Chorus Guitar Pedal project

Discussion in 'The Projects Forum' started by dacart, Jun 16, 2010.

  1. dacart

    Thread Starter New Member

    Jun 7, 2010
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    First, I am a novice and I'm coming back to this project after several months. I'm attaching a pic of the schematic. The immediate question I have is what does the term Vb mean? It appears to have to do with attachments to the two pots but there are three others off of R2 and R3 and most confusingly off of the voltage divider at R12 and R13. I'm also confised by the voltage divider since it doesn't really divide except atVb.

    If someone could explain Vb and the how the voltage divider works in this case I would be extremely grateful.

    Dan
     
  2. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    Vb I assume would be "V bias". Everything labeled Vb is connected together, it's just like labeling ground or Vcc or 9V.

    It is used as a reference created by the divider of R12 and R13. Notice that almost everything else labeled Vb is an op-amp input.

    You need to use a reference voltage like this when using a single power supply with op amps, since the signal needs to go positive and negative but your battery only provides 0 and positive. You use Vb as the middle point and the signal goes positive and negative from that.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Just to add a bit to Ghar's accurate assessment,
    R12 and R13 are likely roughly equal, unless the opamps are TL072 or TL082's; in that case R12 should be smaller than R13, as those opamps can "see" within 1v of the positive rail, but not within 3v of the negative rail; hence some offset is needed. C7 helps a great deal to eliminate resistor noise, much like placing a partially open garden hose nozzle in a bucket of water.

    It would help us help you a great deal if you would post the parts list, so that we can see what values were used in the schematic. It's very likely that we could suggest more modern parts that would result in better performance and less frequent need to replace the battery.
     
  4. dacart

    Thread Starter New Member

    Jun 7, 2010
    13
    0
    Here is what I worked off of. Rather than etching which, when I did this a while back, I wasn't comfortable with, I used buss wire for the traces and soldered it on some PCB. Nonetheless, here is the PCB layout with the parts labeled. Thanks for all your feedback.

    Dan
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    I see.

    A quick Google search for "Zombie Chorus schematic" turned up this:
    http://www.geofex.com/PCB_layouts/Layouts/zombie.pdf
    and this: http://www.hollis.co.uk/john/circuits.html

    Did you get it working?

    I don't see any problems with the basic design offhand. It would have been nice if they'd mentioned to use a TL072, TL082 or LF353 opamp.

    There are more modern opamps that could also be used that consume less power, have lower noise figures, and are rail-to rail - so they could operate on even less voltage. In that case, R12 should be increased to 15k like R13.
     
  6. dacart

    Thread Starter New Member

    Jun 7, 2010
    13
    0
    Oops,
    I forgot that the opamps aren't designated. This particular layout was done by a Fredric Mauro BTW. It actually calls for a TL062, however, I have two TL072CPs that I'm going to try.

    FWIW I've attached a pic of how I wired up the board with buss wire by basically following the layout that I attached previously. I just removed the two TL062s so there are some wires out of place right now but you should get the idea.
    Is this a reasonable way to go about wiring it up or am I asking for trouble?
    Thanks again,

    Dan
     
    Last edited: Jun 21, 2010
  7. dacart

    Thread Starter New Member

    Jun 7, 2010
    13
    0
    Bumpity bump...
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    You can do it that way and make it work. It's a good idea to tack-solder the wires to the board so they can't move around - particularly the longer runs.
     
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