Effects Loop Update

Discussion in 'The Workbench' started by 67Mopar, Mar 15, 2019 at 5:32 PM.

  1. 67Mopar

    67Mopar Well-Known Member

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    There's a slight difference in volume when I switch the effects loop on. If I reduce the value of this resistor, will it increase the return volume? Top-end is cut a bit as well, so maybe the 1uF cap need to be reduced as well?

    [​IMG]
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    effects loopvol.png

    The return gain resistor can be replaced with a pot.
    Example: 50K pot. Then you can turn it up/down as you need.
     
  3. 67Mopar

    67Mopar Well-Known Member

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    Awesome! Thank you. :dude:
     
  4. 67Mopar

    67Mopar Well-Known Member

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    What about the 100pF cap (C6) in-parallel with the resistor (R7)? Would changing the cap value brighten/darken the signal?
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    The cap limits the high frequency response, taking the cap out = maximum treble response
     
  6. 67Mopar

    67Mopar Well-Known Member

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    Thank you. I suppose I could just clip one side, in leu of removing it entirely.
     
  7. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    What amplifier and before you go modifying for nothing, is there effects in the loop? Which effects?
     
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  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes you can clip one side of it.
     
  9. myersbw

    myersbw Well-Known Member

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    Keep in mind that cap is there to reduce hiss. Clip it and you might get a lot more upper freq amplified than you want. To increase the gain, you can also decrease R5, and it's easier to piggyback a resistor to see if you like the result. Take a 50-100K pot...wire the wiper and one end across R5 (make sure it's up full and measure about 50-100K between wiper & end before you tack solder it across R5)...push a signal through and incrementally decrease until you don't hear a volume increase that's about unity when switching the loop in/out.

    That said, do this first...just insert a patch cord at the loop and switch it in/out. Does it change? Note if more or less...use the same pot I described and adjust until you can't hear the difference. That'll confirm whether the loop is functioning at unity gain as compared to the loop being in or out.

    However, pedals differ in impedance and how they match up. One may be fine...another seem weak...and, still another make have a gain boost and not make a difference. I'd leave that cap alone. Pedals are notorious for extra high freq...that's why it's there.

    Oh, and take the gain increase slow when turning that pot. If you drop it to zero, remember the gain ratio = 1+(R7/R5) in this case...drop R5 to effectively zero and you have a high gain amp with all the pretty squeals from oscillation. Reducing R5 preserves the cutoff knee of bandpass that R7 and the cap dictate.

    Cheers!
    Brad
     
    Last edited: Mar 16, 2019 at 11:25 AM
  10. 67Mopar

    67Mopar Well-Known Member

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    Editting.
     
    Last edited: Mar 19, 2019 at 6:50 PM
  11. 67Mopar

    67Mopar Well-Known Member

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    Editting.
     
    Last edited: Mar 19, 2019 at 6:47 PM
  12. JohnH

    JohnH Well-Known Member

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    For something to try, I like Brads suggestion, to splice another resistor across R5 to increase gain at the opamp. No desoldering and its reversible, if you can just splice the new resistor across R5's leads. A good value to try would be around 18k, which in theory should lift the level by 3db.

    I have done this exact mod on my DSL401 for the reverb and FX returns.
     
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  13. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    There is no need to modify the amplifier yet. Why does everyone go straight to that conclusion?

    The GVT15H and combo are the same amplifier, same effects loop.

    Can you show or tell us what the "effects hum mod" is in the amplifier?

    How is the Echo Input Gain set? Is it set as high as possible with no clipping? Is the incoming signal to the Echo set at the point before clipping?
    Are the Echo Volume(s) set to high or maximum level?

    Did you check patch cables or try different cables?

    Did you try cleaning jacks and plugs with an electronics lubricating contact cleaner?
     
    Last edited: Mar 16, 2019 at 5:28 PM
  14. 67Mopar

    67Mopar Well-Known Member

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    Editting.
     
    Last edited: Mar 19, 2019 at 6:48 PM
  15. 67Mopar

    67Mopar Well-Known Member

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    Editting.
     
    Last edited: Mar 19, 2019 at 6:51 PM
  16. ampmadscientist

    ampmadscientist Well-Known Member

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    1.If the effects loop has a hum, try disconnect the ground (shield) from the return patch cable at the effects unit output jack end.
    This will break a ground loop if there is one...

    2. Hum can also be caused by the power supply for the effects.

    3. Hum can also be caused by :
    running the input jack effects and running the loop effects from the same single power supply.
    If you are doing this try using 2 separate power supplies.
     
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  17. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Please clarify this.
    How did you make sure the signals were level? What signals did you make level?

    Every amplfiier, every device, every component and every wire has differences whether they look the same or not.
    This could mean the combo may require the ECHO Levels be set to -3dB while the head only need be set at -5dB due to any differences like internal tolerance differences.
     
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  18. 67Mopar

    67Mopar Well-Known Member

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    Editting.
     
    Last edited: Mar 19, 2019 at 6:48 PM
  19. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Ok, I get it.

    The preamp tube's filaments are feed DC.
    The effects loop upgrade information would be good to see.

    Like i mentioned, any variance in the components can affect the signal gain/loss. It seems its not your cords or gadgets so it must be in the loop itself since the loop bypass provides a higher signal level. In this case @myersbw had the best solution for dialing in the the entire effects loop. That would make the 10k R5 at U1-B your target. Actually a small trimmer instead may be liked or combination keeping it always variable.

    But first we should see this loop upgrade and how it affects the circuit.

    Also make sure you have good connection/continuity at connections VREF-1 and VREF-2 of that effects loop circuit.

    .
    .
    .

     
    Last edited: Mar 17, 2019 at 2:29 PM
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  20. 67Mopar

    67Mopar Well-Known Member

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    Editting.
     
    Last edited: Mar 19, 2019 at 6:48 PM

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