CODA's. The disadvantage is that the output impedance is not as low as a global negative feedback amp. It will not have as good bass control.
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The Ayre V5x and all of the Theta amps(Dreadnaught I and II,) Citadels, etc. are zero feedback amps.
The advantage of the zero global feedback amps is they are just plain more musical, they image better and they are more holographic. They come closer to tube sound than any other solid state design without the tube drawbacks. I have also found the soundstage to be much deeper compared to negative feedback amps. Ayre (Hansen seems to think feedback causes time domain problems.) I don't know about all that but to me they just plain sound better.
I bought a pair of the highly rated JC-1's and yes, they do have better bass control in an absolute sense but the Ayre V5x I use now has better bass (as in musical definition.) The JC-1's did a poor job of sound stage depth even after burning in for months. The Parasounds have good side to side imaging but not front to back. They certainly didn't present the holographic imaging from wall to wall and well out front to way back in the stage with bloom, articulation and an ability to just hang images in space.
I sold the Parasound's and certainly have not missed them. They sounded "Hi-fi" to me. The Parasound is an excellent amp when compared to other moderate to high feedback amps. The JC-1's use about 29db of global feedback I'm told. When compared to some other high dollar amps like Krell and Levinson, it's right in there---just not the zero feedback amps.
I use Vandersteen speakers which take full advantage of the holographic imaging of the Ayre amp. I'm sure the JC-1 would fair better with some other speakers like Revel, Wilson and others that benefit from a "Balls to the wall (all about power) amp."
I also used a 4-ch Theta Dread I that is an excellent amp but like the Ayre better overall.
Negative feedback amps spec better and all that but after you listen to a no feedback amp, for me, game over!
There are a lot of tricks just to screw up your mind in terms of presence or abcense of global Negative Feedback in the amp.
Today's amplifiers give it or take must have low output impedance to drive today's speakers. They also must have a desired power again give it or take. Sometimes designers would add an extra stage(emitter follower in bipolar designs) that only amplifies current(but no power) from previous stage that amplifies both. Introducing extra output stage to decrease an output impedance is another way of avoiding such term as Global Negative Feedback but with shure presence of such on the previous stages to the level of desired specifications.
So without knowing the inner details and schematics of the amplifier you CANNOT judge whether its performance REALY depends on NF. It's realy a good practice just to get rid of the ill-marketable dogma about an amount of Global Negative Feedback or any kind of such completely before choosing your component.
One thing is known to all electronic engineers is that NO amplification stage is STABLE with NO negative feedback -- otherwise the amp will become an oscillator.