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List of Mosfet power amps
- Thread starter halkaloogie
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DaveVoorhis
Addicted Member
Almost all Class-D amplifiers have MOSFET outputs, but they're used very differently from MOSFETs used in Class A and AB amplifiers. In Class A and AB amplifiers, when MOSFET outputs are used they're used as linear amplification devices. In Class D amplifiers, the MOSFET outputs are operated as switches, rapidly turned fully on and fully off.
PabloTincho
Super Member
After doing some research on Mosfet amp, are all mosfet amp pretty much the same ?
They all have the same specs for distortion 0.02 %
The only different ; watts and configuration: dual mono, regular power supply or toroidal power supply ?
maybe quality caps, but I dont see that much difference.
It make no sense to buy an expensive Mosfet amp when a cheap Hafler or even B&K will have the same results in the end.
They all have the same specs for distortion 0.02 %
The only different ; watts and configuration: dual mono, regular power supply or toroidal power supply ?
maybe quality caps, but I dont see that much difference.
It make no sense to buy an expensive Mosfet amp when a cheap Hafler or even B&K will have the same results in the end.
ducati_EL34
Addicted Member
single or dual power supplies. Class A over AB are two features worth spending the extra money on.
PabloTincho
Super Member
I was under the impression that all the mosfet are class A and AB
A MOSFET is simply a type of transistor. It's a metal oxide semiconductor field effect transistor. The other type of commonly used transistor is usually known as a bipolar transistor.
Most mosfet power amps only use mosfets as output transistors and the rest of the circuit is pretty much follows the same topology as bipolar transistor amps. Thinking of mosfet amps as an inherently different breed of amplifier is a mistake.
As mentioned in post #42, a class D amplifier might use mosfets in its output stage. The manufacturers might sell those as "mosfet amps", but they're more likely to label them as "digital amplifiers" or "class D amplifiers" because that's a better marketing strategy.
Similarly with most amps which have the word "mosfet" on the front panel - it's a marketing device.
PabloTincho
Super Member
What is the difference between these popular brands of Mosfet amps, besides power, power supply configuration ?
Hafler
Adcom 5000 series
and B&K ?
Hafler
Adcom 5000 series
and B&K ?
To be honest, I have no idea. The devil is in the detail, and you (or I) would need to see a circuit diagram to get into the nitty gritty. Personally, I believe the differential amplifier at the input (aka the "long-tail pair") is a hugely important part of an amplifier, but it usually gets very little attention outside of engineering circles. It may or may not incorporate emitter degeneration, a current source, a current mirror, monolithic super-matched transistors, etc.,etc., but you'll never see those talked about in sales literature because they're totally meaningless to 99.99% of consumers. It's much easier to boast about 150 watts or 0.001% distortion or such, or indeed write "mosfet" on the front panel as if it's a self-evident badge of merit.
Anyway, the point I'm trying to make is that the performance of any amplifier is based on the combination of a huge number of tiny design decisions and simple descriptions such as "mosfet amp" are a very broad brush which will tell you nothing about how an amp actually performs.
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ducati_EL34
Addicted Member
well yes they are either setup as either a Class A amp or Class AB. A good class A amp will cost more than an AB amp (larger heat sinks, heftier power supplies, etc.), and would be worth the extra money.
Unless they're class D.
ducati_EL34
Addicted Member
I have no need or desire for that class of amp.
j!m
Super Member
How long the amp stays in class A before it switches to AB is important; as is the switching noise between mode, if any (audible).
Be generous with your power "requirements". The wattage needed for transients is quite massive in dynamic music. I run 500 a side and have the amp right next to the speaker. Listening at higher pressure might require more power. Sounds ridiculous but it's true. And it makes a difference.
Damping factor (higher is better), shortest length possible on speaker cables, and tons of clean power (stay well away from clipping the amps) will make you happy.
