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Diy preamp help

Discussion in 'Pickups & Electronics [BG]' started by FiskFisk33, Mar 14, 2013.


  1. FiskFisk33

    FiskFisk33

    Oct 3, 2011
    So, I'm trying to make an Alex Rice preamp http://www.zachpoff.com/diy-resources/alex-rice-piezo-preamplifier/ and I've run into some trouble. I've wired everything up on a solderless breadboard and now I get no signal whatsoever. :banghead: At first I did though, a lot of humm and a very very weak signal from the piezo. I know I've soldered the piezo and the xlr cable correctly, partly because I've tested multiple times with a multimeter and partly because I get sound from the piezo plugged straight in. What I did realize was that I had mixed up the d and s pins on the jfets. Have I ruined them? Have I ruined something else? I've double checked that the resistors are intact but I have no idea of how I would go about testing if the jfets and the capacitors are ok.

    Do any kind soul here have any help for a confused beginner on thin ice? :hide:
     
  2. mystic38

    mystic38

    Dec 4, 2012
    Mystic CT
    i hope you observed all anti static precautions... pull out the jfets and check them out.. make sure there is continuity between each component legs in the breadboard.. sometimes they dont make good contact.

    http://www.allaboutcircuits.com/vol_3/chpt_5/3.html
     
  3. FiskFisk33

    FiskFisk33

    Oct 3, 2011
    I don't think I did actually.. Great link, all my fets seem to be intact. Hm so drain and source are the same on these? And my "mixing them up" actually made no difference?
    I've torn this up and reassembled it like 8-9 times now and I think I am starting to grow grey hairs. :banghead:
    Could I realistically have murdered a 220pf 50v capacitor during testing with Phantom power?

    Could poorly matched jfets result in the total lack of signal or would that just sound not great? My plan was to try different fets and see which ones worked best together or was that just naïve?
     
  4. mystic38

    mystic38

    Dec 4, 2012
    Mystic CT
    absolutely... you are assuming 48V is just 48V.. but even in spec it can be 52V so 50V caps are simply inadequate... should be 100V minimum.

    you need matched devices in any differential pair configuration.. ie the same device number.

    not going to say naive and i applaud your enthusiasm, but just get it working and start using it.. its never going to sound fantastic no matter what..its still a piezo.:)

     
  5. FiskFisk33

    FiskFisk33

    Oct 3, 2011
    I guess I'll order new caps then! Thanks. :)
    About the jfets they are the same make and model so to say, but I've read that their specs can differ a lot (minimum cutoff voltage undefined, typical 3v, max 8v meaning its cutoff voltage could be anything in between >0 and 8v) so for This usage even same device number fets should be tested and matched. My wonders on this subject is whether this kind of mismatch means unusable or just crappy.

    I bought a few more than I needed to be able to test around but that plan won't pan out well if poorly matched fets give zero output.
     

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