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Speaker Ohms rating

Discussion in 'Electronic Basics' started by Rene, May 16, 2004.

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  1. Rene

    Rene Guest

    I understand that speakers come in different flavors; one of the flavors has
    to do with the ohms rating of the speaker. For the most part, it looks like
    the majority of the speakers come in 4 and 8 ohms.

    My question is, what is the benefit between having an 8 and a 4 ohms
    speaker? I know that you must match the speaker to the amplifier but why so
    many ohms ratings? The only thing that I could come up with is that the
    higher the ohms rating the better the sound quality that I will be able to
    get from the speaker, otherwise there would be no purpose in having the
    different ohms ratings, right?

    Any help is appreciated.
  2. The Al Bundy

    The Al Bundy Guest

    The lower ohm rating how higher power output is. In cars 4ohm speakers are
    used, even 2ohm speakers sometimes to make alot of noise...

  3. In the days of tube amplifiers, where they had to have a matching
    transformer between the tubes and the speaker, the transformer could
    be wound to drive just about any speaker impedance, as needed. So the
    impedance selected tended to favor whatever was best from the
    speaker's standpoint. 8 and 16 ohm speakers were very common in this
    era because this impedance range gave a good compromise between mass,
    ruggedness, manufacturability, magnetic gap thickness, etc. When
    direct coupled solid state amps were introduced, this impedance range
    worked pretty good, as long as the power supply voltage could be
    optimized to match the voltage requirements of the speaker. But
    automotive applications are most cost effective if you can use the 12
    volt battery as the supply, instead of converting it to some higher

    From the power formula P=V^2/R it is pretty obvious that a +- 6 volt
    swing applied to an 8 ohm speaker is not going to deliver much power
    (something like 4 watts peak after transistor saturation drops and 2
    watts RMS). Even if you bridge the speaker between two out of phase
    amplifiers to approach 12 volts in either direction you only get about
    12 watts peak. So they either had to step the battery voltage up or
    lower the speaker impedance to increase the power. They do both.
  4. Rene

    Rene Guest

    John, thanks for responding but I would like to ask you two more question if
    it's ok!

    So why not make all of the speakers 4 ohms? I am sure that it wouldn't be
    more expensive or a big deal to design a home stereo system that uses 4 ohms
    (I would think). I am sure even production cost would be lower if you have
    to produce only one type of speaker, right?

    Also, can a 4 ohms speaker be created that could deliver the same amount of
    decibels and clarity of any 8 ohms speaker that could be created?
  5. tempus fugit

    tempus fugit Guest

    Well, it's not really about clarity at all. A lower impedance (i.e., ohms)
    speaker will have less resistance (or impedance) to current flow. Therefore,
    more current will flow through the speaker, thus delivering more power.
    The downside to this, of course, is that the amplifier must be able to
    source (supply) this amount of current. So, if you have an amp rated at 25w
    into 8 ohms, it will deliver more power into a 4 ohm speaker, and even more
    into a 2 ohm speaker. However, it may not be capable of sourcing the amount
    of current required to power a 2 ohm speaker, and burn out.

    Most home speakers are 8 ohms, and I suspect that no standard has been
    established simply because companies are designing their gear with
    compliance in mind. No one is going to design a stereo amp for use at 16
    ohms when all the speakers are 8 and vice versa. That being said, most home
    stereo amps will be able to power 4 ohm speakers, but won't easily go down
    to 2.
  6. Probably so. But these things have a certain inertia. And physics
    haven't changed. The compromises that made 8 ohm speakers a good idea
    before still apply. But it is not much off the sweet spot to make 4
    ohm instead of 8 ohm.
    I don't see why not. But what I don't see might sneak up and bite me.
  7. Jan Pompe

    Jan Pompe Guest

    It's fine for home stereo systems where the voltage of the power supply
    can be tailored to fit the requirements of your output stage but when
    supplied with a 12 volt battery it starts to become messy when you want
    high power output - as in a car.
  8. bench

    bench Guest

    no, no, no,
    putting a 4 ohm speaker instead of an 8 ohm one will *not* deliver more
    power. What will happen is that the current will increase and this will
    cause more voltage to be "wasted" on the internal resistance of the
    amplifier and the voltage on your speaker will actually drop and hence
    the power on your speaker will drop (V^2\R). This is known as the maximum
    transfer theorem and can be prooved with simple calculus mathematics.
    (well, I may be wrong, but I am sure if I am someone will correct me)
    So you need to put exactly the speaker ohm rating which is specified.
  9. If we assume that feedback holds the output voltage constant, then the
    speaker current will go up when you lower the speaker resistance.
    Audio amplifiers do not usually operate with an impedance match
    between amplifier and speaker, but act as low impedance voltage
    sources. So if the amplifier is capable of holding the voltage the
    same, changing an 8 ohm speaker to a 4 will double the current and by
    P=I^2*R, the power will also double. The losses in the amplifier will
    also increase. But assuming the amplifier can deliver the increased
    current while still operating as a voltage source and can handle the
    extra heat, this is fine.

