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Stereo power amps

Discussion in 'Amps and Cabs [BG]' started by ThePaste, May 20, 2001.

  1. Say you have a stereo power amp (1000w per side into 4 ohms), do you have to use both sides, or just plug one speaker into one side?
  2. Angus

    Angus Supporting Member

    Apr 16, 2000
    Palo Alto, CA
    No, you'd want to use 2 cabinets (or 2 sides of 1 cabinet) when using Stereo. That's the whole point. Otherwise, you might as well just run it bridged and get almost twice the wattage.
  3. The be exact, you'd get 2000W into 8 ohms (absolutely no less), when bridged. But yeah it's possible to run a stereo amp one side only. It won't smoke or anything. But you'll be only using half of the amps capacity (duh!).
  4. ubersam


    Oct 12, 2000
    That's what I'm doing right now while I'm saving up for a 1x15 cab to pair [bi-amp] with my 2x10.
  5. Yea I know, but some of the modern power amps put out so much wattage, can the cabinet handle it? The most handling capacity I've seen in a mainstream cabinet is 1000 for most 810s and for the Goliath.
  6. A good cabinet will be perfectly happy with twice its rms power from the amp. In fact, some people even recommend this setup. When you play bass, only the very high peaks will use the "twice-the-rms" power, that's why it's called peak power (this relates to cabs only).

    Professional speakers will take as much as 4 to 10 times their rms power in short bursts. They will distort, but they won't smoke.
  7. Even if you run a big honkin' QSC MX-3000a (3,000 watt) amp plugged into a standard 120Vac 15-amp wall outlet, it will never put out more than 1800 watts RMS to your speakers.

    All these big amps will put out the big wattage *IF* they have sufficient wall power to do so. The MX-3000a draws 58 amps maximum, and will need a NEMA 5-50P connector plus a 120Vac 50 amp circuit, and STILL not have enough power available.

    If you are running a pair of 1,000 watt RMS rated drivers such as the EVX-180B or Eminence Kilomax series, they will absorb everything your amp can provide from a standard wall power outlet.
  8. MikeyD


    Sep 9, 2000
    Not quite true. Such an amp can certainly deliver its rated RMS wattage for a limited time, depending on how much energy is stored within its power supply. RMS, strictly speaking, is the equivalent power (or value) of a waveform over one cycle (or perhaps a half cycle). A sine wave need merely exist for only one cycle to supply its "RMS" value of power. If you are talking about "continuous power", OTOH, that's different. The 15-amp circuit (if source voltage does not sag) can indeed supply 1800 watts RMS *continuous* power, but no amp that I know of is 100% efficient, so its continuous output will be measurably less. So, to illustrate this, if an amp's power supply could store 10,000 joules of energy and the speaker load and output stages were such that it could deliver all of that energy in 1 second, you would see an output of 10,000 watts average over that 1 second. Sorry to be picky - it is a bit confusing.

    In the sense of continuous output, yes. However, I wouldn't be surprised that the output of that amp would be time-limited at full power, as are QSC's bigger RMX and PLX series models.
    I'd qualify that, based on the energy storage factor within the amp. If the amp can achieve short term transient power well in excess of the speakers' handling, then there might be a problem. It all depends. The other thing is, as you know, such a speaker might only be able to withstand *half* that power if fed a signal at 5 or 10 Hz. (or worst case, DC) for a long time.

    What I'm alluding to here is the difference between "peak", "RMS", "transient", and "long-term" (or continuous) power. They are different animals, from both the amplifier's perspective and the speaker's perspective. The speaker has mechanical limits as well as thermal limits, and one needs to analyze the transient and continuous abilities of both.

    - Mike
  9. Agreed. My interpretation is "usable" power, not some potential peak value available for a fraction of a second. Speaker loads will absorb much more than their rated input power for these transient spikes. If the drivers are that close to maximum load, there needs to either be more drivers or less power applied.

    The point I was trying to make was not to fear having a big-boy amp attached to your cabs, cuz the big-boy can't provide any more usable power than it can draw from the wall. It is a given that no amp is 100% efficient. All the dives I play have 15-amp wall outlets, and hair-splitting aside, a player isn't going to get any more than the wall can deliver. Plus, the player may have to share that wall outlet power with other amps.

    As for speakers, those with 1,000w RMS ratings have a "program" power rating of 2,000w.
  10. MikeyD


    Sep 9, 2000
    Okay - yes, your point being that the effective "heating" value of an amp really cannot exceed 1800 watts x its efficiency (from a 120V, 15A outlet). I can buy that.

    What I'd like to do, though, is make sure we differentiate RMS power from continuous average power. They tend to be thought of as the same by most people, and they're not. This is why you'll see in the "big boy" amps big RMS power ratings (the average value of their sine wave output), yet the amps are only able to sustain that for a finite time, so one would argue that it's RMS power, but not really continuous. For all practical purposes, I interpret "continuous" as indefinite or unending.

    I guess that's why some of the early mega-driver designers used to demonstrate their speakers' power handling by plugging them directly into a wall outlet! V^2/R gives approx. 1800 watts for an 8-ohm driver.

    - Mike
  11. That MX-3000a I'm interested in draws 6960 VA and puts out a continuous 2400w into 8 ohms.

    Plugged into the wall... the ultimate in 60 cycle hum...
  12. We got 230 Volts here... YAY! Europe has more power, I tell ya, more power!!!!

