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valve preamp modify to make DI box?

Discussion in 'Electronic Basics' started by Matt, Apr 1, 2004.

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  1. Matt

    Matt Guest

    Can anyone advise me on following?

    I have a simple valve preamp (based on a 12AX7) which I want to modify so
    that it works similar to a DI box. What I want the device to do is provide
    high impedence ( > 1 Meg) input for a device such as a guitar pickup, and
    convert the signal to a lower impedence suitable to feed to a microphone
    input or line input of a mixer. Prefereably the output would be balanced
    but its not a requirement.

    I understand that the output from the 12AX7 might need a transformer to
    convert it to a lower impedence. I have a quality MC step-up transformer
    (Dynavector DV-6A). Could this be used in reverse if the signal was
    suitable attenuated first?

    Can anyone advise me or point to schematics on the web?

  2. May be, but I wouldn't bother. Its best to avoid transformers.

    How is the pre-amp configured. You could use one half, as the gain
    stage, then use the other half as a cathode follower to get the
    impedance low.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "quotes with no meaning, are meaningless" - Kevin Aylward.
  3. Terry King

    Terry King Guest

    A typical DI (Direct Input AKA Direct Injection) box takes an unbalanced
    High-impedance input at 100mV or more from a musical instrument pickup
    (typically with a 1/4" 'phone' plug) and matches a standard professional
    microphone input (with an XLR plug) which is a balanced output at
    microphone level (say 1 to 2 Mv). Most DI boxes are passive and do use a
    transformer for isolation and good common-mode balance.

    Just why do you want to use the 12AX7, and what 'special' features are
    you looking for??
  4. Matt

    Matt Guest

    The amp configuration is shown in this schematic It's actually a
    preamp plus power amp. Since my knowledge of circuit theory is fairly
    limited it's probably best for me to show you the schematic rather than me
    attempt to explain it.

    Thanks for your input.

  5. Matt

    Matt Guest

    The features I want are high input impedance and the use of equipment I
    already own if possible. The main purpose is to feed an electric guitar
    signal directly into a mixing desk without significant loading of the guitar

    I specified the 12AX7 preamp because a) the impedence is high enough that I
    don't lose significant high-frequencies from the single-coil guitar pickups
    I use, and b) I have a spare 12AX7-based amp available to modify, and c)
    I've read of high-end DI boxes that use a valve input such as a 12AX7, and
    use a transformer output stage.

    I've looked at buying an active DI boxes but the ones suited to my budget
    don't have a high enough input impedence for my guitar pickups. The passive
    DI boxes that I've seen don't have a high enough input impedance either.

  6. Spajky

    Spajky Guest

    why not to use simple transistor emitter follower circuitry on the

    -- Regards, SPAJKY ®
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  7. Ian Bell

    Ian Bell Guest

    Becuase it would not have a high enough input impedance.

  8. Spajky

    Spajky Guest

    it would; if not take FET or Mos-Fet one !

    -- Regards, SPAJKY ®
    & visit my site @
    "Tualatin OC-ed / BX-Slot1 / inaudible setup!"
    E-mail AntiSpam: remove ##
  9. Ian Bell

    Ian Bell Guest

    Simple contradiction is no argument.
    Precisely i.e don't use an emitter follower, use a source follower.

  10. Rich Grise

    Rich Grise Guest

    A schematic! Awesome! ;-}

    Replace R3, the 3.3K resistor, with a 1K pot to ground, with _no_
    cap across it, in series with (a 2.2K resistor in parallel with C1).


    12AX7-3 (cathode)
    | |
    \ |
    2K2 / = 20 uF
    \ |
    | |
    / 20 uF
    1K \<-----||----- output
    / +

    That's your low-impedance emitter-follower output.
    If the volume gets too high too soon on the knob, use
    a smaller pot and larger fixed R. The change in feedback
    will probably change the overall response of the rest
    of the amp, but I'm ass-u-me-ing, from your description,
    that you're willing to live with that.

    Good Luck!
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