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2 Ohms?

Discussion in 'Amps and Cabs [BG]' started by Stu L., Jul 16, 2002.


  1. Stu L.

    Stu L. Supporting Member

    Nov 27, 2001
    Corsicana, Texas
    I am about to drop some money on a 4 ohm 2x10, and the lowest my Ampeg runs is 4 ohms. I want to later add a 1x15 cab.

    So, would it be wiser to set my sights on an amp that can run 2 ohms (eg. Peavey), or one that has multiple channel outs (eg. Hartke 7000)?

    And, could I please get some makes/models and their ability (2 ohms or Multiple channels)?

    Thanking you all again,
    Stu
     
  2. lneal

    lneal

    Apr 12, 2002
    Lee County, Alabama
    Well IME most amps don't like to run at 2 ohms even though the manufacturer may rate it for such. Some QSC's will handle it, but I still wouldn't risk it. Better to get a 2 channel amp and run each at 4 ohms. That's just been my experience with power amps, and I've experienced a lot of them.
    I like QSC's PLX line. They're light, powerful and sound pretty darn good. Also, they're not too picky about AC power and are reasonably priced.
    The best sounding amps I've ever owned were my old Crown Macro-Tech's I used in my PA system. But boy were they HEAVY. And finicky about the AC power too, and that's why I got rid of 'em.
    Good luck!
     
  3. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    What Ineal said.

    I wouldn't build a rig so that the poweramp always has a 2 Ohm load. I mean, you don't drive a car with the throttle on full all the time either, do you?
     
  4. Stu L.

    Stu L. Supporting Member

    Nov 27, 2001
    Corsicana, Texas
    What about intergrated heads? Still can't afford a reasonable pre-amp. Power amp would be no problem, and the weight doesn't bother me (thanks for the warning though:D )

    So 2 channel amp it is. Any suggestions? Or what about an inexpensive pre-amp to match a power amp? I'm spending my measley savings on the cabs. If only it was available as an 8 ohm.....
     
  5. *dumb question*

    I`m not too expearienced with amps.How does a 2 channel work?Does that mean you can run one channel to one cab and another channel to another?Kind of like biamping but not separating the cabs into highs and lows...or?:confused:

    I will be getting a 4 ohm cab and had hoped to just add another 4 ohm if needed in the future and go to an amp that can run at 2 or 4 ohms.Bad idea?
     
  6. Golem II

    Golem II

    Jan 4, 2002
    Macon, GA, USA
    "2 channel amp" can mean two different things.

    Some amps, like the Eden Metro and Mesa Bass 400, have two "channels" that have different basic sounds. You switch between them, using a footswitch usually. Sometimes each channel has its own input, so you can plug two basses in and switch between them quickly. Some people like to split their bass signal in two so they can drive both channels at once.

    Other amps with two channels (stereo amps) have two "sides", or speaker output sections. You can plug a 4 ohm speaker into each side. However, you can't run a TWO ohm load on each side. I'm not sure about the technical details, but a stereo amp isn't just two mono amps put together; if you run a two ohm load on each side, you put a total load of 1 ohm on the amp, which easily damage it. These amps usually have built in crossovers so you can send the lows to one side and the highs to the other. Sometimes they don't have crossovers at all(I think the eden wt-500 doesn't.)

    You can also "bridge" a stereo amp, combining the two power sections. This puts out a lot of power but also places a lot of strain on the amp, so you have to watch the temperature and you usually can't run it at less than 8 ohms.
     
  7. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    Yes.

