tube amps and electrostatics

Rodman99999,
this debate on MOSFET vs BJT has been duked out before many times & I do NOT want to get into it again. So, I'm writing this with a bit of trepidation -
both your references for MOSFETs seem to deal with MOSFETs used in a high frequency switching application (this is the key here - switching application) where MOSFETs are deemed better power devices than BJTs simply because MOSFETs are all-majority carrier devices (unlike BJTs which are bipolar carrier devices. hence the word "Bipolar" in BJT - both charges exist in this device - majority carriers in the emitter & collector & minority carriers in the base). Today's class-D amplifiers use power MOSFETs in their final output switching stage & for good reason as this stage operates at 10X the max audio frequency & we want to minimize the losses in this stage. For traditional class-A & class-AB power amplifiers, the output stage is NOT being used as a switching output stage; it's being used a continuously-variable analog output stage with high current capacity. I.E. the output stage is a gain stage & not a switching stage. There's a big difference here. You can definitely use power MOSFETs in the output analog stage as we see with Pass Labs, Threshold & many other brands. Not saying it cannot be done....

MOSFETs make better high frequency switches because at high frequencies we worry about the AC losses (CV^2f). These are the losses that are directly proportional to frequency & the parasitic capacitances of the device. Recovery time from off-to-on & on-to-off is also a big consideration in switching application & in MOSFETs this time is less than in BJTs as one does not have to wait for the majority & minority carriers to move from & get back to home base 'coz in MOSFETs there are no minority carriers!
MOSFETs are self-limiting current output-wise (higher temp means higher resistance, which means lower output current. BTW, if you did not realize this - this is negative feedback! But this negative feedback is localized to within the device) & all that is very well.
Output current-wise I also believe that BJTs do a better job - for the same amount of drain (MOSFET) or collector (BJT) current, the gain of the semiconductor device is higher in the BJT than in the MOSFET. Current in a BJT is linear relationship to circuit parameters like Vbe, circuit resitances, etc. In a MOSFET, the drain current has a square-law relationship to gate-source & threshold voltages.
At the very heart of it, the MOSFET is a voltage device (yes, it does convert input voltage to drain current, hence, the transconductance) but, naturally, it is a Field Effect device. And, intrinsically, the BJT is a current device. Yes, applying voltages to the terminals does create high electric fields but the applied voltages are meant to create electron flow (current) in a BJT & not setup a source-drain channel (like in a MOSFET).
MOSFETs have become very good power devices off-lately but I believe that BJts do a better job of high current delivery in analog circuits.

I am no fan of SS amps(except for driving woofers), but-
when someone makes blanket
statements, regarding MOS-FETs and ESLs; I have to take
exception.
Whatever their application; they still protect themselves
against thermal runaway, unlike BJTs, and can remain stable
in high-current situations. I
will side with those that have used them(past and present),
to great success,
in
their output circuits, ie: David Hafler's SS amps, Acoustat
TransNova TNT 200,
Van Alstine Synergy 450, Accuphase A-60, BAT VK-655SE,
Goldmund Mimesis8, Jeff Rowland M-9, to name a few(all Class
A or A/B). I've
not
heard any of those accused of being unstable, into any kind
of
real-world load. It's been my experience, as well as
others,
that MOS-FETs(in a correctly designed circuit) have no
problems driving ESLs. You mentioned PASS and I'll use the
following article, as my parting shot. Especially pay
attention to the portion entitled, 'Stretching Those
Muscles':
(http://dagogo.com/pass-labs-x350-5-amplifier-review)
Happy listening!

Rodman999999 You mis-read my statement on I vs R.
I said "that can almost double", the app word being "almost" you read what you wanted to see.
And here is the advertised specs of an Australian amp called and ME1500 and yes it can "almost" double all the way down to 1ohm 220-8 430-4 800-2 1500-1. And thats both channels driven at the same time!!

http://www.me-au.com/ME_1500_data_1.jpg

http://www.me-au.com/ME_1500_data_2.jpg

And even the smaller ME amps do the same amount of "almost" doubling. Show any Mosfet that can do these kind of figures, and like I said I'll show you a case for false advertising.

Cheers George

Regardless, the ability of an amp to double power as impedance is cut in half is not an advantage on an ESL. This is because the speaker really wants to see constant power regardless of its impedance at a particular frequency, as the impedance curve is based on a capacitor rather than the resonance of a driver in a box.

Consequently tube amps have been the preferred choice for ESLs since the 1950s and remains that way today- if you really want to hear what the ESL does, you gotta have tubes, else you are leaving a lot of the speaker's capabilities on the table.

I couldn't agree more. I base that strictly on what my ears tell me whenever
I hear electrostatics. What I hear is that the transparency of electrostats
lays bare ss amps' tendency to sound leaner and less dimensional than
tube amps with a resulting sound that can can be sterile and dimensionally
flat; as opposed to the dimensionality and image density that a good tube
amp offers. A couple of examples of this (besides my own) that come to
mind are Martin Logans run by Threshold amps at audio shows,
Quads/Spectral at Lyric, and most recently a friend's InnerSounds with the
InnerSound amplifier. On the very same InnerSounds, my Manley tube
monos sound timbrally closer to real with full and and dimensional images;
but, admittedly, not capable of as much volume or bass extension. In
fairness, original Quads driven by Levinson ML2 sounded very good.