Guitar Boost pedal - transistor help

Discussion in 'The Projects Forum' started by Justincharlesmoses, Mar 19, 2014.

  1. Justincharlesmoses

    Thread Starter New Member

    Mar 15, 2014
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    Hello everyone!

    I was recommended to this forum from a friend because he thought you guys might be able to help me.

    I am trying my hand in building my own DIY guitar stomp boxes and while some of these answers may have to come from a niche crowd, I feel that FET boosters are something that most of you may be familiar with.

    I have the circuit built and it amplifies (boy, does it amplify!) but what is happening is when the pot is anywhere but 100% clockwise, there is no change in volume. When it moves that tiny amount from 99% to 100% clockwise, it turns on the transistor full blast and not only SUPER amplifies the guitar's signal but also, distorts the signal. Now, I don't mind if it distorts at full volume but I would like to have a gain knob that actually…works the way it should.

    The diagram I am attempting to follow is
    unnamed.gif

    Now, I have all the parts listed to the T except for the 62k resistor where I used a 56k and two 6k resistors. At this point, I'd like to mention that I am quite new to actually building circuits; however, I have a pretty decent grasp of the basics. That being said, I am sure to make a few newbie mistakes.

    I will attach two more images in an attempt to help with someone's diagnoses.
    The first is my interpretation of the diagram as I follow the signal path through the breadboard:
    image.jpeg

    I have color-coded the transistor pins to show how I interpreted the info sheet. I know this is normally one of the biggest issues when working with transistors so, I figured I'd describe it as best I could.

    The second image may be of no use but it's the actual breadboard I have currently. Nearly impossible to see connections but I figure it may help?
    image-2.jpg

    Well, that's all from me for now. I appreciate any help or insight you can give me. I am even unopposed to the idea of tearing it all down. Again, I'll say I am very new to this but get giddy when I have new ideas for pedals and audio augmentation! Thank you for your time!

    - Justin
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    You have a lot more components in that design than you really need.

    For starters, you don't need the two extra 3kΩ resistors. The one 56kΩ resistor should work fine. Electronic circuits are not that picky.

    I would have eliminated the R1, R2, C3 components and find a simpler way of biasing the transistor properly.

    R6 is a poor choice for gain control. This also affects the frequency response.

    If you want to reduce the gain a bit, remove C5 and R6.

    Put R6 on the output of C4 wired as a voltage divider. i.e. one arm goes to output of C4, the other arm goes to ground and take the output from the wiper of R6.

    You can play around with the gain of the amplifier stage in order to get the best gain/distortion performance.
     
  3. bertus

    Administrator

    Apr 5, 2008
    16,446
    2,702
    Hello,

    Did you read the text that is used near the schematic on the site:
    http://www.muzique.com/schem/mosfet.htm
    R2 may need adjustment.

    Also you have VERY long threads, wich may cause oscillations of the circuit.

    Bertus
     
  4. Justincharlesmoses

    Thread Starter New Member

    Mar 15, 2014
    16
    0
    Thank you for the quick responses! Took my "tearing it all down" to heart I see :)

    I'll make those changes and let you know what happens. Thanks again
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I second MrChips suggestions. You can bias the FET by connecting the top end of the 10meg resistor to the FET drain instead of R1,R2,C3.
     
  6. Justincharlesmoses

    Thread Starter New Member

    Mar 15, 2014
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    schem2.jpg
    So, here is the updated schematic. I feel like I did something wrong but I'm not sure what. I am getting no output when the circuit is active. Thoughts? was I supposed
     
  7. #12

    Expert

    Nov 30, 2010
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    That capacitor on the output to ground (looks like 100 farads) will stop any output.
    Just remove it.
     
  8. Justincharlesmoses

    Thread Starter New Member

    Mar 15, 2014
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    Hey, I tried just popping that capacitor out. Still no output
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    If you can hang on until tomorrow I can breadboard your circuit and try it out.

    Are you using that exact transistor, BS170?

    What other transistors do you have handy to try out?
     
  10. Justincharlesmoses

    Thread Starter New Member

    Mar 15, 2014
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    That would be amazing of you. I have plenty of time so, there is no rush. I specifically ordered the transistors from Mouser based on this diagram. I'm at work now so, I don't know which other transistors I have banging around but it's not many.

    If it helps, I can switch them but I'll have to make a trip for parts. I'm just looking for a clean boost in volume with no distortion (for now). I'm kind of gunning for as much volume as I can get since I can always turn the gain down. It will just make the pedal more versatile in a show setting.

    Thank you again for all your help!
     
  11. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    The 0.1 uF output coupling capacitor to the pot should be the 100 uF cap. For this out capacitor, large size means flatter frequency response. With only 0.1 uF, you basically had an amp with the bass turned all the way down and the treble all the way up.

    But the main thing is that the gate now is tied directly to the + rail (+9V), so the transistor is turned on full all the time. You have the top end of the 10 M resistor tied to the battery, not the FET drain as Alec suggested.

    ak
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Try this. It uses mostly the components you already have.
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    Alec has presented the same circuit I was going to breadboard. It's worth a try.

    The gain pot can be anything from 5k to 100k. I would use a log taper pot.
     
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Here's a mod with the gain reduced somewhat (R3 increased) but improved bandwidth due to R4,C3.
     
    djsfantasi likes this.
  15. #12

    Expert

    Nov 30, 2010
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    Now you're talking!
    I've done this very well with jfets, but mosfets still have me in the amateur zone.
    It's good to see a design get cleaned up and functional. Good job!
     
  16. Justincharlesmoses

    Thread Starter New Member

    Mar 15, 2014
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    Nice! I'll have to try this when I get home from work today. Thanks guys! I'm really wishing I had gone to school for this rather than music recording. It is still very much gibberish to me for now. I'm hoping that over time and experimenting, I'll figure some of this out, heh. I appreciate everyone's help thus far!
     
  17. Rolf Zetterberg

    Member

    Sep 20, 2008
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    A BS170 needs a gate-source voltage of around 2 Volts to let any current flow,but this value needs adjustment due to spread in the transistors.This is important!
    When I designed circuits with BS170 many moons ago,I always made the gate voltage adjustable,and confirmed it with a simple check of drain and source voltages.
    The last years I've seen so many circuits on the net that are horribly wrong and so few builders seem to measure voltages.Why?

    Read what this guy writes-he clarly knows what he's talking about.
    http://www.geofex.com/Article_Folders/mosboost/mosboost.htm
     
  18. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The adjustment is automatic if the gate bias resistor is fed from the FET drain (as in the circuits I posted), so you can throw almost any N-FET into the circuit and it will self-bias.
     
  19. #12

    Expert

    Nov 30, 2010
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    That link in post #17 goes to a partial tutorial. The author didn't get to the part about self biasing yet.
     
  20. Justincharlesmoses

    Thread Starter New Member

    Mar 15, 2014
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    Just to help me learn, why does it self-bias? That sounds like an incredibly helpful and efficient setup.
     
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