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"Headroom" vs. "Overkill" advice

Discussion in 'Amps and Cabs [BG]' started by Chris Fitzgerald, Oct 5, 2002.

  1. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    The longer I wait for the new EA iamp 800 to come out (been waiting since February!), the more I'm starting to look at the possibility of going with a power amp and pre amp of some sort as an option for when my Clarus isn't enough to cut the louder gigs. After doing a lot of reading, I've narrowed my choices down to one of the QSC PLX series amps, or one of the Stewart World series amps.

    My "Headroom/Overkill" question has to do with the fact that I've never really understood the whole "Stereo/Mono/Bridgedandathowmanyohms" issue, and some of the power ratings on these amps look absolutely ridiculous on paper. I'm looking for an amp that I can use for both my DB and BG running through an EA CXL112 and a Bergantino HT112 (Both 8 ohm). It would be nice to run the amp so that I could tweak each side to balance the speakers, but not essential.

    So, the question becomes, how much wattage do I need to have to have plenty of "Headroom" in reserve without buying a system that has so much power that the speakers I'm using can never handle the power anyway? Please enlighten this technically challenged customer!
  2. Brendan

    Brendan Supporting Member

    Jun 18, 2000
    Austin, TX
    To me, headroom is having about 300-500 watts better than the RMS limit. So, say you have a 4 Ohm load of 700 watts RMS, a 1000 watt amp in plent of headroom.

    For a 2x12 load like you're suggesting, with the EA at 350 RMS and the Berg at 300 RMS, at 4 ohms, I'd suggest a 800 or 1000 watt head at 4 ohms. This would be MORE than adaquate (about 500 watts into each cab). However, that's a bridged load. If you wanted a stereo load, then you'd need to beef it up to a 2000 watt head (which usually put out about 425-450 watts per side at 8 ohms). I wouldn't bridge that load, no way, but itd be a super clean bi-amped rig.

    So, 1000 watt if you plan to run bridged, 2000 if you ONLY want stereo. Personally, I'd go for the 1000. It's got a lower powered stereo mode, but at least you'd be able to use it two ways instead of one.

    Just my .02.
  3. For me, headroom is having a second PLX 3002 in my rack. The problem is finding separate wall circuits to power each.


    Your mileage will no doubt be much better than mine...
  4. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY

    Wow, great post! Thanks for taking the time to answer that very vague question. The next question of course is whether there's any difference in sound between a power amp running bridged into a 4 ohm load and a power amp running 8 ohms into 2 channels. Money is not really the object here - rather, I'd like to spend whatever it takes to get a small, clean, lightweight and hi-fi rig with plenty of headroom that will last me for the next 10 years or so.

    I love the size of the Stewart amps, but have heard that reliability can be a factor with the 1 space models. On the other hand, if it turns out that there is a sonic difference between running at 8 or 4 ohms, I guess that would leave me looking at the QSC PLX 2402 or the Stewart world 2.1 and running each side seperately. Thoughts on either of these two amps in terms of sound, ease of use, and reliability anyone?
  5. there might be a sonic difference because most amps distort a bit more with lower impedances and bridged 4-Ohm is effectively the same as 2-Ohm non-bridged distortion-wise.

    The PA rule of thumb is having 1.7-2 times (if i remember correctly) the RMS power of the cab from your amp.
  6. FretNoMore

    FretNoMore * Cooking with GAS *

    Jan 25, 2002
    The frozen north
    This is a clip from the user's guide for PA speakers similar to those I have (EV-300Sx) that are rated at 300 W continuous, 1200 W peak. I'm not sure if this translates directly to bass cabinets, but I'd think it's very similar.

    Amplifier Power Recommendations

    As noted in the Power-Handling Capacity section above, the Sx300PI has a random-noise power capacity of 300-watts long term (1,200-watts peak) (nontransformer) per ANSI/EIA RS-426-A 1980. The following guidelines will help relate this number to an appropriate power amplifier output rating.

