Thursday, 10 October 2013

DOD Studio Bifet Preamp / 410

Without the pseudo-balanced output and adapted to work as a stompbox. Not really sure if this is worth a build as it is supposed to be not that loud. Simple design with low parts count, so it might be nice thing to experiment on... The pot tapers are not mentioned on the schematic i found, but i suspect that linear should work well enough for both.



31 comments:

  1. Hi Mirosol, This looks very interesting to me however Id like to have seen how the (Pseudo) balanced out would have worked in the circuit. I can think of loads of reasons. I may have to try it but id want to know how to reconnect that balanced out that you've ommited. Also what did you mean by "pseudo balanced output" ? Is it not really a balanced out ?

    ReplyDelete
    Replies
    1. Schematic didn't have the output, so i'm not 100 percent sure how it's done. And no. According to forum posts, it is not really balanced.
      +m

      Delete
    2. Ok, Ive started the build anyway however Ive noticed the top right 4.7uF cap is sitting a trace cut ?

      Delete
    3. Damn. That cut should not be there. My bad. I can't fix it until monday..
      +m

      Delete
  2. Mirosol, speaking of balanced wiring, could you try your hand at an active D.I schematic sometime? Would be interesting to have one of those put up.
    Thanks,
    J.

    ReplyDelete
  3. Theres one of these on sale on ebay right now with some really good photo's in good detail especially of the balanced out, I dont know if some of you folks on here might be able to pick out the detail of how the balanced out is connected up ?

    ReplyDelete
  4. OK Im giving up with this one. voltages at the ic were pin 2 - 4.69, pin3 - 3.31 and pin 6 4.69. No sound at all. I tried a tl071 amongst other single op amps while waiting on the 081.

    ReplyDelete
    Replies
    1. Those voltages don't look too bad.. Have you tried probing it?
      +m

      Delete
    2. Unforntunately I dont have any thing I think that will do that, you mean like an audio probe/ oscilloscope ?. Just got a digimeter (DT9202).

      Delete
    3. http://www.diystompboxes.com/pedals/debug.html
      +m

      Delete
    4. ok feel embarrassed now, thats a great tool thanks Mirosol, wired it up and checked the circuit. There is sound at the chip and it is amplified at the other end.
      There is no signal beyond the 3.9M resistor at the bottom end.
      There is no signal on pin 2 of the volume but there is at pin 3.
      If you set the probe at pin 6 of the chip and adjust the tone, you get volume change.

      Delete
    5. No worries mate.

      You should have the output at the pin 6 and the "tone" seems to work also as intended. If you have the double link placed correctly, your signal should reach the output cap and go through it to the volume pot.
      +m

      Delete
    6. Yeap all good except Im starting to think there is a problem with the grounding in this, Ive tried various set ups but nothing seems to be letting the signal through to the output jack.

      Delete
    7. What I mean is with the probe the signal gets through the volume pot without the ground connected, as soon as you connect ground from anywhere you get no signal at pin2 of the pot.

      Delete
    8. You mean without ground connected to lug 1 of the volume pot? Is that pot ok? As it seems to work ok without that pot. You can measure the pot without signal or power. Turn it all the way clockwise and measure resistance between lugs 1 and 3 (that should be 100K), then between lugs 2 and 3 (that should be zero ohms). If that is ok, i'm guessing you're getting your signal grounded after the volume pot.

      Debugging may not seem like a fun thing right now, but you'll learn a lot by doing it. :)
      +m

      Delete
  5. You are right about the debugging thing - having the probe tool is great.
    Yeap I mean a signal on lug 2 with no ground on lug 1 - with the probe ground to the common ground, however when you connect the common ground to lug 1 then nothing, Ive checked this with 3 100k pots a 22k, 49k and 10k all do the same thing.

    ReplyDelete
    Replies
    1. That sounds pretty strange. The pot is wired as a standard output level control, so i'm surprised there is any issue with that.. From where do you take your ground for lug 1?
      +m

      Delete
    2. At the moment its wired up ground from the power socket to the board then the board to input ground the input ground to output ground then to lug 1 volume pot.

      Delete
    3. This may be a stupid question, but you are using the sleeve of the input jack? Tip connecting to pot lug 1 could explain everything.

      Try taking two wires to board ground - one from power jack and other to pot lug 1.
      +m

      Delete
    4. I think in this case there are no stupid questions !. I swaped the ground and hot wires around and tried it, then wired the ground as you've said and still no luck.

      I think Im going to try to build a new one from scratch over the next few days and compare.

      Delete
    5. Working with ua741, sound is great.
      Thanks

      Delete
    6. I'm tagging this as verified. Thanks multipraktik.
      +m

      Delete
  6. got it working, dunno what was up but the switch seems to have done the trick.

    ReplyDelete
  7. Anybody USED the TL071 in this? I tried a NE5534 since I had them and it's no go. Tayda doesn't carry TL081 but they do handle TL071. Don't see much difference. Mike.

    ReplyDelete
  8. Ne5534 is not a replacement for tl081, tl071 is ok .

    ReplyDelete
  9. Yeah, after looking closer, the 5534 won't work. Ordered some TL071's. Mike

    ReplyDelete
  10. Works fine with TL071's. Ordered some 061's too just because the order was going in. Just have to box it.

    ReplyDelete
  11. Has anyone compared this build against the original?

    ReplyDelete
  12. This comment has been removed by the author.

    ReplyDelete
  13. Would Anyone know how to connect the XLR Lo-Z output ? I would greatly appreciate the help, I am drawing a blank..
    Awesome job mirosol

    ReplyDelete
  14. I seem to have come across a mistake on this layout:

    The 22k resistor connected to pin 3 of the op-amp, i.e. the non-inverting input, needs to be moved one hole upwards, so that it connects to the inverting input. This makes the tonestack work much, much better. It is also consistent with the various schematics of the original that I managed to dig up.

    ReplyDelete