headroom?

[MikeyD]

Originally posted by Spacegoat
There are marked sonic differences between different amps with the same power ratings. Part of it is due to different test criteria as has already been pointed out. I'd like to see standardized test conditions for all amp manufacturers under ISO. {Lots of good discussion snipped...}

Good comments. I don't think there's any magic going on here, though. I believe that most differences in performance can be measured and quantified. As you suggest, a good, common set of standardized tests could be performed to give people a better idea of what to expect from an amplifier.
In your example of the Crowns, I wonder whether damping factors were similar (and your comment about reactive loads was right on, BTW). I suspect differences in power supplies might have the biggest effect on short-term power: a supply which stores lots of joules locally will probably "thump" much more cleanly than one with a switching supply and/or small energy storage. (And then we get into issues about the power line itself - it's part of the system!) Again, I'm sure these could be quantified. However, it gets to the point where one almost needs a degree in electrical engineering to really understand them - which then becomes counterproductive to most musicians. Probably the solution to the problem is to have, say, 5 or 6 performance measures and then some "words" about the design of the amp (tube/SS, switching PS or traditional, dual independent PS or single, etc.). But I think a number of us would like to see the manufacturers give us MORE info than they are giving now. That way, if someone does not understand, he/she can certainly post questions and people with technical knowledge can help.
- Mike

 

[Mark Reccord]

Mike,
I'm working on that EE degree, and they're trying to kill me! Actually the K2's damping factor is considerably higher than the macrotech's(k2-3000, macro-1000), these are Crown's numbers. I usually see DF numbers on the order of 10-40 so I'm assuming that Crown is just using units 2 orders of magnitude down from everybody elses...What are the units anyway? I never see them listed or is DF a unitless thing? The fact that the k2 has a higher DF is sort of how I deduced that there were other factors at work. Then I did some reading about switching supplies vs conventional supplies. You're right about the energy reserve thing, according to the literature I've read. Is it right to say that an amp with a conventional supply reacts faster than one with a switching supply? Slew rates are typically higher on conventional amps, which would semm to support that. But that would seem to be more of a high frequency factor.... I've seen the same phenomenon on other brands of amps too. Qsc powerlite vs MX series for example.

 

[ROCKBOBMEL_old]

I think you guys are going right where I need. I traded uo to a Navigator pre, and a WT 1000 power a,p, but the WT1000 is back ordered. I got a QSC PLX 2402 instead. This amp (switching) gets lost on notes when the band is in strong passages, and on very quiet parts, the notes get away from me. I have a real tough time controlling the notes. I'm going to get the Eden when it comes in. What a lesson!

 

[MikeyD]

Originally posted by Spacegoat
I'm working on that EE degree, and they're trying to kill me!

Been there! I studied acoustics and ME mostly, but took a number EE courses. It's a tough course. Where are you studying?

Originally posted by Spacegoat
Actually the K2's damping factor is considerably higher than the macrotech's(k2-3000, macro-1000), these are Crown's numbers. I usually see DF numbers on the order of 10-40 so I'm assuming that Crown is just using units 2 orders of magnitude down from everybody elses...What are the units anyway? I never see them listed or is DF a unitless thing?

Yes - a unitless measure. Take a look at 2nd-order systems (e.g., resistor-capacitor-inductor or dashpot-spring-mass) and resonance/ringing phenomena. The damping factor is a derived quantity and can express overshoot. A well-damped system has no overshoot. I'm surprised at the numbers you cite. Most big power amplifiers have DF's in the hundreds, if not thousands. For example, QSC's PLX series is claimed to have DF > 500 at 8 ohms, according to QSC literature. I see on Crown's site that the K2's DF is >3000 at 8 ohms.

Originally posted by Spacegoat
The fact that the k2 has a higher DF is sort of how I deduced that there were other factors at work. Then I did some reading about switching supplies vs conventional supplies. You're right about the energy reserve thing, according to the literature I've read. Is it right to say that an amp with a conventional supply reacts faster than one with a switching supply?

I'm not really sure; if the full energy reserve is immediately available to the power stages, then the amp whose energy reserve is greater will be able to handle transient power demands better. This is why people sometimes add big capacitors to augment their power supplies' short-term energy storage. I don't know the topology of switching supplies very well, but my guess is that you are right: they don't have the level of immediate energy available that a similar conventional supply would.

