mosfet vs jfet boosters

Discussion in 'Effects, Pedals, Strings & Things' started by woude, Jan 3, 2006.

  1. woude

    woude Member

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    What's the main difference between those two type of boosters soundwise?

    What are representative (early) examples of both types?
     
  2. wgs1230

    wgs1230 Fully Intonatable Silver Supporting Member

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    IME, you really have to turn up the booster 'til it's reached its headroom ceiling to hear any differences- the MOS tends to be a touch brighter or "more present" once it introduces clipping artifacts to the signal, though both types are brighter in that same sense than the common types of audio IC. In terms of clean signal boosting, the differences in the ways the common FETs handle signal are minor- it's really a question of overall circuit implementation. I can tell you that the MOS design is more sensitive to the harmful effects of static electricity on its field than the JFET and must be properly isolated to avoid component failure (been there, done that).
     
  3. brian marshall

    brian marshall Member

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    wgs is correct, but i will expand on it a bit.

    most mosfet boosts are simple inverting amplifiers based on a single mosfet biased either by a voltage divider (catalinbread, bad bob, AMZ/orman design) or negative feedback (sho) The former usually has a flatter frequency response through the range of its boost knob, and the latter has subtle changes due to how it is biased... good or bad... the jury is out i suppose.


    Most jfet boosters are more complex, and vary much more... the katana and liquid sunshine use a pair of jfets in series as amplifiers... the katana has a voltage doubler to stay clean due to the lower headroom of jfets in this configuration. Other jfet boosters use one jfet as a constant current source, and the other as an amplifier... the constant current source gives the single jfet amplifier more headroom... I know there are boosters based on this design, but not sure which ones.

    In all mosfet boosters tend to have more headroom, but to my ears they dont break up as nicely as jfet boosters. jfet boosters typically have a higher output impedance as well... but that is more due to design than an absolute rule.

    There are also other types of boosters, and different implementations of the garden variety.
     
  4. 58lespaulman

    58lespaulman Member

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  5. brian marshall

    brian marshall Member

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    i already know i made a mistake... the bad bob is based on jack ormans muamp, not the mosfet booster. The mu-amp is the one where the first jfet is used as a constant current source, and the other is used as the amplifier... too many different things floating arround in my head when i wrote that.
     
  6. woude

    woude Member

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    thanks for all the information! much appreciated!

    Is it possible to 'just' swap a mosfet(...) for a jfet(...) in a certain boost circuit like transistors in a fuzz an ic's in an overdive?
     
  7. brian marshall

    brian marshall Member

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    not really... most mosfet boosters use enhancement type mosfets, and they are biased differently. you might be able to use a depeltion type mosfet in place of a jfet, but ive never seen a pedal that used depletion types, and have no experience with them either... in some cases you can replace a bipolar transistors in place of a mosfet, but they typically require a stronger bias current.
     
  8. woude

    woude Member

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    again; thanks for the info. Within a few weeks I've got the new fulldrive here with the mosfet switch; I'm going to do some test with a keeley katana and a HWA fetclone (boss FA-1 clone) and see what happens. Very curious how those pedals compare!
     
  9. brian marshall

    brian marshall Member

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    a customer i have talked to a couple times on the phone has 3 katanas that he runs in series. I've never actually tried them, but he swears by them.
     
  10. nashvillesteve

    nashvillesteve Member

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    bob mcbroom told me that the Hellbaby is similar to the Orman booster design, but uses op-amps for a smoother top end to the sound. Sounds good to me, and so does the pedal, but I'm not so sure I've objectively mastered understanding the differences between these components or anything.
     
  11. brian marshall

    brian marshall Member

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    opamps are what the mxr micro amp used... its actually very different than the orman design, so im not sure how it could be similar except that it boosts.
     
  12. nashvillesteve

    nashvillesteve Member

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    I went back and re-read that email... I had mis-read it. The McBoost was a Jack Orman type, with the JFET, the Hellbaby is an opamp design he came up with after being sort of dissatisfied with the harsh high end of the JFETs. So, it is not the same design... my mistake.
     

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