Mosfet mist?


I have read several articles on the sonics of mosfet amps to include Adcom,B&K,Counterpoint,Perreaux,etc. What exactly is mosfet mist and how would one know what to listen for. Are some amps more prone to this mist than others? I have a Perreaux 1150B doing woofer duty on a pair of Focal 706s spkrs(A modified Parasound 1000A on top)How would say the 1150B compare with a B&K in relation. Thanks.
I kinda prefer the description from Doug Blackburn (SoundStage):
"It’s a bunch of baloney, nothing but made-up cute words drawn from mistaken conclusions. MOSFET mist is no more useful or descriptive of anything real than Triode Tarnish or Pentode Pallor."

For completeness, the whole article is here ->
It's actually quite easy to detect. You'll need a fluorescent lamp, a variac and a pair of wrap around 3-D glasses. Let your amp warm up for 1 hour while driven to output 1/3 its rated power. Place the lamp approximately 1 meter above and pointed at either the amp's or the speaker's binding posts. Turn off all other light sources in the room. Set the variac to the standard voltage in your area and turn on the lamp. Raise the variac's output voltage exactly one half semi-tone above your area's normal voltage. Now the hard part -- rapidly fluctuate the variac's output voltage over a quarter semi-tone range, this is known as "voltage fluttering". With your 3-D glasses you should now be able to see a slightly purplelish mist being emitted from the binding posts. DO NOT BREATHE THIS MIST FOR EXTENDED PERIODS OF TIME! Stop fluttering, turn the variac back to your standard voltage, turn off the lamp, but keep on the 3-D glasses. Remain in the darkened room for 10 minutes.

Seriously, notice the alliteration - Moseft mist. It's a nonsense phrase invented during the cocaine crazed 70s by a writer trying to be clever. It's supposedly a kind of softening and homogenization of the sound. If you think you understand what the last sentence means, then maybe you'll hear it too. Otherwise, it's not something to be taken seriously.
I thought Sam Tellig (Stereophile) came up with it and has since said that improved mosfets no longer display it, or something like that.
I have tried this, but it's hard to be precise with the +/- Variac sweep with the 3D glasses on as it is hard to see, plus I am mostly in the dark here. I thought I detected some smoke/mist coming from the Variac, but nothing from the speaker terminals. Also I have blown a couple fuses in my amp. Are you sure this is the way to do it?
I agree with Dopogue on this.
My friend is currently running a B&K Pro 10 preamp, B&K 2020 amp I think, Marantz cd player.... powering MMG Maggies. I hear no such mist. Sounds detailed and clear to my ears. Seriously, I've heard much worse in systems costing 3 or 4 times as much.
I like canadian mist better!!!
I'm getting kinda misty about this.
Ok I get it. Nothing more than a twist of words. Just thought I would ask. Thanks and enjoy your mosfets : )
There is nothing wrong with mosfets, many Conrad Johnson amps used them. Their rugged and require less circuitry than bipolars plus their tauted as sounding tube-like. Some of the best sounding amps I have heard use them. I could never identify the elusive mosfet mist. Maybe I heard it but it never got in the way of the music.
what do they call a fog over italy ????

a bigamist.

as to mosfets, it's all in the design. odd order harmonics are an important consideration to the sound of a ss amp.

i would conjecture that all mosfet amps do not sound the same, so don't be too concerned. listen and decide if you like the amp, not its component parts.
Play Misty for me! My Meridian 605s have Mosfets and I haven't heard it. I use to sell Hafler and Hitachi power amps and never thought they had sound deficiencies due to Mosfets. I think my CJ Premier 350 has them in its driver stage and my tube using friends think it is a great amp.
The best way to collect and actually see mosfet mist is to first find a powered subwoofer powered by an internal mosfet based amplifier with no heatsinks for cooling on the outside.All the cooling has to come from the inside of the woofer enclosure.The powered subwoofer should also be a ported design with only one port to the rear of the box where the internal power amp controls are located.Leave your powered subwoofer in a damp enviroment such as a basement etc. overnight. Now you are ready to collect some mosfet mist.Take your now prepped sub back into your listening room and hook it up to your system and play some recordings with some deep bass like pipe organ and such.Collecting and examining mosfet mist is now easy.Get yourself a brand new dry and clear ziplock bag and hold it nice and tight up against the port opening of your sub while playing your organ music.It should fill up in no time with this very elusive mosfet mist.You should also now not only see this quite rare mist but feel it inside the ziplock bag.You may also fill a few to send to those unbelievers out there to quite them down.Mosfet mist,yes it does exist!!!
Don't knock Johnny Mathis I like him.
By the way Adcom especially the older ones, like the 555 used Bipolar transistors and they sucked. They were suppose to be the poor mans Krell, no way.
The 'MOSFET mist' is a term coined that points to the problems of driving extremely capacitive inputs of the output devices of a MOSFET-based amplifier. If the driver circuitry lacks the current to deal with this capacitance, there will be a high frequency roll-off, hence the term.
Very well said Atmasphere,that is exactly what happens and I have heard it on speakers with a lot of capacitance in their crossover networks like 18/24db designs and large caps in thier woofers,shunts for instance.Mosfet amps tend to sound quite nice with 6db network speakers.Now back to my collected Mosfet Mist.Anybody out there want any free samples?
Would you rather be bipolar or have a mosfet mist? The choices are unsettling.
Atmasphere is the capacitance of the MOSFET that you describe is it of the non linear kind i.e. does it bounce around at different frequency and amplitudes?
Mustagefan, yes. This was more of a problem during the lateral MOSFET period. These days you see more of the vertical MOSFETs. I think what has happened is that in the old days designers did not always take the input capacitance of the devices into account. Experience is always helpful though so the issue is less common with modern designs.
Atmasphere if you were to make a tube hybrid amp would you pick Mosfets or Bipolars for the output stage.
I've been studying that very question for the last year or so. There are advantages to each, but in my case I think I would go with the MOSFETs for the additional linearity- I like to run things zero feedback.
I think you are right and a hybrid tube amp would be a good idea for home theater. It would have no output transformers which you like. So lets get cracking.
I personally like the more tube like sonic signature of mosfets vs bipolars. I recently acquired a Counterpoint SA100 rebuilt by Greenstreet Audio with Exicon mosfets and with some tweaking and replacing of the coupling caps @ the tube gain stage I'd say there's hardly any mist I could detect and all there is is music.
I currently own Vac Phi 70 mono, Counterpoint Altavista NP220PG, NP100PG, and some heavily modded/rebuilt tube amps.
Mosfet amps are given a bum-rap by designers not going all out to do them justice by using up to par components in the gain and driver stage. Fix all the problems upstream and use modern mosfets that are less lossy and you wont need to burn money with power tubes (esp. those Sovteks passed off as so-called whatever brand reissues)
Lamm does ok with their mosfet based hybrids.
Some early mosfets had a slow decay , it gave them a slow unclean sound , maybe thats where the term came from .
Sounds like the name for a pop wine beverage. It's mythological. I used to own a bipolar amp from Bryston and have been running a JEFT/MOSFET amp from Hafler for no less than 16 years. No "mist" has been detected, but if I ever got back into reading one of the audiophile media mags they might incite this angst in me. Toss this one to Peter Aczel the Audio Critic and watch him go ballistic!
How many posters have spent any time in a fabrication facility seeing how semiconductors are made?
Exactly 37.
Thanks Atmasphere. I learned something today. Pls excuse my uninformed sarcasm. I was caught up in a mirthful moment. I will have to scrutinize my 1995 Halfer 9180 for this phenomenon. It uses lateral MOSFETS. I do LOVE the sound, never tiring.
point is, Woof, that some knowledge of how semiconductor devices is made would go a long way to help people understand what they are buying into. Toss in a dose of device physics and than you can talk.

