The under appreciated impedance curve

Anytime I get asked about the suitability of a speaker with our gear I always look for the impedance curve. All amplifiers make more distortion into lower impedances so I look for that sort of thing- it can be pretty detrimental in the bass where a lot of energy can be needed. The reason its so detrimental is distortion components can extend to some very high frequencies- and we might be talking about some very high orders- not just 7th but well above the 20th and the like. The ear finds this irritating!

You can reduce the amplitude of the harmonics by simply presenting the amp with an easier load. This has the effect of smoother sound with greater detail, regardless of the amp.

Tubes in general like to drive higher impedances solely due to a higher output impedance. Our smaller amps are the most popular, so I impedance curves are very helpful. I'm not a fan of Stereophile but their measurements of speakers have been very helpful in the past.

What do you look for in the impedance curve?

The minimum value and the frequency at that value. Also, whether the curve is flattish over multiple octaves and, if so, the average impedance over that range.
That said, the map is not the territory. This is even more true for amp measurements. No way to know how amp+speaker will sound until you actually listen to the combo.

The whole speaker/amp impedance stuff drives me mad. I just seem to have a brain that doesn’t easily grasp electrical theory (not to mention I’ve never studied it).

I know the general principle for speakers seems to be: wild impedance swings - especially low - hard to drive, not great for tube amps.


Low sensitivity...hard to drive.

But I remain unclear exactly "why" this makes speakers ’hard to drive’ i terms of the actual sonic consequences.

I can more easily grasp low speaker sensitivity requiring more amplifier power/wattage. I can get "hard to drive" to some degree there, where an amplifier is going to run out of steam much sooner in terms of turning up volume.

But what are the sonic consequences of impedance curves in a speaker making it "hard to drive?"

My layman brain translates impedance variations this way: when impedance dips downward, at those particular frequencies it’s like the speaker becomes a lower sensitivity speaker, putting higher demand on the amplifier. So if you just went with a calculation for the demand on an amplifier’s power by looking mainly at the higher, flat points of an impedance curve - say it’s around 6 ohms, you could miss that it actually drops way down at one frequency to 2.5 ohms, in which case what you *really* need is an amp that will drive 2.5 ohms well, otherwise you live with possible frequency response deviations from the speaker. In other words, in figuring out the amp power requirements for a speaker, you should pay attention to the lowest dips in impedance, especially figure out where they happen in terms of their possible sonic consequences and if you need an amp that will drive *that* part of the impedance with necessary volume.

How off am I in that thinking?

I tried to grasp the consequences of impedance in that other great thread about amps, but I’m not sure I really got it.

That’s why I’ve used the comparison of two speakers, one I owned (Thiel 3.7) and one I’m interested in (Joseph Audio Perspectives).

https://www.stereophile.com/content/joseph-audio-perspective-loudspeaker-measurements

https://www.stereophile.com/content/thiel-cs37-loudspeaker-measurements


Looking at stereophile’s measurements and JA’s comments, he measures a pretty low sensitivity for the Joseph Perspectives, 84db, so my mind immediately thinks "hard to drive, needs a lot of power."

But then after the impedance measurements, JA says

"the Joseph is a very easy load for the partnering amplifier to drive. Not only is the phase angle relatively low, the impedance remains above 8 ohms at almost frequencies, with a minimum magnitude of 6.27 ohms at 135Hz. "

Oh...ok..now they are "easy" to drive. What does that mean for my Conrad Johnson Premier 12 tube amps? Not too powerful at 140W per side in terms of that speaker’s sensitivity, but the impedance is "easy" so....good to go?


Then there are the measurements for the Thiel 3.7s. JA measures a sensitivity of 90.7. So my brain says "Oh, easy to drive, doesn’t require much amp power to go pretty loud."
But then I read JA say:

Thiel specifies the impedance being nominally 4 ohms, with a minimum of 2.8 ohms. I actually found the minimum impedance to be 2.4 ohms at 125Hz. The difference between 2.8 and 2.4 ohms is academic, either mandating use of an amplifier that has no problem delivering high currents.

Whoops, now they sound "hard to drive." What does that mean for my tube amps?

Which is "harder to drive?" The speakers with the lower sensitivity and "easy" impedance curve? Or the speaker with the higher sensitivity and "harder" impedance curve? And what are the possible sonic consequences for each speaker in terms of being driven by a tube amp, like my CJ Premier 12s?

(My CJs seemed to drive the Thiels quite well btw...as well as any number of other speakers I’ve had, including even my small MBL speakers which are something like 82db sensitivity and I think fairly low....but somewhat even...impedance).

Thanks for any unraveling of my ongoing confusion...

The quick answer: The weaker the amp, the more its output in the frequency domain will look like the impedance curve.

The under appreciated impedance curve

Under appreciated?? I don’t think so, to those that find/look and know how to use it.

I believe "it" together with the combined "-phase angle" curve (EPDR) if hard to drive, "becomes the no1 criteria" to selecting the right amp to get the very best from that particular speaker. Probably that’s why it’s the 1st thing John Atkinson measures in Stereophile.

Cheers George