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.

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.

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.
More important is finding a pair of loudspeakers that is relatively efficient due to having 40 wpc. 
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? 
Look at speaker sensitivity. Even this is a gray area. My cutoff is 92dB. I simply will not consider any speaker with sensitivity below 92. However, a lot of that is because I enjoy a nice high volume level without having to buy amp with 100-200 or more watts. One time recently we had some really inefficient little speakers in here, 87dB I think they were. My 20wpc Raven Blackhawk not only had no problem driving them to a nice loud level, it made them sound a helluva lot better than a 100wpc ss amp that was also used. Enormously better. So again, gray area, rough guide. 

I have for nearly 20 years now used all tube integrated amps, 20-60 watts, and just like your Prima Luna. Currently using Tekton Moab, and if you go with any Tekton speaker you will be fine. Better than fine. Before this I used Talon Khorus which were right at my cutoff 91-92dB and they were fine. Impedance? Who cares?    

So brief as it was russ69 totally answered your question. Beware the people who recommend paying a lot of attention to technical specifications, because there is no end to it. Next you will have to learn to read waterfall charts, dispersion patters, obsessing about front vs rear firing ports, cabinets vs open baffle, dipole, planar, on and endlessly on.   
Instead of what actually works: filter inefficient speakers out, then focus on finding the best sounding from whatever is left. Yes it really is that simple.
The question is about variable impedance and the genius answer is "who cares?"
Good answer.
Thanks to all, twoleftears's post is useful and yes, my question wasn't meant to trigger a 'who cares?' answer..
BTW, I know about sensitive (not efficient) speakers, and the two links in twoleftears's post are clear about the sensitivity/impedance pattern issue.
I'll read the threads,
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:


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.
Keep in mind that a higher impedance speakers, say one that peaks at 10 or 12 Ohms but is rated at "8 Ohms", will require more voltage to drive it than one that peaks at 8 Ohms.  It won't need as much current at 10 or 12 Ohms obviously, but it will require more voltage.   The only way you can get more output voltage is to use a higher wattage amplifier if you going solid state or use a higher impedance tap if you are using a transformer coupled output stage, such as most tube units.   

Remember the Dahlquist DQ-10's?  They had a relatively low impedance dip but also had a high impedance midrange, so they required both higher current and higher voltage to drive them properly.   Not too many amplifiers back in the day could do both.
“One time recently we had some really inefficient little speakers in here, 87dB I think they were. My 20wpc Raven Blackhawk not only had no problem driving them to a nice loud level, it made them sound a helluva lot better than a 100wpc ss amp that was also used. Enormously better. So ”

How entirely errrrr ....... Believable ... Not
How big is the room, how loud do you listen ? EL 34 is a great sounding tube…..
Post removed 
"...My 20wpc Raven Blackhawk not only had no problem driving them to a nice loud level, it made them sound a helluva lot better than a 100wpc ss amp that was also used. Enormously better. So ”
How entirely errrrr ....... Believable ... Not..."

I believe it. Tube amps generally have good power supplies and generous capacitors and they often play much better than the WPC rating suggests. The WPC rating is a guide but it's not the whole story. Yes I know, a watt is a watt but test out enough gear over a lifetime and you will find out that perceived power is not just about watts. 
Thanks russ69. Anyone experienced will know what you are talking about. Not to mention there was about a half a dozen here who heard it. But we are talking Asperger’s-level MDS here. Oh well at least you tried. Thanks again.
I listen to speakers or read reviews of what they sound like and ignore all but a quick look at a couple parameters. I was a former scientist and analytical person who has spent his career doing highly analytical work. I found most the analytical stuff just leads me away from sonic truth. Not wrong to be interested in it… but for me life is too short.
"...Not wrong to be interested in it… but for me life is too short.."

Exactly this. Impedance matching the speakers and amp is more about how they sound rather than having a perfect impedance match on paper.
The question posed by @tomic601 was the exact one Roger Modjeski asked when a potential Music Reference customer inquired about the suitability of one of Roger's amps for use with a given loudspeaker. He advised taking a SPL reading at the preferred listening level, a measurement of the amp's output voltage at that level, then extrapolating the required increased power requirements for every 3dB of added SPL.

I have been conversing with a fellow Eminent Technology LFT-8b owner, who has been marveling about how well his moderately-powered tube amp (I do not recall the amp, but it's under 100w/ch) drives the 83dB sensitivity LFT-8b. That's the same sensitivity as that of Maggies, which require and benefit from a high current amp. Sensitivity alone tells one only so much about a loudspeaker.

