Critique my onboard project

Discussion in 'Pickups & Electronics [BG]' started by daveze, Nov 24, 2005.

  1. As the title suggests, I'm wondering if a few interested folk could have a look at the preamp I've 'designed' (flogged from various sources). The idea is a differential buffer for each pup into a JFET gain stage, volume for each pup, no tone controls.

    Currently the JFET stage is the same as Kreuzer's, so I want to modify it a little, to operate at 18V being the main motivation (partly to be different, partly because I'd prefer to run the opamps at 18V).

    Its about 1:30am at the moment, so I'm going to bed. Tomorrow I'm going to have another look at my EE textbook and see if I can figure out the JFET stage, then I'll SPICE it up and see what happens.

    Any comments?

    Josh D
  2. Hmm, might attach the schematic...

    Sorry about the big watermark in the corner but like I said, its the same as Kreuzer's . Does anybody have a reasonable freeware schematic drawing program?

    Attached Files:

  3. Tim__x


    Aug 13, 2002
    Alberta, Canada
    The opamps need supply decoupling.
    Coupling caps on the the opamps arn't necessary.
    You need a coupling cap after the JFET.
    Volume pots are connected wrong, use the wiper as the output.
    I can't see all the component values around the jfet, perhaps you could post what they are.
    Either there only needs to be one source resistor, or one source resistor should have a cap in series with it.
    With such high resistor values around the opamps make sure you use a FET input opamp.
    And lastly, why do you want a JFET buffer, for distortion/tube-like warmth?
  4. I haven't drawn in the supply for the opamps yet, it was getting a little crowded and I'm not that familiar with the program to make it neat.

    The source resistors you mentioned are for the opamps, right? Everything I've seen has the inputs symmetrical for a differential amp...Unless of course your talking about the JFET, in which case one of the source resistors does have a cap in series with it.

    I've removed the coupling caps. And coupled the JFET output, I forgot that one.

    Fixed the volume pots.

    The covered up part is a 2k2 resistor in parallel with a 1k2 resistor and 100uF in series (1k2 and 100uF are in series). But like I said, I'm going to change the JFET stage, currently its mostly a filler to demonstrate the idea. I'm opting for the JFET mostly because I've never used one before. Its becoming evident that its a tad more complicated than I thought it would be.

    For the opamp it'll probly be the TL072.

    Josh D
  5. Tim__x


    Aug 13, 2002
    Alberta, Canada
    I was talking about the JFET source resistors. The bottom lead on a JFET is called the source. The differential amplifiers are straight-out-of-the-textbook (not a bad thing) as they are.

    The 2.2k resistor, the 100uf cap, and the 5.6MEG resistor after the two caps need to go to V-, not to ground.
    A TL072 should be fine.

    Using two batterys like you suggested simplifies every thing, connect them in series with the center point as ground.

    P.S. Please accept my apologies if any of this post seems at all patronizing, it's very difficult to guess at how much you know.
  6. I know little but many semesters of sleeping through lectures has forced me to learn quickly and guess well.

    When I hear 'source' my default meaning for source is the source of the sound. Which was why my first guess was towards the opamp. However, I've been reading a fair bit on electronics lately, so I also know where the source is on a JFET.

    It turns out that it doesn't seem I can easily get ahold of the BF245 (they're not in the catalogue), so I'm going to use an MPF102. Of the options I have, it appears to be the most similar.

    I've got an EE textbook, which I've been trying to use to figure out drain and source resistances but its not working particularly well. Using this site and using Vcc = 18 and Ids = 8mA, I calculated 560 for the source and 1.8k for the drain, which seem a little wrong...

    Thats it for now.

    Josh D
  7. ejsagun


    Jan 20, 2003
    use a better op amp like a NE5532 or a burr brown

    instead of the volume control why no use the pots in the feedback loop of the opamps for adjustable gain, so you can push the JFET into overdrive.

    maybe a switch to bypass the JFET buffer?

    ive got a few ideas too. Ill try to draww some up so ican share it with you
  8. Tim__x


    Aug 13, 2002
    Alberta, Canada
    A NE5532 wouldn't work with these resistor values, the Ib is too high. Pretty well any opamp better than a TL072 will reduce battery life. If battery life is of no concern, I'm partial to the OPA134.

    To retain CMR (the whole point of a differential amplifier) you'd need to use a dual pot with unbelievable tracking.

    That's a fine idea.

    JFET tolerances are so bad there is not much point trying to calculate exact values. A rough approximate value is Rs=Vto/Iq, then choose Rd so that Rd=V+/Iq, then choose the capacitively coupled Rs according to the gain you want, it might be interesting to make that last resistor a pot.
  9. Thats something I'd also gathered from Kreuzer' and Till's websites.

    I was also thinking a pot on that last resistor, probly a little trimpot though.

    What Tim said. There are a number of methods for achieving gain at that stage, the problem is they were a bit too involved for what I want out of this (ie different but simple). I'm taking a shining to the overdrive idea though, I'd be thinking more along the lines of another JFET stage though. Problem is, won't I be starting to draw a bit of current if I add that?

    To actually wiring it up, I was thinking of a modular approach. Three boards, two identical ones with the differential stages (including their respective volume controls) and the JFET on the third. It would make easier to contain physically, and would mean that if I wanted to add the extra stage for overdrive, its just an extra module, same if I wanted to add piezo's or something. It would also make a JFET bypass switch a snap to wire.

    Josh D
  10. Tim__x


    Aug 13, 2002
    Alberta, Canada
    If the gain control on the JFET (Rs with the C) doesn't give you enough overdrive, though I suspect it will, you could make the other Rs variable, changing it would upset the DC bias reducing headroom.
    It would be a little funny to use because somewhere near center there would be no overdrive, and at either end there would be different kinds of overdrive.

    If you decide to use another JFET to overdrive the output JFET you can run it at a very low current, 50ua or so would work.