What's with 4 ohm speakers?

It's easier to design a speakers with a steady low impedance than a steady high impedance, something that some amps seem to have an easier time with. Historically there have been more speakers that can produce wave form fidelity with a lower impedance than a higher impedance.

This is entirely false, plain and simple.

To answer the OP question, I suspect the reason is that many designers don't know how amplifiers work, so we often see crazy loads that are 'hard' to drive.

Folks, there is a reason such speakers are considered hard to drive- the amp has to work harder to do the job. You can always see it in the specs of any amplifier- the harder you make it work, the more distortion it makes. Unfortunately the distortion we are talking about is the kind that makes a system harsher and brighter- the odd ordered harmonics, to which the human ear/brain system is very sensitive.

Part of the problem is that speaker designers often confuse Sensitivity with Efficiency. I can point to examples if anyone is interested. You don't get something for nothing in this world. In electronics, this idea is known as the Law of Energy Conservation, or Kirchoff's Law.

But many speaker designers don't understand this. They think that if they put two drivers in parallel, that the speaker gets easier to drive (sensitivity increases). It does not! It gets *harder* to drive, and the amount of power to make it play a certain sound pressure does not change at all!

If lowering the impedance was actually helpful, why not 1 ohm instead of four? Then the sensitivity would be increased by 9 db! Nearly a 10:1 improvement... but of course that would violate Kirchoff's Law. If you are able to violate Kirchoff's Law, FWIW, you will have created a Free Energy Device which, as far as we know, does not exist.

Yet many speaker designers persist in trying to do exactly that, and many audiophiles that don't understand how this works (its really just math when you boil it down, FWIW) follow along in the fantasy.

So here is the bottom line: higher impedance speakers cause amps to make less distortion (smoother, more detailed). Increasing the Sensitivity of a speaker by decreasing its impedance does not affect Efficiency, but it does make the speaker harder to drive (amp will sound harsher, less detailed).

Put another way: If Sound *quality* is your goal, your amplifier investment dollar will be best served by a speaker of higher impedance, all other things being equal. If sound **pressure** is your goal and you have a transistor amp up to the task, then there is an argument for lower impedances.

Using solid state amplifiers without output transformers a 4 Ohm nominal impedance allows peaks 3dB louder than 8 Ohm speakers with the same cabinet size and low frequency cut-off which ultimately limit efficiency per Hoffman's Iron Law. This is generally a better engineering choice than doubling cabinet size (thus halving the spousal acceptance factor ) or choosing a low frequency cut-off 1/3 octave higher.

People don't go too over-board with lower impedance in the home market because of

1. How the FTC requires manufacturers to rate stereo and mono home amplifiers - they must be "pre conditioned" at 1/3 of rated output power and the power dissipated into low impedance loads would make the numbers look bad so the capability to run 2 Ohm loads isn't usually advertised and consumers would be leery of buying such speakers to go with their "4 and 8 Ohm compatible" electronics.

2. Some amplifiers are unstable (they start to oscillate) driving low impedances; and starting with a 2Ohm nominal impedance minimums of 1 Ohm aren't unreasonable.

3. Some audiophile amplifiers have silly high output impedances which interact with the speaker's varying impedance to change the frequency response and this is exacerbated with low load impedances. Output Transformer Less Tube amps are especially bad although single ended triodes without global feedback can also have problems.

For instance an Atmasphere M-60 Mk.II.2 has a 4.1 Ohm output impedance.

Driving a 3-way speaker with impedance varying from 16 to 64 Ohms this would cause a 1.4dB output difference between the minimum and maximum impedances.

With 4 to 16 Ohm impedance the difference would be 4dB. This is not atypical for a 3-way - the reactive components for a Zobel network to counter the bass driver's resonant peak would be too big and expensive so the best you can do is bring it down with a resistor in parallel.

At 2 to 8 Ohms it'd be 6dB.

Apart from this edge case the effects on distortion aren't interesting compared to what the speaker is adding to the sound.

If you do want to run such an amplifier you'll do well going out of your way to buy speakers with high (16 Ohm nominal) and intentionally flat impedance.

I wrote:
Apart from this edge case the effects on distortion aren't interesting compared to what the speaker is adding to the sound.

That's not quite right - my power amplifier biases run towards push-pull solid state class AB for practical reasons although I like building with tubes where I can actually see how a negative charge on the control grid surrounding the cathode limits electron flow to the plate on the outside.

There are output stage device, topology, and biasing combinations which won't play nice with low load impedance.

Point:

Just because a loudspeaker says 4 ohms on it's back panel does not mean it's actually 4 ohms..Perhaps it's 5 or 6 ohms or 2.5 ohms?? Or an 8 ohms loudspeaker is actually 6 ohms simply stating that 4 ohms is hard to drive is not the answer as the answer is all over the board. Speaker builders don't want to post 5 ohms in there stats they use 4 or 8 ohms.

