Don Tillman's discrete FET guitar preamp

Discussion in 'Burnt Fingers DIY Effects' started by wnorcott, Jan 27, 2009.

  1. mmaatt

    mmaatt TDPRI Member

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    I just bought 10x J201's, as the first 3 I had took some messing about about with values of R2 & R3 to bias properly.

    I've just checked all 10 J201's with R2 @ 2k2 and R3 @ 6k8 the drain voltage was between 4.1v and 4.3v. Changing R2 to 3k9 got my highest J201 (4.3v) to 5.19v, which I think I'm going to live with. Reducing R3 also affects the output, so you lose the 3db gain (not this this is important).

    I was wondering what, if any effect these two have on the tone of the thing (my test rig was on protoblock, without in/out jacks).

    Cheers

    Matt
     
    Last edited: Apr 10, 2009
  2. wnorcott

    wnorcott Tele-Holic

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    Changing R2 should not harm the tone since the purpose of R2 is just to bias the J201. Before we can worry about tone the transistor has to conduct. But it will increase the sensitivity of the amplifier and hence the volume: even without having to touch R3, when properly biased the FET starts amplifying at a lower input signal voltage, making it more sensitive to your picking and sound more 'responsive'. I wish I knew why Jack renumbered all the components on his redrawn schematic and starting with R4 etc I did a double take when I saw that. ;)

    If you are getting the preamp to amplify then you are fine. Changing R3 that amount should not harm the tone. It is still medium impedance output.

    I would suggest at this point focus on your ear so time to hook up the guitar jacks and play through it. The ear is the best judge after all when you are done it has to sound good. When I design mods I start with theory but past a certain point it is the guitar in one hand and the soldering iron in the other. :lol:

    Bill
     
  3. zorgzorg2

    zorgzorg2 Tele-Meister

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    It’s me again. I was almost buying the components today but then I startet doubting : would the preamp be sufficient to plug my tele into a line input of a mixing console ? I know the impedance is corrected, but what about the gain ? Is 3dB enough, or will it still sound faint and noisy ?
     
  4. 11 Gauge

    11 Gauge Doctor of Teleocity

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    If you've used mass produced pedals (i.e. Boss, Ibanez, etc.) then you've experienced unity bypass buffering. Actually, it's a bit of a fallacy, because it's actually close to unity. Chain enough buffered bypass pedals together and you'll get signal attenuation.

    Unless there's some sort of radical EQ alteration, 3dB should be plenty.

    Lots of pedal geeks like to tack a jFET onto the beginning and/or end of their circuits to finesse the buffering and input and output impedances.
     
  5. wnorcott

    wnorcott Tele-Holic

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    By the way the transistor pins in this layout are D-S-G reading from top to bottom in the layout.

    The pinout of the J201 is 1-D 2-S 3-G

    So for a J201 you need to orient it with the flat side of the transistor facing the LEFT which is opposite direction it is in this picture. Sorry about that.


     
  6. scitizen17

    scitizen17 NEW MEMBER!

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    I have developed a PC board design that is 30mm x 17mm and includes a 10K trimmer in place of R2. This should help with matching specific J201 devices to the circuit. It includes the required 9v-GND, SIG IN-GND, & SIG OUT-GND solder points. It also includes a location for C3 (I'm using a 4.7uF) which is in parallel with R2 for additional gain if you require it. If anyone is interested I will post the board design.
     
  7. teletin

    teletin Tele-Meister

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    Please do, I would appreciate it.
     
  8. vikki

    vikki Tele-Meister

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  9. FenderLover

    FenderLover Poster Extraordinaire

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    These simple boosters are essential in my book - and too simple to screw up if you build it yourself. You could spend another $100 to have a Keeley or Fulltone name on it, but we all know better.
    Keep in mind that these single stage designs invert the signal. That may come into play if you are recording or mixing your signal. If you use just the buffer version, you get nearly 1:1 gain and no inversion. Even with no gain, a buffer goes a long way to helping you signal.
    If you use a slightly larger box, like a Hammond 1590DD, you can easily fit a buffer, a booster, and a looper in one box for for an essential Swiss army guitar setup.
     
  10. toolguy

    toolguy NEW MEMBER!

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    Sorry if this was asked before and I missed it.

