Saturday 17 July 2021

Telefunken Echomixer "Kanal A" Pre Amp

 A cool little pre amp circuit from an old german reverb unit. It is fairly clean on its own, it adds some volume, lows and sparkles to your sound. When being puched hard, it clips in a very pleasant way. It also works great for pushing a tube amp. And of course it can be used as a studio tool for drums and keys.

The stock transistors were TFK OC603s. After some experimitation I found out that low leakage transistors wih Hfe between 70 and 120 sounded best (....at least to my damaged eardrums) so transistors spec'd for Rangemasters or USSR germaniums works really good.

 

 
 
Layout with onboard voltage inverter. (Still unverified)
 
 
 

 




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8 comments:

  1. Ben Spinks18 July 2021 at 12:15

    Looks really interesting. Will definitely try this one.

    On another subject have you seen the free Alpha Dog pack on VFE’s new site? Has schematics for V1, V2, SPS, junior and Red Wolf editions. Might be worth a look.
    Cheers
    Ben

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    Replies
    1. fuzzhead18 July 2021 at 16:45

      Thanks for the headsup! Will definitely do a layout for Red Wolf.

      Delete
  2. KG18 July 2021 at 13:20

    Thank you, another interesting layout!

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  3. Miguel345 August 2021 at 10:52

    Is it possible to do a silicon npn version of it??

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  4. Jonas Fridell11 September 2021 at 13:42

    Any ideas on how to get a bit more output? Running the version with voltage regulator.

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    Replies
    1. fuzzhead11 September 2021 at 16:57

      I see no easy way to boost the overall output without altering the overall sound. You can try to reduce the 150k resistor to smaller value but if it is too small, the signal might hit Q2 too hard and generate undesried clipping. Worth a try, though.

      The other solution would be having a clean booster after the circuit.

      Delete
    2. Jonas Fridell11 September 2021 at 18:27

      Ok thanks - I’ll try some different values.
      The max output is just barely higher than the amp alone.

      Delete
  5. Miguel3429 April 2023 at 11:50

    I finally built this one. Made it npn with 2n3903 (hfe90). Changed input cap for 100n, input resistor for a 10k and 150k resistor for 100k ( I don't know aything about the maths involved, just messing around with values). It really needs 10kc (used linear). Soundwise, it's very clean if you use it on a clean channel, but it gets hairy once you crank up the knob. It adds "oomph" to the clean sound. It rrally shines used as a booster in front of an fairly saturated amp. I will probably continue experimenting with different cap values and maybe add a standard transistor booster after it

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