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More Obnoxious Impedance Questions

Discussion in 'Amps and Cabs [BG]' started by Deeter, Dec 17, 2002.


  1. Deeter

    Deeter

    Dec 5, 2000
    San Fransico, CA
    Okay, I understand the fundamentals of figuring out impedance when it comes to using multiple cabinets and how those cabinets are connected to the head. My question is more in terms of drawbacks to running at various impedances.

    I've got a SWR Bass 350 head, capable of running at 8, 4, and 2 Ohms. (at 240, 350, and 450 watts, respectively) I'm going to buy a 4x10 cab for it and was thinking that I ought to go for a 4 Ohm cab so that I can get decent power output from it when played on its own. But should I want to add another cab to it, it would make it run at either 2 Ohms, parallel or 8 Ohms, series.

    Will running it at 2 Ohms cause wear issues with the head? I mean, will it shorten the effective life of the head, or have any other drawbacks in spite of the fact that the head is rated to handle cabinets at 2 Ohms?

    Thanks in advance . . .
     
  2. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    Running at 2 ohms makes an amp work much harder, that's for sure. They typically run a bit hotter as well.

    If its rated for 2 ohms, you should be OK. It likely won't ruin its life in the short term, but it might shorten the life very slightly over the course of a decade or more from the added heat and stress.
     
  3. Solid-Body

    Solid-Body Guest

    Nov 10, 2002
    Exiled to Australia
    The advantages of running lower ohms is more kick. The lower ohms mean less resistance, which means more current. Speakers respond to more current with more air volume. More air volume means more uhmph.

    As Eric Moesle said, the balance of that is more heat and work for your amp. But if the amp is rated for 2 ohms, then the effect is negligible.
     
  4. Lord Valve

    Lord Valve

    Dec 13, 2002
    Denver, CO.
    This is utter nonsense. Several factors come into play at very low impedances, not the least of which is speaker-lead resistance. Add in power supply sag and lack of dynamic headroom (not to mention the decreased damping factor) and you have a recipe for flabby low end - the difference between tight, punchy kick and a bunch of elephant farts. Sure, many amplifiers are rated by the manufacturer for 2-ohm operation; this just means that they won't fail when this is done, not that it's a desirable practice tonally. The trend these days is to buy a lower-powered amp that's rated for two ohm operation and then run it into a four-ohm bridge; this maximizes watts-per-dollar but it minimizes tone while maximizing stress on the output devices and the power supply. Certain amps, however, are *optimized* to run the lower impedances ("high current" designs) and these are largely free of the problems (and the negative tonal mojo) encountered when running an amplifier at its lowest rated impedance. One of these is QSC's RMX-1850HD, which makes a killer bass amp as long as you eat your Wheaties. (It ain't exactly light. ;-)

    Consider this - your amp sees the resistance of your speaker cable as being in series with the actual load (speaker) the amp is driving; using round numbers, let's say your cable presents a half-ohm load to the amplifier. (This is not unreasonable, given an "average" quality cable that's been indifferently maintained, i.e., has dirty contact surfaces, plating worn off, etc.) If you're driving an eight-ohm load, the amp will see the total (including the cable) as an 8.5-ohm load. This means that 16/17 of the amp's output power will reach the speaker, and 1/17 will be dissipated as heat by the cable. Use this same cable with a 2-ohm load and things are quite different - that identical 1/2-ohm cable is now going to turn ONE FIFTH of the amp's output power into heat. Now, this isn't necessarily a situation you can expect to encounter frequently (although I've seen this, and much worse) but it does point out that there's more than meets the eye to driving very low impedances in a stage situation, where every watt counts, than just ohm's law vs. manufacturer's rated specifications.

    My take on this is that you should run higher impedances if you can afford to buy an amplifier of the power necessary to do so. If, for instance, you are going to buy a bass rig, and you're looking at a QSC PLX-2402 and a pair of Acme Low Bs, you have two options: First, you can buy two 8-ohm boxes, put them in parallel, and operate the amp in bridge mode at 4 ohms. This gives you 2,400 watts of available RMS power, but it operates the amplifier at design maximum as far as output stage current, cooling, etc. are concerned. (The cautionary tale about speaker-lead resistance, above, also applies.) A better choice would be to buy 4-ohm cabinets, and run the amp in stereo (in this case, dual mono, given a mono signal input), giving you an available output power of 1,400 watts RMS. Now, on the face of things it would appear that you've just spent the same amount of money on an identical rig which is 1,000 watts less powerful than it could have been, but you've gained conservative operation, some dynamic headroom, cooler operation, and greater reliability. (Remember - an amplifier which is not operational won't make any more music than a brick will, regardless of how powerful it was before it crapped out.) Now, if you really feel you *need* all of that power, then you can step up to a PLX-3402, run it in dual-mono mode at 1100 watts/channel/4-ohm load, and have 2,200 watts available. At this point, I can see the boys in the back of the class waving their arms in the air and saying, "Hey, wait a minute - if I'm only going to have 1,400 watts doing it the way you recommend, why can't I get a PLX-1602 and run it at four ohms in bridge mode, so I can have 1,600 watts and save ~$200?" Well, yes, you can do this - but if you do, it just means you weren't paying attention. While the numbers look good on paper, the stage is a different situation. Any time you can gain conservative operation, you gain reliablility. The fact that your rig will sound better at any comparable output level is just icing on the cake.