Sony's new reference speaker is considered very hard to drive and it's rated at 4 ohms but my bet is that it's 3 ohms or less..again it depends on the Frequency as the ohm loads change by the frequency. 16 ohms loudspeaker? good luck... perhaps Avante-garde acoustic: several of there models are rated 16 ohms but they have there own problems such as sounding quacky and having poor bass... no simple answer here.

Drew Eckert wrote:

"Some audiophile amplifiers have silly high output impedances which interact with the speaker's varying impedance to change the frequency response and this is exacerbated with low load impedances. Output Transformer Less Tube amps are especially bad although single ended triodes without global feedback can also have problems.

"For instance an Atmasphere M-60 Mk.II.2 has a 4.1 Ohm output impedance.

"Driving a 3-way speaker with impedance varying from 16 to 64 Ohms this would cause a 1.4dB output difference between the minimum and maximum impedances.

"With 4 to 16 Ohm impedance the difference would be 4dB. This is not atypical for a 3-way...."

In reply, I'd like to point out that, with the same 4 to 16 ohm impedance difference described here, the power that a transistor amp puts out varies by 6 dB, because it is putting out constant voltage rather than constant wattage. Why does the audio world accept this without a blink, and yet think there's a problem when a tube amp exhibits less variance in power output into the same load??

It is because the audio world is accustomed to the way transistor amps behave, and most speakers are designed to work well with transistor amps. The designer designs the speaker to sound right when driven by an amplifier that puts out 1 watt into the 8-ohm portion of its curve, 2 watts into the 4-ohm portion of its curve, and 1/2 watt into the 16-ohm portion of its curve, all at the same time (2.83 volts).

Now, what if the designer's goal was a speaker that works great with an amplifier that puts out approximately constant wattage, regardless of the impedance curve (within reason)? Well that can be done just as easily, but there are fewer amplifiers designed that way out there, so his potential market is smaller.

These two approaches to amplifier and speaker design have a name: Voltage paradigm, and power paradigm. You can read more about the subject here:

http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

Okay, what kind of amplifier sounds best? Boy that's a long debate for another day, but a lot of people familiar with many types of amps prefer the sound of a good OTL or SET amp, assuming a good speaker pairing, and that includes yours truly. Some speaker manufacturers give priority to building speakers that will work best with power paradigm amps because they believe that combination sounds best, and just accept that they are fishing in a smaller pond.

Is it possible to build a speaker that works well with both types of amps?

Yes, by keeping the impedance curve as smooth as possible, the speaker will work well with both types of amps. And with such a speaker, you can really make an apples-to-apples comparison of the different amplifier types, rather than actually evaluating whether a (typically roller-coaster impedance) speaker synergizes best with a voltage paradigm amp or a power paradigm amp.

Drew's numbers above illustrate an argument in favor of using a high impedance speaker with a low-output-impedance amp: The amp's output is approximately constant-power when the speaker's impedance is varying between 16 and 64 ohms, but the amp's power output changes significantly when the speaker's impedance varies between 4 and 16 ohms (same 4-to-1 variance in both cases). Also note, in both cases the amp's power output change vs speaker impedance is in the opposite direction of what happens with a voltage-paradigm (solid state) amp, and this is probably the main reason why simply dropping a specialty tube amp into your current system is a roll of the dice (and the odds are against you) unless you already know your current speaker is a good match.

Back to the original question, what's with 4 ohm speakers if they're harder to drive, well in general they can play louder with a solid state amp, and most people have solid state amps, so they get more sound per dollar with 4 ohm speakers (quantity outsells quality). Also, most woofers are 8 ohms, so if the designer wants to use two such woofers, he has to choose between series connection (16 ohms) and parallel connection (4 ohms). Most choose parallel connection because 4 ohm speakers outsell 16 ohm speakers. I believe that most amps - tube or solid state - sound better into a 16 ohm load, so my home audio two-woofer designs are 16 ohm loads (whereas my prosound two-woofer designs are 4 ohm loads, because there we're trying to maximize available SPL with solid state amps). What do I do about the typical halving of maxium power from solid state amps when driving a 16 ohm load? I start out with speakers that are about 3 dB more efficient, and of course pay a corresponding price in box size vs bass extension. However there is one "free lunch" to using power paradign amps: They generate equal or even increased, rather than reduced, power into the speaker's virtually inevitable bass impedance peaks, and if we keep this in mind and design our box accordingly, we get back most of the bass extension we otherwise would have lost when we traded off in the direction of higher efficiency.

So I take the position that, all else being equal, 16 ohm speakers sound better than 4 or 8 ohm speakers, and that power-paradigm amps are well worth seeking out matching speakers for.

Duke
dealer/manufacturer/power paradigm groupie