Fuzz box guitar pedal project

Discussion in 'General Electronics Chat' started by Roob, Mar 21, 2015.

  1. Roob

    Thread Starter Member

    Mar 21, 2015
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    I am making a Fuzz guitar effects pedal and the schematic requires an AC128 germanium transistor. They are available on eBay, untested or with the hfe value measured. Values available are 70 - 110.

    Can anyone tell me what is a good value or how the value effects the sound?

    I've attached the circuit. Got this via Stoopkid
     
  2. blocco a spirale

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    Jun 18, 2008
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    hfe is not critical in this circuit but I'd lean toward the higher values. How does it affect the sound? Who can tell? Buy a few and try them.
     
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  3. Roob

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    Mar 21, 2015
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    There is an offer of 25 matched pnp/npn pairs of these untested on ebay, am I likely to get 50 duff ones? I've heard they are unreliable...

    They are new old stock from the 70s, the rectangular heat sink k variety...
     
  4. blocco a spirale

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    You may get some duffers but that's the way with old germanium transistors so you will always be taking a chance. They're not expensive so it's worth a shot for NOS devices.
     
  5. Roob

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    Mar 21, 2015
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    Ok, sounds good.

    Best way to check for duffers? Just plug and play I suppose...
     
  6. Alec_t

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    Sep 17, 2013
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    An LTspice simulation of the circuit in post #1 shows very little difference in the AC response of a circuit using the AC128 and one using any common-or-garden low-power silicon transistor (despite a shift in the DC operating point, due to Vbe difference).
    I couldn't hear any difference between the two circuit outputs (recorded as a stereo .wav file).... but then, I don't have golden ears :).
    For the sim I used an AC128 spice model obtained from this reference.
     
  7. Roob

    Thread Starter Member

    Mar 21, 2015
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    I just bought some germanium ac128 transistors.

    I'll set up the circuit and try these and some silicon pnp.

    Will post the results...
     
  8. Alec_t

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    What signal level would you expect to feed into the circuit? I may have over-driven it in the sim (but isn't a fuzz box supposed to dramatically distort the waveform ;) ?).
     
  9. Roob

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    Mar 21, 2015
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    Something like 100 -200 mV
     
  10. Alec_t

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    That's the level I was using, and means that the first transistor (silicon npn, 2N3904) is responsible for much of the distortion! Perhaps you don't need the other transistors :).
     
  11. Alec_t

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    The fuzz circuit shown here might be a fun one to build :).
     
  12. blocco a spirale

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    I have a box of government-surplus NOS flux capacitors up in the loft but have been unable to find a use for them.... up until now, that is...
     
  13. Roob

    Thread Starter Member

    Mar 21, 2015
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    OK, flux capacitors? time travel? May leave that particular and daunting looking circuit for now.

    If as you say Alec t, the 200mV is high is it worth decreasing this and bumping it up after the fuzz?
     
  14. Alec_t

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    I only wondered if 200mV was too high for a typical input level, but your reply in post #9 indicates it isn't. I don't think decreasing the input (pickup) signal level will help at all, since the principle of fuzz generation is to overdrive an amplifying circuit deliberately. Your main (pre)amp will bump up the generated fuzz anyway.
    Have you tried any of the fuzz circuits out there on the web that use an opamp with diodes in the negative feedback path?
     
  15. bertus

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    Sinus23 and Roob like this.
  16. Roob

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    Mar 21, 2015
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    I haven't tried any yet, somehow feel generous feelings toward transistors and cold toward op amps, not sure why, I think it will pass...
     
  17. Roob

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    Mar 21, 2015
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    will have a look, thanks
     
  18. Dodgydave

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  19. Veracohr

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    I've been casually working on a fuzz pedal myself recently, and found my guitar is putting out over 1V peaks on power chords. 200mV definitely isn't too high.
     
  20. Roob

    Thread Starter Member

    Mar 21, 2015
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    Yeah I was wrong, my piezo pickup puts out 200mV, this was going into the preamp which I'm working on at the momen, I was using the 2N2N3055 and getting over 2V out, another reason not use this transistor I can tune the output voltage to suit the next stage, either fuzz or eq.
     
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