    Now, remember - I *sell* amplifiers. It's to my advantage to have you buy larger ones - after all, I make more scratch on 'em. But it's to *your* advantage to buy a larger amp and run it in a conservative fashion, both tonally and (in the long run) financially. Looks like a win-win situation from where I'm standing. You can't violate Lord Valve's First Law of Equipment Purchasing: "Good sound costs big bucks." Spend 'em - you won't regret it in the long run. ;-)

    Lord Valve
    American
     
  5. I think you should run it at 4 ohms. It is really designed to run at 4 ohms or higher. Your manual says " **Running the amplifier constantly at 2 ohms, while technically acceptable, will cause the amp to run hotter than usual, and will cause heat-related wear on components sooner than normal." 2 ohms is the lower limit and could thermal off when you need it the most. The 350 should be fine as long as you buy a fairly efficient cabinet (97 db or higher) Don't expect it to drive a ACME b4 however, it just doesn't have enough juice. If you are really thinking of adding another cab, then do it now. Get 2 8 ohm cabs and be done with it.
     
  6. Solid-Body

    Solid-Body Guest

    Nov 10, 2002
    Exiled to Australia
    <HR>
    This is utter nonsense. ...
    <HR>

    I disagree that my statements were utter nonsense, but I do agree with your argument. I was ignoring several factors for the sake of simplicity. (BTW, I came across a cable with 1.5 ohms of resistance. Wasn't soldered properly.) The driving force in a speaker is current, not voltage. The result of more current is (assuming the speaker can handle it) more cone displacement, hence air movement. This is, of course, all theory. Reality, as your examples point out, may vary.

    I didn't mean to imply that you should run at 2 ohms, only that it was possible since the amp is rated for a 2 ohm load and the damaging effects to the amp are negligible. (If the leads have a high resistance, the damage to the amp is even less.) With the wrong setup it will definitely not sound good. But with the right setup it may sound superb. I simply meant to state the advantage of running at lower ohms.

    I digress, as you obviously have more experience with these cabs than I do. (My experience in this field is more with car and home audio.)
     
  7. geshel

    geshel

    Oct 2, 2001
    Seattle
    It's utter nonsense because:

    1. 8-ohm load
    8 volt signal
    1 amp current
    8 watts

    2. 2-ohm load
    4 volt signal
    2 amps current
    8 watts


    Voltage and current work together. The end result is POWER. According to your theory, Setup 2 is preferable because there's more current. But this just isn't true. The power output is the same. And, for the reasons Lord Valve mentioned, there are actually advantages to shifting things to the voltage side a bit (within limits).

    Another way of looking at it: yes, the electromagnetic operation of a voice coil is dependent on the amount of current in the coil - times the number of turns of wire. The biggest part of making a speaker 4 ohms instead of 8 ohms is to cut the number of turns - nearly in half. So, it equals out in the end.
     
  8. It's not exactly wrong what you're saying. But your description of what happens is strange.

    A lower impedance means more power. There's really nothing more to it (the fact that voice coils respond to current is not important in this context). More power means a higher sound pressure level. Period.

    ...and all the things Lord Valve said :D

    BTW Lord Valve, if you bash someone's post like that, I hardly think he'll listen to what you have to say, even if it's true (which it is).
     
  9. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    . . . and the Lord hath spoken . . .

    Don't bother lecturing him on etiquette. Nobody has EVER accused him of being anything less than blunt. ;)
     
  10. Solid-Body

    Solid-Body Guest

    Nov 10, 2002
    Exiled to Australia
    Geshel. That's exactly the opposite of what I'm trying to say. You're scenario 2 will output a larger volume acoustic wave (what i referred to as uhmph originally) because there is more current, at least in a speaker and amp that can handle that kind of output. (That's also assuming the same speaker and amp for both scenario 1 and 2, which I realize is difficult, if not impossible.) Its somewhat like trying to compare apples and oranges with two different speakers (especially two with different impedance values) because of differences in the materials, craftsmanship, and design.

