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Valve preamp

Discussion in 'Electronic Basics' started by Dave.H, Jun 18, 2008.

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  1. Dave.H

    Dave.H Guest

    I have found a valve preamp kit in an online store, to go between an
    amplifier based on an LM380 chip, and a crystal or regen radio, but it
    uses a microphone where I want an audio input. If anyone can tell me
    how to connect an audio in one this or give me a schematic for a low
    voltage preamp using a single 12AU7 tube, I'd be grateful

  2. Dave.H

    Dave.H Guest

    Oops, forgot the link to the preamp kit,

    They do have (had) another similar preamp with an audio input, but
    none are in stock now (discontinued)
  3. Eeyore

    Eeyore Guest

    Forget. It's designed for nuts who like to add a special type of
    distortion to their hi-fis'

    For God's Sake buy the National Semiconductor Audio handbook (I think it's
    back in print now) and/or the TI Bifet manual. Or download them.

    And /or also 'op-amps for everyone' (IIRC) from TI.

    And stop looking at crackpot sites using 30 year old chips ffs.

  4. Eeyore

    Eeyore Guest

    Fascinating in its own way but utterly irrelevant to what you require.

    Stop looking for 'kits' and do some *learning*. Then you can design your own
    kits. Less than one day's appropriate study should do it.

  5. ian field

    ian field Guest

  6. Dave.H

    Dave.H Guest, it's the 1951 Improved Hiker's one
  7. ian field

    ian field Guest

    Replace the headphones with a logarithmic pot - the kit instructions should
    indicate the resistance value to use, you need to make sure the pre-amp's
    input capacitor's working voltage is rated to withstand the full HT on the
    regen, or you could add a coupling capacitor to the pot's wiper.
  8. Dave.H

    Dave.H Guest

    I just built a two transistor (BC549,BC558) preamp kit that works
    well, will use that. I have a question regarding another regen I
    have. The 45 volt HT is placed across the headphone terminals, how
    would I connect an IC based amp to the radio, without frying the chip?

    Schematic of the radio is at
    (bottom of page, not my website). Nice looking vintage style radio,
    the top corners have been rounded off since (not by me), updated
    pictures are @

  9. ian field

    ian field Guest

    Put a resistor of the same resistance as specified for the phones in place
    of the phones, then couple the plate/anode to the top of a volume pot via a
    non-polar capacitor (0.1 - 0.47uF) rated to withstand the HT and take the
    O/P to the IC from the wiper - the bottom of the pot is obviously taken to

    You may find the IC input impedance is a little on the low side (often less
    than 47k) resulting in not much volume, an easy quick fix is to add a simple
    JFET source follower between the wiper and the IC amp chip - take the Vdd
    from the IC supply rail.
  10. neon


    Oct 21, 2006
    the last time i looked up microphones it was up to 20-20kc that is audio as far as i am concerned. a tube means hi voltage to low voltage lm380. that is easy to fix just a cap in between. but lets not forget this whole lame idea which ridiculous.
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