Are low impedance speakers hard to drive?

Paul attempts to explain why impedance in speakers is important and how it works.

[frankvee]

Lower impedance means the speaker is less of a roadblock for current, thereby drawing more current from the amplifier and driving the amplifier harder because it has to output more power.

[Landoverse]

I love how this gentleman from India took it upon himself to try and educate Paul and all us hopeless Americans about the world. Sharing knowledge like for like, sounds like a fair deal. Well done Sir👍

[erod]

Simple analogy: It takes a lot less water (power) to get a steady stream out of a thin hose (8ohm) than it does to get that same stream out of a larger hose (4ohm).

[thelawman]

I also always struggled with this concept, for the following reason: I understood impedance to mean "resistance". Therefore, to me, it always seemed logical that the higher the resistance / impedance, the more difficult it must be for the amplifier to drive. At school in physics, a parallel was often drawn between electricity and water, with voltage being the pressure in the tap, amperage being the amount of water flowing, and ohms / resistance being the narrowness of the pipe. Therefore, to my mind, the narrower the pipe (higher resistance) the more pressure (voltage) would be needed to get the water / current to flow. While I do "technically" understand why the opposite is true, it remains counter-intuitive to me - even 50 years later! 😀

[darrenweight]

Another way to put it -
A lower impedance speaker doesn’t “allow” or “let” an amplifier deliver more power; it “forces” it to deliver more.

[Peter_S_]

I think I lot of people find Ohms law counterintuitive because of the perspective. Another way to look at it is that a lower impedance speaker "forces" the amp to work harder.

[Philgrafton]

Most Manufacturers say, you can run an 8ohm speaker on a 4 ohm amp but not a 4ohm speaker on an 8 ohm amp.
Did I say that right? Did they change that, I haven’t bought a speaker in a few years. I know this probably doesn’t help any more than any other comment but me, I think that if I wanted to learn electronic engineering from somebody, I’d want that some body to be Paul. The coolest teacher in the Electronic Audiophile World. Thanks Paul!

[NathanStorer-RC]

It makes sense that less resistance = more watts. I think where I, and possibly others, struggle on this topic is that it seems counter intuitive that it is MORE difficult to drive a lower impedance speaker when you have less resistance and get double the power output. It would seem logical that if it's capable of delivering double the wattage at half the load, it would be easier to drive a 4 ohm speaker to 100 watts than it would be an 8 ohm speaker at 100 watts as you're only at 50% volume with 4 ohms at 100 watts rather than 100% volume with 8 ohms at 100 watts.

I've, over the years, learned less impedance/resistance = more watts and just really stopped concerning myself with how difficult it is for the amp to drive them so long as it's rated for that load with an acceptable level of THD.

[Rational]

Part of the problem explaining why low impedance speakers are harder to drive is not as simple as running a battery through a wire, etc., the answer is much more complex. How much power an amplifier can deliver to a speaker depends on the power supply, the current capacity of the output section of the amplifier, the stability of the amplifier, etc. Some power supplies adjust the voltage that goes to the output section as the demands for power change, others don't. Some amplifier designs are very stable at very low impedances, others will distort or oscillate. An amplifier may be able to deliver double the power at half the impedance but it may have a lot of distortion. The power rating is at a specified percentage of distortion so an amplifier that is rated at 100 watts at 8 ohms at .1% distortion, and delivers 200 watts at .2% distortion at 4 ohms, would be rated at less than 200W at 4 ohms even though electrically it may be delivering all 200 watts. Also, the output section of the amplifier has to dissipate heat. A class AB amplifier output section has to dissipate more heat when it delivers half power than when running at full power because it has to dissipate the same amount of heat as it delivers to the load. The lower the impedance of the speaker, the more the internal impedance of the output transistors become a factor. In essence, the output transistors in a class AB amplifier work as variable resistors in series with the speakers, connected across the power supply. Their internal resistance change in accordance to the input signal but transistors are not perfect and will have some resistance even when driven to their maximum current. So for example, (using very unrealistic numbers to make the explanation simple) if you are driving an 8 ohm speaker and the internal resistance of the output transistor is 8 ohms, and it's delivering 10 watts to the speaker, then the transistor will also have to dissipate 10 watts of heat. Change the speaker to 4 ohms, now the transistor would have to dissipate twice as much heat as gets delivered to the speaker. That's one reason it's harder to drive lower impedance loads. The lower the impedance of the speaker, the more stress on the output transistors. Before protection circuits became common it was very easy to destroy an amplifier by shorting the speaker wires together while the amplifier was playing. Better designed amplifiers will normally have more output transistors in parallel, heftier heat sinks and better power supplies.

