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Speaker impedance patterns - how to read them?

I know this has probably been discussed ad nauseam, but here's my question. I'm grateful for any sensible opinion.

Premise.
A speaker system may have a complex impedance pattern that can vary from 3-ohms to 30-ohms depending on frequency. Some speaker systems with similar impedance patterns may be stated as being 8-ohm loads because the speaker is 8 ohm in an important part of its frequency response, but only there.
A tube amp, as a voltage amplifier, likes a regular impedance pattern. Almost no speaker system has a constant, regular impedance pattern. I am aware that solid state, powerful amps are built to compensate for modern speakers' complex impedance patterns, but there are tube amps around and people who love them. It's difficult to understand what average load a given speaker system will present to the amp.

Question.
When looking for suitable speakers for a tube amp - mine is a PrimaLuna EVO 100, 40w x 2 from EL34s - what must be looked at? How to best interpret the impedance pattern, whenever available? Assuming that one listens at normal levels in a medium-sized room, what are the criteria for matching speaker and amp?

Thanks for your views. M.
martinguitars

Showing 4 responses by erik_squires

erik_squires
16,695 posts
 
@timlub

We are clearly talking past each other and it's unnecessary so I am stopping now. 
erik_squires
16,695 posts
 
@timlub You are mixing up the impedance of a driver for the impedance of a completed speaker, meaning 1 driver on your desktop vs. multiple drivers attached to a crossover.

Of course anything with a coil will eventually behave like an inductor, and woofer’s are the easiest of these to use to make an example. However to tell an end-user that has much to do with the end result is really not a good characterization.

Here’s a typical impedance curve for a simple 2 way:

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

It’s very very different than the impedance curve for the woofer it used, which can be seen in this impedance chart:

https://www.blogger.com/blog/post/edit/8271724650517523711/3931230143413830127

To say all speakers rise with frequency ignores everything that happens in between, and ignores both frequency and impedance equalization, not to mention the basic high and low pass filters as well.

erik_squires
16,695 posts
 
Most conventional cone speakers have a rising impedance as frequency goes up.

I’m sorry that’s not really true. You are describing the impedance of a coil.

It is true that larger cone drivers have this, and that this effect needs to be dealt with but it is not true for a speaker as a whole. A quick survey of impedance curves at Stereophile would show this.

Here’s a random sample:

https://www.stereophile.com/content/stenheim-alumine-three-loudspeaker-measurements

It is also true that the most challenging areas for a conventional speaker tends to be in the bass, but the impedance curve of a raw woofer is not at all a good indicator of the impedance curve of the speaker as a whole.
erik_squires
16,695 posts
 
A tube amp, as a voltage amplifier, likes a regular impedance pattern.


All power amplifiers are actually voltage amplifier, but tube amps typically have higher output impedance, which makes the output vs. frequency load dependent. Please see the first graph’s black trace for a very typical example:

https://www.stereophile.com/content/doshi-audio-evolution-monoblock-power-amplifier-measurements

I am aware that solid state, powerful amps are built to compensate for modern speakers’ complex impedance patterns,


I would not use this phrasing. Solid state amps tend to be more ideal voltage amplifiers, with a lower output impedance. There is no particular compensation, unless you mean a feedback loop, which tube amps can have too. They aren't compensating, they are lacking things like output transformers, which reduces the output impedance of the amp. 

When looking for suitable speakers for a tube amp

This is actually a little complicated as a lot goes into this. The sound of your room, and your personal preferences may complement each other quite well.

There are some general guidelines, which is to avoid speakers who dip below 4 Ohms, and those with sensitivities lower than 92 dB or so. Generally also avoid ESLs or you’ll lose all the treble. Of course, you may have an overly bright room, or amps which are less sensitive to this.
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