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Discussion Starter · #1 ·
I currently have a 5 channel GFA-6000 from adcom. Are there any MOSFET multichannel amps out there, and is the difference really that big between non MOSFET amps? What advantages do these types of amps have?


Thanks,

Peter
 

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What advantages do these types of amps have?


In the hands of a good amplifier designer, one can make a quality amplifier

if it uses bipolar or mosfet transistors. If you want to design your own amplifier you will

have to understand the difference between transistor types in order to make

a functional circuit. People make claims that certain types of transistors have a

sonic character but usually the sonic character is based on the design as a whole.

A bad amplifier design regardless of technology will be bad.
 

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Quote:
Originally Posted by thylantyr
What advantages do these types of amps have?


In the hands of a good amplifier designer, one can make a quality amplifier

if it uses bipolar or mosfet transistors. If you want to design your own amplifier you will

have to understand the difference between transistor types in order to make

a functional circuit. People make claims that certain types of transistors have a

sonic character but usually the sonic character is based on the design as a whole.

A bad amplifier design regardless of technology will be bad.
Man. Talk about trying to impress with one's knowledge without attempting to answer the question asked? Where did you get that he wants to build an amp?
 

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Quote:
Originally Posted by baller99
I currently have a 5 channel GFA-6000 from adcom. Are there any MOSFET multichannel amps out there, and is the difference really that big between non MOSFET amps? What advantages do these types of amps have?


Thanks,

Peter
You don't mention budget or the power you are looking for. Have a look at B&K and Parasound just off the top of my head. I think Adcom still has some MOSFET designs, as well.


Bob


EDITED TO ADD: Audiosource, Sherbourne and Classe have MOSFET designs.
 

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Sounds Simple,

Part of the question did relate to advantages\\disadvantages.

I think the answer was a very good one.

Pass Labs uses MOSFETS.
 

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Adcom has always been considered a very good value for the money in amps.


I have a 2 channel Adcom amp (which I like) and I know many people who swear by Adcom.


So, yes there are many different designs and manufacturers all have their strengths and probably weaknesses.


Many people who are into 2 channel music claim to hear these differences and talk about how each of the manufacturers sound and many have switched to the old style tube amps (very pricey).


I'm into mostly HT and I think for movies its is very hard to tell the difference between very good amps for watching say LOTR although some will probably disagree with me based upon their speaker selection.


But, for two channel - the skies the limit on money and I'd guess most would buy more expensive amps than Adcom but like I said I have some long time friends who have B&W Nautilus speakers and Vandersteen Signatures and they both use Adcom amps.


If you get upgraditus, let me know, I'll take that Adcom off your hands and be very happy.


Mike
 

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Quote:
Originally Posted by Milt99
Sounds Simple,

Part of the question did relate to advantages\\disadvantages.

I think the answer was a very good one.

Pass Labs uses MOSFETS.
So neither has any advantages or disadvantages? I was under the impression that each has its own advantages and disadvantages but I could certainly be wrong. I did just find this from the Classe site:
Quote:
Most designers who use solid state devices tend to choose a favorite type of device (bipolar, MOSFET, J-FET, etc.) and design around its known strengths and weaknesses. It’s a logical approach, since knowing everything about one type of device is easier than knowing everything about all of them. But the Classé Design Team have decades of experience working with many different devices. They have intimate knowledge of the strengths and weaknesses inherent in solid state devices of every major category. This knowledge translates to a more creative design approach, where devices of different type are used to maximum advantage in each amplification stage.


In order to achieve consistently high performance across the entire range of amplifiers, Classé designs are derived from a common platform, sharing common circuit blocks and in some cases, entire channel modules. J-FET devices are utilized in the input stage because of their high input impedance and low offset current. As a result, Classé amplifiers are easy to drive and low noise, they don’t interact adversely with interconnect cables, and with J-FET input stages, require no sound-degrading coupling capacitors.


MOSFET devices, like J-FETs, are voltage driven. They were chosen for the driver stage, where they deliver the necessary voltage gain while also being well-suited to meet the current requirements of the output stage. Bipolars, if driven with sufficient current, are excellent high-current output devices for amplifier output stages. Bipolars are also capable of superior low frequency performance and stability compared with other alternatives. All Classé amplifiers share the benefits of these and other design choices where tradeoffs are balanced to achieve uniquely compelling sound.
Marsh Design stereo amps are MOSFET and their 5-channel is bipolar. Marchand uses MOSFETs in most their designs.


