Why are low impedance speakers harder to drive than high impedance speakers

So by and large, @atmasphere re-expressed my explanations for output impedance with a lot of discussion about tubes and distortion.

I may not agree with some of it, but the truth is, voltmeters don’t get pleasure from listening to music. Humans (and cats) do. So while I can say with precision that most solid state amps are more accurate, as far as measurements are concerned, I cannot tell you which you should buy instead of a boat, gun collection, or second home. :)

Tube amps behave and sound different, and some sound quite wonderful. If that’s the sound you are entranced with, you should go that route, but your range and choice of speakers will be different.

I agree that the idea of picking a speaker first is not the right way to go. You should find out which team you want to bat for first. Overall do you like very good solid state, or do you like tubes, and if either, what are their qualities you like?

Also, what is your real listening style? Do you really sit in a throne for 3 hours at a time in front of two speakers alone? Is it usually background music? Do you sit with your partner, friends, etc?

All these matter. :)

Best,


E

Not sure anyone cares about this, but I thought I would chime in with more information now.

In general, many good drivers are available in 4 and 8 Ohm versions. When this is true, the 4 Ohm is usually 3 dB more sensitive. The low impedance causes extra current flow which provides for increased force against the same magnet. The choice here usually has to do with whether designers will put them in parallel and matching other drivers in the system with less wasted heat.

Some speakers really can't help it. Like ESL's. Essentially they are moving capacitors. The Apogees of old were/are essentially just wires suspended n a magnetic field. Getting up to 1 Ohm impedance was as high as they could get!  Any decrease in the current (i.e. increased impedance) would cut the force down proportionately.

In some cases I have seen crossover design deliberately lower impedance to below 4 Ohms to make speakers seem more "discerning" or "demanding." This added nothing to the sound quality of them. They just added to the snobby appeal.

Best,

E (yes, I'm a snob)

@mapman

Ahem, please see the example I posted, above. :)

The drop at resonance is no lower than Re. :)

I'm sure @Atmashpere will chime in, but depending on the amp, it's really the difference between the peaks and Re that makes a tube amp misbehave, as the high impedance will track this. If Re is high enough, it will be minor.

Best,

Erik

Because of these properties of amplifier and speaker systems:

1. Amplifiers have a non-zero output impedance
   
2. Voltage is proportional to impedance in a series circuit.
3. Speakers usually present complex impedance, with a range of impedance magnitude and voltage / current phase angles

Let’s simplify this to a couple of statement:

Amplifiers are current limited. This limit defines the maximum voltage at any given impedance.

As the amplifier output impedance rises, or speaker impedance drops, the electrical frequency response at the speaker inputs goes from flat to tracking the speaker impedance.

Want to understand why? Read on.

As an introduction, see the first graph on my blog post on speaker impedance for a very easy to drive speaker:

https://speakermakersjourney.blogspot.com/2016/12/crossover-basics-impedance.html

We’ll ignore phase angle, as it’s hardest to grasp. Let’s assume instead:

• Tube amp output with about 1 Ohm impedance,
  
• An ESL such as a Martin Logan electrostatic, which has a panel impedance that varies from 4 Ohms at the bottom of it’s range to 0.3 Ohms at the top end.

At low frequency, say 300 Hz where the panel is 4 Ohms, the voltage at the panel is

4/(4 + 1) = 4/5ths = 80%

of the amplifier output.

At high frequencies, say 10kHz 0.3 Ohms the voltage stays at the amp, and the speaker gets

0.3 / (0.3+1) = 0.3 / 1.3 = 23%

of the amplifier output, or about 25% of the amp output!

But look what happens with an amplifier with very low impedance of 0.001 Ohms (aka high Damping Factor):

At 4 Ohms:

4/(4 + 0.001) = 4/4.001 = 99.97%

At 1/3 Ohms:

0.3 / ( 0.3 + 0.001) = 99.67%

So in the range of speaker impedance from 4 to 0.3 Ohms, the amplifier output remains nearly rock-solid.