Talking about a particular tech having certain characteristics is generally a no-win with many other factors tossed into the mix.

I worked in-fab since the early 70s, first making quartz lo-frequency oscillators and later CMOS / BiPolar and other integrated devices and still later making discrete HexFets, IGBTs and FREDs. It was a lot of fun, with the first ICs I worked on having 6 or 7 micron geometries on 3" wafers!

But anyway, show me a good design using any device and you'll be hearing good sound. Bad sound can come from any technology.

If anyone has any curiousity about how any of this stuff is made, I'll do my best to answer or find out.
Who you calling "woof?" I've also worked on wafers...I spent the holidays working hard on drinking beer while eating only seeded wafer crackers.
Magfan what probers and testers have you worked on;I am in ate final test.
Never ever worked test /probe. Did some Reliability (1000 hour burn in) on some tests I ran with induced leaks in a vacuum system, but NEVER probe.
I think that would be some valuable information about some of the differences between MOSFET and BiPolar designs.

I was into diffusion.....oxide growth (insulator) and implant diffusion. Also spent lots of time with metalization. Aluminum interconnects and various backmetals and some platinum silicides. I also worked in photo / etch but that wasn't where my heart was.

One entire fab I worked in was sold to the Chinese. Every machine, valve, pump, filter, bit of plumbing and microscope. The works right down to the flooring. I even caught 'em photographin the Specification Book.....A VERY NO NO thing to do. BUT, the bosses didn't give a XXX.

Sorry Wolf.....a typological error.
"Mosfet Mist" is a phrase coined by that noted 90's adult film star, Linn Klimax.
Magfan all that sounds familiar as it seems all engineering and prototyping is done in the states then once the wafers have a proven yield it seems to go offshore.
There is a fab within 25 miles of here with a capacity of 20,000 (!!) 6" wafers per week. At least. The class 100 area is almost 2 kidding.
They make power devices, FREDS, IGBTs and who knows what else.

I carried a pedometer with me for a week or so and even before the expansion, never walked LESS than 3 miles per day.

You simply would not believe the electric bill......and the low power factor surcharge. Each furnace, for example, has a 100 amp breaker. The same as the service to my house. And....they come 4 to a 'stack' and there quite a few stacks. Each implanter has a transformer the size of a Smart Car.....only a slight exageration...but you get the idea.

Many companies use 'foundaries' for overflow capacity. Also, companies like Intel simply don't want the mask set (layer by layer blue print, if you will) out of there grasp. All Intel is made in the US of A.

Many comodity OPAMPS are made in these foundaries and pretty much sold by the bushell.
I own a B&K ST-140 that I use as a backup amplifier. Stock, it does sound really warm, and a little undetailed.

However, a quick look at the schematic and you'll see a gross little electrolytic used as the input capacitor. I replaced this (and the power supply bypass caps) with some Elna Silmics. It was a big improvement in the sound, making the amplifier sound more open.
I seem to be hearing the same thing inre electrolytics.

After living with my CJ ART preamp (no elecs anywhere), every pre (with elecs) that I've tried since seemed to have a certain "sour?" sound. Latest experience was with a DAC. A friend offered me one of his personals. He helped design the DAC but in his personal ones, he took out all the elecs and replaced them with polys. I've tried a few well reviewed (fwiw) DACs since then and each one, while slightly different, had that "sour? sound. Coincidence? Don't know, works for me.

I'm also a fan of bypass caps on the big PS elecs of power amps.