Slew rate and rise time

I personally have read enough to understand Sean's analogy and even though "technically" it needed a "Slight" bit of refinement, I thought it was an excellent post allowing these parameters to be interpreted in an overall scheme. I also enjoyed Herman's post(s).
I would like to ask this, when looking at the effects of negative feedback in an amplifier, how does local feedback vs global feedback effect the overall musicality of the amp or overall performance (is there a compromise in here somewhere?)
I have seen a few amps that featured no global feedback but used local feedback and then, according to Charlie Hansen at Ayre, use no feedback, either local or global. Which type of feedback would be most audible or does it all equal up to about the samething?
I feel there is something to this feedback thing. Even though the no negative feedback amps do not offer up the "Slam" of feedback amps, they do seem to sound(for lack of a better description) more tube like. However, I can certainly see why someone might pass on the zero feedback amp if running it full range. The bass is a little light compared to some.
A really good example was when I compared the Parasound JC1's(39db of global feedback) to the 0 feedback Ayre V5xe amp (yes, I owned both and still have the Ayre.) Yes, the JC1 sounded more visceral but it also sounded more "Hi-fi-ish" overall. The mids and highs portrayed by the JC1 just didn't sound totally realistic. The Ayre was more liquid and harmonically rich. Also, the portrayal of the soundstaging cues seemed more fully developed through the Ayre (deeper stage cues vs a somewhat flatter stage presentation.)
I use subs, so the bottom end is not that much of a concern with me. Soundstaging, natural mids/highs and liquidity is.
Can this be directly related to the feedback or is it more of my personal preference to one sound over another or do I need to look at other parameters? I kinda would like to know why I hear what I hear or if it is truly just a personal thing and has nothing to do with design.
For general information, the amps were both broken in and used on 20 amp dedicated lines(2 in the case of the JC1.) I listened to both at around 85-90db or lower and my impressions stayed the same. I guess what I'm trying to say is the JC1 was kind of like a GTO, Camaro or whatever (lots of power but a touch short on refinement) where the Ayre was more BMW like(The Theta Dreadnaught also seemed this way-another zero feedback amp.)
BTW, this is not to disparage anyone who owns the JC1's but is related in context to design and its effects. This was my take over an extended period in my room with my equipment.

Local feedback is a correction network for only a small segment of the circuitry whereas global feedback compares the signal at the output of the amp to the input of the amp and tries to correct accordingly. As such, there is less to monitor and less to subjectively correct in the local feedback design. On top of that, by taking care of each mess individually at a local level rather than trying to fix everything collectively at one time, less total feedback can be used and that amount is spread out amongst the various gain stages. In effect, local would be deemed "faster" and "less detrimental" overall. To my ears, this is both audible and desirable from a sonic standpoint.

The drawback here is that the output impedance of the amp will always be higher when using less or no feedback in the output stage. The output impedance being higher directly relates to a reduced damping factor. While some think that damping factor corresponds to how much control the amp has over the driver, that isn't really technically correct.

Damping factor corresponds to how much the speaker can modulate ( temporarily distort or even take control of ) the output of the amp. A higher damping factor means that this potential is reduced. While some would say that this equates to increased amplifier control, and it does in an indirect sort of way, the only thing that keeps the amplifier in control of the speaker is the ability for it to load more voltage and current into the driver than what the driver is generating in reflected EMF ( Electro-Motive Force or "voltage" ).

All drivers generate some amount of "electrical backlash" when a signal is applied to them, hence the term "reflected EMF". Larger drivers with more mass and motor structure and / or drivers that are making longer excursions generate more reflected EMF. That's why the theory goes that with big speakers, one needs a big amp. A big amp typically has more voltage and current potential than a smaller amp, so it theoretically should offer greater driver control too.

The thing with tube amps is that they typically have a very high rail voltage, which means more resistance to the reflected voltage from the amp. The only problem is that most tubed amps suffer from a lack of current capacity and have a higher output impedance. The lack of current can cause the output rails to sag, lowering the amount of voltage available to "ward off" or "fight back against" the reflected emf. On top of that, the higher output impedance makes the amp far more susceptable to having the output stage "modulated" by that same reflected emf. As mentioned above, the more excursion that a driver takes, the more reflected emf that you have to deal with. This is why many tubed amps sound "warm & rich" at low volumes but "loose & flabby" as spl & excursion are increased. As a side note, the more that the output stage of the amp is modulated by the reflected emf, the more likely distortion is to occur further up in the driver stages too. In amps with poor stereo separation, the one output stage can actually modulate the other channel of the amp, from beginning to end.