    A 'stereo' amp has 2 power sections and usually can be used the following ways:

    dual mono (often called stereo): each side puts out the same fullrange signal, minimum load is 4Ohm or even 2Ohms each side - recommended if you want to use two 4Ohm cabs without bi-amping or stereo effects (you can also use two 8Ohm cabs of cause)

    stereo: if you have stereo effects (e.g. chorus) and a preamp which can send a stereo signal, each side of the amp puts out one part of the stereo signal, you should use 2 identical cabs for each side.

    bridged (mono): both power amps work into the same load. This way you have the full power available for one cab, but minimum load is also doubled (e.g. 500W@4Ohm each side -> 1000W@8Ohm bridged). But not every amp is bridgeable, e.g. the Hartke 7000 is not.

    bi-amping: if your preamp features a crossover, you can send the lows to one side and highs to the other side.

    Matthias
     
  8. A Carvin R1000 has two power amps that each go to 2ohms so they work with about any cabs,2 or 4ohm.They sound pretty good too.Also for a preamp the Sans Amp RBI is great and goes for around $300.You could get a power amp for around $400,Carvin or QSC and have a killer rig.
     
  9. Richard Lindsey

    Richard Lindsey

    Mar 25, 2000
    Metro NYC
    No, this is wrong. Putting a 2 ohm load on each of the two channels of a stereo amp does NOT add up to a "total load of 1 ohm." If an amp is rated to deal with 2 ohms per channel, then that's what it does. Each channel has its own "total load," in effect.

    If you were to use the exact same complement of speakers and then run them in parallel off the *bridged* output of the amp, then yeah, you'd have a 1 ohm load, but not under normal running circumstances.
     
  10. lneal

    lneal

    Apr 12, 2002
    Lee County, Alabama
    Well actually there isn't a total load of one ohm on the amp. Because it is two independent power amps in one unit, there is 2 ohms on each channel, not one ohm total.
    Bridging does not put any more strain on an amp than running stereo.
    As far as running at 2 ohms, I will reiterate: most amps don't like it. This may be an arguable point, but if a Crown Macro Tech, which has 2 separate power supplies (one for each side) doesn't like a 2 ohm load, well I doubt seriously your Carvin will like it either.
    Don't get mad at me, just speaking from experience. Here is my rant about amp manufacturers.
    http://www.talkbass.com/forum/showthread.php?s=&postid=518187#post518187

    PS: Maybe Bob Lee could stick his head in here?
     
  11. 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
    Somebody call me? ;)
    Going further on what lneal said: The only thing inherently more stressful about running an amp in bridged mono (versus in stereo) is that if you're driving one channel hard, you're driving the other equally hard at the same time. In stereo, though, that may or may not be the case.

    Running at 2 ohms per channel requires the amp circuitry to put out 4× as much current as at 8 ohms, and 2× as much as at 4 ohms, for the same output voltage. Granted, you also get 4× and 2× as much power, respectively, for the same input signal and gain settings.

    But putting out that much more current is more stressful on the amp's circuitry, and the amp's internal losses (which generate heat) also increase accordingly. Thus, at 2 ohms/channel an amp is more likely to overheat or to reach the maximum safe amount of current its output transistors are capable of if it is driven hard. (If it's not driven hard, it's actually not much of a concern.) A good amp will have protection circuitry that prevents damage when that happens, but even good protection circuitry is not 100% foolproof 100% of the time.

    Some amps can handle 2-ohm loads better than others. I know of two current models in particular that can even carry an FTC power rating under the older, more stringent preconditioning rules (60 min. @ 1/3 of full power, followed by 5 min. @ full power, without current limiting, overheating, interruption, etc.): the Crest 10001 and the QSC PowerLight 6.0PFC. But those are massive, expensive amps.

    And not all 2-ohm loads are the same. Neither are all 4- or 8-ohm loads, either. A speaker's nominal impedance--e.g., 8 ohms--is just an approximation; over a wide range of frequencies, its impedance might actually range as low as 5 ohms and as high as 30 or 40 ohms. One 8-ohm speaker model might tend to have slightly higher or lower actual impedances than another 8-ohm model. So four of the slightly-lower speakers in parallel would be more stressful on the amp than four of the slightly-higher models.