    In reality both of these assumptions are problematical. If you use a
    speaker with a resistance lower than specified, you will usually
    increase the distortion (as the amplifier current limits at the peaks
    and thus, is not a voltage source, so the peaks sag) and the output
    stage overheats and possibly is destroyed, eventually.
  10. Rich Grise

    Rich Grise Guest

    AFAIUI, that's not true any more. In the days of TOOBz, and mongo
    output transformers, yes, you had to impedance match for pretty much
    the reason you've cited - but these days, they make power amps with
    an output impedance that's almost negligible. Transistors _do_ have
    their place, after all. :)

  11. Sir, I respectfully disagree. The amplifier will certainly deliver
    more power else why does it run hotter? It appears you're assuming the
    output voltage will 'droop' under the higher current. Any halfway
    decent amplifier has a fixed gain (just a big opamp) so the lower ohms
    will double the current doubling the power.
    Henry Kloss used that 'trick' on the Smaller Advent loudspeaker in the
    early '70s. The woofer was mass loaded to reduce the system resonance.
    The mass reduced the efficiency so the impedance was dropped raise the
    power level. A 'down' side of the reduced impedance is the damping
    factor drops because the resistance of the wire becomes a larger
    factor. (damping is the ratio of the actual amplifier impedance/load
    impedance) Phase Linear claimed damping of 1000 on the old 700 watt
    amp. That amp couldn't possibly work into the implied 0.008 ohm load
    to 'match' the impedance to the load.

    Now if you're working with a transformer coupled amplifier, the power
    will not double because you use a different tap on the secondary.
    However, these are definitely the minority.

    Sorry my 2 cents turned into a nickel.
  12. It was *never* true for power amps. For example, the choice of
    transformer in tube/valve amps has nothing whatsoever to do with
    obtaining a power match in the sense of the maximum power transfer
    theorem. Obtaining a "maximum" match is pretty daft. You would throw
    away half the power.
    Misleading, at best, incorrect at worst. Impedances for power amps are
    chosen based on the maximum current and voltage capability for the
    output transistors. Even for RF amps one designs the load so that it is
    something like Vmax/Imax of the output devices at the device output.

    Its obvious that if the transistor can take 100 volt, and only 100ma,
    then to get its *real* max power of 10W requires a load of 1k. This load
    needs to be transformed to say, 8 or 4 ohms.

    This maximum power theorem is usually churned out wily nilly with no
    idea of just what it means. For the most part, it is meaningless. The
    "maximum" power of an audio amp with 0.01 ohm output resistance at 50
    volts would be 62.5KW, clearly irrelevent.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  13. John Fields

    John Fields Guest

    Considering that an audio amplifier is a voltage source with a very
    low output impedance and since power, in watts, is equal to:

    P = ---

    If you were to connect an 8 volt source to an 8 ohm load, you'd get:

    E² 8²
    P = --- = --- = 8 watts dissipated in the load
    R 8

    If, however, you were to connect the 8 volt source to a 4 ohm load,
    you'd get, instead:

    E² 8²
    P = --- = --- = 16 watts dissipated in the load
    R 4

    So, by halving the load resistance you double the power it dissipates.
  14. bench

    bench Guest

    well, it seems I was wrong, thanks for the corrections.
  15. Chols

    Chols Guest

    I have one question here.
    Back in my engineering study days, we had to design an amplifier using
    op amps for the first stage and equalizers, and then use power (though not
    too big) transistors for the output stage, attaching a heat sink as needed
    to prevent it from overheating. We had to drive a fixed load, that is,
    deliver a maximum power without a given distortion.

    But, and here's the trick, we also had to design the amplifier with a
    protection so that if we short circuited the output stage, and we had to
    prove this, the amplifier would NOT burn. I don't remember how exactly we
    did it, but it was some sort of current controller in the output stage that
    would shut off the power transistors if conducting too much current. It
    worked (well, some in the class failed to work, beautifully turning the
    transistors bright red and then pufff.... ;-) but the question is...
    Aren't these short circuit protections added to comecial amplifiers as a
    standard? :-o
  16. Just as with your class, there are a wide variety of protection
    schemes with varying degrees of ruggedness. Production amplifiers
    range from just poor designs to very cost reduced designs to well
    proven and tested designs to absolutely iron clad. bullet proof, money
    is no object designs.
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