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Power amp question

Discussion in 'Amps and Cabs [BG]' started by fadlan bassman, Aug 20, 2002.

  1. fadlan bassman

    fadlan bassman

    Oct 23, 2001
    What is the difference in power amps as far as using one to power a PA and using one to power my bass cab? is the power all the same your just preamping the bass?
  2. Matthias


    May 30, 2000
    Vienna, Austria
    It's the same.
    In fact only very few manufacturers offer dedicated 'bass power amps' (Eden and SWR come to my mind [edit: also Aguilar and Glockenklang]).
  3. fadlan bassman

    fadlan bassman

    Oct 23, 2001
    so this does not affect tonal quality? the power amp will just amplify what the preamp shapes right?
    what would be the benifit of the 'dedicated' bass power amp?
  4. A higher figure on the manufactorer's bank account.
  5. BryanB

    BryanB Moderator Staff Member Supporting Member

    I am not so convinced of this. Let's say you are designing an amp for PA use. You would want it to sound great across the entire audio spectrum, 20-20k, with as flat a response as possible. On the other hand, if you were designing an amp specifically for the bass guitar, you would focus on a narrower range of frequencies and specifically taylor the frequency response and other factors for bass.

    I think this explains two things:
    1) Why power amps with the same specs sound different.
    2) Why companies like Aguilar is able to make a 750 watt SS hybrid amp that sounds much louder than any 1500 watt PA amp I have ever heard.
  6. I'd respectfully disagree. I think those "bass-specific" power amps don't convey any extra value of significance. I may be cynical, but I think the main reason SWR and Eden sell their own bass power amps is so that you don't spend your power amp dollar with QSC, Crown, Crest, Stewart, Carvin, or whomever.

    IMO a great PA amp would be great at bass as well, because it includes all the frequencies a bassist needs. First, basses actually can generate a surprising amount of higher-frequency harmonics. It isn't just, say, 3 kHz and below. Not everybody focuses on those, but they're often there, as you can tell if you fiddle with the tweeter settings on a cab that has one (e.g., an Eden XLT). This means that to cover all bass tastes and tones, you actually need a rather wide frequency response, not a narrow one.

    Second, just in principle, outside of maybe guitar amps and SVTs, a power amp is really not supposed to add anything to a signal--it's supoosed to make what's there louder, that's all. No amp does this perfectly, which is why they sound different. But to me at least, an amp that deliberately predistorts your sound along the lines you suggest--and that's what emphasizing (or focusing on) certain frequencies amounts to, because any deviation from the signal you put into an amp is distortion by definition--is inherently more restrictive than one that is more faithful. This for me makes such an amp not better for bass (or anything else) but actually worse, because it limits the tone shaping I can do. Which IMO is best left to the preamp anyway.

    My guess is that your perception of the loudness of the DB750 doesn't result from with any emphasis on bass frequencies but on the physical construction of that particular power section amp and Aguilar's system for rating power.
  7. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    I don't think any manufacturer designs a (solid state) power amp that way. They might tell you so in their ads, but that's just marketing.
    Can you elaborate? When amps sound different, the specs are obviously not the same.

    Did you test the PA poweramp with the Aguilar pre? And with the same cab(s)? Sensitivity mismatch or not? Otherwise this A/B test is pretty much useless.
  8. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    First of all, let me point out that it's no longer difficult to make a power amp's frequency response flat from below 20 Hz to well up above 20 kHz, despite what some marketing pitches claim. "Focusing on a narrower range of frequencies" doesn't offer any performance benefits in a power amp, so it doesn't make any sense unless you subscribe to the "it's not a defect, it's a feature" school of marketing.

    1) Two power amps with the same published specs--even the same identical model--can easily be made to sound different just by turning one up even just a little louder than the other.

    2) You can make even a 50-watt amp sound louder than a 5000-watt amp simply by making its gain higher than the bigger amp's. Do that, put the same signal into both, and the little amp will sound louder, at least until it hits clipping, which won't be difficult to do. If you match the gains, they'll sound equally loud, as long as the little amp isn't clipping, but if you start boosting the input signal, the little amp's going to run out of juice well before the big one does.
  9. fadlan bassman

    fadlan bassman

    Oct 23, 2001
    Wow, thanks for all the great information. It is very helpful.
  10. gnome01


    Oct 30, 2001
    Bronx NY, USA
    It doesn't seem to me that power amps have a huge variety among them when hooked up to the same preamp - maybe a few exceptions. I use a Samson, which not too many people seem to care for - but with my Alembic F2-B, sounds just as good as the SWR SM400s that I got rid of (because it didn't always produce sound:( ).
  11. BryanB

    BryanB Moderator Staff Member Supporting Member

    I am not going to try to go tete-a-tete with Bob Lee on this issue. I am sure that he can provide a much more convincing intellectual argument than I can. I will tell you this, when I compared a Kern/QSC 1602 PLX in bridged mode, to a Ashdown 500 watt amp, the Ashdown kicked it's a$$. The QSC shrunk like balls in ice-water.

