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difference in impedence, bassist point of view?

Discussion in 'Amps and Cabs [BG]' started by SirPoonga, Feb 24, 2005.

  1. SirPoonga


    Jan 18, 2005
    I've been trying to get the idea of what the differnce in 4ohm and 8ohm are for a bassist. Most of what I have searched for is technical discussion. In layman's terms, what is the difference?
  2. Happy MurphDay

    Happy MurphDay

    Mar 9, 2004
    the resistance of the speaker, the lower the ohm, the more power an amp puts out,

    amps can only handle a certain ohm, most heads can take a total 4 ohm load, so two 8 ohm cabs gives a 4 ohm load, thats why most cabs are 8ohm,
    two 8 ohm cabs = 4ohm
    two 4 ohm cabs = 2 ohm
    a 4 and 8 ohm cab together = 2.67 ohms

    if you combine cabs that have a 4 and 8 ohm cab, you need a amp that can handle a 2 ohm load because its to low of a load for a 4 ohm setup
  3. SirPoonga


    Jan 18, 2005
    Right, I know the technical side of it and have seen all the formulas. But would I want to choose 4ohm over 8ohm? Is there a difference or just get a cab to match an amp for whatever amp you get?
  4. Ben Clarke

    Ben Clarke Liquidating to fund a new business. Buy My Gear!

    Jan 6, 2005
    Western NY
    There really isn't a subjective side to it. It's really just a matter of amp/cab matching.
  5. SirPoonga


    Jan 18, 2005
    Cool. I'm a newbie to amp and cabinets. Going to be getting my first one. Just wanted to make sure I knew what to look for. Thanks.
  6. Not sure if this helps but here goes:

    My Ampeg V4B can run at 8 or 4 or 2 ohms. I've run it with an 8 ohm cab, two 8 ohm cabs for 4 ohms, one 4 ohm cab, and two 4 ohm cabs for 2 ohms. Regardless of the config. it still sounds like a V4B and puts out about 100 watts.

    That being said, not all amps behave this way.
  7. Happy MurphDay

    Happy MurphDay

    Mar 9, 2004
    Yeah, but you have a tube amp, tube amps can take different loads unlike a solid state amp.

    It is up to you to decide which cab ohm youll want to get, if you plan on getting a cab, and adding a second cab later, get the 8ohm, unless your amp can handle a 2 ohm load.

    but if you want just a single cab, get a 4 ohm so that your amp is using more watts, it wont be a huge difference, but youll get a little more volume
  8. My intent was to show that the ohms number does not necessarily influence power or affect tone.

    It was in response to the original question of "I've been trying to get the idea of what the differnce in 4ohm and 8ohm are for a bassist" posted by the thread starter.

    I chimed in with that angle because I occasionally hear people imply that 4ohms is louder than 8ohms (and vice versa) as though the ohms are what make the power!?! These usually are the same folks who believe 12" speakers are always louder than 10".

    At the end of the day the best one can do is to determine what is the minimum wattage they need to be happy and then connect there favorite amp providing said wattage to the load its designed for.
  9. Thor

    Thor Moderator Staff Member Gold Supporting Member

    While I whole heartedly agree with your last paragraph, I
    must disagree with this statement.

    The following statement is always true:
    V = I x R Ohm's law volts = amperage x resistance
    so is this:
    P = I x V Power (watts) = amps x volts or:
    P = I(Squared) x R

    Thus, Power and Resistance are closely related by definition.

    A 4 ohm speaker does perform more efficiently in the SAME
    circuit than a 8 ohm load would. Whether that is desirable is
    subject to the individual application and rig and an altogether
    different question.

    This article has a very interesting discussion of one of the
    finer points of the desirability and application of a small
    4 ohm driver in a practice rig, at 30 hz and high SPL's.

    I revised my thinking on 4 ohm drivers after reading this article which was published in Speaker Builder about 10 years ago.
  10. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Actually, in a way, tube amps always drive the SAME load.