Originally posted by Spacegoat
Slew rates are typically higher on conventional amps, which would semm to support that. But that would seem to be more of a high frequency factor.... I've seen the same phenomenon on other brands of amps too. Qsc powerlite vs MX series for example.

I agree that slew rate is probably more pertinent to high frequency response. Another thing I've learned about recently has to do with the ability of a high-powered amp to deliver rated power continuously. When I was checking out the bigger QSC and Crown amps, I found that there was no way they could indefinitely provide rated power, given that a standard outlet (120v, 15A) can only provide 1800 watts RMS steady state. Turned out that this rating was really for short-term power (or, more to the point, the ability to handle transients of a typical program signal averaging 1/8 the full-power rating). A long, sustained note at full output would probably start to weaken once the power supply exhausts its reserves and its internal voltages sag.

Thanks for your reply. Good discussion!
- Mike

 

[MikeyD]

Originally posted by ROCKBOBMEL
I think you guys are going right where I need. I traded uo to a Navigator pre, and a WT 1000 power a,p, but the WT1000 is back ordered. I got a QSC PLX 2402 instead. This amp (switching) gets lost on notes when the band is in strong passages, and on very quiet parts, the notes get away from me. I have a real tough time controlling the notes. I'm going to get the Eden when it comes in. What a lesson!

Thanks for the feedback. It's kind of hard to imagine the 2402 getting "lost" - at least on stage (unless you have horribly inefficient speakers) - it's a powerhouse. If you are saying that the amp seems to compress loud passages - that's very interesting. See my response to Spacegoat. If, during the loud passages, the amp is not getting sufficient voltage from the power line, I could definitely see its output fading after some finite time at or near full power. This is an issue I've had to explore recently myself. When I ran both my Carvin heads bridged simultaneously (output potential 1350-1600 watts), I watched a nearby nightlight dim noticeably during loud passages or transients. This showed the line voltage sagging. That meant that the power line could not remain "stiff" under such heavy load conditions. The message here is that when we bass players start thinking about mega amps (upward of 1000 watts or more) as a solution to our power problems, we now have to start thinking about whether our outlets are truly up to the task of feeding these voracious monsters. As well, the power/extension cords must have lots of copper to minimize the line drop. This is no different than racers' putting huge engines in their cars to make them go faster. It's one piece of the puzzle - but then they have to install bigger fuel pumps, stronger transmissions, etc. to deal with the additional power output. This is why I'm satisfied that I'm done with seeking more power from my amps. If I continue to have trouble getting enough sound, I must now turn my attention to speaker efficiency.
- Mike

 

[Skip]

Originally posted by brianrost

Originally posted by MikeyD
I agree - wouldn't it be nice if BP or a similar publication commissioned such lab tests on bass amps and speakers? That would be ultra cool!

It might be counterproductive. It works in hi-fi audio publications because in general you're looking for transparency in the product.

If BP tested an SVT and a Walter Woods, the SVT might look pathetic spec-wise compared to the Woods and the specs don't help explain what the amp SOUNDS like very well.

(clip)[/B]

Actually, one of the big nitches in High End right now is SET amps. That's Single Ended Tube amps. These amps are usually driven by tubes designed before WWII. They are very low power (some as low as a watt or two) and don't measure anything like flat.

But they produce lots of low order distortion, especially second order distorsion. And, as the technical reviews of these amps point out, there are studies showing that distortion that decreases with each order sounds quite good to most people. That doesn't mean they recommend these amps to everyone - there is no bass and it makes everything sound warm and fuzzy regardless of the type of music. But for some people they make magic.

The same is true for bass amps, a SVT is not for everyone, nor is a Walter Woods. But I want to know if they can actualy deliver what they promise in wattage and if a "high end" amp is as clean as they say they are.

I'm not asking for an "Audio" style multi page layout. Or even a "Stereophile" stlye multi-page sidebar (especially since the reviews are a paltry one page long). But a short sidebar would be nice. Saying what power levels it tested to and if there were any other things we need to know as a user or in auditioning the amp. You could do a world of service in the space where they summerize the scores and list the price - which are all repeated elsewhere in the article anyway.

Sorry about being so far off topic.

