seems sensible).

    Also, with the correct wrong bias this cicuit will give the so much loved correction
    of the waveform, sometimes by some non musicians bluntly referred to as 'distortion'.


    This aside, in the old vidicon (camera picture tube) days, great effort was made to make
    a high impedance input amplifier, one day I just stuck the signal in a cicuit as above,
    and it worked just as well.. (vidicons have many MOhms output impedance, makes it pretty
    much a current source, ideal to drive a base of a transistor, if impedance is high enough
    then no distortion.


    Anyways I have no guitar now, so I cannot test it, maybe somebody can?
     
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