  13. Hey guys! I'm pretty familiar with the mx 3000a. I have a PA in an arena right now that is using 16 (!) 3000s plus 4 1500s. They're powering 32 EAW KF850 series cabs. I've found that a 30 Amp circuit is sufficient for each 3000. My sub racks have 2 3000s in them (one channel per 2x18 sub) and I'm powering them with single phase (4 wire) twistlocks on 30 amp breakers. I've never had a breaker trip even under the most extreme conditions (or more than a couple volt drop) The whole rig even when driven hard (plus 10 mx 2000s and 10 mx 1500s powering monitors) rarely draws more than 100 amps a leg. The big current draws are on transients, although I might be a bit nervous running a 3000 on a standard wall circuit. :D Bgavin, the 3000 is a great amp. It runs pretty cool and has been reliable. I've only seen one of them go down in 6 years using them. The weight is a bit off putting but the power and sound quality is worth it.
    Joris' comments on overpowering cabinets is dead on! I totally agree. An undistorted transient at many times the cabinet's continuous power rating will cause less damage than a distorted signal at way less than rated power. another intersting discussion!
  14. That is exactly what I wanted to hear. There are several on eBay right now. They seem to seel around $950 which is right in line with 66% of new discount price.

    Yes, the weight is a factor, but that is life in the bass fast lane. Another few pounds added to a quarter ton isn't much additional weight.

    Joris, you lucky fellow. The Europeans were much smarter going with 240v circuits. I run as much 240Vac at my house as possible, but not my instrument amps.
  15. I know all about the weight thing, I own a 70s SVT, 90 odd pounds. Ouch. Yup 240 would make more sense.....
  16. MikeyD


    Sep 9, 2000
    Well, when it comes to getting buzzed by a ground fault, I'd rather get buzzed with 120 than 240. Safety is an important consideration, for sure. Joris, what are typical amperage capacities for common single-phase circuits over there? Is this common through western Europe?
    - Mike
  17. 16 amps per circuit, and the main breaker in my appartment is 35 amps, I believe (not sure). All 50 cycles, by the way.

    3.84 kW per circuit, 8.4 kW total. That's at 240 volts, but we're not there yet, it's around 230. Over the years, the line voltage has been very very gradually raised from 220 to 230 volts, and eventually we'll be at 240 volts.

    Whether these breaker values are standard for Europe, I don't know. I do know that the UK has 120 volts 60 cycles. Those guys will do anything to not be a part of the European mainland :D:D

    At Para, the club I (volunteer to) do the light show, we have 3 pcs 3-phase wall outlets, with 3x25 amps breakers. We use them to power 3 dimmer packs with each 6 channels of 2 pcs 1 kW spot lights. Now that's serious power consumption! 36 kW put to good use. I mean, there are instances where all lamps are actually on, for a 36 kW amplifier system this is very rarely the case.
  18. Power consumption? The PA and lighting rig that I have up in an arena right now is using: one 200A 3 phase service for audio, one 400A 3 phase and one 200A 3 phase for lighting. 168 kW of dimmers plus 24 moving lights! Audio is using 45 power amps. Heheh more power please :D I like watching the built in ammeters on the main lighting dimmer hit 350A per leg. There's just something about all that power......
  19. MikeyD


    Sep 9, 2000
    Thanks, Joris. Sometimes I wish there were more 240v outlets available here, particularly in clubs - then one could have better power. 240v is commonly fed to our buildings via two 120v legs that are 180 degrees opposed. Many houses provide 240v circuits for electric ranges, dryers, or water heaters.

    I've played more places whose power seems just an afterthought - a single 120v circuit (or worse, a single outlet pair) for the entire stage area! My bass amp alone just about sucks that dry on peaks. That's why I've become more interested in speaker efficiency now. Efficiency is the thing to go after when one gets close to the limit of power availability. (Yo, California - hint, hint!). :)
    - Mike
  20. We here in California get what we deserve as a consequence of who we elect. No new dams since the 1960's, generation permits take a full year, no upgrades to sewage, water, and yet millions of building permits issued. In 2003, there is another gasoline law becoming active, and 20+ refineries are shutting down and leaving rather than pay the enormous costs to comply. This will put us easily into $3+ per gallon gasoline. Today, diesel fuel can only be sold if it is produced in California, which is already at 100% capacity. Our diesel is the highest in the nation.

    Efficiency will not happen here until the PAIN arrives. That is why we have to let the rates float and turn up the PAIN for short term conservation. This is political suicide, and the Prince of Darkness (our wonderful governor) hasn't the balls to take bitter medicine, and favors mortgaging our grandchildren with bond burdens (read: taxation).


    As for speaker efficiency, whaddya think about looking at the maximum SPL the driver is capable of providing, and not just 1 watt at 1 meter? For example, the Eminence Kilomax Pro 15 will do 125 SPL at 1,000 watts (calculated), compared to the ElectroVoice EV15 which is 125 SPL at 400 watts.
    As noted, this is calculated, and I have no way of determining if the driver will actually reach its calculated SPL potential or not.

    The situation becomes more unclear when bass response is factored in. For example the JBL E-145 provides 119 SPL calculated at 150 watts. But... this driver is AWFUL below 65 Hz. Applying 2nd order EQ at +9dB to give it flat response to 40 Hz, and this driver can only accept 20 watts to produce 109 SPL at 40 Hz, and 137 watts for 119 SPL at 200 Hz. Again, all are calculated. Note the maximum power SPL figures, at 40 Hz. the JBL is still down -10dB (half as loud) with EQ applied.

    MikeyD, if you have some suggestions for efficient drivers, please post them here. I am at a loss trying to find efficient 15" drivers and flat bass response, as this combination appears to occur only in 20+ cubic feet cabinets. My solution is using inefficient drivers in smaller cabs, and applying LOTS of power.

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