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Watt's Watt with Power amps ratings?

Discussion in 'Amps and Cabs [BG]' started by Petebass, Jan 5, 2003.


  1. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    This one is for the guys that use a pre-amp + power-amp. I've had a question burning for a while now. I bought a new amp recently but before I parted with my hard-earned cash, I borrowed as many amps as I could get my hands on and gave them all a bit of a go at an all-day rehearsal - everything from 200w combo's right through to massive power-amps.

    The results with power amps were wierd. I used the same pre-amp for all of em (yamaha). The power amps were rated between 600w per side up to 1,100w per side at 4 ohms. My speakers have a total load of 4 ohms.

    No matter what I tried, none of the power amps would go as loud as a standard 400w head, not even the 1100w so-called-monster. I'm talking brands such as QSC, Mackie and Yorkville, all reputable. It wasn't even close. I even tried bridging to double the power and still no cigar.

    I ended up buying a standard 400w head and before long found myself being able to overpower PA stystems rated at 3000 and 4000 watts. I had sound engineers getting ****ty with me and insiting I turn down so they can put me back in the mix.

    This leads me to believe that PA power amps have a different system for rating their wattage to other musical instruments? Is this so? All thoughts welcome.
     
  2. A watt is a watt, no matter what device supplies it.

    I think the culprit here is a too weak output signal of the preamp. Power amps need a HOT signal on their inputs, because their "volume" dials are always only attenuating the signal, not boosting it.

    It's a shame you came to the conclusion to buy something else, while all you needed was a signal booster :(

    At least, I think so.
     
  3. bassotus

    bassotus

    Nov 24, 2002
    Finland
    I couldn't get an answer in my own thread so I'll ask again; Can I feed a power amp (Phonic MAX1500) with the post-EQ fixed-level balanced D.I. output of my Trace Elliot head? I'm forced to use the TE for a while still before getting the cash for an EBS pre-amp ...

    Thanks,
    --
    Mikko
     
  4. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    I doubt it. A DI is designed to provide mic level signal at low impedance rather than pro line level signal. You're more likely to have luck with the FX send.

    Alex
     
  5. There are a lot of factors involved in what you're hearing. Different manufacturers do use different methods to rate power but it's fairly random and from my experience bass amp manufacturers are more guilty of inflating their numbers than PA amp manufacturers. Were you able to drive the power amps to full power? the odd flash of a clip light would indicate that. Many bass preamps cannot produce enough voltage swing on their outputs to drive professional power amps to full power, as per Joris' post. Especially if you're using the XLR out. As alexclaber said, it's geared to drive the mic input of a mixing console and generally won't be able to drive a power amp. On a side note, the impedence isn't the problem. Most amps like to see balanced Lo-Z signals on their inputs.

    How an amp is voiced can have a large effect on its perceived loudness. Sounds with a lot of midrange in them sound a lot louder than sounds with attenuated midrange. So if you had lots of mids going on with the bass amp head and the mids scooped on the preamp, the one with more midrange would sound louder. Also, 1100W wouldn't be all that much louder than 400W anyway. It would be about 4dB louder at most. This is noticeable but not a huge difference.

    As for being able to be heard of 3000-4000W PAs. Just think about this: The only thing that your amp is amplifying is your bass, the PA amps have to amplify everything. This means that they have to deal with everything from kick drum to vocals with all the transient peaks that go with them. This eats up more amplifier power than running a single instrument. It's also possible that the guys running the PA weren't driving the PA as hard as it could be driven but were running as loud as they felt comfortable with in the room for the type of audience. Maybe you were too loud to sit in the mix in that way. Maybe they were just stage volume nazis.... Many guys are;).

    EDIT: erroneous information removed...
     
  6. Like everybody else has said, your pre-amp wasn't driver the power amps hard enough to bring them up to a decent volume. If you get the chance, try putting a line level device like an EQ between your pre-amp and the power amp. Use the amp in the EQ to boost your signal to the power-amp. I think this will change your opinion of power-amp ratings.
     
  7. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Mark, I can't think of any amp manufacturer that uses--or even could use--pink noise for power ratings. You have to use a signal with a controllable level and no harmonics (so you can identify the onset of clipping by the increase in THD). The only signal to fit that bill is a sine wave.
     
  8. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Petebass, even a 10,000-watt amp pushed to only 100 watts is not going to be as loud as a 500-watt amp being pushed to full power. Don't confuse power with gain.

    Your bass puts out a small signal voltage. Your amp head, or your preamp and power amp, each multiply that voltage by some factor (that is controllable by one or more gain controls). That multiplication factor is called gain. Power ratings of amps are not indicators of their gain structures; if you're working with millivolt-level signals, a 10-watt amp made to accept millivolt-level input signals will sound louder than a 1,000-watt amp designed for input signals of a couple volts.

    One common problem with preamps is that many models just have wimpy outputs that can't even deliver a volt. As a result, they won't be able to drive many, if not most, power amps to full power.

    But the bottom line is, 600 to 1100 watts into 4 ohms is louder than 300 watts. You just weren't putting out that much power when you tried out the amps.
     
  9. D'OH!

    I actually knew that....:oops: Early morning post! Sorry for any confusion.
     
