If I could make a suggestion, listen to the QSC amp and listen to a Krell amp. If you like the QSC amp better buy it, if you like the Krell amp better buy that. It all comes down to what you like best. Many people like different gear for different reasons. There are some very good amps out there that I will not buy because of poor service, etc. It really comes down to your personal choice because when you take it home and power it up it has to please you.
It all depends on the application. Pro gear has different goals in its design than much hi-fi gear.
I think the line is becoming blurred between so-called "pro" gear and consumer products -- at least in terms of sound quality. There are companies that have made mostly "pro" gear that are now making consumer products, and vice versa. Two examples that comes to mind are Bryston, which for many years was regarded mainly as a "pro" gear manufacturer (but is now highly regarded as a maker of high-end gear for the home), and AudioControl, maker of equalizers now used in cars, recording studios, and the home.
With the widespread success of home theater, many buyers want the reliability of "pro" gear with the features and sound quality of good home gear. I think we will see this cross-fertilization between pro and consumer gear continue.
Pro gear typically has far more protection circuitry, is narrow bandwidth, much slower in transient response, uses a lot more negative feedback, etc... resulting in what most consider to be "sterility". Most pro designs and design philosophies that are in current use are the same that they used in the "THD wars" i.e. "if it measures fabulous, it will sound good" or "if they measure good, they will all sound the same". This approach is what gave much of SS a bad name in terms of sounding "hard, lifeless and sterile".
To be fair though, there have been changes amongst many pro designs. Most of the changes have been to the power supply i.e. making them lighter, smaller, more efficient, easier to transport and cheaper to produce. As such, bass attack and sustain have suffered as well in most of these designs. There are pro power amps rated for 200 wpc @ 8 and 200 wpc @ 4. What does that tell you about the power supply and current capacity of the output stage ??? Yet it is one of the best selling pro amps to come down the line as it is relatively cheap, highly reliable and comes from a major manufacturer with a lot of industry clout.
When it comes to hi-end audio reproduction, there is no substitute for balls out circuitry that doesn't need protection "band aids", is designed with finesse and uses high grade parts. Pro power amps simply have to turn on, stay on and take the abuse. If it sounds like crap, you've got compressors, EQ's and a rack full of effects to try and make up for it. Different products for different markets.
As such, we have two different types of products that are marketed according to wants and needs of that specific industry. If you are strictly looking for "oomph" and rock concert reproduction, buy and use a pro amp and the associated equipment to take advantage of such power levels. If you are looking for something that sounds far more refinined, is capable of finesse and detail at any given listening level, you'll have to step up to the plate and buy an amp that has more thought put into it than just assembling a power supply, output stage and adding protection circuitry. Sean
Here's a pro audio guy who tried an "audiophile" Pass power amp. He was "astounded"pass amp in a "pro" audio review
Cdc: That's a good link. If you want to see what "pro's" think about an amplifier and what they expect out of them, read some of the other reviews on that website. Like i said, so long as they power up, stay up and don't present a hassle to use, they are "good enough". Sean
Sean, I would have to agree that QSC and Krell sound different from each other. They are both meant to perform different tasks, but you saying that "pro" gear have slower transient response and narrower bandwidth is ridiculous. In contrast, all pro amps have a very high damping factor. In fact, pro amps should have a very high damping factor because it should be able to control high mass cones at high volume. I don't know what you meant by "narrower bandwidth" but I know that power-bandwidth is different than frequency-bandwidth. Lets talk about the difference. Frequency-bandwidth corresponse to an amplifier's frequency response at 2.83 volts input. On the other hand, power-bandwidth corresponse to an amplifier's frequency response at high output level(usually below clipping). In reality, the one is most interest is the power-bandwidth. Yes, Bryston was used as a "pro" amp. In fact, some studio today still use them as there "reference amp". Cdc, I have read your article on the Pass Lab X-250 I was impressed. Judging by his statement, he probably use it at a very low level. Go read about QSC Powerlight Series, Crown K Series and Macro-tech, and Crest amps. Just like there is alot of hi-fi amps, there is pro amps.
Just in case you weren't aware, high damping factors are a direct result of high negative feedback. A serious no-no in audiophile reproduction.
Twl, now why is it a no-no to audiophile reproduction? Do you know what negative-feedback does? Mostly all audiophiles hate to have negative-feedback because it effects the sonic characteristic of sound. An open-loop gain in an amplifier rely mostly at the input stage. A disadvantage about open-loop gain is that a substantial change in input voltage can dramatically change the output stage into nonlinar. This is where negative-feedback comes into play. If an increase of positive feedback is fed back to the input stage, it increases the output gain. In contrast, if there is a increase in negative feedback it reduces the output gain porportionaly to input. Negative feedback is key answer. Negative feedback, also called degenerative feedback, reduces THD, improves the frequency response, and the stablility of an amplifier.
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
I have very limited experience with negative feedback, but I'll share it anyway. On my VAC 70/70, there is an adjustable negative feedback (either 0 or an assortment negative feedback, I think 6, 7, 8, 9 or something like that).