    1. To use the Sx300PI to full capacity, skilled experts in sound system installation and operation will obtain the best results if the power amplifier is 2.0 to 4.0 times the long-term average noise-power rating of the speaker system. For
    the Sx300PI this is 600 to 1,200 watts (nontransformer).

    The caution cannot be made strongly enough, however, that this arrangement is only for experts or for those who can discipline themselves against “pushing” the system for ever-higher sound levels and who can avoid “accidents” such as catastrophic feedback or dropped microphones.

    2. A more conservative, “normal” amplifier size, which will produce audible results nearly equal to those of the “expert” recommendation, is 1.0 to 1.4 times the long-term average noise-power rating of the speaker. For the Sx300PI, this is 300 to 420 watts (nontransformer).

    3. To be very conservative, one can use an amplifier rated at 0.5 to 0.7 times the long-term average noise-power rating of the loudspeaker. For the Sx300PI this is 150 to 210 watts (nontransformer). desired output wattage.

    So, I guess an amp with 1.5 to 2 times the RMS power rating of the cabinet wouldn't be far off the ideal, if you know what you are doing with that volume knob...
  7. Brendan

    Brendan Supporting Member

    Jun 18, 2000
    Austin, TX
    Haven't used those two poweramps, sorry Fitzy. I usually use my poweramp bridged at 8 ohms or bridged at 4, so I never turned it up in single channel mode. My thoughts would be that any GOOD poweramp worth it's salt, it wouldn't matter if you ran it stereo or bridged. Of course, the neat thing about stereo, is that you could plug different preamps into each channel, and mix and match sounds.

    I think if you shot a PM to Bob Lee (QSC) he's be able to tell you about the respective specs and questions you have about poweramps. The man's a cornucopia of info.
  8. jerry

    jerry Doesn't know BDO Gold Supporting Member

    Dec 13, 1999
    I use a lot of speaker configurations for different gigs......2/12+ 3/10 .....2/12+ 1/12.....1/12 +2 /10 etc....and the 2402 has been great for all of them,
    and only 21 pounds:)
  9. FunkySpoo

    FunkySpoo Supporting Member

    Feb 6, 2002
    I just recieved a PLX 1602 that I run bridged at 4 ohms if I use both of my VL210's for 1600 watts or I run it bridged at 8 ohms with one of the 210 for 1000 watts. Very loud with lots of headroom. I didn't hear any difference when I tried in stereo at 8 ohms per side.
  10. Isn't headroom having enough power to produce a particular note accuratly with proper articulation, and without clipping the output stages of an amplifier.

    So, if you play quietly, then I don't see why 50W shouldn't be ample headroom enough.

    I thought at one time this might also apply to speaker systems, and overdriving them (distortion). However, noted in Anders Östberg's posted article, 2-4 times RMS power handling. I'm guessing that is why they mention more power is for professional use only, and also peak power is usually 2-3 times that of it's rated RMS power handling, SO I imagine that running more power wouldn't be a problem.

    I personaly like running Eden's 10's really hard. :D
  11. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    headroom is no fixed value. Important is the difference between the maximum (clean) power and the amount of power you normally need in everyday situations - it's the power reserve so-to-speak.
    It enables the system to process short peaks without clipping or distortion.
  12. I'm a headroom freak and my rig requires a huge amount of power just because of the way I choose to run it.

    I run 8 ohm Eden 210XLT (350w) and 115XL (400w) cabs. Power amp is a QSC RMX2450 (500w/channel into 8 ohms).

    I could have saved a bit of money by getting a smaller amp and bridging the output - but I don't like running dissimilar cabinets on the same amp channel. I'd prefer to have some control over balance between the cabinets.

    I'm sold on high power - In the last year I've gone from a 200w mono amp to a 300w per channel rack to a 500w per channel rack. I've never cranked the QSC up more than halfway - and even that produces oppressively loud stage volume.

    A lot of g*itarists want clipped waveforms and a lot of bassists don't. For me, the difference between 600w and 1000w is all headroom - I don't play any louder than I did before but I can hear a lot more of my bass than I could with a smaller amp.