[cablebrain]

Paul, I think you need to go back and remake that one. :P

[andydelle]

It all comes down to Ohm's Law. Very basic algebra. Google the equations. You have two known values to compute the third unknown. If I have an 8 ohm speaker and put 10 volts into it, it will draw 1.25 amps or 12.5 watts. If I put 10 volts into a 4 ohm speaker it will draw 2.5 amps or 25 watts. So with a four ohm speaker I can get the same output power with half the audio voltage than at 8 ohms, but need twice the current. This was an advantage to use 4 ohm speakers with car stereos before the age of cheap DC/DC converters. You have say 100 amps but only 12 volts to work with. Of course in both cases the amplifier must be able to supply the voltage and current for the desired output power. A disadvantage to 4 ohms is you need a heavier wire gauge for the same power at 8 ohms so with 8 ohms you can have longer speaker wires with less resistance loss. Yes, impedance is not identical to resistance but for this simple example they are interchangeable.

[michaelbeckerman]

It's obvious that this is one of the most difficult concepts in the entire "world of audio" to both properly explain and for most people to grasp. All you have do is read though the comments below and that becomes abundantly clear. A lot of different factors feed into this concept and that's largely what makes it so difficult to convey and to understand. I get the feeling that you could probably teach an entire college-level course on just this one subject alone.

[AudioNaut]

Think of it this way…

The amplifier rated for 100 watts into 8 ohms is like a top fuel dragster racing down the 1/4 mile with the parachute deployed the entire time.

The same amplifier producing 200 watts into 4 ohms is like the top fuel dragster racing down the 1/4 mile with no parachute or brakes whatsoever.

A speaker(the dragster) with a lower impedance is going to ask for more fuel and power than the amplifier(the engine) is capable of producing without running the risk of overheating.

[kevenharvey]

Depends if the amp is limited in the voltage or current it can put out. If it's limited by the voltage, power capabilty will go up with lower impedance, if it's limited by current, power capability will go down with lower impedance. Higher current requires beefier components, and higher current usually means more heat and more loss so it's less power efficient. Higher voltage only requires better electrical insulation.

[andrewt]

Paul you did an incredible job explaining this I'm forever grateful that I came across the world of Paul McGowan!

[johnnytoobad]

A lot of this depends on the internal design of the amplifier. ALL Audio amps are designed around Q-point and something called an SOA (Safe Operating Area). Most linear (class AB/B amps) audio amps have their Q-point between 6 and 8 ohms. Now these amps also come in two basic flavors, bi-polar and MOSFET. In the case of Bi-polar amps, when the impedance drops low (approaches 4 ohms) the amp will run hotter and will have to adjust its internal bias circuits dynamically or "thermal runaway" will occur and the amp may "self destruct".
Bipolar transistors conduct more with increase in temperature. MOSFET amps, on the other hand, become more resistive as the internal component temperature increases (ie. the load impedance decreases). So MOSFET (and class D amps) amps seem to "tolerate" low impedance loads better than conventional bi-polars. The exception to this are amps that have several "speaker taps" marked 4, 8 (and sometimes 16). These amps (like the MacIntosh) are using output transformers and are designed for this. Other (newer) class D-amps are "optimized" around 3 ohms and like to be run between 60-80% of full power at that load ! However very few (if any) amps are stable if/when the speaker impedance drops to around 1ohm at high power. In either case the stability and reliability of any amp can be improved using an internal or external fan when driving low impedance loads at high power.

[datadaveangelstravelsadven]

As a former teacher I remember one of the hardest things to do as a teacher is to explain something so incredibly obvious to someone who doesn't understand the concept AT ALL! This matter is further complicated when dealing with a topic that doesn't inherently make sense. You talk about it being easier to drive current through something that has greater resistance. THAT MAKES NO SENSE! It reminds me of times when I would tell the students that matter with a higher heat capacity retains heat longer. They would ask, "Why?" And I would say because it just does.

[societyofhighendaudio]

I didn't ask this same question to you the late engineer Stanley Iskandar in Jakarta before he passed away. Thanks Paul

[Belas_Photography]

Great explanation! Certainly one of the most lucid discussions of impedance v. power demand for those of us who are not electrical engineers

[richardsoffice]

I'm surprised that Paul didn't mention speaker sensitivity. This is also important. I'm sure that he & his designers keep this in mind, when they engineer their speakers for the best sound, in their price ranges. If you're worried about too low speaker impedance, please see my reply to Dan Donna, below. Regardless of impedance, when a speaker is efficient, less power is needed for a certain 'loudness' (S P L - Sound Pressure Level, the end result, assuming acceptable fidelity.)