    Building this tonight and I had a question on the inputs/outputs.
    Do the ground leads from the jacks get connected to the ground circuit of the preamp or connected to each other (lug to lug).
    Searched google but can't find any good pics of a built unit.
    Thanks
    Jd
     
  11. RomanS

    RomanS Poster Extraordinaire

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    All ground connections (plugs, circuit, power plug, etc.) have to be connected to each other - doesn't matter which point you use to connect them to, for practical reasons, the ground lug on one of the jacks is a good place for soldering all the ground wires to...
     
  12. jkstigma

    jkstigma TDPRI Member

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    hello my friends!!!
    i just build this box and its sounds great

    is there any problem if i use this preamp with a 9v adapter and not with a battery?
     
  13. droscigno

    droscigno TDPRI Member

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    I was thinking that my iPhone might make a handy practice amp since I always have it with me anyway. I looked for an interface cable, but the one that is commercially available has many reviews that say it does not amplify the signal up to mic levels, so it does not work so well. Is the 3DB gain of the Don Tillman circuit the right amount to bring an electric guitar output up to the level expected at the iPhone mic input?

    Edit: From further reading it is likely that the real issue with the commercial cable is an impedance mismatch, not the output level of the guitar pickups. From reading Don Tillman's design considerations, the input impedance of his circuit is 3 Mohm, and the output impedance is 6 Kohm. One replacement headset for the iPhone lists its mic as having 2.2 Kohm impedance. Does this sound close enough to people who know what they are doing :) (I don't!)

    Thanks,
    Dan
     
    Last edited: Mar 31, 2010
  14. limbe

    limbe Tele-Afflicted

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    If the adapter works with stompboxes it will work just fine
    Avoid adapters made for calculators or office machines.
    Usually they aren´t filtered enough.The preamp will hum.
    Limbe
     
  15. Mytelesastrat

    Mytelesastrat NEW MEMBER!

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    Hi Wnorcott - you seem to be the electronics guv'nor around here so could I pick your brains for one second? Don says to measure the bias at the drain. Is that between drain and ground across the 2.2k R2?

    Incidentally for future builders, Vishay Siliconix seem to have replaced the J201 with J201-E3. The specs look identical though, so I've bought 10. They come in packets of 10, that's why...
     
  16. danelectro2501

    danelectro2501 TDPRI Member

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    Does anyone have the lauot pics for this? I've built 2 from other sites and neither works. I'm new at the diy pedal thing and need a butt simple layout that shows all the jacks wired etc. Any help is greatly appreciated as I an dying to make one that sounds like the video above. if anyone has the pics or a layout could u email to me at danelectro2501@yahoo.com ???Thanks!!!
     
  17. lukasoft

    lukasoft TDPRI Member

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    danelectro2501, I actually just finished making one of these the other day, and its still on a breadboard, not yet soldered. When you say you built from other sites, do you mean you built this specific preamp? If so, where did you get your JFETs from, and are they J201s? If not, some of the properties change as far as amplification is concerned, and your headroom may be affected and you may have to bias them differently, and they still may not amplify at all. An actual picture isn't going to help you much more than a schematic, like the one shown by the OP, as the schematic is very simple. pretty much the only thing you can mess up is the wiring to the transistor, all other parts can be put forward or backward and the preamp will still work (if the capacitors are placed incorrectly, the amp will still produce sound, but possibly not as good).
     
  18. njbraskey

    njbraskey NEW MEMBER!

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    Having Issues

    im trying to build the circuit from

    http://www.till.com/articles/GuitarPreamp/

    my input is a strip of piezo material from here

    http://windworld.com/products-page/electronic-hardware/piezo-cable/

    i built it on a protoboard first before making the pcb and all im getting out of it is either nothing or a fuzz. i havent really worked with transistors before, so im not familiar with the pins or how they work. when i check the voltage from the first pin to the third pin, i get about 8V. im not sure what im setting up wrong or if i need to change a resistor to a different value.

    if anyone can give a better explanation on which pins of the transistor go to where in the circuit and any resistor changes i can try that would help a lot,

    thank you

    Nick
     
  19. Armo

    Armo Tele-Holic

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    Fingers crossed this should work.
     

    Attached Files:

    rawdatalab likes this.
  20. Crawfish

    Crawfish Tele-Afflicted Silver Supporter

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    Hi Bill -

    Great job on your build. The link to the PCB layout isn't working...do you have another link? About to make my first PCB and this would be a good one to work with.

    Thanks!
    -Kevin
     
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