This is for Georgehifi especially but others can chime in.

I just can’t stand seeing BS put up, and have to give the truth. There is no such thing as an amp that can double it’s wattage the way he quoted. And if you believe Mosfets can do current better than BJT’s then your just as bad.

As far as Ralph he great and we agree on many things, except for his instance on Zero auto transformers being a fix without any negatives, which they are far from, and are just a bandaid fix for a problem of not having the right amp for a given speaker.

Cheers George

Kalali, I think your right. He just makes it up. Clearly he's never seen the measurements on a Pass amp or any number of MOSFET amps. He doesn't even know that the limiting factor in an amp doubling it's power isn't even the active devices. 

@georgehifi 
Since you're hung up on the doubling power, which you say is impossible, I'll tell you how to do it. Remove the source resistors in the source follower output stage of a MOSFET amp. Guys do that all the time in the DIY world, but you practically never see it in production gear because the line between stable and unstable is pretty fine. You don't really want to try that with BJT's because of their more aggressive thermal runaway characteristics. But, if you use MOSFETs with a big enough heatsink, there's a point of equilibrium and you can build a fairly stable amp. 

You two really need to update your semiconductor knowledge.


Upvoted by, MSEE Electrical Engineering & Circuit and System, Master of Science in Electrical Engineering (2015) and M.S Electrical Engineering, University of Southern California (2017)

Pros for BJT:
"BJT’s at the same physical dimensions and price can usually give you a lot higher speed, as they have very little input capacitance.

BJT’s can give you a lot higher gain. Just take a bunch of components and compare them, and you’ll find the BJT’s give you better gain characteristics and therefore require fewer gain stages.

BJT amplifier stages are much more linear than MOSFET amplifier stages, as the gain doesn’t depend on the bias voltage. This gives better fidelity.

BJT’s are capable of handling higher output currents for signal outputs and can have lower output impedance. In amplifiers intended to drive a low input impedance load or deliver significant amounts of power, this is a huge advantage. Many of the highest quality op amps are made with a BiCMOS process using a BJT pair for the output buffer stage."



Cons for MOSFET:
"Not as high of fidelity as BJT, since the gain will vary slightly as you increase the input voltage (that is, it will generate some very weak harmonics).

Input capacitance. The higher the gain, the greater the input capacitance thanks to the Miller effect.

Can’t drive a low-impedance load very well.

Low gain per part, which often means more amplification stages are necessary for higher gain, even when using advanced design techniques. Each amplification stage adds noise -- that is, you can never, ever get a better signal-to-noise ratio at the output than at the input."



I believe the OP has everything he needs, and doesn’t need the BS from the F5/Mosfet lovers, which I’ve heard and are great amps, if used within their comfort zone with preamps with high’ish gain and speakers not too low in EPDR load impedance’s and moderately efficient, because of wattage limitations.

Hope you got everything you needed bubba12
Unfollowing this corrupted thread.
Cheers George

@georgehifi 
Oh.... We're going to just resort to half-truths and omissions, huh? Just going to ignore the thermal issues, the low base impedance which is a much bigger problem than a little gate capacitance. And how do you make a simpler circuits with devices that basically demand a driver stage?