Highend64: I have worked with many of the amps that you have mentioned. I would not consider any of them suitable ( sonically ) for a hi-end audio system. However, they do work fine for PA type situations and that is where i've used them.
As far as frequency response goes, that is typically taken at 1 watt. Power bandwidth is the same as frequency response but the test is conducted at the rated output of the amp at a given impedance ( typically 8 ohms ). For an amplifier, power bandwidth is what one is looking for.
Having said that, most pro amps will have limited response above 20 KHz and below 10 Hz ( give or take ). There are reasons for doing this in pro situations that one typically does not run into in home audio. If an amp is purposely filtered or rolled off at 20 - 30 KHz, the limited high frequency response will cause measurable distortion at frequencies as low as 2 - 3 KHz. Not only is this audible in many different ways, it is clearly visible on an oscilloscope. Typical audible signs of this type of design are that the upper midrange sounds cold and hard with increased glare and sibilance, treble is brittle and piercing with increased smear, etc...,
Just like the high frequency response problems introduced by using filters or "narrow bandwidth circuitry", the results in the bass range are also audible and measurable. If bass response is purposely filtered or rolled off too early, bass impact and definition are reduced and ringing is increased. The upper or "mid-bass" tends to sound "bloated", "congested" and seems as if you keep hearing "one note".
For the record, reduced bandwidth instantly means reduced transient response. Wide bandwidth and fast transient response walk hand in hand with each other. You can't have a wide bandwidth circuit with good linearity and stability if it is not "fast enough" to reproduce a signal that is very high in frequency. The better the amp responds above and below the audible range, the easier it is for it to respond in-band. Kind of like a car that can easily travel at great speed. Such a design is literally "coasting" at anything that most of us would consider "normal" operating ranges.
As far as negative feedback goes, it can be used in a positive manner to increase circuit stability, linearity and reduce distortion. If used in very limited quantities, it can be beneficial. The problem is that many engineers / designers count on the "benefits" of negative feedback, design a sloppy circuit that is both cheap and easy to produce and then expect the errors to be corrected by swamping the design with gobs of negative feedback to obtain "good" measurements. Like anything else, too much of a good thing becomes a bad thing. Since pro amps are more worried about stability and linearity ( in terms of gross errors such as overdrive and clipping ) and less about sonics, this approach is okay but unsuitable for "quality" audio reproduction. As such, negative feedback is NOT the "key" but only a microscopic portion of good amplifier design.
It should be noted that many amps ( both consumer audio and pro audio products ) make use of all of the above. As such, you can end up with the problems that each type of circuit ( extreme lows rolled, extreme highs rolled and gobs of negative feedback ) brings with it PLUS various combinations of all three. This is the kind of SS gear that measures "good" ( by most common specs ) but sounds "cold, hard, lifeless, sterile, unmusical", etc... You get the idea.
As far as the Pass X-250 review goes, i came away with the impression that he was listening to the amp / system at the same volume that he hears bands play in the studio. Sometimes this is loud, sometimes it isn't. He specifically stated that sound coming from the Pass was directly comparable to what he heard when the bass player was actually playing those notes plugged into a sizable bass rig in the same studio.
Besides that, the sound heard through a Pass as compared to a Crown, Crest, QSC, etc... of similar power rating would be MILES better in every respect. Not only is the circuit more linear over a wider bandwidth, the amp is capable of far greater current output at any given impedance than any of these designs. This results in more control over the drivers, increased dynamics, lack of compression due to increased headroom and reserves, greater bass authority ( especially as impedance is dropped ), etc... This is besides the far smoother, airier and delicate mids and highs that newer Pass amps are famous for. Believe me, there is a reason why he was impressed with the X-250, especially if he's only been exposed to pro type amps. The reason that these amps would never make it as a pro sound piece is that they are very expensive, very heavy and run hot as hell. All of these factors are things that NOBODY in the pro sound arena would be willing to work with when they can get 200 wpc in a package that weighs 20 lbs, costs $250 and runs cool as a cucumber.
Twl: High damping factor is not necessarily a result of high negative feedback. A high damping factor equates to the output impedance of the amp being VERY low. The greater the contrast between the output impedance of the amp and the terminating impedance of the load, the higher the damping factor. Since impedance varies with frequency on a loudspeaker, so does the damping factor of the amp. As such, running an amp with a "nominal" 8 ohm speaker would result in a higher damping factor than if running the same amp with a "nominal" 4 ohm speaker. In effect, the damping factor would be cut in half with the 4 ohm speaker since it is only 50% of the load impedance of the 8 ohm speaker.
As a side note, most SS designs have an output impedance that is WAY, WAY below 1 ohm. SS amps that measure into the 0.0X range are not uncommon when it comes to output impedance. on the other hand, many tube amps have output impedances as high as several ohms. As such, there is less of a difference between the output impedance of the tube amp and the speaker than there is with the SS amp and the speaker. This can "load down" the tube amp, resulting in muddier bass that lacks definition and suffers from "bloat" or "roundness". The closer the two impedances ( amp output impedance and speaker load impedance ), the greater the chances of this occuring. Hence, most tube amps do worse with lower impedance speakers than higher impedance speakers in terms of bass response / impact / defintion. Many well designed SS designs are relatively immune to the differences in load impedance although there are always exceptions to any given "rule" ( more like "generalization" ). Sean
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