I still confused abut impedance high is 8 ohms and is low is 4 ohm? help! and if amp has hard time at 4 ohms does the manufacture account for that 4 ohm load in the design of the amp so it can handle that load?
4 ohms is lower impedance than 8 ohms.
4 ohms is harder to drive for an amp, all other things being equal, as it allows more current to flow. an amplifier has to be designed to allow more current to flow else it’s output circuits can over heat.
most amps are designed to allow 4 ohms as well as 8, but not all.
also a 4 ohm speaker (like all speakers) is not going to be a constant 4 ohms, it will vary up and down in different parts of the audio spectrum i.e. bass, middle, and treble. an amp that can only just cope with 4 ohm night then struggle, and be at risk of overheating if, for example, a 4 ohm speaker’s impedance dips below 4 ohms at certain parts of the spectrum. this is surprisingly common, and requires an amp designed for, say 2 or 3 ohms just to be safe.
hope that makes some sense?
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Thank you, I have seen some speakers 300 or more watts into 6 or 4 ohms but compatible with 8 ohms martin login is 1 of few others variable impedance on speakers how hack i does this work??
Dan here is how it works. The cross-over and drivers have reactance (capacitance and inductance) not just resistance so we see a variation in those numbers as each is FREQUENCY dependent.
Xc= 1/(2 x PIE x F x C)
Xl= 2 x PIE x F x L
This generates a curve like you see below in this speaker. Here is the rub, amplifiers are NOT tested into dynamic loads, but an 8-ohm RESISTOR, and all amps vary their performance into reactive loads so this is why amplifiers can sound so different from one another. Capacitance shifts when voltage is delivered and Inductance shifts when CURRENT is delivered. Work, or watts, is AMPS x VOLTS or CURRENT SQUARED X RESISTANCE. The common “memory triangle” shows all the relationships.
We don’t convert energy to HEAT (work) until it is purely resistive and STORED energy as Xc or Xl “reactance” causes amplifiers to be nonlinear in dynamic reactive loads like speakers and cables. How well an amplifier stay linear depends on a lot of things, one of which is the CURRENT delivery at low impedance, especially low impedance’s that are highly PHASE reactive. You DO NOT want a speaker to be a LOW impedance with a big peak in the reactance. The model shown here has impedance PEAKS that closely match the reactance peaks, and this is a good thing. This speaker has HIGH reactance and LOW impedance at about 15Hz. Not too much information there so not a big deal.
CABLE also adds to the reactance equation, too, so differing cable designs can change how amplifiers work, too, and this is what we hear with cables…it is the cable AND the speaker we are hearing with a given amplifier. Change amplifiers and the sound differences will be altered with the same speaker and cable as each amplifier is going to act differently dynamically.
Notice that STEREOPHILE uses a resistive load and a “simulated” speaker cord to try to get something consistent. But, it doesn’t really tell you what the amp does at all playing music.
Best,
Galen Gareis
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Posts like this give me a lot of info and a headache. Why can’t we just go to “11”?
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And now hear this…ELEVEN.
Best,
Galen Gareis
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@rower30 Galen, thanks for taking tour time to explain these technical details. Could you please elaborate a little more about the relation between phase and reactance, and It is harder for the amp to drive a region that presents a dip in impedance and a peak in phase/reactance?
I’m sure the OP is going to understand that perfectly 
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It is not all about the OP.
Well it should be. He’s the one asking. At least until he’s satisfied anyway and then the thread can devolve how ever it likes.
Howz about this for simpler answer.
Speakers provide a load, or resistance, to an amp and this is measured (loosely) as ohms.
4 ohm speaker load is more difficult to drive than an 8 ohm load. To get the same volume level, you need twice the watts (more current) for a 4 ohm load than 8. Said another way, you need to double the power when you move from an 8 ohm load to a 4ohm load to keep the same volume.
One way to read specs on an amp, and determine if it is well built, is that it even denotes watts into 4 ohm. They usually publish only 8 ohm specs. Great amps publish their spec into 2 ohm. The most powerful amps will say something like:
100 watts into 8 ohm
200 watts into 4 ohm
400 watts into 2 ohm
An amp like this is usually big heavy monster… and expensive. They are also nicknamed “arc welders”.
Why should you care? Well, as I noted “loosely” above, a speaker does not present a single value, or load to an amp. The load changes with frequency… so when a speaker maker denotes 8 ohms… well not really, it will change and music is all over the place. When I buy amps, I buy beefy amps… what is that? Amps that can dump current into difficult loads such as down to 4 ohms or lower. If an amp maker does not spec how many watts it delivers into a 4 ohm load… I skip it… it is not targeted to real audiophiles and I am sure they skimped on something.
Stereophile publishes a speaker’s impedance curve. This is the load it presents to an amp at different audio frequencies. I look at this and go straight to how low, lowest ohms, it goes. If it hit 4 ohms or less (regardless of how the maker rated it), it is a tough speaker to drive and requires a beefy amp.
Peace
Bruce in Philly
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In the simplest possible terms, an amplifier needs to deliver more current to a lower impedance speaker to produce the same sound level, than it would to a higher impedance speaker.
