Home Forums GuitarPCB Build Support Add Verb volume

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  • #1670
    Anonymous
    Inactive

    Hi,

    I’ve just build the Add Verb, everything works fine but I have a loud output volume. To be more precise when the volume knob is full anti-clockwise (no volume) the pedal is louder than in off position. So I can’t manage to have no volume change when the pedal went on and off. Is it normal?

    I’m glad to see this new forum! Is it planned to have the old one as static pages for archives?

    Thank’s

    #1671
    Cybercow
    Moderator

    That is not a normal behavior. After a quick look at the schematic, I’d double-check the value of R7 and ensure that it is a 6K8 and not a 68K ohm value. Hope this helps.

    #1673
    Barry
    Keymaster

    Did you hand wire the Volume?

    If so you may have wired it backwards if it is loud when off and you get nothing when turning it up.

    Reverse Lugs 3 and 1 at the potentiometer (not the board) to change how the Pot works.

    Pictures are much more helpful as I am blindly guessing here.

    That said you do have a build issue as we would never release a circuit that worked in that manner.

     

    Regarding the “Old Site” question see our Guides section and you will find the link to the old site which will be kept as an archive.

     

    #1676
    Anonymous
    Inactive

    Thank you for your answers,

    I’ve checked R7 and it’s a 6k8 and my pots are on the board, here are some photos (I didn’t wire the led yet…)

    I hope you could find some mistakes!

    Thank’s again!

    #1678
    Barry
    Keymaster

    I cannot see the color bands easily in the photo so the next thing to check is all resistor values by color band since you cannot test them in circuit. You can use the card in the guides section or you can use an online calculator.

    The problem you are describing sounds to me like a possible resistance issue.

    There are not that many so it should not take too long but do not rely on packaging, labels etc and do them all by color band with the build guide photo in hand and check them off one at a time.

    One other thought is the Output Jack looks very close to shorting on the back of the pot where there is no tape especially when inserting a cable which would expand it. Just an after thought.

    #1679
    Anonymous
    Inactive

    I’ve just checked all resistors values and it seems ok.
    I’ve put some more tape on the pot so there is no problem in this place.
    I also tried to swap the TL072 with a NE5532 (didn’t have another tl072 to test) with no more change…
    I will continue my investigation.
    And to be sure of the correct function of this pedal, when the volume pot is at 0 (full anti-clockwise) there should be no sound at all, right?

    Thank’s

     

    #1680
    Barry
    Keymaster

    Zero does not equal no sound. – Many pedals do not go “completely quiet” when the volume is at zero.

    Should the Volume Knob at Zero be louder than unity gain as you described above… No.

    Three Questions:

    • When your Volume is at Zero and you rotate it up full does it get extra loud or softer?
    • Is there tape on the back of the dual-gang pot?
    • R7 = Blue, Gr, Black, Br, Br

    Next check and verify all capacitor codes. Once you have verified all capacitor codes then re-flow all solder joints to be certain there are no cold solder joints causing the problem.

    #1681
    Cybercow
    Moderator

    elfanatic – I built an Add Verb just a few weeks ago just as outlined in the Add Verb build document. It works just fine. Barry’s recommendations are sound.

    For grins, giggles and a quick little test, temporarily tack a jumper across R7. When the volume pot is turned to zero (fully counter-clockwise, the result should be that pins 7 & 6 of IC1B will be shorted together. When pins 7 & 6 (or pins 2 & 1) are shorted together, the opamp yields “unity gain” by design. The theory and documentation behind these types of opamps states that when pins 7 & 6 are shorted together, the opamp is in “unity gain”. The 6K8 value of R7 is going to provide a minute amount of gain from that opamp stage; but I also believe that the 6K8 value is used to elevate the gain slightly to recover the signal from the previous stage. So, in theory, shunting R7 should lower the output of the final opamp stage.

     

    #2623
    Anonymous
    Inactive

    Hi!

    Sorry but I had the same problem as elefanatic. Seems like the input stage has too much gain (Av = -(R2/R3) = -(200000/27000) = -7,41. The gain for the dry path of the mixing op amp, when the volume pot is turned fully ccw, is Av = -(R7/R6) = -(6800/22000) = -0,31. Then I think we have a total voltage gain of 7,41*0,31 = 2,3 and can never reach unity gain. The solution was to replace R2 with 100k to get a gain of -2 in the input stage, this gives unity gain around noon at the volume pot (when R7 + Vol. Pot = 11k). The effect sounds nice and as expected now. Hope this helps!

    Kind regards

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