Ohms law question

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  • rattmann316
    Junior Member
    • Aug 2005
    • 24

    Ohms law question

    Sorry to "nerd it up" in here but I have a question about hi-current amps. Can anyone explain how some amps are "hi-current" while others arent? If you apply Ohms law to any basic hook up there is a finite amount of current that can be used for any device, especially considering (at least in the US) that the reference voltage will always be around 120. So I just dont see how a 100watt "hi-current" amp can be louder and drive a speaker more efficiently than a 100watt amp that is of generic quality. The only reason I ask is because the person doing a demo of these amps could not explain it to me.
    RB-1070
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  • Kevin P
    Member
    • Aug 2000
    • 10809

    #2
    "hi-current" is a marketing term as much as anything, but in any case an amp's current capacity determines how well it can handle low impedance loads.

    If all else is equal, with 8 ohm speakers, a 100 watt amp is a 100 watt amp, regardless of current rating. Higher current capacity allows that 100 watt amp to push more power into a 4 ohm speaker, or handle impedance swings without clipping. Ideally, an amp rated at 100 watts into an 8 ohm load would be capable of 200 watts into a 4 ohm load, or 400 watts into a 2 ohm load. In reality, this is rarely the case except in really well constructed amps with robust power supplies (thus, higher current rating).

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    • Sim reality
      Senior Member
      • Aug 2005
      • 173

      #3
      Not exactly... The output to the speakers is basically an AC current which is them subject to resistance due to inductance (ie: the voice coil) and capacitance (ie: crossover). Whereas the higher the frequency the greater the resistance.

      The problem becomes at high frequencies the amp needs to provide high voltage and at low frequencies it needs to provide high current to maintain linear power. It is possible to design solid state electronics to provide high voltage (to a point before the voltage will overcome the dielectric in the IC) it is simply difficult to get a large amount of current from a circuit that is a few nanometers across (why people still like tube amps).

      "hi-current" is a marketing term as in there are no standards to define what exactly is a "hi-current" application but the concept of a system being able to provide "high current" to low frequencies is valid.

      BTW: I have exaggerated the nanometers as you would run mutilple circuits in parrellel but then you have to take into phase variations due to different circuit path lengths (which is not critical at low frequencies but you might be able to tell at high frequencies)

      Comment

      • taz13
        Senior Member
        • Jun 2004
        • 930

        #4
        Your mistake is using 120v as your ref. That is only your input voltage. You have a transformer and power supply that are going to determine whether you have a voltage source or a current source. If your power supply is current limited then your voltage will change across your load, if your power supply is voltage limited then your current will change across your load. In both cases the watts will remain constant for the same output. Current limited power supplies are smaller lighter and less money. Understand I am using a lot of generalities here, but those are the basics.
        The day is not complete if something new is not learnt.
        Taz/Rick/Richard/Ricardo

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        • Taito
          Senior Member
          • Sep 2004
          • 226

          #5
          Hi! I'll also take a crack.

          Say an amp(lifier) can deliver 10A. This given amp is rated at 100W (continuous) into an 8 Ohm load. If the amplifier's maximum output voltage (limited by the supply rails) is 50Vrms, then the current (for this voltage) delivered into the 8 Ohm load is 6.25A.

          Now consider the same amplifier driving a 4 Ohm load. As the amplifier cannot deliver 13A, and Ohm's law MUST be satisified, the maximum voltage that the amplifier can output is now 40V (for a 10A limit). Compression (non-linear distortion) becomes a problem - the signal literally is compressed.

          Note that this argurment only considers purely resistive impedances. Loudspeakers present a more difficult, or complex, load :rofl: (Sorry, I couldn't help myself). When reactances (due to capacitance and inductance) are thrown into the mix, things get significantly worse -as Sim Reality pointed out.

          Bottom line: Max. power output and max. current delivery are not intrinsicly linked, for two amplifiers with the same power rating (and all other things being equal), it is better to have one with a higher maximum current delivery.

          Hope this helps, Ben.

          Comment

          • VikingP
            Junior Member
            • Feb 2005
            • 28

            #6
            Originally posted by taz13
            Your mistake is using 120v as your ref.
            Exactly, Ohm's law must be used after the transformer and bridge rectifier in order to obtain the power supply's maximum current capabilities (also depends on the transformer's VA rating).

            I totally agree with Taito's "crack"

            Paul

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            • David Meek
              Moderator Emeritus
              • Aug 2000
              • 8938

              #7
              Warning, warning, EE humor! :scratchhead:

              :B

              .
              .

              David - Trigger-happy HTGuide Admin

              Comment

              • whoaru99
                Senior Member
                • Jul 2004
                • 638

                #8
                All great technical details, but for the average guy about the best indicator you can get is to compare the 8, 4, (and somtimes 2) ohm output ratings. If the amp doubles down it should have good current capacity. This certainly is not a hard rule because there are many good amps that don't double down.

                My .02 is that "high current" is mostly an advertising hook.

                It really does not matter if an amp could deliver 1,000 amps of current if the speaker only asks for 5 or 10 amps since the amp will only deliver what the speaker "requests" (based on it's impedance curve). That's a pretty broad statement, but I think the essence is to the point.
                There are some things which are impossible to know, but it is impossible to know which things these are. :scratchhead:

                ----JAFFE'S PRECEPT

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