    Everything in life involves trade-offs, and amplifiers are a perfect example. Running an amp into lower load impedances like 2 ohms per channel offers the advantages of economy--more power from a smaller amp--with the disadvantage of possibly lower reliability. But if you learn how to manage advantages and disadvantages, you can really reap the benefits.
     
  12. Stu L.

    Stu L. Supporting Member

    Nov 27, 2001
    Corsicana, Texas
    Wow. So, any reccomindations (sp?) on a 2 channel amp? It doesn't have to run 2 ohms. I'd only need a 2 ohm amp if it was a one channel amp.
     
  13. Golem II

    Golem II

    Jan 4, 2002
    Macon, GA, USA
    Sorry, what I said was mostly based on an earlier thread ("swr bass 750 vs. sm-500" or something like that.)
    According to one post on that thread, running a stereo amp in bridge mode into 8 ohms is equivalent to running each side at 4 ohms, 4 is equivalent to 2 per side, and so on. Is this accurate? In that case, an amp shouldn't be bridged into a 2 ohm load, and one should be careful about bridging into 4 ohms as well, correct?

    I've heard that with tube poweramps you're supposed to follow different guidelines as far as impedance goes (you generally have to manually switch the amp to the impedance you want to use, and if you don't match it correctly you can destroy the output transformer.) Some tube amps can be run at 2 ohms, does this generate extra heat and risk of failure just like with a solid state amp?

    As for recommendations, make sure when you look for a "2-channel" amp that it's a stereo amp and not just an amp with a channel select option. I'd suggest finding a used SWR SM-400 or sm-400s (they can be found on ebay at any time, but you should be careful that you're buying one in good condition, with protection in case the seller sends you a dud.) for around $400. I think it can run 200w@4 ohms per side.
     
  14. geshel

    geshel

    Oct 2, 2001
    Seattle
    This is correct. But I interpreted what you said earlier as: running an amp at 2-ohms per side is equivalent to bridging it into 1 ohm. Whereas it would really be: bridging an amp into 2 ohms is equivalent to running it at 1 ohm per side. :)
     
  15. lneal

    lneal

    Apr 12, 2002
    Lee County, Alabama
    No. A tube power amp has the transformer, which when properly loaded is kind of a buffer between the output stage and the load. The tubes are seeing the same load impedance regardless of the actual speaker impedance.
    Solid state power amps, OTOH, have the load connected directly across the output devices. This is why you have to be careful loading them.
    Yes, you can cause serious damage to a tube amp by mismatching the load. Most solid state amps have protection against this, tube amps don't.
     
  16. Pacman

    Pacman Layin' Down Time Staff Member Gold Supporting Member

    Apr 1, 2000
    Omaha, Nebraska
    Endorsing Artist: Roscoe Guitars, DR Strings, Aguilar Amplification
    My Aguilar DB750 will run down to 2 ohms all day long.


    That being said, you don't get much of a difference in volume by dropping the resistance to the head. Your money would be better spent on a more powerful head than one that can be run will a smaller load. The volume difference between 750 watts and 975 watts, for instance (in the case of the Aguilar) is negligible.
     
  17. lneal

    lneal

    Apr 12, 2002
    Lee County, Alabama
    Agreed.
     
  18. 44me

    44me

    Jun 17, 2002
    Bedford, NH USA
    A lot of good comments on amplifier compatibility with a 2 ohm load, but I’d go at it from the other side. Stu, why not buy an 8 ohm 210? A small 4 ohm cabinet doesn’t leave you a lot of options for expansion.
     
  19. Stu L.

    Stu L. Supporting Member

    Nov 27, 2001
    Corsicana, Texas
    It's only available in 4 ohms. Life would be so much simpler if it was available in 8 ohms.

    DID I MENTION I'M A MORON?

    On a 2 channel amp (P.A.), how do you split the signal to the inputs?
     
  20. 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
    If the amp has a switch or switches for paralleling the inputs, use it/them.

    Another way to do it is to use a Y-cable to send the signal into both input connectors.