Bob
 

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Quote:
Adcom has always been considered a very good value for the money in amps.
Mike-


I am upgrading my system and, since most the speakers I am considering are 4 ohms, I am also shopping for an amp (at least for the fronts). As much as I love the looks of the new Classe or would love to get some Brystons, I will most likely "settle" for an Adcom. Everyone I talk to says these are really nice amps for the money. I just have to save the Brystons for the next upgrade I guess.


Bob
 

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Discussion Starter · #9 ·
Well my main questions still stand. Are there any MOSFET multichannel amps out there? From what I have seen, these amp circuits are only in 2 channel amps for some reason. And what sort of difference there is between MOSFET and regular bipolar amps.
 

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Quote:
Originally Posted by baller99
Well my main questions still stand. Are there any MOSFET multichannel amps out there? From what I have seen, these amp circuits are only in 2 channel amps for some reason. And what sort of difference there is between MOSFET and regular bipolar amps.
How many channels? What price point?


Parasound Halo A51 and A52

B&K Components Ref 125.x and 200.x where .x is 5 or 7 channels.

Adcom GFA-5503 3-channel for your fronts
 

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So neither has any advantages or disadvantages?


Not on the end user level. The end user only cares about the finished product

and it's performance specifications, akin to saying. I'm not choosing a Corvette

over a Mustang because they use different nuts and bolts.

I was under the impression that each has its own advantages and disadvantages


If you are designing amps, yes. You need to know how to design with

each transistor properly so the amplifier as a whole performs accordingling.

You just can't blindly install mosfet or bipolar transistors in a circuit and let

them do their magic, you have to design around their weaknesses to make

a quality circuit.

Most designers who use solid state devices tend to choose a favorite type of device (bipolar, MOSFET, J-FET, etc.) and design around its known strengths and weaknesses.


What I said.


The rest of the message is maketing stuff ...
 

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I would agree with thylantyr on mosfet vs. bjt. In this discussion, we are talking about output devices, and the classe (marketing) paragraphes are talking about either input or VAS stage (actually MOSFET VAS is rarely used in commercial amps tho. I do use it quite often in my own amps).


for output devices, mosfets win hands down in terms of ruggedness. they have much wider safe operating areas and can handle very large peak current. Lateral types have negative temperature coefficient so they are ease to deal with in setting up and maintaining bias.


MOSFETs suffer from a few issues: they tend to have large gate capacitance, which makes them tough to drive, and stablize. they also have low gain, so you may need another (bjt) driver stage which adds complexity and worsens stability.


BJTs are much easier to drive, don't have as much rail loss as MOSFETs. However, they are very easy to destroy, and require many more pairs to achieve the same output power.


In the hands of a competent designer, I doubt anyone of us can tell them apart, sonically.
 

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In Star Wars, they use BobaFET transistors in their designs :p
 

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Quote:
Originally Posted by thylantyr
In Star Wars, they use BobaFET transistors in their designs :p


Hmm..

My understanding was that they used Dark Side output devices.. :rolleyes:
 

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Prior to BobaFET, they used JangoFETs :p

.. reason was... too much Bipolar Disorder :p
 

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Quote:
Originally Posted by baller99
Well my main questions still stand. Are there any MOSFET multichannel amps out there? From what I have seen, these amp circuits are only in 2 channel amps for some reason. And what sort of difference there is between MOSFET and regular bipolar amps.
A google search will net you plenty of detailed discussion about the differences between MOSFETS and other devices. As others have stated, most won't be able to distinguish any sonic differences between MOSFET and other solid state designs.


As a practical matter, however, MOSFET designs can produce quite a bit more heat than a bipolar design, for instance. Plan to leave much more breathing room for a MOSFET design, particularly a multichannel amp.

All of B&K's amps use MOSFET output devices. Not all of Adcom's amps use MOSFETs.
 

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you will also need to distinquish between which types of MOSFETs. the older, lateral types are very easy to set up for analog / audio applications. However, they have larger voltage loss thus are less efficient. Laterals also don't handle as much power. However, they have a very nice attribute: at high enough current (200ma and above), they Vgs goes UP with temperature.


The later vertical types are mostly for switching applications. They can handle very large current and are much cheaper than the lateral types. In addition, the vertical types tend to have higher gain. Their temperature coefficient doesn't go negative until Ids is very high (in excess of 10amp usually), making biasing adjustment necessary.


Vertical types are very inexpensive (1/2 - 1/4 of a comparable BJT, if the BJT exists), and the lateral types are very expensive (2 - 5x of a comprable BJT).


It should also be noted that finding complimentory power mosfets is a lot harder than finding complimentory power bjts.


All in all, this BJT vs. MOSFET thing is more or less of a marketing game for most domestic audio applications. MOSFET, in my view, holds an edge in high-power large current pro audio applications.
 
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