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Discussion in 'Amps and Cabs [BG]' started by zeh, Oct 10, 2003.
Whats Mosfet ??Tubes??
From a search:
faux tube tone.
Naw, anyone who falls for the "tube tone" thing with mosfets has been scammed. They DO NOT make tube tone....they CAN NOT make tube tone.
A Jfet acts like a tube, much more closely, and even IT does not make "tube tone".
I did see a device at Namm once that was a solid state tube. It even sounded right, it was a fet-type device, acted just like, sounded just like a 12AX7.
Guy had them set up as 12AX7 replacements. NOT the Boogie thing,,,something different.
But I never saw the guy again. Good idea that didn't get financing I guess.
MosFets are just nice tough devices....and incidentally, regular power transistors are now obsolete.
They are much less available than they used to be, they are going out of production except for a few types, so designers use MosFets.
The "tube tone" is all in the minds of the marketing turkeys.
I think the tube tone idea is all about the fact that mosfet circuits dont clip as hard as other solid state circuits. In that sense it is more like a tube.
I will state that I prefer the tone of a mosfet preamp clipping than most tube amps clipping.
Metal Oxide Semiconductor Field Effect Transistor
The tube analogy comes in because a FET uses a small control voltage to throttle current through the device, just like a tube, whereas a standard Bipolar Junction Transistor uses a tiny control current to throttle current through the device.
Also, there is a portion of a FET's curves where it acts just like a triode tube, and that area is called the triode region of operation if I recall correctly from about 1994.
Unfortunately they really act more like a pentode, not a triode, in the linear region.
At least jfet has a "grid conduction" area which can do a better job of tube simulation.
That Namm guys was about perfect. Just like a triode, NOT a pentode. Sounded great.
Not a cheezy jfet simulation deal, something else. I dunno what happened to it, maybe Peavey bought him out and buried the technology.....
Yes FET curves look very similar to pentodes, which is why when FETs first came out people thought that would be the end of tube power amps. Of course we all know better than that.
A MOSFET is just a type of transistor. For some reason manufacturers like to use the fact that they use MOSFETs as a selling point. It's one of those catch phrases like "punch" or "growl". Don't fall for it. A well designed amp that uses BJTs can be just as good as an amp using MOSFETs and vice versa.
"I will state that I prefer the tone of a mosfet preamp clipping than most tube amps clipping."
I prescribe some Q tips, peroxide and an appointment with a good ear specialist. No accounting for taste, I suppose.
Well taste is one thing, BUT
Who in heck makes a MOSFET preamp?
Never heard of one....Mosfets are normally power devices and are not as a rule found in preamps...And the characteristics are kinda bad for preamp use, even for small ones.
If someone is using them in a preamp, please educate me...tell me who.
JFETs are indeed found in preamps, and would have (if decently made) a lot of the sound of pentodes...some might like that, even if you or I don't.
Gosh, Bob Lee is going to be so disappointed when he hears this!
Power MOSFETs have some advantages for output stages, especially in Class D amps. Some audiophool designers think they sound nifty in Class AB. But they're hardly bulletproof.
Standard bipolar power transistors are still cheaper and easier to design with. And some argue that bipolars actually sound better!
I have to disagree with you completely.
Power mosfets (are there non-power mosfets?) allow for much more simple designs, because they don't require drive stages other than a simple opamp to couteract their gate capacitance.
99.9% of the analog amplifiers are class AB. What those audiophool designers are so lyric about, is class A; they have their reasons. Class A designs have negligable intermodulation distortion, which apparently can be heard. I myself wouldn't bother with class A.
Mosfets are as bullet proof as bipolars. Back in the 70s, mosfets had unprotected gates, making them sensitive to static discharge. Nowadays, I estimate 99.9% of all transistor produced are fets, and 95% of all power transistors are mosfets.
Bipolars are not easier to design with, but engineers who are used to designing with them, well, are more used to them.
All in my humble opinion.
As tough as bipolar and more so.....there are less failure modes in a mosfet than a bipolar.
The mosfets are a lot different than when they started out...actually pretty cool.
They will take a lot more abuse with fewer mosfets needed than bipolars for the same job, another reason they are getting used in A and even AB amps.
Drive starts to be an issue with AB amps, check out some of the better makes, and the schematic has a heavier duty driver shown. It seems to have to do with crossover notch...more drive is required to reduce distortion, but not as much s bipolar takes.