In an attempt to short circuit the inevitable questions:
--Yes, you can increase the bias as long as you have sufficient heat sinking/power supply/etc.
--Yes, you can decrease the bias.
--Yes, you can substitute the usual MOSFETs for the IRF outputs.
--Yes, you can substitute the input JFETs, but the field of candidates is much more limited.
--I don't have the datasheets for the input JFETs with me at the moment, but I don't think you're going to want to increase the rail voltage much. The outputs can take it, but the inputs can't. If someone knows where to find 100V low noise, complementary JFETs with reasonable current capability, I'm all ears.
--Yes, you can decrease the number of output MOSFETs, but watch the heat dissipation per device.
--Yes, you can increase the number of output MOSFETs, but bandwidth and distortion will suffer as a result--though not fatally.
What'd I miss?
Yes, you are missing my interest.
I'm a bit tired of doing DIY these days, and busy.
I will not care of any scratch on the surface
as long as there is the original finger-print of Nelson Pass.
What is the retail price for the F4?
Much much higher than 2.5K USD . . . ?
I remember seeing a reference that the IRFP140 is a better complement to the IRFP9240 than is the IRFP240. Is this true? Would anyone have spare IRFP9240? I have spare IRFP140 I would like to trade off for 9240. maybe we could help each other.
Amongst the guys that know jfets - Please don't be shy in suggesting suitable replacements (and hashing out which is best, and why). I am sure more than myself could benefit in how you come to the conclusions you come to.
heatsink on the jfet?
I promised myself no more amps. This one will be the last
Hehehe....I have built ZEN V9 and an F2 lite. Great! Very friendly amps to fullrange speakers. At least I don't need another amp! But...what could be better than a fullranger on the Open Baffle, driven by Zen v9 or F2?
Uhm....you bet-F4 driving big woofers on OB!
But...Yea, no more amps....
Someone on diyaudio once said "Stop pushing my Woo-woo button!"
Very, very nice amp. It has the JFET sweetness, like the F3, but also has push pull control and is direct coupled. An awesome little amp for those you can live with it's limitations (low gain and low power).
If your system does not work with these limitations consider this amp to be a great excuse to build a better pre-amp and get into high efficiency speakers. Yes, it is that good.