Don Tillman's discrete FET guitar preamp

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

  1. wnorcott

    wnorcott Tele-Holic

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    I have built Don Tillman's superb and easy to build FET guitar preamp. I would like to share a PCB design and layout I created for it.

    Don Tillman's FET preamp is designed to always be in the signal path, and not a "sound effect" pedal per se. It is a beautiful sounding preamp one of the best I have heard. The only thing that makes it stomp box like is that it is portable: you plug your guitar into it first, and then into a regular guitar amp.

    The Tillman FET preamp uses a single J201 field effect transistor. The J201 JFET is a common part that should be familiar to many of you. What is different here is Tillman is not trying to drive it to saturation, he is keeping it clean and linear. The circuit has a very high input impedance of 3 Megohm which means it does not suck the tone out of your guitar and in that respect it also acts as a buffer. The tone quality reminds me of the very best of high end tube amplifiers, flat response and no noticeable tone coloration but still has that je ne sais quois that we know it when we hear it. It is just amazing that such a simple, spare design sounds so good. It only has seven parts and one of them is optional.

    First the schematic this is from Don's site.

    [​IMG]

    These can be built real small but my days of building a ship in a bottle are over so I scaled it so it is easy to build not clumsy. It will fit in the smallest project box that can hold 2 guitar jacks and a 9V battery.

    Here is my parts layout design for it. As seen from the component side.

    [​IMG]


    Here is my PCB mask to etch your own printed circuit boards, the image may display different sizes in your browser. When you print the finished size should be about 1.2" tall and 1.5" wide. The easiest way to print might be open the image in Paint and print on plain paper first, scaling it until the scale is correct. The size of the PCB is not so critical.

    [​IMG]

    The preamp is intended to be always in line and always on. There are no controls it is a fixed 3dB gain amp, and operates on a single 9 volt battery. I wired mine with a simple SPST toggle switch to power it on. IF you forget to turn it off I would not sweat it: A 9V battery will last about 300 hours.

    I built it with 1% tolerance metal film resistors. There are very few components in this build so use good quality. There is only 1 capacitor in the signal path, C1 is a DC blocking capacitor on the output. So it is not cricitcal, but I used good quality electrolytic. Use shielded cable on the input.

    No project page would be complete without a demo so here it is. I have a YouTube demo of it very clean guitar sound:



    Stereo link for the above since it is a guitar duet one in left speaker the other in the right:
    http://www.youtube.com/watch?v=RDeJXaxguqU&fmt=18

    It would be interesting to use the Tillman preamp as building block for a DIY amplifier head, I would say right after the input jacks and before the tone controls. You would probably need and additional clean gain stage to drive a power amp since the Tillman is fixed 3dB gain. Just a thought.

    And last but not least, here is the link to the original project page by the master himself, Don Tillman:

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

    I HOPE YOU LIKE IT!


    Bill
     
  2. pgambon

    pgambon Tele-Meister

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    I've tried this curcuit and it sounds great! 3db is a good boost. Really lets your amp tone come through.
     
  3. udimet720

    udimet720 Tele-Holic

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    I amde one of these for an acoustic that had no built in pickup/preamp. Very easy to build. I've wanted to do the preamp cable version, but I don't really have any need for it.

    It worked great with the RadioShack piezo I got for a pickup.

    Also, you can include a volume control by making R4 a 50K pot. This is handy in some situations....
     
  4. mmaatt

    mmaatt TDPRI Member

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    Hi,

    I'm interested in making this and found this http://www.muzique.com/schem/multi.htm looks quite good, has any one tried these PCB's.

    Finally how do you identify the pins on the J201 and which one goes where ie Ground, Source & Drain.

    Many thanks

    Matt
     
  5. wnorcott

    wnorcott Tele-Holic

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    Anything made by Jack Orman is good: Jack is the Godfather of pedal design. Orman is the most experienced guy out there by a factor of ten as far as I am concerned.

    To identify the pin out on J201 you can refer to the chart that I added earlier for that purpose:

    Transistor Pin-outs Chart Transistor Pin-outs Chart

    Bill
     
  6. mmaatt

    mmaatt TDPRI Member

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    I'm waitning for my board to arrive from Muzique and was thinking of putting a switch then an LED (check light) after the 9v+ will this affect anything?

    Many thanks

    Matt
     
  7. wnorcott

    wnorcott Tele-Holic

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    Well as you can see I put a toggle switch on mine which is fine. A LED will draw a lot of current. I am not a big fan of LED's. On the Tillman an LED will use up more power than the Tillman preamp does: without the LED a fresh battery will last up to 300 hours.

