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MOSFET power supply??

 

Bronze Member
Username: Comet345

Orlando, FL USA

Post Number: 65
Registered: Sep-04
What does this mean when an amplifier has this?
 

Gold Member
Username: Jonathan_f

GA USA

Post Number: 2052
Registered: May-04
Nearly all amplifiers utilize this in some form or fashion, ALL switching amps use MOSFETS. MOSFETs stand for Metal Oxide Semiconductor Field Effect Transistor, basically, it's a J channel transistor, the difference is that the gate is external and isolated from the channel. Mosfets are ideal switching devices, and A/B, B, C, D, etc. amps switch to save power and heat because it uses seperate transistors for each wave. Basically, one transistor is used for the positive wave, and one is used for the negative. Mosfets are what turn these transistors on or off, switching between the two, and do so by the bias that they are wired. Other transistors are J channel (not used in car audio, at least not to my knowledge or not often) and bipolar transistors (rarely used, but better sonic performance than a mosfet b/c it's more linear, with the cost of being less reliable). Vacuum tube or "valve" amps are another type of amp, and are rarely used in car audio.
 

Silver Member
Username: Fishy

Tamarac, FL USA

Post Number: 353
Registered: Sep-04
If I remember correctly Mosfets just started to become popular when I really started getting into car audio like around '86 I guess. "Mosfet" was a huge buzzword back then. If you had a mosfet amp you were stylin. There were a lot of amps out there that sounded decent but weren't utilizing them.

What kinda FET's were being used in these guys?

-Fishy
 

Silver Member
Username: Fishy

Tamarac, FL USA

Post Number: 354
Registered: Sep-04
I know in a galaxy far far away they use BoboFET's

oops, I apologize for that.

-Fishy :P
 

Bronze Member
Username: Comet345

Orlando, FL USA

Post Number: 69
Registered: Sep-04
well, what are the advantages/disadvantages of an amplifier having a mosfet power supply, does this mean it doesnt require power from the alternator, etc .. ?
 

Gold Member
Username: Jonathan_f

GA USA

Post Number: 2076
Registered: May-04
you wish :-) You still need an alternator, it's not that easy. Nearly all car audio amps use a MOSFET power supply b/c the voltage is low (12-14.4V typically). This needs to be stepped up to create meaningful power. When MOSFETs are used in a power supply, it means a switching power supply. How it works is the mosfets switch at high rates (20,000-50,000 pulses per second) creating pulsating DC, this creates a magnetic field (similar to AC). Then, that is sent to a transformer, and the transformer increases the voltage. The secondary windings of the transformer are in AC, and they are then smoothed via a rectifier (diodes) to power the amp. Transistors used for the audio signal are different. Transistors are designed flow one way(like a diode), the difference from a diode is that they add another junction, they have either two positive or two negative(depending on transistor type) and one of the opposing junctions(two positive and one negative, or two negative and one positive). MOSFETs are also typically used as the transistors for the input signal as well. Power is applied to the transistors, and this power flows electrons through the transistor and is controlled by the input signal (RCA signal, or music). When the music has a positive wave, that wave will go through the transistor and electrons will be "pulled" from the power supply, resulting in the same signal, just at a higher power level. Think of it like the venturi effect. This is amplification. The reason that MOSFETS are so good for it is that they work well off of low signals, meaning they are very fast to respond to a signal because their internal (gate) voltage is low, meaning you need less power to overcome the internal voltage of the transistor, and thus, power will be amplified more quickly.
 

Gold Member
Username: Jonathan_f

GA USA

Post Number: 2077
Registered: May-04
And Fishy, earlier car audio amps didn't use FETs, they used capacitors and diodes.
 

New member
Username: Krazy_karl

Kansas City, USA

Post Number: 2
Registered: Oct-04
I'm probably not saying this correctly, but I was heavy into car-fi when they became the "buzz-word" and seem to remember that pretty much everything that "amplifies" uses a FET (Field Effect Transistor) and what made these special was the fact that they were a MOS (Metal Oxide Semiconductor) version of the standard FET. I thought that every amp since the dawn of time (not really) used FET's and the MOS portion was a technicalogical breakthrough in the mid 80s and originally only available on high end gear as Fishy said. I could be wrong though.
 

Gold Member
Username: Jonathan_f

GA USA

Post Number: 2078
Registered: May-04
"Mosfets are what turn these transistors on or off, switching between the two, and do so by the bias that they are wired"
When I said that above, I meant to say MOSFETS are what turn on or off and switch according to the bias they're wired for. MOSFETs don't turn other transistors on or off. Just clearing up if I confused anyone.
 

Gold Member
Username: Jonathan_f

GA USA

Post Number: 2079
Registered: May-04
They started appearing earlier than that, but were rare. Really, a MOSFET is worse than a bipolar transistor for audio output, but they're more rugged than bipolars, and in a car that's a good thing,. Their audio performance isn't very linear and they have to have a lot of feedback to get low distortion. They also can't be perfectly matched, making crossover distortion worse. But, they switch fast, meaning they work well w/ switching power supplies and A/B, B, C, D amps, etc. because they can control power with a low gate voltage. FETs were used a while back, but today MOSFETS are the primary transistor of choice in car amplifiers. A select few high end companies use bipolars b/c of the better sonic performance, but bipolars are very rare in car audio. About as rare as car tube amps.
 

Silver Member
Username: Fishy

Tamarac, FL USA

Post Number: 359
Registered: Sep-04
So if they are mainly used for switching amps whats in my Class A 50?

Its in my truck now so I can't look.

-Fishy
 

Gold Member
Username: Jonathan_f

GA USA

Post Number: 2092
Registered: May-04
They may be used as a switching power supply. Could be used as the transistors too, can't remember. A lot of A/B amps are "deemed" Class A from companies b/c of a high bias voltage, meaning the transistors have little to no crossover distortion b/c they're always on the verge of turning on, but literally, they're still Class A. Not sure if that Soundstream is one of companies that does that, though. Pretty much all car audio amps use either a MOSFET power supply, some high end amps use pulse width modulation.
 

New member
Username: Krazy_karl

Kansas City, USA

Post Number: 4
Registered: Oct-04
That's all I meant was to try to clear up the difference between a standard FET and a MOSFET. I knew that Jonathan could sort it all out for us beyond that.
 

Silver Member
Username: Fishy

Tamarac, FL USA

Post Number: 365
Registered: Sep-04
Nope, I believe that Soundstream was among the first(along with Alphasonik I think) to provide a "Pure" Class A design in their Class A 40 and 50 and later the Class A II 50 and 100.

Grrrr...... I miss my Class A 100II. One went for over 300 dollars the other day on ebay and it only listed at 700. The thing had to be at least 13 or 14 years old as well.

-Fishy
 

Gold Member
Username: Jonathan_f

GA USA

Post Number: 2100
Registered: May-04
Wasn't sure, wasn't trying to make them look bad :-). Pure Class As are rare.
 

Silver Member
Username: Fryguy

Nova Scotia Canada

Post Number: 243
Registered: Jun-04
a MOSFET has a oxide layer that acts as in insulator to the gate region and makes a MOSFET have a near infinite input impedance unlike the standard FET. Thats the main difference between them.
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