Home Forums GuitarPCB Build Support Bass Overdriver bias voltage setting

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  • #20780
    Michael
    Participant

    Hello,

    I am new in this forum and I need your help 🙂

     

    I have build a “Bass Overdriver” pedal and I have a question regarding the bias voltage.

    When I setup the voltage of Q1, Q2 and Q3 to 5V, the sound is very terrible. It’s a very hugh distortion.

    So I found a voltage for Q1 and Q2 to ~ 8,4V and Q3 ~ 5V.

    That sound for me more smooth and enough to have a sound like “tube amp”.

     

    Question:

    Could that be normal?

    Does some other have the same problem with 5V bias ?

     

    My other problem, what I have with the Bass Overdriver is, when I play with the JazzBass the sound is good so far (with the 8,4V bias on Q1 and Q2). But when I play with my P-Bass and play the E-String, the sound with the Bass Overdriver is very “boooommmy” (Booooooommmmm). The other strings are good and playing without the Bass Overdriver is also good with all strings.

    For me that looks like the input level, what the Bass Overdriver get, is to high. (I use an compressor, but also without a compressor I have the problem)

     

    Question:

    Is there a circuit (like a “-X db pad”-switch), what I can add into my Bass Overdriver to reduce the incoming level. Maybe in combination with a switch to toggle between my JazzBass and my P-Bass?

    Do you have any example circuits for a test?

    Does anyone have this problem, too?

     

    Many Thanks and greetings from Germany.

    Best regards

    Michael

     

     

     

    #20781
    mybud
    Participant

    Hi Michael and greetings from South Africa. I built one awhile back which biased correctly and sounded fine on both guitar (passive) and active bass guitars, not too much like you experienced when biasing yours to 5V. I suggest checking the orientation of your FETs for starters and also posting some good quality pix as the forum generally prefers. There may be a wiring mistake that can be resolved fairly quickly by the collective wisdom. Good luck and best wishes, mybud

    #20787
    Michael
    Participant

    Hi mybud,

    many thanks for your quick response.
    Yesterday evening, I double checked my wiring, solder points and FETs. As far I can see that looks good.
    I can not find any problem.
    I bias the pedal again to 5V and did a sound check. It sounds like a fuzz, not like a “SVT” like sound.
    Then I switched back to my 8,4V for Q1 + Q2 and 5V for Q3.

    I also checked again the Bass Overdriver docu and saw in the “Party List” the R4 + R7 are 68K Ohms (what I use) but in the schematic picture the R4 and R7 have the value 33K Ohms.

    Is that maybe a possible issue?
    Do you use for R4 + R7 33k or 68K Ohms?

    I also saw I have solder the R10 resistor (1K8 Ohms) on the board, but I use the LED soldered on the footswitch. I do not think that R10 is responsible for that “fuzzy” sound at 5V bias voltage, correct?

    I made some picture, I hope the quality is good enough.

    I hope we can fix that issue.

    Many thanks
    Best regards

    Michael

     

    #20788
    mybud
    Participant

    Hi again Michael: Well spotted for the differences in R4 and R7 between the schematic and the BoM. I can’t recall exactly what I used but I suspect that I might have gone with the schematic and installed 33ks for both. I’ll dig the board out and check. Give me a while to confirm this please.

    You could try as an experiment connecting a 68k in parallel with the existing ones at R4 and R7. If you have crocodile clips at hand connect them across the two resistors in question and insert one 68k across each one. That should give you more or less 33k in each case and would then align with the schematic values. That may or may not make the biasing closer to 5V as suggested in the build doc.

    R10 should have no effect on biasing AFAIK. It’s only there to prevent burning the LED out. Perhaps the collective widsom will advise in due course. Best of luck with the experiment. Regards, mybud

    #20807
    Michael
    Participant

    Hi mybud,

    many thanks again for feedback.
    As you suggest, I have soldered 68k Ohm resistor parallel to R4 and R7. But that does not change the result.
    Also, at 5V on Q1, Q2 and Q3, the sound is like a “fuzz” ant not as a “SVT” style.
    I change the voltage back Q1 + Q2 to 8.4V and Q3 at 5V and I have the feeling it’s a little bit more “aggressive” as with the R4 + R7 at 68K Ohm.
    So I will remove the temp. experiment.

    I will test if it helps to increase the R1 (1st resistor after “input”) from 33K to 68K Ohm.

    I let you know about the result.

    Does someone have an idea, why my Bass Overdriver sound like a fuzz at 5V bias?

    Many Thanks
    Michael

    #20811
    Michael
    Participant

    Hi mybud,

    ok, I have increase the R1 to 101K Ohm and the sound with the P-Bass, you remember the “booommmy” sound on the E-String, is much more better. I think that higher resistor decrease the incoming signal from the bass.
    I will say that problem is so far fixed.

    The only thing which is not solved is, to set Q1 + Q2 = 8.4V and Q3 = 5V, to get a acceptable sound.
    The fact is if I bias Q1-Q3 to 5V it is a “fuzzy” sound and not like a “SVT”.

    I am not sure if it would help to increase the resistor R3, R6, R9 from 1K to a higher value?
    I saw something in the forum. https://guitarpcb.com/forums/topic/3-time-champ-hot-bias-and-pots-are-reversed/
    The “3-time-champ” pedal is more or less the same as the “Bass Overdriver”.

    But before I de-solder and solder the 3 resistors, I will wait that you or someone can tell me if that make sense, because I do not have a deep knowhow about that.

    Many thanks in advanced.
    Best regards
    Michael

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