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MOSFET vs. Class D

Discussion in 'Amps and Cabs [BG]' started by terbay, May 5, 2005.


  1. terbay

    terbay

    Apr 13, 2004
    Colorado Springs
    Can someone point out the differences and/or benefits of a Class D amplifier vs. MOSFET amplifier.
     
  2. I'll take a stab. MOSFETs are a type of transistor - the device that generates the power to the speaker. The other types of devices that do the same thing are tubes and bipolar transistors. Each have their pros and cons. they are usually run in a class A or class AB types of amplifiers. The class is they way the transistors/tubes used to generate the power.

    Class D is a different animal altogether. A and AB are analog amps - sine wave in = sine wave out only bigger. A class D amp, also known as a digital amp, turns the sine wave in put to a series of pulses whose width correspond to the amplitude of the pulse then the pulses are amplified and run through a low pass filter to get rid of all the nasty edges and turn it into a sine wave only bigger. The benefit is much higher efficiency so the power supply can be smaller and the heat generated is less, which theoretically will translate in smaller and/or cheaper amps, such as the EA and Clarus models (smaller but *not* cheaper). Class A and AB amps are easier to design, and to most people sound better (always subjective), and usually don't have noise and RF issues like class D amps can.

    That's the basics, hope it was somewhat understandable.

    On a side note, digital power supplies and all-ditigal amps are different too. The digital power supply is again smaller and more efficient. A digital amp, say Line6 pod, is an amplifier that turns the input from the instrument into a digital signal and does all of the effects and tone shaping digitally (in a computer basically) and then turns it back into an analog signal at the output of the amp.
     
  3. SirPoonga

    SirPoonga

    Jan 18, 2005
    The people at diyaudio would know.
     
  4. 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
    Good answer, mostly. Class D is not necessarily digital, though, and in fact it usually isn't.

    What is a digital power supply?
     
  5. Class D - digital - PCM I guess is the usual term I associate it with. Since it's all ones and zeroes it is therefor digital. To me anyway.

    Digital power supply - oops I mean switching. Same as above in a way.
     
  6. 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
    PCM is a coded digital stream, like data from a CD. You're thinking of PWM--pulse width modulation. The vast majority of pulse width modulators are analog and not digital in any way. Most class D amps operate not as switching between 1s and 0s but between +V and -V.
     
  7. Thanks for pointing that out.:oops:

    As you can well see, I am a digital engineer and have forgotten most of what I learned in engineering school about analog. "it usta be i coont even spel enjanear, now i are one!" :rolleyes:
     
  8. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Typically Class-D will be smaller, lighter, and more efficient than an equivalent standard "analog" power amplifier, whether the analog amp uses mosfets or bipolar transistors, or tubes.

    Good Class-D is very good fidelity-wise. The best class-D is as good or better than the top audiophile Class-A, class-AB, Class-H etc amplifiers.

    A Mosfet used in the linear mode as a traditional "class AB" power amp is, well, an analog amplifier.
    In general, they are larger, hotter, heavier, and less efficient. At worst, well over half the input power simply becomes heat that must be dissipated, so the efficiency can be well under 50%. So they require a larger transformer (or other power supply), large heatsinks, usually fans, etc.

    Most, if not all, class-D amplifiers also use MosFets*. I don't know of any current one that does not.
    A class-D amplifier produces output pulses instead of a continuous waveform, and uses an output "reconstruction filter" to re-create the analog waveform from the pulses. Since the pulses occur at rates in the range of several hundred kHz to over 1 mHz, to your ear it is continuous.
    For various technical reasons, it is more efficient to do that, it wastes less power, creates less heat, etc. Efficiency can easily be well over 90%.
    Transformers or other power supplies can be smaller, and heatsinks smaller, fans smaller if used at all, etc. Less heavy stuff to carry.

    While class-G and class-H multi-level power supply type amplifiers can be more efficient than class-AB, the efficiency is still not that of class-D, and the units are considerably more complex.

    Techie quibble: for purposes of argument, the PWM output stage can take only one of two states.....either +V or -V. Therefore it is "binary", and can be expressed as being either a 1 or a 0......
    To that extent, PWM is "digital". You can make a workable, although rather low power, PWM amplifier output with a TTL "gate".

    The catch is that instead of the voltage being analog, the pulse time duration is. So there is also a sense in which it is analog.

    However, there are a number of patented, commercially available ways to go directly from PCM to PWM without any analog stage, so no part of the signal chain has to be analog from the input thru to the actual speaker output.


    * IGBTs are not fast enough to be used in a credible Class-D amplifier yet.
     
  9. 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
    Hi Jerrold,

    IMHO, "digital" implies more than just on/off or 0/1, but also some sort of numerical process of discrete steps, whether it's using a clocked processor or Boolean logic or something along those lines.

