Informations on wideband Preamp

Discussion in 'The Projects Forum' started by johnsy, Jul 3, 2014.

  1. johnsy

    Thread Starter New Member

    Jul 3, 2014

    first post on this forum, i hope this is the right place to ask this.

    while searching for a small (portable) wideband (below 15hz) preamp i found one but alas no details on the components values (probably have everything to build it, but i have no infos on these transistors)


    The specs:

    Operating Voltage: 12- 30 V/DC

    Super Wideband:
    10Hz - 150KHz (approx)

    Input Sensitivity: 2 - 20mV (approx)

    Output: 200 mV - 2 V

    Structure: 2 - Stage


    Any ideas on those values, especially the transistors!

    I'll really appreciate any help!

  2. MrChips


    Oct 2, 2009
    There are 1000 ways to build a microphone preamp.
    If you are interested in the circuit you have presented why not purchase the kit?
  3. johnsy

    Thread Starter New Member

    Jul 3, 2014
    Thanks for your reply!

    Yes i know, but i didn't find any schematics for a preamp with those specs (10hz is required up to 100khz)

    I first contacted both the vendor and ebay sellers but the shipping charges are very high (three to four times the price of product) which is absurd.

    Knowing that i might have the components already here, it makes sense to ask if someone might help on this.
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
    Only a guess - 2N3904.
  5. DickCappels


    Aug 21, 2008
    D1: Don't connect the power backwards and you can leave this out. Other way use a 1N4001...1N4007

    Reading the color code.
    R1: 560
    R2: Mystery 160k?
    R3: 47k
    R4: 560
    R5: 560
    R6: 100
    R7: 2.7k

    Guessing at the capacitors:
    C1: 100 uf
    C2: 22 uf
    C3: 22 uf
    C4: Maybe 47 uf

    T1, T2: Low noise audio NPN such as 2SC3199, BC414, BC549, 2SA1048, or KTC3200
    absf, johnsy and bance like this.
  6. johnsy

    Thread Starter New Member

    Jul 3, 2014
    Thanks ronv for your reply (personally i thought of the bc549)

    Thanks a lot DickCappels!

    Are you sure?

    I couldn't find R2 in the color codes (not really sure about that one)

    Really? it seems to me that "47.." can be read on the side (i might very well be mistaken)

    Ah! i have everything then, this is great!

    I wonder why this schematic isn't very popular (might need to reduce gain but not much beyond that)
  7. DickCappels


    Aug 21, 2008
    About R2: No not sure, but 160k is a reasonable value based on how its used in the circuit. 150k should work fine as well.

    About c3: Its the power supply bypass capacitor so not critical. Generally, using the larger capacitor is more conservative.

    As Mrchips pointed out, there are a thousand ways to make a preamp, and this is merely one of them.