Aragon 4004 mk2 vs Adcom 555 mk2

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.

"Way more musical" is a nice way of saying it has more harmonic distortion. Ok if that's what you like.

Kool39...My observation about "way more musical" was general in nature, interpreting Johngalt47's observation.

With regard to the particular Aragon amp: no I do not have experience with it. From what I see and read it appears to be similar in concept to Adcom products, and I am sure that I would be quite happy with it. (But how do prices compare between comparable models?) I only see one power transformer, so I can't agree with the "dual mono" description. Adcom and some others went from bipolar to mosfets because of inherently better distortion characteristics, but bipolar can be good, and Adcom has returned to bipolar for their multichannel HT products because of lower heat dissipation.

My next amp, if there is one, will be digital. Maybe the Carver ZR 1600. By all accounts it will blow all this stuff away, for less money.

Aball...What do you know about the Carver 1600? Its distortion specs are quite respectable, especially when measured well below the 600 watt level, which, of course, is where it will operate. I have not seen anything but good words about this amp, which sells for about $900.

Kool39...The photo I saw of a for sale 4004 showed one toroidal transformer. Pop the cover and take a look, or admit it doesn't matter :)

If the toroidal transformers are stacked that could explain the disagreement.

By the way, think that the Aragon amp costs more than twice the Adcom price, so it ought to be better.

My Adcom 5503 (like a 5500 but with three channels) is hooked up to Maggies. Although Maggies are often called low impedance speakers, they actually are a very uniform 4 ohms regardless of frequency, which makes them an "easy" load for the power amp. Maybe that's why you can get good results with a less exotic (expensive) amp. (But they do like lots of watts).

Gmood1...Negative feedback just means that the amp output is compared with the input, with appropriate gain and phase compensation, and the resulting "error signal" is applied to the input (or at some intermediate stage) so as to reduce the error signal to zero (nearly). There is no fundamental reason why negative feedback is bad, but if an amp requires too much this probably means that the circuit is not too hot to begin with.

Tube amplifiers that use transformers must have negative feedback to get flat frequency response, quite apart from the issue of distortion. The widely used "ultralinear" configuration has taps on the output transformer specifically for the purpose of negative feedback.

If the feedback signal were perfect with regard to gain and phase it could only improve the overall performance of the amp. High amounts of feedback become a problem when the feedback is less than perfect, either because of circuit design or real-world component tolerances. "Too much of a good thing".

You probably have less problems with feedback phasing around one stage (local) than across the entire circuit. So in the real world, local feedback may be less problematical than global feedback, even if the net total dB is the same. In an ideal world, both would be perfect.

It is unfortunate that we use the word "Negative" to indicate polarity of the feedback. That word has negative connotations.