    I compared a Mackie 1400i, a Crown K-2, a QSC 1602 PLX, and a Stewart World 2.1 through the same system. Now, the specs are different, but they are in the same ballpark. Considering that doubling the output gives a barely noticeable increase in volume, I didn't sweat the details about slight differences in volume, etc. Despite their similarities, they all displayed very different characteristics. I am sure you the numbers guys will attribute this to speaker efficiency, gain staging, slew rate, whatever. That doesn't really matter to me. The specs mean doo-doo, use your ears to find out what the real deal is.

    Oh, by the way, my Aguilar DB 728 400 watt TUBE BASS AMP slays them all.
  12. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Apparently user error. ;)

    Use your ears, as long as you use your brain, too. Recognize your shortcomings in hearing and perception.

    Since you didn't care about differences in volume, you shouldn't care about the resulting differences in "characteristics." Had you actually matched the gains, you'd probably have heard insignificant differences among the power amps, as long as all were operated within their linear region.

    Specs are meaningful only if they actually and truthfully describe something important, and if you know what they mean.
  13. BryanB

    BryanB Moderator Staff Member Supporting Member

    Please Bob, don't treat me like some ignorant fool. I know what I am doing. I had considerable input from people who are very well respected in the field of pro audio systems as well as my own direct experience. If you want to claim the superiority of specs, then feel free to do so. I am sure you will find plenty of mindless sheep willing to follow. For me, I continue to listen and learn.
  14. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Please don't offer the distortion that I claim the superiority of specs. I do not.

    Based on your posts, I am not convinced that you know much more about power amps than the average newbie. You have claimed several things that are just plain inaccurate, and I pointed them out. There's nothing wrong with that, given the amount of misinformation and misunderstanding there is in audio and music technology. I've gone into music stores and heard sales guys tell whoppers that you wouldn't believe. Well, maybe you would.

    And if a 500-watt amp is louder than a 1000-watt amp, it's simply because it's being pushed harder. It's like putting my elderly aunt behind the wheel of a Ferrari and saying, "Hey, I thought these cars are supposed to haul ass! What is this? A Toyota could whip this car!" ;) To think that it's a characteristic of the amps indicates a lack of understanding of how amplifiers work. If you consider yourself knowledgeable, that's okay with me, but if I see a false point being made, I'll still speak my mind.
  15. BryanB

    BryanB Moderator Staff Member Supporting Member

    You seem to be taking the approach of an engineer. Like most engineers you would like to believe that all of this is quantifiable and deterministic. I am sure that you could measure this and that and come up with a good explanation as to why it behaves like it does. But this does not alter the my perception or the perception of others who have come to same conclusions. I don't really care if one amp has more gain than another. It don't care about the slew rate or other factors. I care that they are able to deliver the sound I want at a loudness that I need. I realize that this is may be an uncomfortable position for an engineer. After all, by calling into question this reliance on quantitative information, I am questioning the underlying philosophy of your profession. I understand this apprehension, I am an engineer too.

    Looking back on my original post, I can see where a reader may have mistook my musings as fact. I do not claim to present a scientific theory. It was intended as a hypothesis about why I experienced a much greater loudness from an amp that has half the volume. I didn't think anyone would take my view as gospel.

    I have to say to you, Bob Lee, that I have read your comments in this forum many times. I infer from your tone that you speak as if yours were the only opinion worth holding and that anything else is the whinings of a child. I respect the company you work for and the products they produce. However, I do not appreciate the fact that you take my offer of a different point of view as an intentional distribution of dis-information.

    Please take a cue from Richard Lindsey, who quite politely started his response with, "I'd respectfully disagree..."
  16. Bob wins, end of discussion.:p :p
  17. BryanB

    BryanB Moderator Staff Member Supporting Member

    That was not very helpful. :mad:
  18. geshel


    Oct 2, 2001
    Do you want him to bow when you enter the room, too?

    From what I've seen, Bob just states the facts. There's an undercurrent of annoyance that stems from the fact he has to state them so frequently :). But I've often admired his stability when people start going off in his general direction.

    To jump in the fray on the main question, while I fully agree that if it sounds good and fits your need go with it, more power = more power, no two ways about it. If you took the preamp out of that Ashdown, and plugged it into a PLX1602, you'd at least be closer to a valid comparison. But then, perhaps the power section of the Ashdown head adds its own tonal qualities.

    Anyway. . .if a 500w Ashdown head could really be louder than a 1600w QSC amp w/ comparable preamplification, then the Ashdown head is putting out more than 1600w. End of story. But, that's unlikely.
  19. BryanB

    BryanB Moderator Staff Member Supporting Member

    If you read carefully, my statements have, for the most part, been about my experience. I don't appreciate it that when I convey my experience, someone comes along a tells me that my experience is meaningless and that I am an idiot.

    Bowing is not necessary, but a little courtesy would be appreciated.

    Also, keep in mind that volume, (gain), and loudness, (as measured in phons), are not necessarily synonymous. (IMO :D )
  20. notduane


    Nov 24, 2000
    Let's keep this friendly (and moving) fellas.

    So far, so good :).

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