    The transformer causes the tubes to "see" the same load regardless of nominal speaker impedance. So an 8 ohm load on the proper 8 ohm tap "looks" the same to the tubes inside the amp as does a 4 ohm load on its proper 4 ohm tap.

    The voltages and currents are all the same right through the amp, regardless of 4 vs 8 ohm load. They only change in the output transformer's secondary winding, which is to some extent "outside" the amplifier.

    By contrast, in an SS amp, for a 4 ohm load, currents are different all through the amplifier than they would be for an 8 ohm load. Internal loading changes, there may be internal gain changes, etc, etc. For 2 ohms, the changes are more extreme. Good design minimizes the sonic effect of the changes, but they are there.

    Of course, the SS amp also tends to produce more power into a lower impedance. This approaches 2x the power for 4 ohms vs for 8, and 4x the power for 2 ohms vs 8. How close that gets to theoretical depends on the power supply and various circuit features.

    Sonic differences are real at different load impedances for SS amps.

    Look at many power amp specs. They tend to give different specifications for 8 ohms than they do for 2 ohms, if the amp can drive 2. Often, the 4 or 8 ohm specs will be very complete, including power, frequency response, distortion, damping, etc. The 2 ohm spec may be 1 kHz power and very little else, depending.

    That does not mean the amp won't meet the specs at 2 ohms. It does not mean the amp will sound bad at 2 ohms, either.

    But it may mean that the manufacturer does not want to have to guarantee to meet the same THD specs at 2 ohms as at 8, for instance. it can be done, of course. But it may require a different design and more cost to make an amp that will drive 2 every bit as well as 8, versus one that has "2 ohm capability".

    And not every amp will drive 2 ohms well as a subwoofer amp in the PA, either. Again, design differences, likely in the power supply.

    Bottom line:

    For an SS amp, I'd tend to NOT select the lowest possible impedance load. Usually sonic performance at the "next step" up is better. So a "2 ohm capable" amp will run longer without shutting down, and may sound better at 4 ohm load, or even at a combination giving 3 ohms, than it will at 2.

    For a tube amp, do whatever, so long as you use the right output for your load. Makes no difference.
  11. Happy MurphDay

    Happy MurphDay

    Mar 9, 2004
    King, I wasnt trying toinsult you, I was just trying to mention that your amp was a tube amp, and I wouldnt think that a "newbie" would know the difference, and you didnt mention that you have a tube amp. i wasnt trying to come across as argumentative, but the above info clears up a bunch
  12. I do not know Ohm's Law. I will tell you that up front. So with that in mind, why does my V4B output the same wattage when connected to my 8ohm B40 cab as it does when connected to a 4ohm SVT cab as it does to two SVT cabs?

    That's not a trick question; I don't know the answer. And I'm not trying to dispute or challenge Ohm's law. Maybe my amp is just broken and I don't know it!?! :eyebrow:

    In the end though, I was trying to address the original poster's question which was to describe "in layman's terms" the difference ohms make to the bassist. I was trying to do that not by showing how much they (ohms) can make a difference in a circuit but by showing that the difference need not matter as much to the bassist as it does the circuit.

    And I felt like my amp (assuming it's not broken) was a good example to cite to make this point because it puts out the same 100 watts into 8, 4, and 2 ohms.
  13. I did not take it the wrong way and thanks adding that. You remind me of me. :)

    It's good that you made the point about the tube part because I did not know that was a factor.

    I'm not the king of amps, my SVT is! :p :bag:
  14. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Very simple equation.

    For tube amps, "always match the rated impedance". There's some leeway there, but not much (+/- 50%).

    For solid state amps, the "minimum rated impedance" is what you need to pay attention to (and that's usually labeled, right next to the speaker jacks).

    Lower speaker impedance, means more power, and more volume.

    But go below the "minimum", and you'll fry your amp.
  15. Ben Clarke

    Ben Clarke Liquidating to fund a new business. Buy My Gear!