    Take note of <a href="http://melhuish.org/audio/article5.htm">this</a> article (its a bit long, but note the 7th to the last paragraph where it states "We do not hear the voltage signal across the amp's output terminals, we hear the current through the cables, crossovers and voice coils." A speaker is really a linear motor. Electric motors gain torque (turning power) by current. Linear motors don't have torque because the don't turn, but they do still have motion. The three factors that affect this motion are magnetic flux strength, current, and the angle at which they cross. In this regard, power (including the voltage variable) is irrelevant.

    In other words, 2 amps of current through a voice coil will displace the cone more than 1 amp of current through the same voice coil, regardless of the voltage potential across the coil's leads.

    I enjoy a good discussion and this one has had me thinking long and hard.
     
  11. geshel

    geshel

    Oct 2, 2001
    Seattle
    No. If the speaker was EXACTLY the same, of course it'd be 8 ohms too, and double the current would be double the power, and you'd get 3dB more output.

    If you change the impedance of the speaker, and EVERY other parameter is the same (this includes efficiency at 1w/1m), the acoustic output between 1 and 2 will be EXACTLY the same.

    What does "larger volume acoustic wave" mean? You're getting off into the land of make-believe here. There's SPL, then there's SPL. You can't get the same SPL with more or less "oomph" at a constant frequency. This is like the "tube watts are louder than solid-state watts" discussion.

    If you put one watt at 1kHz into a speaker that produces 90dB with one watt at 1kHz, you get 90dB out. It doesn't matter if it's a 4 ohm speaker or 8 ohm speaker. Really. Trust me.
     
  12. geshel

    geshel

    Oct 2, 2001
    Seattle
    P.S. I *don't* necessarily disagree with anything in that article, and my point doesn't contradict anything it says.
     
  13. Lord Valve

    Lord Valve

    Dec 13, 2002
    Denver, CO.
    Sure he will - depending on how *smart* he is. The fact that I'm an ******* has nothing to do with it - the Internet is full of 'em. BTW, all of you "current = tone" freaks out there should really consider running a zero-ohm load. After all, if current = tone, then the infinite current that your amp would put across a zero-ohm load would have to be the best tone possible. A little smoke never hurt anybody.

    Lord Valve
    American


    edit: Moderator. Language
     
  14. Solid-body, an amplifier output is a voltage source, although there are exceptions. It's more convenient to look at a speaker as a voltage-input device because of that. Also, impedance varies widely over the freq range, and current varies with it. It would really complicate matters.

    The fact that you're an ******* (I'm only quoting you) might get in the way of expaining technical issues, which you do very well, but most people will not read through your bluntness. I know you don't care (I read the other thread in which you participated) but I think it's "unfortunate". More people would value your knowledge.

    I once read an article for an amplifier design with a true current output. It was to drive a ribbon speaker with a single strip of aluminium as a ribbon. Is that close enough to 0 ohm? :D


    edit: Moderator. Language
     
  15. 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
    No.

    SB, you're treating voltage, current, and power as if they were independent things that just happen. They are not independent; they are tightly and irrevocably interrelated, as Georg Ohm first published way back in 1827 in Die galvanische Kette, mathematisch bearbeitet.

    In a linear load, such as a loudspeaker, the current through it is exactly proportional to the voltage applied across it. Current is the rate of electron flow, and voltage--or potential--is the pressure forcing the flow. In order to double the current in the load, you must double the voltage. If 2 amperes of current is flowing through the voice coil, you can be assured that the voltage across it is twice what it takes to push 1 ampere through it.
     
  16. Solid-Body

    Solid-Body Guest

    Nov 10, 2002
    Exiled to Australia
    Bob Lee:

    I agree. But what your assuming is that the resistance is fixed. I'm talking about varying the resistance. [I realize that the speaker's resistance does vary, but it is more frequency dependent. It relatively centers around the rated resistance. For simplicity, I'm ignoring that fact.]

    If you have 1 amp through the coil at x voltage and y resistance, then y/2 resistance and x voltage will induce 2 amps current. Thats roughly what we have when we put the speakers into a 2 ohm load, but its not exactly true as not all of the current is flowing through both speakers.

    Lord Valve:
    It is convienent to think about speakers as voltage-input devices, but the fact is they respond to current, not voltage. And if the amp is a voltage source, that means the voltage will remain somewhat constant. By cutting the resistance, you increase current flow. And I'm not talking about tone, I'm talking about volume.

