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What is MOSFET?


Jono Bolton
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I'm looking at an Ampeg B2RE, and it says on the GAK site that power amp is MOSFET? Is this good or bad? I really have no idea about amps really beyond solid state and Valve, is there something along the lines of Bass Amps for Dummies on here? Something that covers stuff like this, possibly ohmage, bi-amping, separate pre and power amps, that kind of thing? Is the B2RE a decent amp?

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It's more or less an industry standard for solid state power amps. You can have solid state pre-amp with a solid state power amp, or a valve pre-amp and valve power amp or any combination of solid state / valve hybrid.

All have their pros and cons but the main difference has got to be in the quality of components, circuit design and assembly skill.

The music you play and the sound you want to achieve will be a deciding factor in your amp choice.

IMHO with a B2RE you are paying for a chinese built amp with a sexy name. You can probably find a chinese built amp with a less sexy name for half the price which will sound just as good.

I'll now run away before all the Ampeg fans arrive :)

Hamster

Edited by Hamster
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That's what I was hoping wasn't the case. I reckon I might try to get up to Glasgow soon to try out some heads, but I'm pretty set on an Ashdown. The main reason I started this thread was because I didn't know what MOSFET was. I've seen the term bandied about a bit, but I don't really see the point in telling folk it's MOSFET if it's just standard solid-state transistors.

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I'm sure I read somewhere that MOSFET, although solid state, has behavioural similarities to valves. I'm sure someone who knows more about electronics than me will correct me if I'm wrong.

It could explain why MOSFET is highlighted/promoted in marketing literature otherwise, as you point out, who'd really care? Maybe it's to appeal to people who don't want the cost of an all valve amp but want an approximation of the sound. Or maybe it's just marketing fluff.

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The two main solid state power amp types are Bi-Polar and MOSFET. There's no "good2 or "bad" other than component selection and design, they're both good at their own thing. Higher power amps are almost always MOSFET I believe, earlier Trace stuff was BiPolar but I think they changed to MOSFET in the 90s as power levels increased. I think you'll find most solid state power amps are MOSFET thesedays.

As for MOSFET sounds or behaving like valves, well.... In preamps that is kinda the case, but I think you'll find these are BiFET rather than MOSFET. Different sort of Field Effect Transistor, y'see :)

Bored?

Good :huh:

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I have a vague Knowledge about this from reading Hi Fi magazines, so apologies in advance if an electronics expert here can correct me:

Mosfets can be made to sound more like valves if they are used in class A opperation. This means that instead of being used to 'push' and 'pull' the signal, they are biased. This means that with no signal, they are resting at half their voltage, and create a wave by increaseing or decreaseing the voltage, as opposed to normal amps where the voltage is changed from positive to negative to achieve alternating waves.
The Idea is that a smoother more valve like tone can be made with transistors, The downsides are that they produce LOTS of heat. With no signal the transistors are running at half power, and that the power that the transistor can output is halved. They are popular for Top End HiFi kit and are used in the classic Musical Fidelity A1 amp. This is low powered, sounds lovely, and you could fry eggs on top of it if you wanted to! I believe it would be impractical to use class A operation for High powered Amps, they would be too big, heavy, hot, and would demand more power than is safe on a 13amp plug. However, as stated above, they can be used in preamps to simulate valves without the transformers and glasswork hassles.

To be honest, saying that and amp has MOFETS does not really tell you much, - Its a bit like saying it has a plug.

This is probably more info than you need but I cant help myself from rambling sometimes.

Edited by Moo
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[quote name='Moo' post='118186' date='Jan 10 2008, 09:44 PM']They are popular for Top End HiFi kit and are used in the classic Musical Fidelity A1 amp. This is low powered, sounds lovely, and you could fry eggs on top of it if you wanted to![/quote]
I have that amp, it is gorgeous.

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[quote name='jono b' post='118066' date='Jan 10 2008, 06:51 PM']That's what I was hoping wasn't the case. I reckon I might try to get up to Glasgow soon to try out some heads, but I'm pretty set on an Ashdown. The main reason I started this thread was because I didn't know what MOSFET was. I've seen the term bandied about a bit, but I don't really see the point in telling folk it's MOSFET if it's just standard solid-state transistors.[/quote]

If you tell people what:- your bass & cabs are; the music you play; the sound you want and your budget then you'll get some pretty sound advice and options you may not of thought of.

Hamster

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[quote name='Hamster' post='118253' date='Jan 10 2008, 11:37 PM']If you tell people what:- your bass & cabs are; the music you play; the sound you want and your budget then you'll get some pretty sound advice and options you may not of thought of.

Hamster[/quote]

Will do sir :)

Basses are Mexican Jazz and Precision, both with SD 1/4 Pound pickups for a big fat bassy sound. No cabs just now, I had been playing a Peavey TNT 115 combo, but thats gone to fund my new amp. Music is heavy rock, a bit prog, like Foo Fighters crossed with Pink Floyd. I enjoy the bass sound of Reuben's Jon Pierce and Brand New's Garrett Tierney, listening to them is what's currently inspiring me to play bass these days.

As for the sound I want, I've recently been given the opportunity to play both a MAG 300 and ABM 300, and loved the sound of them both, it suits my Jazz sound and sounds great when I've got my Boss ODB-3 running too. IMO, of course. Budget is about £400 for the head just now, but possibly higher by the end of the month.

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The original transistor designs, with bipolar devices, let more current through as they heated up, so in a high powered amp you could get yourself into a destructive cycle where it runs hot, so it lets more current flow, so it runs hotter... then it burns out.

MOSFETS let through less current as they get hotter, so they do not go into thermal run-away, and they have become the industry standard for high powered amps.

Interestingly (for geeks) bipolar transistors have a smaller voltage drop accross them than mosfets do, leaving more voltage available to push current through the speaker, so for a supply of +&- x Volts, a mosfet amplifier will be less powerful than a bipolar one.

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If memory serves, HH were the first major amp builder to advertise that they were using MOSFET technology in their amps. I'd guess this was around 1979/80. I still have their brochure from then (sad I know). In their day they were pretty good amps although somewhat underpowered.

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The first SS amps that I was aware of in the late 60's and early 70's were Burns, Selmer, HH, Vox and Carlsboro and in my opinion they were uniformly awful. By the mid to late 70's they seemed to be getting it right, for example with the Musicman HD series of bass amps.

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MOSFETS= warmer sound....in most applications....they restrict current as they heat up...but used within their limitations sound "warmer" than bi polars.....

[url="http://www.livesoundint.com/archives/2001/julyaug/mosfet/mosfet.php"]http://www.livesoundint.com/archives/2001/...sfet/mosfet.php[/url]

Edited by jammie17
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