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solid-state power (mosfet vs. switch mode vs. other)

Discussion in 'Amps and Cabs [BG]' started by Golem II, Jul 31, 2002.


  1. Golem II

    Golem II

    Jan 4, 2002
    Macon, GA, USA
    First of all, I was wondering if anyone knew what the transistor setup in my solid-state poweramp is called, and whether you would say it is superior or inferior to mosfet power for bass guitar amplification. The amp is a Yamaha p2201 (at the time of this amp's production there was also a model called the p2200, which was identical except that it had VU meters for each channel.) I think it was designed mainly for PA use but may have been commonly used for studio reference as well. It puts out 200w per side at 8 ohms. It's huge (4 rack spc., about 45 lbs,) has no fan and is cooled by heat sinks on either side. Is anyone familiar with this amp? The previous owner told me that it was considered superior to mosfet power in its day (I presume its production was discontinued in the early 90's.) Is it just considered "ordinary solid state", or does it have some other name?

    I want to know if I'd lose anything by switching to a smaller, more convenient switch-mode style amp like a Stewart World series or QSC PLX, or whether a mosfet-powered bass head would improve or hurt my sound. I've read posts here saying that some people prefer mosfet-powered heads to PLX-style poweramps, claiming they offer a greater punch or presence in the low end.

    Unfortunately, I don't have any other poweramps I can use for A/B comparison, and trying them out in a store won't do me much good because I need to know how my rig will function onstage, pushed to the limit with a loud rock group.

    I think that my amp doesn't provide enough headroom to justify its size and weight. If I were to change, do you think I should go with a mosfet-powered head, a switch-mode poweramp, or should I save up and buy an all-tube monoblock-style poweramp? (I'm willing to put up with tube weight for tube sound, but will it be loud enough?)
     
  2. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Switch-mode refers to the power supply—as opposed to a conventional supply with a big 50 or 60 Hz transformer.

    MOSFET refers to the type of transistor in the output circuit—as opposed to bipolar transistors or tubes.

    Neither one is locked with the other, so a question of "switch-mode or MOSFET?" is like asking "do you walk to work or do you bring your lunch?" ;)

    QSC PLX amps have switch-mode supplies and bipolar outputs. Stewart amps also have switch-mode supplies, but I don't remember whether they have MOSFET or bipolar output transistors. Hafler amps have conventional supplies and MOSFET output stages. Most Crown amps have conventional supplies and bipolar output devices.

    Advantages of one type over another depend much more on the designs and implementations than on which type you choose. There have been excellent switch-mode supplies in amps, and there have been really horrendous ones. Same with conventional supplies; however, it takes less engineering knowledge to design a good conventional supply than a good switch-mode supply. Likewise, some MOSFET output sections are great, and others are crap; some bipolar amps are superb, while others are dismal. Because of the power-handling capabilities of the individual devices, very high-powered amps are more likely to use bipolar transistors than lateral MOSFETs. Ironically, ultra-high powered amps like some Cyberlogic models or the PowerLight 6.0 and 9.0 use vertical MOSFETs in the output sections because they can handle even higher power than the largest-die bipolar transistors can, but vertical MOSFETs require significant auxiliary circuitry to make them operate linearly.

    A good power amp doesn't have any more or less "punch" than is in the signal you put into it.
     
  3. Golem II

    Golem II

    Jan 4, 2002
    Macon, GA, USA
    Ah, thanks for clearing that up. So I'm assuming most solid-state bass heads have standard power supplies and mosfet output sections. Is this correct? I think the Euphonic Iamp 800 and the AI Clarus have switch-mode power supplies, but I don't know about their output (do any bass heads use bipolar transistors?)

    Are MOSFETs inherently lighter than bipolar transistors? If not, why do they seem to be used in more bass amps (ie. what advantages do they offer to amp makers and players?)
     
  4. fast slapper

    fast slapper

    Dec 11, 2001
    Fresno, CA
    I'm not sure why MOSFETs are used so much, but I do know that bipolars are used in bass amps like SWR, Eden, Hughes and Kettner, and others that I can't remember.



    [Edit] One reason MOSFETs might be used is because they feature certain characteristics that are found in tubes, while being cheaper, and more reliable.
     
  5. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Most have conventional power supplies, and I don't know if most have MOSFET outputs.

    I'm sure that a number of them do, particularly the higher-powered ones.

    Not that I know of. It depends on the transistors you're comparing. They range from probably a few tenths of an ounce up to an ounce or two apiece for either type.

    They're really just another way to make an amp. "MOSFET" might have sort of a marketing cachet to it.
     
  6. Golem II

    Golem II

    Jan 4, 2002
    Macon, GA, USA
    Maybe it's a "marketing" thing. Advertisements for bass amps sometimes make a big hullaballoo about MOSFET power (as if their amp is somehow superior to others because of the mosfets,) but if it uses a bipolar output section they don't even mention it (it's just "solid state".) I guess MOSFET has a better ring to it than Bipolar (nobody wants to buy an amp that's prone to mood swings ;) ) . As for the notion that they're cheaper, more reliable, or that they share some characteristics with tubes, can anyone else confirm this?
     
  7. fast slapper

    fast slapper

    Dec 11, 2001
    Fresno, CA
    Who knows if it's true, but I read it from the EBS site. www.ebs.bass.se
     
  8. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    JFETs and depletion-mode MOSFETs work in a way similar to how tubes work. But in power amp outputs, MOSFET circuits and tube circuits bear little resemblance to each other, so you don't get tubelike performance from MOSFETs, bipolar transistors, et al, or anything else but tubes.

    Probably the best place to get tube "sound" is in the preamp, especially in the stage where the really high gain is; that way you can get the "warmth" in the sound and not in the room. ;)
     
  9. jokerjkny

    jokerjkny

    Jan 19, 2002
    NY / NJ / PA
    great thread! we should archive this somehow...
     
  10. Ah, Transistors. I'm just finishing up a heavy engineering course on them.... Actually, all my courses are heavy this term ;). Ten days and I'm free.
    I've heard tell of people saying that MOSFET power amps have some tubish warmth to them, but I'm not sure if that's actually the case. I know the old HH V-800 power amps have a very warm colouration and they are MOSFET amps. I don't think generalizations could be made based on that though. MOSFETs work better in switching applications like logic (ie our computers, CMOS, etc) because they switch faster than BJTs, I'm not going to go into why:D I suspect that any class D power amps would have MOSFET output sections because of that. Actually, most digital stuff will probably move to the ultra-fast MESFET technology soon.....