There are ways to play games with the output impedance on any given amp without introducing gobs of negative feedback though. The simplest and most common method is to use more outputs. Since the outputs are in parallel with each other, the output impedance of each device is reduced by a significant percentage each time another device is added to share the load. Only problem is, now you've got a ton of output devices, making it harder to get them all to work in unison at the same time.

In this type of situation, the poorer the matching of output devices, the more smearing of signal with a reduction in "blackness" between notes. Various types of distortion can climb too, so many amp designers will simply tack more negative feedback into the circuit to help keep things under control. In effect, one bad implimentation in the design requires more detrimental circuitry to cover it up. This is one of the main reasons why many people think that smaller amps sound better. They can have shorter signal paths, require less error correction and there aren't as many parts to worry about.

Another approach to lowering the output impedance of an amp is to lower the value of the emitter resistor. This increases the current flow through the output stage and develops more heat though. On top of that, if the output devices are running out of balance due to poor parts matching, the increased current will aggravate the situation and cause an early death. Like most things in life, free lunches are hard to come by and most everything requires some type of decision / design compromise.

As one can see, there is somewhat of a reason why we have the sonic differences that we do between no / low negative feedback amps and those that use quite a bit of it. Obviously, the best way to do this would be to build and impliment the best circuit that you could using all hand-matched parts of extremely high quality and then use feedback only as needed in very small quantities at the local level.

Wanna guess what the drawback is here ??? Cost and labor. This is why the amps that sound REALLY good tend to cost a LOT of money. While i'm NOT trying to justify the massive prices on some of this gear, you simply can't make a hand built and tuned component as cheaply as you can a mass produced component.

In terms of spaciousness of presentation, my experience is that low / no negative feedback amps ALWAYS sound more spacious. As mentioned above, feedback tends to slow down the response of the amp. That reduction in signal processing speed comes in to play when trying to reproduce the harmonic overtones of the original signal. The reduced harmonic overtones not only changes the timbre of the instruments, our ears use the time differences between the primary note and the harmonics to place the origin of each note within the soundstage. If we lose the harmonic structure of the note, we also lose spatial cues too.

One of the ways to get better bass out of any amp is to use an iron core transformer, a very short signal path and gobs of power supply reserve ( filter capacitance ). On amps that use output inductors, increasing the gauge of that inductor also allows it to pass more instantaneous current, giving it better bass slam. Having said that though, high frequency response can suffer, so you have to juggle the gauge vs desired bandwidth. This is why some amps are great for bass but less desirable for treble, etc... Many low / no feedback amps do not use an output inductor, which can be both beneficial in some areas and detrimental in others.

It has been my experience that amps that have all of these design attributes ( with a reasonably low output impedance ) all have pretty solid bass response. Lowering the cut-off frequency of the amp or DC coupling it is also a good thing in this regards, as the higher the cut-off frequency of the amp in the bass region, the less articulate the bass is due to ringing from the filters occuring.

Obviously, a lot of the above information is subjective at best and how much of it applies to any given amp or circuit depends on how that amp or circuit is designed, implimented and constructed. There are so many variables involved here that it could make your head spin. As such, i'm hoping that this has covered some of the basics of that area and will give you folks more to think about and digest. There is no "magic circuit" at this point in time, only design variations that each designer / manufacturer thinks is worth giving a chance. Sean
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Makes sense. I know in the Ayre amp, Hansen does use a lot of handmatched output devices. On the other hand, I don't know how he gets the amps noise floor so low and the "Blackness" between the notes with this particular zero feedback design. I do know this is one of its strong suits along with a simply holographic soundstage. He recommends against using any power conditioner.
The other point is it clips like a tube amp, fairly gradual and the distortion figures are relatively high for a SS amp at max output (something along =>1% at max power.) The amp does double down and is stable into low impedances(a good thing for me since my speakers do dip near 3 ohms at a few points.)
Now, another question, I have heard that feedback alters the "Phase and time" relationships of the input signal(global feedback) if you look at what arrives at the output in comparison. Is this true? Is this what you are refering to above talking about delay? Would no neg. feedback amps pass a truer duplicate of the input signal(based on other parameters remaining the same?)
Just in passing, I was in the smaller amp camp until a year or so ago. I biamped using smaller amps. They just sounded better overall(provided they were high quality to start with.) It seems bigger amps have caught up now ( at reasonable prices) and some are truly excellent.