    Otherwise, a simple light switch is also digital. In a very minor way it may, be, but it's far more useful to think of it as simply an on/off switch than as a digital device.

    Yes, you can process digital audio data directly into a class D stream. That's what DSD audio does; in theory you can just put the stream through a passive low-pass filter and have audio. Most class D modulators, though, are based on one or more analog comparators and an analog reference signal, together with the analog input signal.

    Now here's my idea of making class D digital: take two audio signals, run them through class D modulators, and XOR the outputs to drive the amp output devices. ;)
     
  10. 44me

    44me

    Jun 17, 2002
    Bedford, NH USA
    I'm with Bob on this. I think the marketing guys, have applied the term 'digital' to any switching amplifier, but digital means something more specific to the engineering community. 'Digital' when used with 'amplifier' or 'power supply' implies the control method, not the output stage. Most switching amplifiers and power supplies are using analog control on the front end. Pure digital amplifers and power supplies are much more on the leading edge. Digital signal processors are just now becoming powerful enough and cheap enough for this to be a cost-effective approach.
     
  11. Digital = 0/1 , on/off, +,-

    Analogue has to be a constant waveform


    Not sure if this is nessisary, but hell, might as well check my physics is still right 3 years down the line :)
     
  12. 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
    It's a bit more than that. Could you, say, sell a 68 VW Beetle and honestly say it has digital headlights? They're on or off.

    If you have a pickup switch on your bass, does switching it on and off while you play make the signal no longer analog, because it is no longer a constant waveform?
     
  13. MJ5150

    MJ5150 Terrific Twister

    Apr 12, 2001
    Lacey, WA
    All I know about class D is that EA uses it in their iAMP500. The 500 is an awesome amp, and I am going to be the owner of one in a few days. If you check out the EA website, they do some explaining of class D amps.

    -Mike
     
  14. Tash

    Tash

    Feb 13, 2005
    Bel Air Maryland
    The only thing I remember about class D amps is that I had to build one for an electronics class back in college, and I ended up setting the lab on fire somehow....
     
  15. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    getting warmer
    Was the fire digital though?
     
  16. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    ROTFL!!!
     
  17. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Not particularly bass-related, but fun anyhow....if we dont take it too long...

    What you are really saying is that you expect the 1s and zeros to combine into values according to a base 2 system to be "digital". OK, that is one possible definition.

    What the PWM does is to compare the current signal at the output node to the input, and "decide" whether during the next time period the unit should present a "1" or a "0" to that output node in order to follow the input.

    Since the decision can only result in one of two states, the decision is "digital" using a binary system.

    I am willing to let the system be called "digital" for purposes of description....but I may be biased as we make a number of Class-D products at various power levels.

    After all, you can't really call it analog either.....
     
  18. 44me

    44me

    Jun 17, 2002
    Bedford, NH USA
    Digital = 0/1 , on/off, +,-

    That's a valid argument – certainly one made by marketing people trying to get some bang out of the word ‘digital’ – but it’s kind of a broad term. It much like saying linear or analog amplifier. Class A, AB, B & C are all linear amplifier topologies that are commonly implemented with MOSFET’s, bipolar transistors, and tubes, but each had quite different characteristics. The same is true with switching amplifiers (the preferred general term). Class D, pulse width modulation (PWM), is the most common, but there are many others. Most are not given class names like linear amplifiers. Since almost all early switching amplifiers used analog circuits to generate the switching waveforms, analog control is assumed unless it is specifically stated to be a ‘digital switching amplifier’.

    As far as what this all means to sound characteristics, there are good and bad examples of most every topology. Each has it’s tendencies, but there can be quite a bit of variation from one design to another. As always, let your ears be the judge.
     
  19. Okay, it sounds to me like we're really battling semantics here - what a word means to me is different than what it means to you. All good information though. terbay, I hope you haven't been blown away by all this engineering mumbo jumbo. So I like what 44me says, there's good and bad in every topology. Nuthin else like 500 volts across 8 6550's and a transformer that weighs 65 lbs. though. :)
     
  20. Funkengrooven

    Funkengrooven Turn it down? You gotta be nuts!!

    For my ears I have not heard a Class D that I like.
    Not that there ain't one, I have just not heard it.
    I don't much like switching supplies either, not for music anyway.
    Maybe I'm a throwback.....(Thor is probably gonna jump me here, I can just feel it)
    But a transformer coupled linear power supply is what sounds good to me and mosfets vs transistors is a non issue
    Mosfet means
    Metal Oxide Semiconductor Field Effect Transistor
    Still a transistor but acting more like a tube if I remember correctly.