4 ohm vs 8 ohm

i was reading in another group about speaker impedence. what was written has me confussed. i've taken 4 years of electricity. the statement was that it takes a higher wattage amp to push 4 ohm speakers. if this is so why is the wattage on an amp almost doubled when it is rated compared to an 8 ohm speaker. this just goes against everything i've been taught. if it is true can you explain? or was it someone who doesnt know what they were talking about?

Absolutly. I mean, it's total BS.

If you have four years of electricity, you must remember this :

I= V/Z or compared to Ohm's law I=V/R

where I and V are the rms or "effective" values. The quantity Z is called impedance. For a pure resistor, Z = R. Because the phase affects the impedance and because the contributions of capacitors and inductors differ in phase from resistive components by 90 degrees, a process like vector addition (phasors) is used to develop expressions for impedance.

Also, it should be noted that the loudspeaker is not a simple resistor - it contains a coil or coils with significant inductance, and is typically composed of two or three speakers with a crossover network that has capacitance and inductance.


In modern electronics, the integrated circuits of an amplifier have at their disposal hundreds to thousands of active transistor elements which can with appropriate creative use of feedback make the performance of the amplifier almost independent of the impedances of the input and output devices within a reasonable range.

And yes, i'm talking about solid state.

that makes me feel beter. i was thinking my meds must have killed my brain cells. i laid in bed last night going over theroies and i couldn't grasp what he was saying. i've been off work for years and hadn't did theroies for over 20 years. thank you clint for straightening out what i thought.

I think that the person was probably trying to explain something that is true, but was using the wrong terminology, mostly because they didn't really understand the electronic principles involved. What is true is that at any given signal voltage at the output of an amplifier, a 4 Ohm speaker will draw a higher current from the amplifier, than an 8 Ohm speaker. Or perhaps stated better, a given voltage (or voltage differential to be more exact) will pull (or push if you prefer) a higher current across a lower impedence/resistance load, than across a higher impedence/resistance load (BTW, I am not going to get into the discussion on whether voltage pulls or pushes current, and whether current is the flow of holes or the flow of electrons - the debate between technicians and engineers will probably go on forever). So if signal voltage stays the same and current increases, wattage also increases (voltage * current = watts).

What is important is not to focus on watts, but on current. This additional current must not only be handled by the output device of the amplifier in question, the amplifier must also have a power supply capable of delivering it in the first place. The lack of ability to deliver this additional current is why most amplifiers today cannot double their continuous 8 ohm output into 4 ohm loads.

So generally speaking I think the guy was trying to say that you need a "stronger" amplifier (one with a higher current capability) to drive 4 ohm speakers compared to 8 ohm speakers with the same volume setting (i.e. the same output voltage). Stronger in this case, means a power supply capable of delivering more current, and output devices capable of handling the increased current.

Do such amplifiers tend to be more "powerful" models when comparing their wattage into 8 ohm loads? Yes. However, that is not always the case. For example a Krell KSA-200 can continuously deliver 200W into 8ohms, 400W into 4ohms, 800W into 2ohms, and 1600W into 1. While my Adcom GFA-5802 kills the KSA-200 at 8ohms delivering 300W, at 4ohms I just barely maintain my lead on the Krell with 450W. However at 2ohms, my 5802 can only deliver about 600W but not continuously. And just one powerful note at 1ohm drains the power supply of my 5802 totally dry, causing the signal to completely break up. So is my Adcom GFA-5802 "more powerful" than the Krell KSA-200 into 8 ohms? Definitely! But it is clearly not the better "more powerful" amplifier that you should use for driving low impedence loads. The HUUUGEEE power supply and high current capable output stage of the Krell give it the clear advantage.

I hope that made sense.

Rob

excellent explanation of this often confused topic. pinned for future reference.

the p is watts in the formula p equals IxE or pie p stands for power. in common practices if you cut your resistance by half your output will double depending on the circuit you are using. this if i remember correctly is what happens in a series circuit not a parallel. in a parallel you resistance is lower than the lowest resistance.
i dont know what circuits are used on the output of any given amp. in most crossovers it is a series circuit. if you believe as i do you wont use resistors in your xover. you use inductors and caps. so your drivers are what gives you the ohms to which it operates.
this is fun.

But, let me tell you that the impedance is not a rigid value, sometimes an 8 ohm speaker can typically have an impedance dip to 4 ohms in the upper bass and rise to 20 ohms in the treble.
Simply because there's another factors that it must be consider:
- Frequency response and the other is controlling the motion of a loudspeaker cone.

The function of a speaker is to produce a constant sound pressure level for a constant drive voltage across a defined frequency range. Power is calculated as voltage multiplied by current. If the loudspeaker impedance drops and the amplifier voltage is held constant, then the current in the loudspeaker increases (current = voltage ÷ impedance). If the current increases, then doesn’t the power consumed increase? Yes! That is exactly what happens. If the power in the loudspeaker increases, then why doesn’t it play louder? Because the speaker efficiency is changing. As the speaker impedance decreases, its efficiency decreases. All that matters is that the acoustic output remain constant.

Speaker enclosures are “tuned” to “load” the woofer cone at a particular frequency- the frequency of system resonance.

This is the way loudspeakers behave. Loudspeakers are not constant power devices. Loudspeakers must have a constant drive voltage to provide a constant acoustic output with changing frequency.

to which impedence are we talking about. there are several. http://en.wikipedia.org/wiki/Impedance

i added a reply but it didnt load. isn't the caps in the crossover to keep a constant voltage going to the drivers. the drivers are taking a voltage and turning it into a motion.

Uggggggggg!!!! I just wrote about two pages of reply and accidentally hit the back button before I posted it. Sorry, guys you will have to wait until tomorrow to hear what I had to say. It was good too... IMHO

Rob

what i meant was it turned into a mechanical action. now that ive slept on it the cap is to aid in the xover point where the woofer and tweeter are acting together at the point where the voltage is turned into sound. so how many types of impedence are we talking about.

Impedance between amps and speakers

but we have several types in the conversation. sound, electrical and others.

Rob
I can't wait to read what you lost. I like to learn. This conversation is a good one. One question how do you find the effency of the amp?

Efficiency is a value related to speakers, not amps.