  10. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Thanks for all the replies guys. A couple of things though:-

    I don't think the pre-amp was the problem. I must admit I thought of that at the time and tried a couple of different things. We had a spare Mackie mixing desk on hand and I used that as a pre-amp for a while. I even tried my Sansamp Pre-amp/Driver thingy. All of them could produce enough of an input signal to get the red clip lights to flicker, but I wasn't keen on send clipped signal to the speakers for obvious reasons. I tried using a couple of different equalisers as an additional signal booster (by now the whole band and sound crew were involved). Still nowhere neas as much volume as a standard bass head.....

    If a watt is a watt, why where some of the 400w amps considerably louder than other 400w amps?

    MR's comment about the PA's makes sense. But for the record our crew aren't volume nazzis. The opposite in fact. We're just a really farken loud band (blame the guitarist and his Marshall quads).
     
  11. monkfill

    monkfill

    Jan 1, 2003
    Kansas City
    Equalization and frequency response.

    Low-mids will be punchy and cut through the mix and will appear louder. Cutting the midrange will result in a transparent, scooped sound that sounds good by itself but results in you getting lost in the band.

    I have a 100 watt Fender BXR that totally blows away my 350 watt SWR 350x-> 4X10 volume-wise. But the Fender sounds like complete ass, and it all has to do with the frequencies that are getting the 100 watts versus the frequencies that are getting the 350 watts.
     
  12. zoran

    zoran

    May 10, 2002
    croatia
    On my Trace I usually use "line out" (unbalanced) for feeding separate power amp.
     
  13. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Also, heads and combos rarely, if ever, have clip indicators to make you think about turning down, so you might well run well into clipping, enough to sound really loud but not quite enough to detect lots of distortion.
     
  14. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Robs right. Which is why so many people blow speakers. Yet I have trouble convincing people that a 200w amp can easliy blow a 1000w speaker. I think I've developed a clipping paranoia, but I've never blown a speaker in 16 years of playing.

    I've got a clip light in one of my effects units which actually lights up 3db before clip. That's according to the manual but we proved it correct on the big test day.

    Oh and I've got a pretty good head for frequencies and EQ. So when I say one 400w amp is louder than the other 400w amp, that's after I've set the EQ to produce a similar sounding result. If you ask me EQ is one of the most powerful effect you can buy, but that's another topic...........
     
  15. monkfill

    monkfill

    Jan 1, 2003
    Kansas City
    Adjusting the EQ is one thing, but what I'm thinking is that different amps put out different levels of power at different points along the spectrum. I think the Fender BXR100 I have that is super-loud is pushing almost all of its 100 watts in midrange. No real lows, no real highs. As a result, its loud, and you can crank up the bass and it gets boomy, but the low-end doesn't have the definition and feel of the low-end of my SWR 350x.

    Some of this variation I'm sure has to do with the frequency response of the speakers/cabinets.

    One question I have though: My SWR amp is rated as 350 watts RMS, which I believe translates to 700 total available watts. What I'm wondering is, is the QSC or other power amps rated in this same way? This could make a difference.

    Another thing. . . I keep hearing that it takes a lot of watts to make a substantial difference in apparent volume. Its more about the tone of the amp and cabinet, case in point, the Fender BXR vs. the 350x.
     
  16. bassotus

    bassotus

    Nov 24, 2002
    Finland
    ..I wish mine had one. I guess I'll have to try the full range effect send, that at least has a level setting, and maybe putting the MDX2200 between the TE and power amp can be used for boosting the signal...
     
  17. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    One of our younger tech dudes pulled out a behringer DI to see if it could feed enough power to the power amps. He was confident because it had 40db worth of padding available compared the usual 20db. He reasoned that it must therefore have a stronger than usual DI signal - He was wrong!

    Do you run any bass effects Basotus? I know several guys who own multi-effects units that also have decent pre-amps built in but didn't even know it.
     
  18. bassotus

    bassotus

    Nov 24, 2002
    Finland
    Yeap, but no multi-effects though, just some EBS pedals. I have a self-made Aguilar 924 (a OBP-1 circuit enclosed in a box, has a "low impedance output buffer stage", is that good for feeding a power amp?) that doesn't see much use. I've used my TE head for the last seven years and I've kinda grown used to what sounds I can get out of it and like it's tweakability. Although I might dump the TE if I hear enough good things about the GK 2001RBP..

    Since it's hard to get some products for testing here in the northern Europe, what should one look for when browsing just the specs of some interesting pre-amps to avoid the ones with wimpy output? The Phonic power amp I have on the way has <i>1.15v(+3.4dBu) Input sensitivity@8ohms</i> and <i>Input inpedance of 10Kohms (unbalanced)</i>; can I make up any requirements for the pre-amp output based on this information?

    Thanks..
    --
    Mikko
     
  19. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Yes. Your preamp should be able to put out a maximum RMS voltage of at least 1.15 volt (with a reasonable instrument-level voltage on the input). If it can do more, that's great; you can always turn the amp gain down.

    If that voltage is too high for your preamp to deliver, you're going to have some unusable headroom in your power amp. ;)
     
  20. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    This is something I've always wondered about - if clipping 200W power amp will damage a 1000W speaker, what happens when you put your bass through a fuzz pedal and then through a 200W power amp into a 1000W speaker? Is the type of distortion different and less damaging? Or have I got the wrong end of the stick entirely?

    Alex

    P.S. This isn't really hypothetical, I have regularly maxed out my M1400i (500W RMS/ch) into my allegedly 350W rated Low-B2's, whilst playing my bass through a Big Muff. Nothing's blown yet, but I'm getting scared.