Aragon 4004 mk2 vs Adcom 555 mk2

Ritteri here's an section of an article I came across done by Doug Blackburn of Soundstage.You can go to this address and read the entire article .Of course this is one persons opinion. http://www.soundstage.com/maxdb/maxdb101998.htm

Feedback in low-level gain stages – It is becoming increasingly clear that less feedback rather than more is musically desirable. However, there is a point where you have to stop removing feedback due to some element(s) of the whole presentation falling apart, like the quality of bass. Too little negative feedback is big trouble for bass quality. How much is too little and how much is too much? I’ve never been impressed with zero negative feedback in an amplifier. A little bit of feedback, even as small an amount as 2dB to 4dB in one gain stage (out of three) is enough to keep the bass in line while not enough to do harm to the loveliness of the mids and highs. Too much is easy to identify. Think sibilants. You hear too much emphasis on sibilants? Chances are the amplifier you are listening to has way too much negative feedback. In fact, a good portion of what at one time was known in high-end land as "solid-state sound" was strictly an artifact of silly-large amounts of negative feedback. It is not entirely inaccurate to think of amps using a lot of negative feedback as the uptight conservative businessman of the audiophile world -- too inhibited to ever have a really good time. They are too controlled, too unforgiving, too dry, too emotionless, too uptight. They are certainly lower in total harmonic distortion than low-negative-feedback amps, but what does it matter if the music is as crisp and colorless as fall leaves in winter?

While zero negative feedback is too little, 10dB of global or even local (within a single gain stage) is often too much. Think 2dB to 6dB of local feedback for most amplifiers to do their thing with high levels of musicality across the entire musical spectrum. You can do "stupid amplifier tricks" playing with less or more feedback, but in the end, they all eventually sound "tricky" while the very moderate amount of feedback sounds like music. It’s typical for solid-state amplifiers to have 20dB to 60dB of negative feedback. Are those guys actually listening to what they are building?

There may very well be some wailing about my pronouncement that 2dB to 6dB of local negative feedback with no global negative feedback is "enough" coming from the amplifier manufacturer/designer community. Don’t trust them. If they think they need more feedback than that to get good sound, they just aren’t listening, or they don’t know how to resolve the problems that will crop up (sonic problems) when they eliminate most of the feedback they are used to using. That doesn’t mean my concept of the right amount of feedback is incorrect. It just means that some manufacturers/designers aren’t going to know how to incorporate that small of an amount of feedback into their products. The manufacturers that can build great-sounding solid-state (or tube) amps with low amounts of negative feedback without suffering muddiness of sound and woolly bass have a major sonic advantage that you will/can recognize once you have heard it.

Is having adjustable negative feedback a good idea? I have seen several tube amplifiers which have user adjustable negative feedback. On the surface, this seems like quite a worthwhile feature. It certainly is educational to hear what happens as you change the amount of feedback. However, once you begin trying different discrete resistors that are soldered into a negative-feedback loop -- well.... Let’s just say that the resistors in the negative-feedback loop are impressively obvious when changed, either in value, manufacturer or material. I cannot imagine any potentiometer added to the feedback loop would do anything but sound really bad compared to a single properly selected high-quality resistor. In fact, having heard how critical the resistor in the negative-feedback loop is, I can’t imagine a worse place to locate a potentiometer than in the negative-feedback loop. Compared to a good resistor, the best potentiometers sound quite bad. Use of a stepped attenuator might eliminate a lot of problems caused by sonic limitations of potentiometers. However, the negative-feedback loop is a very sensitive area of the circuit, I’m not sure if mechanical contacts in the stepped attenuator could ever be good enough to be sonically inconsequential in this location.

Two things:

1. The feedback loop would not use a pot if the design is worth anything as the guy suggested. The switch, as a designers opinion, switches high-quality resistors in and out - doesn't use a pot so that argument is not worth reading. He is right - pots sound terrible, that is why they are rarely used or done so as to no affect the circuit.

2. The feedback switch is so that if you are using inefficient speakers, the amp will be stable with higher feedback. A zero/low feedback design can become unstable very very easily if the right (rather, wrong) conditions arise. So the switchable feedbacks are to match speakers - not correct sound. Arthur

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".

Yep, anything in excess is bad, feedback included. Yet, I don't understand the hang up that audiophiles have with feedback. When I say feedback, I mean GLOBAL feedback. It has a zillion advantages that any engineer can name off for you because in the rest of the electronic world, feedback is absolutely critical in just about every circuit I can think of. In audio, I don't think feedback causes any detriment to the sound per se. I have looked this up several times and the post Gmood pasted above has too many inconsistencies to be reliable IMO. Local feedback is present in 99% of amps. I talked to Nelson Pass and what he calls "zero feedback" means he has local feedback but no global. This is fine however no one ever seems to complain about local feedback sounding poor. Kind of funny that guys who buy zero feedback amps for the name are actually listening to high local feedback designs and proclaiming its benefits. An amp with no feedback at all, will crash and burn with most speakers. I guarantee it. I have seen it in the lab many many times. So, before complaining about feedback sounding bad, consider what I have said. There is probably an exception out there somewhere (meaning no NFB at all) but we probably wouldn't buy it due to audible oscillatory reactions with the speaker's reactance. Arthur

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.