Anyway, the zero setting sounds WAY better. Negative feedback sounds dry and constricted, even after the volume is adjusted to compensate. The zero NFB has so much more life, is richer, fuller, so much more enjoyable. I would never use the NFB, and wonder why there is even a switch on the amp to increase it.
Dennis: It is possible that a circuit can become "overly dynamic" sounding due to non-linearities in gain characteristics. In other words, very small differences in input drive levels can be amplified at a much greater rate than levels just barely below that. This has to do with how linear the gain curve of the circuit is to begin with. Since most all circuits will have some type of "knee" in the curve, there is a point where more drive equals more output ( sometimes drastically so ) and then you start to hit the point of diminishing returns ( compression ).
To top off the potential for exaggerated dynamics, this can take place at various rates across the frequency band. Bass might REALLY tend to jump out as drive is increased whereas treble might not be amplified at the same rate. As such, it can change the entire presentation of a recording and a system.
Bare in mind that there are different ways to impliment negative feedback and not all are created equally. I would tend to agree that it should be used extremely sparingly though, if at all. Most all amps do have some type of feedback or "error correction" circuitry built into them though. This is even more true for professional designs as they tend to face harsher environments in terms of the load presented to them, the volume required and drive conditions.
Like i said, the "average" commercial / pro sound amp is a very different beast from the "average" hi-end amp. And for good reason. Sean
Sean, How is it possible to have measureable distortion at 2-3khz if the amp's response can go up to 20khz? You know that we hardly ever hear 16khz. I understand that a musical program around 1-3khz will have harmonics in the upper-frequency. Having odd harmonics in the upper-frequency is not a good because it can sound harsh, lifeless, and colored. In contrast, even harmonics is what makes the presentation more musical and life-like. An oscilloscope is not really use for distortion but it can be use to analyze the wave form. If you want to find the THD, SID, or TIM, you need a RTA and a Distortion Analyzer. Of course, your ears are your main priority. BTW, I'm not against you or any thing, I just disagree with some of your answers so don't take it personal.
Sean, have you heard some type of a transformer placed between high output impedance amp and speaker. Audio Asylum had a thread on this maybe 2 weeks ago. Looked like a good way to improve damping factor. Any thoughts?
I will try to find the thread but it may not be easy.
Also, I tried and tried to find your comments about line arrays but couldn't. You had mentioned some inherent problems with the design - don't know if it was imaging, comb filtering, etc. but could you describe again. Thanks!
Highend64: An amplifier that is bandwidth limited to just above 20 KHz will show "rounding" of the leading edge of a square wave at appr 2 KHz. That is a distortion as far as i'm concerned as it is not replicating the input signal. Passing a sine wave can be "gravy" but square waves are much faster, require greater linearity and are closer to the dynamic impulses that we call "music".
Cdc: Line arrays are like any other speaker i.e. they have some advantages and some disadvantages. If done correctly, they can have some very strong advantages such as increased power handling, lower distortion due to reduced excursion, high spl capacity, sustained spl levels at greater distances, etc... Obviously, this design works best if not sitting right on top of the speakers.
If i can ever get my act together, i've got a very large line array that i'm working on for a friend's system. He is brutal on audio equipment to say the least, so after thinking about it, i decided on a line array for the above reasons. The mains ( this is a combo 2 channel / HT system ) will consist of thirty six tweeters, eighteen 4" mid-woofers and eight 12" woofers per side. This is a "budget" project in terms of the drivers selected, but i'm trying to do the best i can for him and his specific system needs and give him something that sounds pretty decent, will hold up to the chronic abuse that he dishes out and can literally shake the house on demand.
As such, i've had to do some studying on the subject and am learning more as i go along. One thing is for sure though, crossovers need to be done actively OR passively at a very sharp slope rate if you want to avoid comb filter based problems. I've been able to learn quite a bit about this subject courtesy of the "speaker nuts" on Madisound and their own personal websites that they have provided links to.
If i'm lucky, i may be able to get these speakers done in a few months. As you might guess, it is going to be VERY labor intensive in terms of just cutting all of the holes let alone installing and wiring all of the drivers. To top it off, i've got to do a center and two surrounds for him too. Needless to say, the other speakers will NOT be anywhere near as big as the mains but they will all make use of identical multiple drivers in order to maintain consisting "voicing" within the system.
As far as using a transformer between an amp and speakers, i don't like the idea unless it is a necessity. I personally prefer active multi-amping as this allows one to bypass passive components between the amp and individual speaker drivers. It has been my experience that "direct drive" sounds far more natural and dynamic than what you can get out of even the very best passive crossovers. Sean
Interesting posts, Sean. Thanks.
Thanks Sean, I agree on the active speaker vs. passive.
Cdc: I was not so much talking about active speakers as i was active bi / tri / quad amplification i.e. no passive crossovers between amp and speaker drivers.
While i do think that active speakers could offer benefits in the fact that specific ( corrective ) equalization curves could be factored into the amplifier design. The only problem with this is that one is stuck with the design attributes of that specific amplifier. Personally, i prefer the options of picking and choosing amplifiers as i don't think that they are all created equally or sound the same. Active speakers don't allow you to "pick and choose". Sean