    Huge amps are highly recommended :)
  13. bben


    Feb 28, 2002
    Santa Fe, NM
    I am running a pair of 1x12 cabs, either a couple of Bergantinos or a Bergantino-plus-Epifani. I had a QSCPLX1602 for a while and now have a Stewart 1.2. I use bridged mode and think either of these is a good match for a pair of 1x12s.

    I like using bridged mode - very punchy - and haven't had any problems. For the amount of power, the rig has very low levels of hiss, something I am fussy about.

    I like the sound of the Stewart a little better than the QSC, but the QSC is a good amp, too. The fan on the QSC was just too loud for me.

    Nothing wrong with getting a bigger PA amp and running the channels separately, but not necessary IMHO.

    I am intrigued by the iAmp800, too, but I paid $500 used for my Stewart, $425 used for the Avalon U5 I use as a preamp, and $70 for a three-space ProTec rack bag. $1000 all told, still less than the $1275 street price for the iAmp800. Stewart prices have come down since I bought mine, and you can sometimes find the Avalon for less, or a used Demeter or Alembic pre, for that matter.

    The iAmp is smaller and lighter than my rack system, and has a built in tuner. But my rackbag only weighs about 35lbs all up, with various cables and such stowed inside, still an easy one-hand carry. And I have had good luck with Korg $20 chromatic tuners. I attach one to the top of the rack bag using velcro while playing, and stick it in the storage pouch for transport.
  14. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    A couple of questions:

    What's the power rating on the 1.2? I love the size of it, but I've heard that it runs really hot, and some advise leaving an open rack space when using it. Also, I take it you haven't had any problems with this unit? There have been some reports on the board about that issue.

    Does it fit in a standard rack?

    Last, you mentioned that you like the sound of it bridged, describing it as "punchy". Have you noticed a difference in sound between bridged and mono?
  15. bben


    Feb 28, 2002
    Santa Fe, NM
    The power rating of the 1.2, bridged into 4 ohms, is - 1.2! Kilowatts, that is, or 1,200 watts. That may be optimistic, but whatever it is, it seems like plenty for a pair of 112s. Un-bridged, it's just 200 watts per channel. Both the Bergantinos and Epifanis like a lot more than that, in the range of 300 to 600 watts, each.

    It fits in a standard rack, one space, but I think you definitely want to leave a space above it. It does run hot. I have my Avalon centered above the Stewart, so there are two open rack spaces on either side above the heat sink fins, open both in front and in back.

    I haven't had any problems with mine, but I don't drive it that hard most of the time, just lighting up the -20dB light.
  16. Mara


    Jan 11, 2001

    Why do you think Glockenklang adviced me to get an 8 ohm 2x12" (500W rms) rather than 4 ohm cab (also 500W rms) to go with my poweramp (2x750 @ 4 ohms or 2x500 @ 8 ohms)?

    You think they don't believe in headroom or are scared of me blowing the 4 ohm version?

    Just curious.

  17. Brendan

    Brendan Supporting Member

    Jun 18, 2000
    Austin, TX
    I dunno, Mara. I'm not familiar with Glockenklang or their way of doing things.
  18. A speaker manufacturer probably wouldn't tell you to buy an amp that could blow the voice coils through the grillcloth.

  19. Brendan

    Brendan Supporting Member

    Jun 18, 2000
    Austin, TX
    This is true. Not that it would, but that it MIGHT if you were using A LOT more power than nessicary.
  20. Mara


    Jan 11, 2001
    Again, just out of curiosity. If all things being equal, what would be the difference in sound, db's, etc. between these two setups with the amp cranked up to 50%?

    a) poweramp running 2x750W @ 4 ohms into two 500W @ 4 ohms 2x12" cabs

    b) poweramp running 2x500W @ 8 ohms into two 500W @ 8 ohms 2x12" cabs

    I can't see myself ever needing to use the amp cranked up to 10.

    It's just that a lot of people here seem to really appreciate having headroom. Is it really worth it?


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