    Jan 6, 2005
    Western NY
    Ohm's law is a step removed from SPL. Efficiency or sensitivity of the speaker system is too often overlooked as a vital factor in output and resultant viability of a bass rig as a practical and creative tool.

    How bout a sticky for that? I see way too many questions asking if a 400W speaker will have enough power! Speakers don't have power, and thermal ratings sure don't have anything to do with anything but how much power they can withstand.

    Question to illustrate the point:

    Which will go louder driven by a 1000W amp?
    A 1000W speaker system, or a 200W speaker system?
  16. Happy MurphDay

    Happy MurphDay

    Mar 9, 2004
    no, but you worship them lol
  17. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Because the transformer output "matches" the speaker to the tubes. It makes ANY impedance look exactly the same to the amp as long as you use the appropriate tap.

    The different taps vary the output voltage to put out the same wattage at each impedance. That is inherent in the "matching" of speaker to tube's best load impedance.

    An SS amp always puts out the same VOLTAGE, regardless of load, so its power varies with load impedance. A lower impedance draws more current....meaning more power.

    Power is volts x current. Same volts and twice the current is twice the watts.
  18. Thor

    Thor Moderator Staff Member Gold Supporting Member

    I must apologize, I missed the V4B reference. I think Jerrold
    Tiers addressed your comment much better than I could, but
    to YOU with a TUBE amp, it makes much less difference than
    to a first amp buyer who will likely end up with a SS amp.

    In essence, the two circuits in the your tube amp are virtually
    separate, connected through a set of transformer windings,
    primary and secondary. ( And in reality, not actually
    mechanically connected ...)

    Which leads us back to the original question. Obviously, the
    answer to this is dependent on whether he ends up buying
    tube or SS. Perhaps we should address his question from the
    point of view that it is rather likely he will buy solid state as
    they are generally more modestly priced for a first amp.

    Then the discussion may be more relevant to him as a first
    time buyer as to what the effect of using certain components
    may be.


    And just for kicks, here we go, a tiny mini-tutorial for you,
    and others who would like a very simplified explanation of
    basic electricity. :help:

    Ohm's Law is easy, I deem that you now officially know
    it after reading this:

    V = I x R
    Volts = Amps x Ohms

    Example 1:

    120 Volts = 15 Amps x 8 Ohms

    If you had a constant voltage going through the circuit,
    and cut the load to 4 Ohms
    you would have

    Example 2:
    120 volts = I x 4 Ohms

    Dividing 120 by 4, the answer is 30
    120 volts = 30 amps x 4 ohms.

    This tells you that in the SAME circuit 120 volts draws
    30 amps at a 4 ohm load. See? Easy!

    Then to calculate the power in the circuit we use

    P = I x V

    power (ie watts) = amps x voltage

    for Example 1 it is
    P = 15 x 120 multiplying the answer is 1800 watts.

    In the SAME circuit, the Power for Example 2 (4 ohms ) is
    P = 30amps x 120 volts
    P = 3600 watts.

    I hope this example illustrates how simple these laws really
    are, do not be daunted, that is really all there is to Ohm's law
    and its uses.

    Congrats, so now you know it! You use it to calculate the
    unknown variable, if you know the other 2. An example would
    be that you can measure (with a voltmeter) the voltage
    flowing through a circuit. You can also measure the
    amperage flowing through a circuit. Knowing these 2 things,
    you can calculate the resistance in the circuit.

  19. Tash


    Feb 13, 2005
    Bel Air Maryland
    They will be exactly the same volume until the amp gets to 201 watts, then all the fire and smoke demons that live in the speakers will escape from the 200W speaker and it won't work anymore :p

    What do I win?
  20. SirPoonga


    Jan 18, 2005

    V^2/R = I*V

    Using subsitution
    P*R = V^2

    See kids, you didn't know your high school algebra class would come in handy :)