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any difference in solid state and moss fet ?

Discussion in 'Amps and Cabs [BG]' started by jacobm, Oct 30, 2002.

  1. jacobm


    May 31, 2002
    the way it was explained to me is that mosfet is just a type of "premium" solid state being much more reliable. i was wondering if theres any truth in this but what i really want to know if there is any tone differences. ive started to wonder about this becouse it seems amp companys go out of there way to tell you solid state, moss fet, or tube. so i would think there would be more of a difference than that "premium" factor, any help would really be appreciated.
  2. A MOSFET is a type of transistor. The other common transistor type in amplifiers is the BJT. So a MOSFET amp is a solid state amp. Their internal modes of operation are different (and pretty complex so I'm not going to go into it here ;)). The main difference in physical operation is that MOSFETs can switch on and off faster than BJTs, but this isn't important in analog audio amplifiers. MOSFET amps are not inherently more reliable or better than BJT amps.

    People say that MOSFET amps sound a bit warmer and more "tube like" than BJT amps, which is certainly possible.

    I certainly wouldn't base a purchase on what kind of transistors an amp has in it, go with whatever amp sounds best to you .
  3. pbassfreak


    May 2, 2001
    long beach
    Having played both types a gk800 and svt350h the mosfet is a lot warmer sounding..very tube like..as far as which is more dependable id have to say ampeg ..my gk was nothin but a headache..
  4. Nick man

    Nick man

    Apr 7, 2002
    Tampa Bay
    In my experience, they are a lot warmer.

    Alot of it has to do with the way that the amps clip when overdriven.

    Ive found that Mosfet amps clip more like a tube amp in that they have a smoother distortion with some compression while non mosfet solidstate amps have a harsher distortion that seems to cause much more stress on the speakers.

    I feel much more comfortable with an amp that will distort in such a way that it would not cause extreem excursions for my speakers than one that sends high powered square waves.

  5. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    MOSFETs don't clip like tubes. Unlike solid state output sections, tubes are loaded with high impedances, and their power supplies also have relatively high impedances. Tubes start distorting a ways below the point where the waveform actually flattens out.

    MOSFETs aren't inherently "warmer" sounding than BJTs, although it's not impossible to design in that kind of distortion/coloration into any kind of amp. However, most power amp designers try to avoid arbitrary alterations to the sonic characteristics of the signals passing through.

    It's not wise to run your amp into clipping, whether it's tube, BJT, or MOSFET.
  6. Rockbobmel

    Rockbobmel Supporting Member

    Amazing!!! That's why Marshall, Mesa, Ampeg, Etc......tell you NEVER drive your amp into clipping!, Especially with the distortion channel.....I know, I won't have any part of it.
  7. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
  8. Agreed, Bob.

    In my experience, MOSFETs and BJTs clip in exactly the same way. I haven't specifically checked this with audio frequencies (yet), but that shouldn't affect the clipping process. I thought it might be unlikely that MOSFETS were actually inherently warmer sounding.....

    It should be noted that in Class D (so called 'digital') amplifiers, MOSFETs are the preferred devices because of their high speed switching ability.

    I think a big part of the reason tubes sound different than transistors when overdrive has to do with negative feedback loops. Tube amps need very little negative feedback to remain stable, while SS amps need quite a lot. When devices are overdriven they produce a lot of harmonics. The feedback circuits in SS amps allow a lot of high frequency harmonics (up to and beyond the audible limit) to be produced giving the classic SS distortion sound, ugly and harsh. The lack of negative feedback in tube amps has the effect of rolling off the harmonics as frequency increases. This results in a "warmer" sounding distortion.
    Also, the duty cycles of the respective amps differ under clipping. SS amps tend to stay at 50% duty cycle, while tube amps can shift to 55-65% duty cycle, depending on the drive level. This allows more even order harmonics to be produced.
  9. Rockbobmel

    Rockbobmel Supporting Member

    You know, I fixed my mosfet with my trusty Zircon Encrusted Tweezers last nigt. Now it sounds warmer. And NO clipping!
  10. When you break out the Zircon Encrusted Tweezers all bets are off!
    Did you buy 'em with a $40 bill, and did you sterilize 'em?
  11. Rockbobmel

    Rockbobmel Supporting Member

    Let me sterilize them.... gimme your lighter. Then she looked over, and with a glazed eye, and some bovine...well you know what happened. The $40. bill didn't matter no more.
  12. rok51

    rok51 Supporting Member

    Sep 2, 2002
    Crawfordville, FL
    I like MOSFETs. Bipolars have been very reliable for me, as well, but are harder on the LiCO3.

  13. You known I thought this was a fascinating topic. I own a Aguilar DB924 (the outboard pre-amp) and I'm just about to buy a combo for my upright with a class D amp and MOSFET pre-amp. So, this is all very applicable for me.

    But, the "Diana Mo Humm" reference was more then I ever expected.

  14. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    I thought most modern solid-state bass amps use metal oxide semiconductor field effect transistors in the output stages. Their output follows voltage applied at the gate, while the output of a bipolar junction transistor is regulated by the current at the base (the analog of the MOSFET's gate). So there you have it.
  15. VicDamone


    Jun 25, 2000
    He he...gotcha. Nice one Bobby.
  16. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    ... or was it input stage?
  17. Rockbobmel

    Rockbobmel Supporting Member

    I learned at a very early age...Nevva clip yo moss feta..

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