running 4 ohm min heads at 2 ohm

Discussion in 'Amps and Cabs [BG]' started by Crockettnj, Oct 26, 2005.

  1. Crockettnj


    Sep 2, 2005
    North NJ
    I was always curious as to whether this was discouraged because after a time the life of the head will be reduced, or if you very likely will blow the head then and there.

    does the level of "crankage" on the gain knob matter? can you run a 4 ohm min amp into 2 ohms at low levels?

    some heads of seemingly similar design form the same manufacturer have different minimum ratings. for example, the older eden 300 and the newer eden 550. the 300 is rated to only 4 ohm, while the 550 is rated to 2. they are basically the same head though, arent they>?

    I guess my questions is, does anyone run their amps into lower-than-rated-impedance, and to what effect?

    this questions IS hypothetical... iam not experimenting.
  2. Trevorus


    Oct 18, 2002
    Urbana, IL
    Here's a good way to look at it. You have to give the amp a certain amount of load to put it's power into. If you don't give it enough, it will overheat itself, and most likely self destruct.
  3. I'm sure the techies will get in here and give you the exact scoop. I've always been interested in this general issue.

    So, to your question, running an amp into a lower load than it is rated is, as you know already, not a good idea. However, to your point, I think the whole issue is heat. If you really aren't pushing the amp so that it's not performing at 'full power' at the inappropriate load, it should be OK. HOWEVER.... you won't really ever be sure at what level the amp is operation... so, obviously, not a good idea.
  4. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Tube amp load mismatch has been discussed.... Too low a load leads to increased power dissipation, and shorter tube life, usually.....

    For solid state amps, there are three problems.... That lead to bad sound or failure.....or both.

    First, current output.... The parts are rated for a certain amount of current, and probably have some sort of protection circuit.. if you operate at a lower load than expected, the protection may operate a lot, which often sounds very bad..... loud cracking noises.... Also, if the amp wasn't designed well, the protection may not be OK with more than occasional operation.... could cause a failure....

    Second, heat... More current cause higher power losses, and unless the amp has some form of thermal protection (against too high a temperature), it may overheat and fail....

    Third, if the designer didn't do their job right with protection, a low load may lead to a condition of excessive current at high voltages.... That leads to a problem often called "second breakdown" by engineers, which means it fails... shorts the output devices. Mosfets don't have that problem, old style "bipolar" parts do.

    So tell me again, why did you want to do that?
  5. BuffaloBob4343

    BuffaloBob4343 Supporting Member

    Jul 3, 2005
    Buffalo, NY
    Power= Voltage (Squared)/Resistance and current I = V/R.

    If you decrease R, the amount of current that the amplifier draws at a given voltage will increase proportionally (and so will power produced). If you cut the load impedance from 4 ohms to 2 ohms (i.e. cut it in half), you double the current that the output section of the amplifier tries to draw.

    A power rating for an amplifier is simply a way of telling you how much current the output section of the amplifier can safely draw at a given voltage. This is continuous power versus peak power of course. As the current increases beyond the physical limitations of the devices that make up the power section of the amplifier, the materials break down and become damaged.

    Some amps have beefier output devices than others (for MOSFET and other solid state outputs stages, remember these are TINY devices formed on the surface of a semiconductor), often commensurate with the expense of the amp. The beefier they are (usually the bigger they are in area on the chip), the more power they can dissipate (the more current they can handle at a given voltage).

    Some amps also have limiting circuits that kick in as the current exceeds some threshold that can actually collapse the supply voltage as the current increases beyond that threshold. This essentially keeps the power dissipation from increasing much further, or may even even lower it some.

    This is the equivalent to a balloon that starts to overfill with air approaching its bursting point and when it gets too close, a valve kicks open and starts to let air out at a rate that equals or even slightly exceeds the rate its flowing in. If the valve flow exceeds the inflow rate, eventually the amount of air ion the balloon falls below the threshold that opened the valve and the valve closes until the threshold is exceeded again.

    If an amp is designed to handle a 2 Ohm load, it either has beefier output devices (e.g. the balloon is thicker) or the maximum power is limited or both. Some mfgrs just don't know what long-term use at a certain power rating will do to the reliability and/or longevity of their product, so they guardband the specification (i.e. they cover their ass) and won't spec below a certain load impedance (e.g. 4 ohms) that might lead to eventual, even if not catastrophic, failure.
  6. Crockettnj


    Sep 2, 2005
    North NJ

    I never told you i wanted to do that in the first place. I was curious if anyone did this and if, in practice, it was as bad as was thought to be (as per p=i^R , decreasing r boosts i and all) , OR if in the real world if amps had some wiggle room built in.

  7. BuffaloBob4343

    BuffaloBob4343 Supporting Member

    Jul 3, 2005
    Buffalo, NY
    Most will guardband (cover their ass) and so it may be that your amp can to handle a 2 Ohm load. However, woe to the person who exceeds that spec and destroys their amp, for you will have operated outside the spec'd operating conditions and will have no recourse with the mfgr (i.e. warranty null and void).
  8. 4Mal

    4Mal Supporting Member

    Jun 2, 2002
    Columbia River Gorge
    In my experience musical equipment that is pushed to extremes tends to fail. For the most part, I'm happier running my SS amp, rated at 4 ohms, into an 8 ohm load. The idea of ignoring the manufacturer's advice seems ... well, not a well thought out approach to me.

    There are intellectually interesting issues raised by the topic I'll guarantee. More from a design and manufacturing point of view though. As manufacturing a product takes a seriese of compromises. The ways that various companies make those compromises defines how we eventually come to view their product and by extension their company. Not many of us are going to confuse Hartke with Epifani or Yorkville Tube amp with an Aggie right ? It's all about the price point. Which from a practical application standpoint, renders the topic moot (IMO).
  9. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    OK.... I asked as a sort of a "rhetorical question" anyway....

    There usually IS "wiggle room".... But the problem is if you use up all teh wiggle room, you might have a problem....

    Different companies differ in the use of 'guard bands" or "wiggle room". Most designers would set it up so it won't "die" or sound bad at the next level down, i.e. 2 ohms vs 4... But they are not expecting someone to use it there....

    The CYA action is because speakers differ, and speakers can "look like" a lower resistance than they measure, when driven by an amp. So the extra range is really to assure good operation at the rated impedances.

    Sometimes you will see a 2 ohm rating with different or restricted specs... like a higher distortion, or the like... That is a clue that they really don't want you to do that, although it is *possible*....

    An exception to that is QSC... they are pretty conservative, and they do often give restricted specs, but if they quote a usable impedance, you can run there.
  10. Crockettnj


    Sep 2, 2005
    North NJ
    " CYA action is because speakers differ, and speakers can "look like" a lower resistance than they measure, when driven by an amp. So the extra range is really to assure good operation at the rated impedances."

    thats pretty much where i was oging with this ultimately. It was my understanding that drivers have difference impedance at different frequencies (or do they!?) and therefore isnt it possible that at some point, depending upon the style/type/song/notes you play, you could be operating below the minimum rec's.

    thanks for the replies all!