Another way to look at this is, my 2 watt amplifier can play at a reasonably high volume level with my 16 ohm speakers. But that same amp would have a hard time reaching the same volume level with a 4 ohm speaker. The lower impedance requires higher power to achieve the same output.
whew—now I have to lie down.
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Manufacturers absolutely do take the range of impedance the amplifier might see into account. For example, you will note multiple speaker output taps on a transformer coupled tube amplifier. Usually always 4 ohms and 8 ohms at a minimum. The reason in that case is to optimize the amplifier output impedance to speaker input impedance match by selecting a different transformer secondary tap. Impedance matching maximizes power delivery.
As Galen already pointed out, though, the actual speaker input impedance is complex with a modulus and phase angle that can vary widely, particularly in ported speakers at the port resonance. That’s why even with a transformer coupled tube amp with multiple taps my experience is the only way to find the best tap is subjectively by trying each.
One thing I want to note, advocates of panel speakers such s Magnepan make the valid point that certain driver types can come pretty darned close to presenting a purely resistive load. Minimizing the reactive component can tame those big peaks in the sample plot Galen posted.
The OP will chime in if he has questions. In the meantime, the rest of us are busy learning and discussing.
It would be a waste if the thread just stopped, waiting for an OK from the OP.
In simplistic terms, 4 ohms pushes back less than 8 ohms and, as a result, the 4 ohm speaker needs to receive more current.
This is a bit tricky to wrap one’s head around.
Perhaps an analogy is comparing providing enough water to fill a garden hose (8 ohm) v. sufficient water to fill a fire hose (4 ohm). The fire hose is much less restrictive and this “wants” more water/more current. The water pump needs to be more powerful to fill the 4 ohm fire hose than to fill the 8 ohm garden hose.
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So help me understand a riddle I never quite understood.
So 0 ohm is a short circuit… no? Current then flows totally free around and around. no? Why does that blow up an amp?
When you hook up a speaker, you are doing the same… connecting the positive to the negative on an amp and the wire runs continuously to the speaker and back… but that doesn’t blow it up… you get work (the speaker moves) but it still is a closed circuit.
Peace
Bruce in Philly
As I understand it, a short circuit results when the connection between the two conductors is too low to prevent excessive current flow. That is, it need not be zero resistance.
In a home circuit a 5W light bulb provides enough resistance to prevent excessive run-away current flow. The same bulb is insufficient in a more powerful circuit to prevent a short.
As an aside, arc welding harnesses a sort circuit where voltage determines the length of the arc; current, the heat generated.
Good question.
The trick is to understand what we HEAR. We don’t hear Volts and we don’t hear current, we hear WATTS, or WORK.
If you imagine two sign waves one represents VOLTS, the other represents AMPS. If they are in phase we see that what we hear, watts, is at a maximum. Watts = amps x volts. That’s the MOST efficient we can be in a speaker or an amplifier.
The problem, is we have REACTANCE called Inductance and capacitance that MOVES one variable or the other such that the magnitude at a point in TIME is VARIED. How is this working? The little ditty memory saying is ELI the ICE man.
The first says E (volts) leads I (current) in an inductor (L). That’s how inductors shift the CURRENT around in the time base.
The next term says I (current) lead E (voltage) in a capacitor ©. That’s how capacitors shift VOLTAGE around in the time base.
When voltage and current are not in PHASE, we see a DECREASE in efficiency. When a circuit is NOT purely RESISTIVE (we have one or both reactance’s) and I = E/R that means that I (current) has to get BIGGER for a value of signal voltage. R decreases because some of it’s VECTOR magnitude was stolen away to create the REACTANCE magnitude so R gets SMALLER.
The constantly shifting reactance causes CURRENT to jump all over and it is WORSE at a low impedance load for a given voltage. We hear WATTS so we want the current and volts to be at a maximum. E= I x R so as R component goes DOWN the Volts go down, too. All the energy is traded around, nothing is added. Now we have a smaller signal voltage that produces less WATTS and higher CURRENT. Not so good.
AMPS don’t like current because of I squared R losses. The current is SQUARED making it VERY influential, times the circuit resistance. This is the amount of signal LOST in the signal path. Cables, wire, and speaker voice coils and all that. Running stuff with lots of current is a hard way to do things.
This can be confusing but the main issue is the PHASE of the current and voltage has to be exactly right to give the most efficiency, WATTS, as that’s what does the WORK in our speaker. Change that, and the proper DYNAMIC response of our music just isn’t right. It is not PHASE linear and our ears are real good at picking this out.
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Hopefully the sum of this thread is helpful for not only OP but also the other readers. Like Elk mentioned, this stuff can be darn hard to wrap one’s head around. I have definitely found a few of these posts helpful with good points and well written. It’s one thing to understand it. It’s another thing to eloquently describe it. Thanks!
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Thanks once more for your patience. I think I van grasp most of It, specially the phase issue, and that means a lot of new knowledge for me. I imagine that one of the main causes for such behaviour must be the x-over design