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Homemade blend and bypass pedal

Discussion in 'Effects [BG]' started by higgi, Mar 24, 2006.


  1. I have built a passive true bypass pedal for an effects loop useing a dpdt switch. I tried a blend circuit with it but kept getting feed back. All I did for the blend circuit was to put the effect on one end of a potentiometer and clean signal on the other, then the output came from the middle variable conection. I worked great except for the occasional feed pack problem. Anyone have a solution. I have been searching the internet for plans, but have not found any. I wanted to keep this passive and simple if possible.

    effect---^^^|^^^^^-----clean
    output
     
  2. tommy.genes

    tommy.genes

    Feb 16, 2006
    Yo! Philly
    The blend pot as you show it provides a feedback path from the effects output back into the effects input (via your "clean" signal connection). Since electrons will take the path of least resistance, making the value of the pot as large as you practically can will minimize the amount of signal that "feeds back" rather than flowing to the output. Try a 1 Mohm pot or bigger. What size did you use initially?

    To completely eliminate the possibility of feedback, you pretty much have to go active, although you can still keep it simple. Check out Sean MacLennan's 90% B Blender. It uses a buffer - either an opamp or a JFET - to make sure that no output signal feeds back to the input.

    You should also look at the full B Blender schematic because 1) the 90% schem makes a reference to "Vr" that you will need to examine the full schematic to understand if you don't already, and 2) once you make the decision to go active, you might just want to go all the way.

    -- T. G. --
     
  3. [SD]

    [SD]

    Feb 24, 2004
    what does +q means on the full blender schematic?
     
  4. it's +9 in a terrible font.
     
  5. [SD]

    [SD]

    Feb 24, 2004
    damn you are right! how could i not came up with that:confused:
     
  6. Thank you for the info. I may just have to go active to prevent feed back. The circuits look pretty easy. Anybody have a recommendation on a good quad op amp that works well with low frequencies.
     
  7. tommy.genes

    tommy.genes

    Feb 16, 2006
    Yo! Philly
    You'll find a lot of mojo arguments about op-amps.

    For this application, you want something pretty low-noise and transparent. The TL074 should do just fine (about $0.80). If you really want to be "hi-fi" go for a Burr-Brown/TI OPA4134 (about $4).

    Your op-amp choice is not really affected by the fact that you want to use this pedal with bass. The size of capacitors elsewhere in the circuit has a much bigger impact. Fortunately, the B. Blender design already had bass in mind, and is already fine in those areas.

    -- T. G. --
     
  8. As to the first reply, I used a 500K pot. Anyone have pro's or con's of FET version verses opamp blender? You guys are a great resource.
     
  9. tommy.genes

    tommy.genes

    Feb 16, 2006
    Yo! Philly
    I've built a very similar circuit (you may have heard mention of moosapotamus' "paralooper") using two TL072 opamps with great results, but I've never tried the FET version.

    If you do, please post a report.

    -- T. G. --
     
  10. [SD]

    [SD]

    Feb 24, 2004
    texas intruments make some of those hi fi op-amps for 4 euros or so...i bought a couple to test them against the TL's and see how good to the ear are both of them
     
  11. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    Hmmm, that B. Blender looks familiar.....

    The FET vs opamp is a personal choice. Many people prefer the sound of a FET. Many people like working with opamps. Your choice.

    Will the blender be in a fairly dedicated application? The real advantage of the "100%" version is that it works under almost all conditions. It also is really good for designing pedals. However, if the 90% pedal works for a given application, there is no need for the larger version.

    For the 90% solution, I would usually go for the FET.

    I always recommend breadboarding it first. If it works, you are good to go.
     
  12. I think I will build the 100% blender, four gain stages. I am going use FET. I may add the volume and or gain control also. This is for fun and learning, no hurry no stress.
     
  13. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    Actually, they are not gain stages. They are unity gain buffers.

    If you are building the 100% blender you might want to use the opamps. The opamps are almost perfect unity gain devices. The FETs have a little loss. The loss dosen't matter for the 90% blender but it might matter a bit for the 100% since the signal always goes through at least two buffers.

    However, go ahead and try the FETs. The losses won't be huge.
     
  14. Sorry for the calling the buffer gain. I am learning alot as we chat. I have made a few electronic things. Everytime I have to start from scratch and relearn. The last thing I built was a quadrature counter for a linear cable encoder. I know very little about analog stuff. I will use the op amps as advised. I thought maybe the FET trans had less distortion or something. I was going to try the TLO82 dual BiFET op amp from radio shack p#276-1715, does that sound good or should I order something?
     
  15. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    I have never used the TL082 but it looks good. Expensive but good.
     
  16. tommy.genes

    tommy.genes

    Feb 16, 2006
    Yo! Philly
    You'd pay $4 for two TL082 at Ripoff Shack, but you could order a TL074 from SmallBear Electronics for $4.50 ($0.75 for the part plus 3.75 shipping).

    I should caution you though, by the time you order all the other parts you see and want, ordering from SmallBear could end up costing a lot more ;)

    The TL07X series is a low-noise relative to the TL08X series, so it's a better choice.

    -- T. G. --
     
  17. Just added small bear to my favorites. I wish we had a better outlet than Radio S. in town. They hardly carry electronic parts anymore. It's like going to Lowe's for hardware, great place for bathroom fashions but not so good for all thread or coupler nuts.
     
  18. Thanks Sean and Tommy. I built a version of the 90% blender useing a dual biFET op amp. It works great, I really appreciate the info.:hyper:
     
  19. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    Congrats!
     

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