I'm partial to my Counterpoint amps, but they are quite fragile and difficult to repair when needed. Output devices need to be closely matched (long discontinued), or the load is not equally shared between them, and all the devices on that side blow.
But they sound nice...
Be generous with your power "requirements". The wattage needed for transients is quite massive in dynamic music. I run 500 a side and have the amp right next to the speaker. Listening at higher pressure might require more power. Sounds ridiculous but it's true. And it makes a difference.
Damping factor (higher is better), shortest length possible on speaker cables, and tons of clean power (stay well away from clipping the amps) will make you happy.
I'm partial to my Counterpoint amps, but they are quite fragile and difficult to repair when needed. Output devices need to be closely matched (long discontinued), or the load is not equally shared between them, and all the devices on that side blow.
But they sound nice...
ducati_EL34
Addicted Member
Both of the comments above may be true, but they relate to all amps, not just mosfets.
In my opinion, "mosfet" isn't a class of amplifier like A, AB, D etc. are classes of amplifier, nor are they really a distinct sub-set of anything. The fact that some of the transistors in an amp are mosfet rather than bipolar isn't an indicator of very much at all.
j!m
Super Member
I'm a fan of pure class A.
BUT, the downsides of heat, and needing dedicated 20-amp circuits for each amp can be a deal breaker for many...
My counterpoints are not fun to run on a hot summer day, and they are not full class-A either. 500 watts of class A would be... well would probably be best with a remote heat exchanger attached. Who sells a 500+ watt class-A amp with external heat exchanger?
I had actually planned to build a pair of mono blocks based on the old Krell KSA-250, with two amps per chassis, run balanced. This effectively creates a 500-watt balanced class-A mono block. Transformers and heat sink surface area becomes a problem fast... A copper slab with water running through it for the devices to mount on, plumbed to a remote heat exchanger (with radiator, fan and circulation pump) actually makes a lot of sense.
Mark Levinson's old home system used six amplifiers (three per channel). They were mounted on shelves in the basement, under his speakers. This is actually a great idea, if you can pull it off. All his old amps ran extremely hot as well.
BUT, the downsides of heat, and needing dedicated 20-amp circuits for each amp can be a deal breaker for many...
My counterpoints are not fun to run on a hot summer day, and they are not full class-A either. 500 watts of class A would be... well would probably be best with a remote heat exchanger attached. Who sells a 500+ watt class-A amp with external heat exchanger?
I had actually planned to build a pair of mono blocks based on the old Krell KSA-250, with two amps per chassis, run balanced. This effectively creates a 500-watt balanced class-A mono block. Transformers and heat sink surface area becomes a problem fast... A copper slab with water running through it for the devices to mount on, plumbed to a remote heat exchanger (with radiator, fan and circulation pump) actually makes a lot of sense.
Mark Levinson's old home system used six amplifiers (three per channel). They were mounted on shelves in the basement, under his speakers. This is actually a great idea, if you can pull it off. All his old amps ran extremely hot as well.
ducati_EL34
Addicted Member
No mosfet is not a class of amp. It is a type of amplifier circuit, Mosfet vs Bipolar.
I am more interested in Mosfet for a power amp because it is closer to Vacuum tube sound, as opposed to Bipolar.
It isn't even a type of amplifier circuit, it's just a type of transistor.
Mosfets can be used in place of bipolars with very little change to the bipolar circuit. Here's one such example where Nelson Pass takes the Harmon Kardon Citation 12 and uses mosfets instead of bipolars.
https://www.passdiy.com/project/amplifiers/build-a-mosfet-citation-12
Nelson Pass likes mosfets and uses them in his other designs too. Here's another one of his, though it's entirely different from the first:
https://www.passdiy.com/project/amplifiers/the-zen-amplifier
"Mosfet" definitely isn't a type of circuit.
ducati_EL34
Addicted Member
You are right. I am here at work trying to read the forum in tween other people’s emergencies. I was over simplifying my reply, in haste.
