Aball and Kool139....I think that this FAQ writeup from Adcom, who uses both mosfets and bipolar output devices is a fair description.
QUOTE
My amplifier seems to run hot. Is this a problem?
Probably not. Most Adcom amplifiers use output devices called "MOSFETs" (which stands for metal oxide semiconductor field effect transistor, if you're curious). We've chosen these devices for their superb 'musicality'. They do, however, run hotter than more conventional bipolar transistors. We have designed our amplifiers with large 'heat sinks' made up of aluminum fins that dissipate excess transistor heat into the air. Just make sure you provide adequate room for ventilation as opposed to stacking other equipment directly on top of your amplifier.
What's the difference between your old amps and your new ones?
Well, the biggest difference goes back to MOSFETs again. Most of our newer "4 number" amplifiers (i.e. the GFA-5802) use MOSFETs while all our older "3 number" amplifiers (i.e. the GFA-555), used bipolars.
So, are bipolar amplifiers bad?
Actually bipolars have some advantages. As noted above, they produce less heat for a given power output. This makes them suitable for more compact components or for multi-channel, home theater amplifiers. As such, we have chosen to use them in our custom oriented amplifiers, the GFA-5002 and the GFA-5006, where size can be at a premium, and in our multi-channel, home theater amplifiers, the GFA-7500, GFA-7400 and GFA-7300.
Bipolars can have a very detailed, up front sound quality, and with our 2 decades of experience in getting the most out of these devices, we think you'll find that the bipolar's inherent immediacy is nicely tempered with circuit touches that let you hear a rare level of musical integrity.
END QUOTE
Of course there is a lot more to amp design that the type of transistor used. The mosfet requires less complex drive circuitry, and because of their thermal characteristics multiple devices can be more successfully paralleled, as is usually necessary for power amps. Perhaps I should have said that the distortion produced by mosfet circuit is typically low and the spectrum of harmonics somewhat resembles tubes (more "musical"). This last point is my recollection of Hafler literature when they introduced the use of mosfets to the audio world. Hafler still uses and touts mosfets. I haven't made a survey of the entire industry.
I only looked at a photo of a 4004, and it only had one transformer. If a power supply is properly designed it doesn't need to be separate for each amp. Use of two transformers is probably a practical choice, because one would be heavy to handle, and hard to mount on a chassis.
Actually, IMHO, and based on my own experience with monoblocks and multichannel amps, the greatest advantage of the mono approach is to avoid having an amp that requires a forklift to move around. Electronic advantages are insignificant.
