Turntable modification

Discussion in 'General Electronics Chat' started by Lagwagfx, May 29, 2010.

  1. Lagwagfx

    Thread Starter New Member

    May 29, 2010
    3
    0
    Hello all, first post here.

    I recently bought for cheap a used turntable to play some records (<snip> yeah!)
    quickly figured out I needed a phono preamp. I bought one and it's pretty small.
    However it doesn't have a knob for the volume and it works on a 9V battery.
    Its a nexxtech stereo preamp.

    What I was thinking to do was to incorporate the little preamp in the turntable and somehow (this is where I need a lil help) to connect it to the power source of the turntable.

    I am concerned that if I plug the preamp in the turntable it will drain a little power from the motor, is there a way ? Also for extra I would really like to add a volume knob, any suggestions ?

    Thank you very much,
    LP
     
    Last edited by a moderator: May 29, 2010
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Please keep the language clean. These forums are strictly [G] rated.

    Preamps don't have a volume control knob. They are designed to convert minuscule signals from inputs such as tape heads, phono cartridges (magnetic or piezo) and amplify them to line-level audio signals.

    Line-level audio signals typically have an impedance of 10k Ohms, and a peak-peak value of around 1v. You cannot use a preamplifier to drive speakers directly.

    As far as powering the preamplifier from the turntable; that may or may not be possible. We have no information on your turntable. However, to present a minimal risk to your turntable, I suggest that you use a "plug supply" aka "wall wart" rated for 12v, and a 9v regulator with appropriate caps on it's input and output to power the preamp.

    "Wall warts" are available everywhere. You probably have a half-dozen of them already.

    You have not posted your location in your profile (hint). If you are in the USA, your local Radio Shack stocks the LM7805. That, a 390 Ohm resistor from the GND terminal to ground, and a couple of 0.1uF caps from the input to ground and output to ground will make for a pretty good 9v preamp supply.

    If you want a better preamp supply, use an LM317 (RS also stocks them) with a 100 Ohm resistor from OUT to ADJ, and a 620 Ohm resistor from ADJ to GND.

    Use a 0.1uF cap and a 1uF cap from IN to GND, a 10uF cap from ADJ to GND, and a 10uF cap from OUT to GND.
     
  3. Lagwagfx

    Thread Starter New Member

    May 29, 2010
    3
    0
    SgtWookie,

    thanks for the reply. I will keep the language clean.

    My turntable is a sears LXI serues model 29673. So far it's playing nicely, steady speed. I live in Ontario, Ca ,will edit profile soon.

    I am willing to go with the wall wart idea but I'm still interested by powering the preamp with the turntable.

    Also I do not understand how to connect the LM317 to the preamp.
    On the preamp there are the 2 wires that connects to the 9V battery. the red and black wire. (Im sorry I am a little confused with the OUT, ADJ, GND.
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    The LM317 is an adjustable regulator. Depending on which resistor you place between OUT and ADJ (adjust) tells the regulator what voltage you want it to supply.

    So that will take your 12v wall wart supply, + from the wallwart to the IN pin on the LM317, (-) from the wallwart to the black wire of the preamp

    A 100ohm resistor between OUT and ADJ on the LM317

    Then OUT will go to your red wire and of your preamp battery plug.
     
  5. Blackbull

    Well-Known Member

    Jul 26, 2008
    70
    6
    Something odd about this posting, the language seems clean to me, his location is there and the attachment comes up as invalid?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Have a look at the attached schematic/simulation.

    Blackbull,
    The language was cleaned up after the fact.
    The location was added after it was requested.
    There was not an attachment that I recall prior to this schematic.
     
  7. Blackbull

    Well-Known Member

    Jul 26, 2008
    70
    6
    Right, Sgt, that's two of the three mysteries cleared up -these things worry me!
     
  8. Lagwagfx

    Thread Starter New Member

    May 29, 2010
    3
    0
    Allright thanks all.
     
  9. flat5

    Active Member

    Nov 13, 2008
    403
    17
    Use Google to find the data sheet for the LM317.
     
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