    The speaker must have some resistance or it will not respond. So a 0 ohm load is not ideal.

    Well prove myself wrong or not, I'm going to setup an experiment. I'll let ya'll know how it goes.
     
  17. geshel

    geshel

    Oct 2, 2001
    Seattle
    You don't sound like somebody who has any education on the subject. The fact remains: whatever impedance the speaker is at N Hz, you can a) put X volts across it, and get Y amps current; or b) put Y amps through it, generating X volts across it.

    You misunderstand the fact that the force inside a voice coil is generated by current. The current and the voltage aren't independent (I'm repeating what's been said several times here). By saying "the speaker responds to current", you're propagating a misconception. It responds to voltage, too! If the voltage source can provide the current that will be drawn. Likewise, you can try to pump current through it, but if the source can't stomach the resulting voltage, it'll distort.

    Finally, the speaker could very well have 0 resistance (superconducting wire). It would still have impedance, the inductance, capacitance, and physical impedance of the cone motion would add up to some non-trivial (over 1 ohm at most frequencies) amount.

    And, even if the total speaker impedance was zero ohms, you of all people should realize that if you hooked a current source up to it, it would produce sound. Current would flow through the voice coil, producing force. Doesn't matter that the voltage across it is zero.

    You probably think I've contradicted myself here, but I haven't.
     
  18. Solid-Body

    Solid-Body Guest

    Nov 10, 2002
    Exiled to Australia
    Now I understand how we keep sliding by each other's intentions.

    I think the problems lies in our interpretation of my word "uhmph". I never meant to call it volume or SPL [read loudness], because that is not what I'm talking about. I meant that the sound is felt more. SPL does not factor this in. I did not intend to say that if you put the speakers in parrallel that they are twice as loud. I meant to say that they would produce more feel, or as best as I can describe it, move more air.

    I left out the voltage factor to a speaker, but after some research I understand where it factors in. It has nothing to do with Ohm's Law or the source (amp), it has to do with creating the magnetic field around the coil through which the current repsonds. On this point I was incomplete. In this respect, 8 watts is 8 watts. Both create (roughly) the same SPL.

    But, this is an imperfect world. Here is the meat of my generalization. High current is better for low frequency sound reproduction because it implies low voltage. With a low voltage across the coil, the magnetic field is weaker. But the high current makes up for this difference. A weak magnetic field resists movement less. This allows the high current to move the coil more, which in turn moves more air. The opposite is desired for high frequency reproduction. A stronger magnetic field (because of higher voltage) and lower current means the speaker moves less, and therefore distorts less. The movement is desired for high frequency because of the shorter wave lengths.

    This is why, generally, tweaters are high impedance ~32ohms and sub-woofers are low impedance ~4ohms.

    BTW:
    A shorted (0 ohm) coil will not respond as expected because the voltage drop across the coil is 0, thus only straight wire magnetic flux lines occur. These cause the coil to move in one direction only, making the speaker act more as a solenoid. Therefore, a 0 ohm coil is not ideal. Again, I was not exact. The coil does move, but not like a speaker.


    Wrapping up:
    I have taken some side-roads in my previous reasoning because of time contstraints on my replies (I was at work) and therefore did not say exactly what I meant. For that misdirection, I appologize. I've had time to think about and discuss it with others. I most certainly did not mean to incite any flame war, which I feel that this discussion has avoided. There may still be some disagreement with my argument, which I will say may not be 100% accurate, but I feel it's not too far off the mark.

    Simply put, I was just trying to offer a reason why anyone would even offer a 2 ohm load amp. Granted, that does not mean the 2ohm setting is ideal for every situation. 2ohm is usually more distorted than 4ohm. But the amp is still rated for a 2 ohm load, and while working harder, it's still not working outside of its rated specs.

    Side Note:
    The only way to know for sure is to try it. But to try it, you must know the capabilities of your equipment. Since your head is rated for 2 ohms, you shouldn't have any trouble there. What I cannot tell you is whether or not your speakers can handle it. They do play a big factor. Check the specs and see what they can handle. I have tried it with an Orion Red 1600Wrms Dual Voice Coil Sub and a Sherwood BP2400 amp. I can feel the difference, but, sadly, I cannot quantify it. As I said, my experience is more with car and home audio. I can not garuantee that your cab will perform similarly. I was giving a generalization.
     
  19. 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
    Hey, man, you still don't get it. I think you should stop now because your explanations keep going off into uncharted territory. If you're interested in learning how amplifiers and speakers work, there are some good books on the topic. I would be happy to recommend any.
     
  20. After reading this thread, all I can say is "Sweet merciful crap!"

    Chris