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Ohm my god.

Discussion in 'Amps and Cabs [BG]' started by Joe Nerve, Jul 31, 2004.

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  1. Joe Nerve

    Joe Nerve Supporting Member

    Oct 7, 2000
    New York City
    Endorsing artist: Musicman basses
    I must be electonically challenged, big time.

    I've asked this before but either forgot, or simply didn't understand the answers I got, so I'm going to aske again. Please explain how the ohms thing works, what you can and what you can't do regarding them, and explain as though you were telling a first grader.

    If an amp says its 4 ohms per channel what speakers is it safe to connect? Why? What if it says use only 8 ohm or below (above?) per channel???? Is it safe to go higher in ohmage (is that a word) or lower???? What causes a head to burn out as our PA head did last year?????

    Please offer as much info as possible and I'd appreciate a control on the sarcasm. Last time some people got a little snippy at my lack of understanding. No snippy dammit! I don't like snippy. Please tell all you can now regarding ohms so I can get this down once and for all and be able to plug our PA head into club speakers without any fear or worry.

  2. natrab


    Dec 9, 2003
    Bay Area, CA

    The way I look at it, is Ohms dictate how much power goes to your speakers. Amps can be set to put out their full power at different ohm levels (my bass 400+ can put out full power at 2, 4, or 8 ohms using separate jacks).

    I basically add it up like balancing a scale. On the left side, you have your amp's recommended load (we'll say 4 ohms for now). On the right side you have your speakers load. These have to balance out or else you'll either fry your amp or be drastically underpowered. So if you have a single 4ohm speaker cab, that will balance it out perfectly. If you put on two 8ohm cabs, then you will also have 4 ohms (see, we're in the magical world where 8+8=4 and 4+4=2).

    Same thing goes for any ohm level. If you amp goes down to 2 ohms, then you can connect two 4ohm cabs to equal your amps rating.

    Now if your amp says 4ohms per channel, then that means you have two separate channels that can each have a 4ohm load hooked up to it. That way you can hook up two 4ohm cabs to an amp rated at 4ohms (but with two channels).

    What causes an amp to burn out? Say your PA amp put out 800 watts at 4ohms. You plugged two 4 ohm cabs into one channel on your PA. Now your PA is doing 2ohms which is a lot more than 800 watts (probably around 1000 or so). Since your amp was not built for that, it got too hot and fried.
  3. Okay, here's how it works. An "ohm" is a unit of resistance, how much resistance something has with respect to electricity flowing through it. An analogy would be a garden hose. You can't flow much water through a 1/2 inch garden hose, but a 4 inch fire hose can pump alot of water. So if the hoses were wires, the tiny one would have a lot of resistance, so it would have a large number of ohms, let's say 8 ohms. The large hose doesn't offer much resistance, so it has a lower resistance, let's say 2 ohms.

    Now lets hook up these hoses to a big tank and hook a 1 inch water supply pipe up into the top of the tank (the 1 inch line will try to replenish the water in the tank.) Turn on the 1/2 inch hose, water comes out...but the 1 inch supply pipe can keep filling the tank up as fast as the 1/2 inch hose can let it out. So far, so good. Ya with me? But suppose you turn on the 4 inch fire hose....the fire hose is really big, has very little resistance, lots of water flows out. In fact the tank will be empty pretty soon because the 4 inch fire hose will let water out faster than the 1 inch supply pipe can replenish it. So the tank will run empty.

    This is what happens when you hook a 2 ohm speaker up to an amp than can only supply 4 ohms. The low resistance draws more current than the amp can supply...the tank runs empty...or in this case, the amp overheats and either shuts off or burns up.
  4. so, I have an amp classified as a "little 1/2 inch hose." now lets say I'm planning on getting a larger amp that pumps out four times the amount of "water" at "four inches", but it's main hole is around two inches. is it going to spank out more percieved loudness or be around the same??
  5. In my analogy, the hose would be the speaker, not the amp...the tank is the amp....

    I'm not sure if I understand the question, but in an amp, when you turn up the gain (volume), the amp will increase the voltage going out to the speaker. The speaker's resistance--let's say it's 2 ohms--will stay (almost) a constant 2 ohms. Ohms law, V=IR, says that for a constant resistance, if the voltage increases, the current will increase also. Eventually the amp will reach a point where it cannot put out any more current, it will then shut off or burn up. If the amp is not designed to run into a 2 ohm load, merely plugging in a 2 ohm speaker can fry or shut off the amp,even at lower volumes....

    Now let's say you have two amps. Both are rated 1000 watts into 4 ohms, but one of them can also run a 2 ohm load. With a 4 ohm speaker, either amp will put out 1000 watts into a 4 ohm speaker. Either amp could run an 8 ohm speaker.
  7. well, allow me to correct myself....on the back of my amp for example....I was using the "1/2 inch hose" terminology because on the back of my amp it does not say that the amp will put out 65@8 ohms, it merely says that's the powering of the speaker, but I believe that is what the amp will push, considering it's pretty loud.
  8. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA

    Jeez, you're not asking for much, are you? :)

    Okay, here's the skinny.

    The "meaning" of ohms (for speakers relative to amps) depends on whether you have a tube amp or a solid state amp. I'll cover the tube amp case first, and then the solid state case.