 

[Mark Reccord]

Mike, I'm studying at Memorial University of Newfoundland, in Canada. The school's best known for its Naval Arch and Marine Dynamics programs, but the EE program is also quite good. I'm only in my first year (I must be nuts starting this at 28!). Been working in pro audio for about 8 years. My mistake on the DF thing, one paper used numbers like 10, 20 etc. The author said it was for simplicity?!?! So I read some different papers, hehe. They All used the larger nubers like the ones Crown quotes. Long cable runs apparently have a huge effect on DF. I know all about signal loss in speaker cables, but I didn't know it affected DF! I gotta run, exams next week, crazy assignments due! Cheers!

 

[Mark Reccord]

Oh, we used two single-phase 30A 110/220 circuits per rack of macrotechs!

 

[ROCKBOBMEL_old]

Sorry, I omitted speaker mention. I using a Hartke 4.5xl 8 ohm cab, (good eff.) and an Acme LowB-2 4ohm (bad eff.) They are about the same volume with the balance in the middle. I would blame them if I didn't already have a taste of the Eden WT800 (400 x 2 @ 4ohms). So what I mean is the PLX does not hold together as well as the Eden for low end power reserve. The PLX probably has more overall, but the Eden is a better sounding bass amp, including tone.

 

[ROCKBOBMEL_old]

I mean the PLX probably has more overall power.

 

[Brad Johnson]

Originally posted by brianrost

Originally posted by MikeyD
I agree - wouldn't it be nice if BP or a similar publication commissioned such lab tests on bass amps and speakers? That would be ultra cool!

It might be counterproductive. It works in hi-fi audio publications because in general you're looking for transparency in the product.

If BP tested an SVT and a Walter Woods, the SVT might look pathetic spec-wise compared to the Woods and the specs don't help explain what the amp SOUNDS like very well.

On the other hand, I wish there was SOME testing, like does the unit actually meet the manufacturer's published specs (GP used to do these sort of tests on amps up till the late 80s).

One good example of total confusion (for me) is the claims made by BP about Acme speaker cabs, namely they need "gobs of power". In recent reviews they found a 60 watt (!!) Allesandro tube amp (not a cheap box, but decidedly LOW power) drove one to "acceptable levels" and they said the same about the 120 watt Acoustic Image Contra amp. So what's with these amps that's different than other amps with less than 1000 (!!!) watts that supposedly can't drive those cabinets? Or is it just that the reviewer's idea of "acceptable levels" is poorly defined? I would sure as hell like to know!!!

This is exactly why I advise people to take printed equipment reviews, especially BP's, with about a pound and a half of salt. Since you rarely have a base line (who is testing the gear, what are their personal preferences? (this is unfortunately key)) and all the other variables, you don't know way too many things. The same feature that's missing on one amp or bass is not an issue on another one...why (or why not?). Too much of a moving target for me. The BP reviews are good for giving me a photo or two, the retail price and some manufacturer info, and a basic exposure to a product. The ultimate judge will be you but some of these reviews are borderline. Some aren't bad. You just have to figure out which is which.

 

[Erick Lam]

It means having the ceiling high above your head to have enough room to stack a bunch of cabinets on top of each other.

 

[Mo' bass]

LOL!

 

[MikeyD]

Originally posted by Spacegoat
Mike, I'm studying at Memorial University of Newfoundland, in Canada. The school's best known for its Naval Arch and Marine Dynamics programs, but the EE program is also quite good. I'm only in my first year (I must be nuts starting this at 28!).

No - it's never too late if it's something you're really interested in. And you're still a young whippersnapper compared to me! I'd love to check out your Province someday; been to Nova Scotia, though, and loved it.

Originally posted by Spacegoat
My mistake on the DF thing, one paper used numbers like 10, 20 etc. The author said it was for simplicity?!?! So I read some different papers, hehe. They All used the larger nubers like the ones Crown quotes. Long cable runs apparently have a huge effect on DF. I know all about signal loss in speaker cables, but I didn't know it affected DF!

Yeah - the whole "ringing" 2nd-order system is probably very sensitive to any resistive components, which is why the DF is rated at a certain speaker load. The low numbers may have been some kind of "normalized" value - perhaps compared to the DF of a given reference amp. Speculation on my part. In other words, maybe the 10 means DF = 10 x DFref (DFref might be of a particular amp, say 150). Good luck with your studies!
- Mike