MOSFETs have some very good properties for audio amps. Lateral MOSFETs are more thermally stable than BJTs thanks to their negative thermal coefficient. They are easier to design with too - no beta droop, no secondary breakdown, automatic current limiting, and are voltage driven. However BJTs have a much higher transconductance and thus higher linearity so they are not to be totally dismissed. Linearity is after all one of the most important properties we want and look for in audio amplification. In the (near) future however, there will be some lateral MOSFETs that will rival the best BJTs in terms of linearity, at which point I will go with a MOSFET amp.
JFETs are quite different and are not used much, if at all, in output stages. However they have excellent input impedance and low drift so are more suitable for input stages and current sourcing.
Balanced connection has nothing to do with transistors so it is definately not the only way to go.
Hope this helps. Arthur |
Hi Jayme,
I wouldn't get hung up on the technical details if I were you. Yes...MOSFETS can sound great in a great design, but a poorly designed MOSFET amp can sound awful. The same is true for balanced circuitry and all other aspect of audio design. Most equipment was designed by different people with varying ideas of what music should sound like. Unfortunately, the only way to know what is "the only way to go" is to listen to different amp designs and decide for yourself. |
My understanding is that generally MOSFETs have a tendency to have sweeter, warmer, even darker characteristics than other transistors. The potential downside of MOSFETs is that they can be attributed to a slightly more polite or recessed midrange.
Balanced connections seem favorable to me in my system as they appear to provide a wider range of macro-dynamics. Perhaps this perception is due to the increased voltage generally associated with balanced connections. But this may not always be the case.
-IMO |
You cannot really pin them down as which is better or what's the advantage of one over another. Each have their own operating parameters and do basically the same thing in the end: amplify a signal. How they are implemented for a given topology, the skill of the designer, and overall engineering from power cord to speaker terminals will determine their perfomance. Just one cog of many in a wheel.
For example, Nelson Pass used JFET's in the Threshold "e" series, IGBT's in the Forte's and MOSFET's in the latest "X" series. All different topologies and all, IMO, one hell of a good sounding product history. |
"I've done a "search" here and elsewhere and not gotten a good answer as to why it may be superior to JFET"
You need to seek out some good books on the subject and explore the facts on your own (that would be my definition/approach of getting a "good answer"). I don't think the answer is within the scope of these forums, especially not in 5k posts.
www.audioxpress.com carries alot of books. |
Thanks, everyone: as a layperson w/limited time to devote to the hobby, and VERY limited science IQ, I depend on my "friends" for help. Amazing what Nelson P has accomplished, BTW: can anyone beat this guy's electronics résumé? (Ez: if reading books would help, I'd have gotten a degree in a scientific field, and not gone through employment hell through these many decades; if we all had the time/innate ability...this forum would vanish!) Happy Holidays! |
IMHO, it's not necessarily the parts, but the sum of the parts that matters. |
A lot of folks think using mosfet drivers and bipolar output transistors give you the best of both worlds. McCormack and Theta are just two prime examples. People use to gripe about the mosfet haze but I guess times change. I think Jfet's are used more as an input amplifying device. I know there impedance stays constant over a wide range. |
Hafler claims that MOSFETs have several advantages over bipolar transistors: better thermal stability, faster switching speed, lower output impedance and better linearity. hafler monitorFrom Stereophile December 2002: Musical Fidelity uses bipolars because "they have a higher current yield than mosfets". |
If the Plasmatronics are the speakers I remember seeing, won't they set the tree on fire? Aren't these the speakers that needed a gas supply to run? Or was it that they emitted a gas from their plasma tweeters?
Happy Holidays |
Cdc - the "current yield" is the transconductance I talk about in my post above. There are a few more details about it there. Arthur |
I have owned both types and it depends on what kind of speaker's you match them with IMO.Bipolar's do a great job at delivering needed current,but with exotics ,like maggies I found Mosfets to be better.Maybe that's just my experience.
The MOSFFET Haze issue was answerd for me by Ralph ,the designer of Atma-Sphere Amps.Mosfets are more linear BTW and having an amp forward bias'd to run closer to Class A clears up the problem of the haze.It did in my case as I had my B&K modified and it sounds alot richer and clearer.An experience recently with Rotels top of the line reciever made me aware of how bad the haze can be or put another way how noticable it is after eliminating it from my setup.
There are several manf.'s that swear by using JFETs one of which I recently contacted. Sonogy!They claim that JFETs are more Linear.Not so sure about that.
All in all ,it matter's what you have for speaker's IMO and what serves them best.Found that out the hard way.Some speaker's will do great with certain amps then fall apart using other speaker's. There is no clear solution or golden rule ,but I bet measurement test would bear it out.Then you get into the testing equiptment use.
Frustrating thing about audio is there will always be a divergence of opinions,therefore trust your ears. |
