Mosfet Power Amp

Discussion in 'Amps and Cabs [BG]' started by pshep68, Jan 13, 2015.

  1. pshep68


    Sep 21, 2006
    Is there any such thing as a mosfet power amp that can be purchased separately that can be used with a tube preamp? I know there are plenty of power amps available, but is there one available with mosfets?
  2. RoadRanger

    RoadRanger Supporting Member

    Feb 18, 2004
    NE CT
    While I'm a fan of MOSFET amps I don't see the need if you are using your preamp for overdrive? I have four of the Tube Works amps (2 combos, one head and one rackmount power amp) and an Ampeg Micro VR, all "suffer" from the typical issue with non-master amps - they only sound awesome at one <usually too loud> volume :( . I did have a chance to fully crank the Micro VR into an 8x10 last summer at an outdoor venue that holds ~1000 - it kept up with the stoopid loud PA they had that had its subs cutting in and out <d'oh> and sounded awesome :) . I also have three tube amps - the stoopid heavy SVT clone (Bugera BVV3000) stays in a rehearsal space with the master on "1" competing with up to three Marshall half stacks ;) .
  3. rigtec


    Jan 30, 2012
  4. Troph


    Apr 14, 2011
    Kirkland, WA
    Not sure why you care about the type of transistor in use inside the amplifier. Most complex amplifier circuits have reason to make use of multiple types of transistors, be they MOSFETs, BJTs, or even JFETs.

    If all you're looking for is a "non-tube" power amplifier so you can hook up a separate tube pre-amp, then there are lots of choices on the market today. PA power amps are the most popular type, such as those produced by Crown, QSC, Peavey, etc. Many (most?) of them will make use of MOSFETs for their power supplies at minimum. Heck, virtually all of the new class D power amplifiers are entirely based on high-speed MOSFETs.
    Last edited: Jan 13, 2015
  5. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Darn near every pro grade Class D amp made uses MOSFET output devices.
    BurningSkies and Bob Lee (QSC) like this.
  6. B-string

    B-string Supporting Member

    If you asked for Bipolar if would be a little harder.
    john m likes this.
  7. beans-on-toast


    Aug 7, 2008
    When power mosfets first came out people started using them like a triode equivalent of a tube. They sounded close to a tube amp and were advertised as such. They had a good reputation.

    Then, maybe around '80, IR came out with hexfets and more companies started using these devices. I started designing switched mode power supplies with them. Using them as switches. They were quite expensive and it didn't take much of a static discharge to damage them. Which I promptly did with the first sample that I got. The IR devices also made good power amps.
  8. Troph


    Apr 14, 2011
    Kirkland, WA
    Sure, but the point is, just because it happens to be using MOSFETs doesn't mean it's been designed to sound like a tube amp or to have any sort of "characteristic" sound at all. It's not a good reason to base a purchasing decision these days.
  9. beans-on-toast


    Aug 7, 2008
    Yes, you make a good point. How the mosfet is used in the design is important and there are many options available.

    I have a Marantz amp that I bought around 77 that just keeps working as good as it did when it was new. There was a period of about 25 years when it was never shut off. Only a couple of times for maintenance. I wish that all amps performed as well.
    Bob Lee (QSC) likes this.
  10. Are you looking to "ride the rails"? If so just get a RB series GK and use the power section.
  11. cfsporn


    Aug 20, 2011
    New York City
    Just about all [solid state] amplifiers use Metal-Oxide Field Effect Transistors in some way, shape or form.

    What I think you are asking about are power amplifiers that are class A/B, G or H instead of D? If so, there are plenty of those, but I am still not 100% sure what you're asking.
    john m likes this.
  12. Crater


    Oct 12, 2011
    Dallas, TX area
    I'm sure the OP wants to buy a class A/B amp with power MOSFETs for the final stages. (Like the Mesa M9 bass amp) He probably read somewhere that MOSFETs "sound /amplify like tubes" but that's only partially true. Yes, MOSFETs, like tubes are voltage controlled devices; standard bi-polar transistors are current controlled. What's different though is that a modern MOSFET power amp uses relatively low voltage and high current. The output to the speakers is right off the MOSFETs, or on older designs, coupled with giant electrolytic capacitors. A tube amp uses high voltages at lower current, hence the need for a speaker transformers.

    Any MOSFET power amp is going to be designed for lots of clean power, and may or may not distort in a pleasing way when it's power limit is exceeded. Class-D is a different animal because the MOSFETs in class-D are not used as linear amplifiers, but as on/off switches.
  13. B-string

    B-string Supporting Member

    Incomplete description, they are not just on/off. The time they are on and off varies with signal. Sounds like splitting hairs, but it really is not. PWM is not "digital" as in a logic state switch that has a controlled constant on/off time to form "words" or code within the clock cycle.
    Vacuum valves can also be digital switches as in the first mainframe computers.
  14. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    FWIW Ampeg mentions MOSFET power amps for some of their bass heads.
  15. ThisBass


    Aug 29, 2012
    Independently of the fashion/method how a SS class AB / class H output stage is designed.
    That's totally independent of any presence either bipolar or mosfet transistors at the output stage:

    Voltage Gain: =1
    Current Gain: >1

    Even a tube output stage has to gain current rather than voltage.
    Last edited: Jan 14, 2015
  16. Grissle


    May 17, 2009
    I use an old MosValve sometimes in a similar fashion and it works well.
  17. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    That's strange that the Micro VR would suffer from the issue that all non-master amps have since it's got separate gain and master volume.
  18. john m

    john m Supporting Member

    Jan 15, 2006
    Purchase the amp based on whether or not you like how it sounds----and its quality.

    Stay away from the cheap, poorly designed stuff. If its built well and sounds good, it's good-----trust your ears.
    ThisBass, beans-on-toast and fdeck like this.
  19. RoadRanger

    RoadRanger Supporting Member

    Feb 18, 2004
    NE CT
    Oops - I forgot it did because my basses can't overdrive the preamp anyways. If I remember correctly using a boost pedal just gets you carpy solid state dstortion. OTOH with the limiter off it does some pretty good "tubey" overdrive from them MOSFETs but alas at just the one volume AKA WFO on the master.

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