    You should not put a LED in series with the +9V the best way to do it is you have to connect the LED beween the power rails i.e. between +9V and ground, after soldering a suitable resistor to one leg of the LED. Another thing to keep in mind is the current draw of an LED can cause a popping sound when you switch it on

    Bill
     
  8. mmaatt

    mmaatt TDPRI Member

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    Hi,

    I tried to make one these using a Jack Orman mulit purpose PCB and followed the componts/values as below.

    PCB layout here: http://www.muzique.com/news/tillman-fet-preamp/

    R4 - 3.0M
    R5 - 6.8k
    R6 - 2.2k
    R7 - 51k
    R9 - Jumper
    C1 - Jumper
    C2 - 4.7uF
    C4 - 10uF
    Q1 - Jfet transistor (2N5457, J201, MPF-102 or similar)

    It just buzzes, I wire a switch between battery and P+ rail, but it doesn't seem to make any difference to signal. what should i check first.

    Many thanks

    Matt
     
  9. wnorcott

    wnorcott Tele-Holic

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    Did you check the pinout on the transistor and make sure it matches the proper orientation on the circuit board?
     
  10. mmaatt

    mmaatt TDPRI Member

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    Got it working, the problem was with one of the jack sockets :oops:

    I had to change R5 from 6.8k to 2.2K to get the J201 to bias a 5.7v, with 6.8k it was just over 4v measured between drain and gate.

    One thing I have noticed, which I'm not sure is right is, if I play through the preamp, the volume is lower than if I play straight into my amp?? I thought this would be the other way round?

    Apart from that the tone is great.

    Many thanks

    Matt
     
    Last edited: Mar 31, 2009
  11. wnorcott

    wnorcott Tele-Holic

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    No it should be louder but only 3 dB louder, when you play through the Tillman. But the volume does not matter much, as your point out the Tillman is about its beautiful tone.
     
  12. mmaatt

    mmaatt TDPRI Member

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    Thanks wnorcott,

    I've realised what I've done, by trying to get the bias right I've reduced the output inpedance, the drain resistor should be 6.8k for this reason.

    I'll reduce the source resistor which will increase the drain voltage (I think!!).

    Thanks again

    Matt
     
  13. Jack Knife

    Jack Knife Friend of Leo's

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    Would this circuit work to warm up the sound of a SS amp? I've got a nifty little Yale Reverb that's slightly sterile sounding played clean.

    mmaatt, did you get the JFET that was recommended in the article or did you use another brand?
     
  14. mmaatt

    mmaatt TDPRI Member

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    I used a J201 - N-Channel JFETs - Vishay Siliconix.

    I've read that the tolerances of these things are quite big, I ordered three an they were all similar, I had to use 4.4k resistor on the source (R2) and 5.6k on the drain (R3) to get just over 5v between the drain and ground. I dont know if this due to the varying tolerances of J201's.

    R3 dictates the output inpedance so bias R2, I had R3 at 2.2k and this lowered the output so much that guitar straight amp was louder.
    Hope this helps


    Cheers
    Matt
     
  15. 11 Gauge

    11 Gauge Doctor of Teleocity

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    JFET's are notorious for having gain levels that are all over the map. This makes trimpots worth their weight in gold. As a plus, you might even be able to get right up to around unity with a 2N5458 or similar unit.
     
  16. Jack Knife

    Jack Knife Friend of Leo's

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    How about warming up a SS amp? Does it work good for that?
     
  17. wnorcott

    wnorcott Tele-Holic

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    It makes nearly everything you play sound better on nearly every amp. I recorded it the demo (above) on a 1973 Peavey theClassic

    It makes the tone clearer but if you pick sloppy it makes the mistakes clearer too so it is a double-edged sword.

    Bill
     
  18. zorgzorg2

    zorgzorg2 Tele-Meister

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    Hi guys,

    I looked at this preamp for a while now, but what I would like is that it would accept phantom power (48v)... I know the preamp cable of the same author does this, but what about this preamp ? Is it possible to adapt the schematics to get 48v instead of 9v ?
    Sorry if this is a stupid question, but my electronic level is not great...

    M.
     
  19. wnorcott

    wnorcott Tele-Holic

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    48V to 9V DC using voltage divider

    You can drop 49V down to 9V with a simple voltage divider I would use 1% metal film to get as accurate as possible since this is a divider.

    Vin = 48V
    R1 = 1K
    R2 = 240
    Vout = 9.29V

    [​IMG]

    http://www.raltron.com/cust/tools/voltage_divider.asp

    That is pretty close to what a 9 volt battery actually puts out.

    If it were me I would us a 9V battery instead of expose my expensive phantom power supply to the smoke test but the numbers work out. :lol:

    Bill
     
  20. zorgzorg2

    zorgzorg2 Tele-Meister

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    Thanks for the answer !

    To clarify a little, my thoughts were to have phantom power as an extra option, so that the battery doesn’t get empty when recording on a mixing console...

    If I end up with something else than smoke, I’ll keep you updated ;-)

    M.
     
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