    Every tube amp has a "rated" impedance, which has to do with the way the output transformer "transforms" the high voltage from the output tubes. The general rule for tube amps is, that you can connect any load impedance that's between 50% and 200% of the amp's rated impedance. So, for instance, let's say you have a vintage Bassman head that's rated at 4 ohms. You "should" connect a 4 ohm cab to it, but you can safely connect either a 2 ohm cab or an 8 ohm cab. I wouldn't go outside of that range though. For tube amps, the optimal power transfer will occur when the load is "exactly" matched to the amp, and will drop off in either direction away from that.

    Solid state amps are different. Most solid state amps have a "minimum" rated impedance, below which it's not safe to go. So, for instance, if you have a Walter Woods Ultra, and it says "minimum impedance 4 ohms", you can use any cab with a 4 ohm or higher impedance. But do NOT connect a 2 ohm load, 'cause you'll surely fry the amp. For solid state amps, a lower impedance generally means a louder signal. So, for instance, continuing with the Ultra example, your amp will deliver about 1200 watts into 4 ohms, but only about 800 watts into 8 ohms (and 600 watts into 16 ohms, and so on).

    When connecting multiple cabs to an amp, one has to be aware of the wiring inside the amp. In the vast majority of cases, if the amp has multiple speaker jacks, they'll be wired in parallel. That means, when you connect two cabs, the total ("aggregate") impedance will obey the rules for resistances in parallel. So, for example, two 8 ohm cabs in parallel will result in a 4 ohm aggregate load impedance.

    But some amps use "clever" wiring methods to play games with the output impedances when multiple cabs are plugged in. This would be true, for instance, with a late 70's Twin Reverb. When only one speaker jack is used, the amp uses the output tranformer's 4 ohm tap. When the second cab is plugged in to the extension speaker jack, the amp switches over to the 8 ohm tap, and the two cabs are actually connected in series.

    So here's the things to be aware of. I'm going to assume you have a solid state PA (haven't seen too many tube PA's lately). First, you need to know the "minimum rated impedance" of each channel of the PA. From your first post, it sounds like that might be 4 ohms. That means you must only connect a 4 ohm or higher "aggregate load" to each channel. If you're using one speaker per channel, that speaker could be either 4 ohms (loudest), or 8 ohms (not as loud), or 16 ohms (pretty quiet). But don't try connecting a 2 ohm load, that'll surely fry the amp. If each channel has multiple speaker jacks, verify that they're wired in parallel (most of the time it'll say so right on the back of the amp). If they are in fact parallel, then you can connect two 8 ohm cabs to each channel (which would result in a 4 ohm aggregate load for each channel, which is safe), but don't connect two 4 ohm cabs to each channel (which would result in a 2 ohm aggregate impedance for each channel, which is unsafe).

    Does that answer your question?
  9. BruceWane


    Oct 31, 2002
    Houston, TX
    We might want to mention the actual formula to calculate total load............

    R = individual cabinet impedance
    T = total load at the amp

    1/R + 1/R + 1/R = 1/T

    So let's say you've got two 8 ohm cabs.........

    1/8 + 1/8 = 2/8, which equals 1/4, total load is 4 ohms.

    Let's say you've got two 4 ohm cabs.........

    1/4 + 1/4 = 2/4, which equals 1/2, total load is 2 ohms (CAUTION - Many amps won't handle 2 ohm loads)

    Let's say you've got three 8 ohm cabs......

    1/8 + 1/8 + 1/8 = 3/8. which equals 1/2.6666; total load is 2.6666. Note that you must reduce the fraction to 1/T

    You can always run a load that is higher than an amps minimum. Practically all amps are OK down to 4 ohms, some down to 2, and remember when working with 2 channel amps that this is per channel.

    Look at the speaker load as if the speakers are actually going to suck wattage out of the amp; more speakers are going to suck more wattage, too many speakers are going to suck more than the amp can provide. The lower the calculated load, the more suction you'll have.
  10. TheChariot


    Jul 6, 2004
    Boston, MA
    Ohms (or impedance) gerenally means "Electrical resistance". That tidbit might help you understand a lil more... and the rest I'm not gonna repeat, because a lot of it is stated above.

    It takes a while.. but you'll catch on... and a lot of it depends on if your using a SolidState or Tube amp.. cuz they're kinda 2 different ballparks.

    Let the information soak in :bassist:
  11. jondog


    Mar 14, 2002
    NYC metro area
    It might be easiest/safest if you tell us what equipment you've got and what it says next to the jacks, then we can tell you what you can and can't do.
  12. emor


    May 16, 2004
    I'm still trying to figure out where I'm supposed to hook up the garden hose to the back of my amp ....
  13. IvanMike

    IvanMike Player Characters fear me... Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    not to mention how to water my lawn with my cabinets..........
  14. emor


    May 16, 2004
  15. IvanMike

    IvanMike Player Characters fear me... Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    does it filter too?
  16. E-ROC


    Feb 17, 2009
    Endorsing Artist: NS Design
    Does anyone know if the outputs on my Walter Woods are wired in series or parallel? It's a 2003 Electroacoustic 450 @ 8 ohms 650 @ 4 ohms model. Any info would be greatly appreciated.
  17. Walter Woods does... :)
  18. uaudio


    Apr 11, 2008
    This is possibly the most random thread revival of all time.
  19. Vakmere


    Sep 6, 2007
    the hose connects to the speakon connectors via hose adapter. Home depot sells them in the garden section.
  20. Joe Nerve

    Joe Nerve Supporting Member

    Oct 7, 2000
    New York City
    Endorsing artist: Musicman basses

    I completely understand the ohms thing now too. :) But what will happen if I put a 4ohm and an 8 ohn speaker with my new carvin head that says it can carry a limited (whatever that means) 2 ohm load.

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