Maggies are a nominal 4 ohm load (dropping to 3 ohms at some frequencies), the LFT-8b 8 ohm. And if you bi-amp the ET (the speaker is fitted with dual binding posts, one for the m-p panels the other for the sealed box dynamic woofer), the magnetic-planar drivers present an 11 ohm load to the amp, great for tubes. By the way, the owner prefers the LFT-8b to the Maggie 3.7 he previously owned. The most under appreciated, under-acknowledged loudspeaker on the market. $2499/pr. VPI's Harry Weisfeld declared the LFT-8b to have the best midrange he has ever heard, regardless of price. Yet it continues to be ignored by most. Brooks Berdan chose Eminent Technology as his shops' magnetic-planar loudspeaker. 
So here you go:
Most conventional cone speakers have a rising impedance as frequency goes up. What most speaker designers should do that do not want to impedance compensate their designs is rate the impedance based upon the operating range of the individual drivers. Most companies do fudge numbers, if a speaker that they are currently calling an 8 ohm speaker showed that dipped into 3 or 4 ohms, many tube aficionado’s would not buy them. A realistic example would be: if you have a 2 way and the woofer is rated at 8 ohms but actually varies from 4 at dc to 30 ohms at 4000 hz. Realistically that woofer will range from say 6 ohms at its 3db down point of say 35 hz or so to a 15 ohms at a crossover point of 2k.... So in the woofers operating range, it varies from 6 to 15 ohms. it is called an 8 ohm woofer.
Rising impedance can easily be controlled. So in the example above, that 30 ohms could be controlled to stay within 6 to 8 ohms. My last design is a 15 inch 2 way, it is 95 db in sensitivity and varies from 5 to 6 ohms at all frequencies and yes, even though a solid state amplifier handles rising impedance easier, it is still better for an amplifier to see a very consistent load and if your tubed amplifier had 4 ohm taps, it would much prefer a impedance compensated speaker. Normally overall, phase angles are tube amp killers as much as just impedance, a tubed amp lover should look at phase angles as well as impedance curves.
There are the basics with a very basic example. There are times that other scenario’s could happen, but the above is the most common for conventional drivers. I hope this helps, Tim
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:


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.
You are starting at the wrong link in the chain  start with speakers you like what if you don't like any speakers with your amp and you can crunch all the numbers you want its not going tell you how they sound demoing is the only way if your dealer don't have your amps bring the amps there 

Enjoy the Music
Actually Eric,  it is absolutely true.  You provided one driver.  I've measured hundreds of impedance curves not a couple.  
Yes,  I used a woofer as an example, but this happens in many mids, tweeters etc and their individual impedance curves do show up on a graph.  Of course, I've stated often on this site, overall, I do prefer and recommend impedance compensation networks. 
Unless you are describing some sort of ribbon/amt's which tend to have a very flat impedance curve. 
Basically the frequency response of an amp with a low damping factor(high output impedance) will be affected by the peaks and dips of the speaker. The amp even if it's flat into a test resistor will not be flat into a reactive speaker load. The amp response will have dips and peaks at the same places as the speaker impedance curve. How big these will be depends on the damping factor(which can vary with frequency by the way). The higher the damping factor the smaller the changes in response until the damping factor is high enough so the amplifier frequency response is flat with the speaker.

I recall a letter years ago to Hi Fi News that said that using very low resistance speaker cables minimized the affect for low damping factor amps. Frequency response was shown for normal resistance cables which exhibited the frequency variation. But with very thick cables(about 4 gauge if I recall correctly) the frequency variation essentially was gone. But this was before modern cable technology and just changing to very low gauge cables will probably also affect other variables in cables which may or may not be good.
Published loudspeaker frequency response is virtually always measured with a solid state amp which puts out increased wattage into an impedance dip, and decreased wattage into an impedance peak. Most speakers are "voiced" for this type of amplifier.

Relative to a solid state amp, a tube amp puts out more wattage into the impedance peaks, and less wattage into the impedance dips. So the speaker’s frequency response will be modified accordingly. In general the lower the speaker’s impedance, the more pronounced this effect.

Ime a relatively high and smooth impedance curve tends to work well with tube amps. The bass impedance peaks can sometimes be manipulated a bit by lowering the port tuning frequency, and/or adding damping material inside the cabinet, if necessary.

Occasionally you find a loudspeaker whose impedance curve zigs where its frequency response curve zags, and that can work well with tube amps. I recall seeing such "complementary" measurements for one or two Silverline Audio models.

I’d suggest asking the loudspeaker manufacturer whether they recommend tube amps for their speakers. Audio show reports can also be a source of information about amp/speaker pairings.

See this paper by amp designer Ralph Karsten:  


tube amp dealer, tube-friendly speaker designer
Your drivers impedance will peak at its resonant frequency. Enclosure (size and type) and crossover (not simply the crossover point, but the design of the circuit) will both have a direct effect on the impedance curve of the speaker. Amplifiers (some) like to see an easy load. My ESL-57’s do not present an easy load, and are fussy when it comes to the type of power supplied.

Ralph sent me this link in my thread on the ESL57’s. Found it to be extremely helpful.

Your example of the ESL is an Electrostatic speaker. The op is trying to learn how to read impedance chart of a loud speaker.  Duke did give an excellent representation of how a solid state amp reacts to a speaker load vs a tubed amp. Which also shows why impedance compensation on a speaker design can be helpful. 
Your analysis of a crossover design can be true of how it effects a impedance curve, but this is normally due to how an inductance coil reacts to the EM field and natural inductance of the drivers. Depending on drivers,  it may be very beneficial to use a series or parallel design based on crossover point and what type of inductors are needed. But for the op's question and in general learning to read an impedance chart, my original response is less confusing for that purpose. 

Last I checked, the 57’s are indeed a loudspeaker. And they have an impedance curve. Brought them up because of the demanding impedance curve they present to an amp. They are a good point of discussion when dealing with amp/loudspeaker matching. 
The amp needs to deal with peaks and dips properly. So as not to over drive the low impedance, and under drive the high.

pretty simple.

Here's an interesting view on Understanding Speaker Sensitivity.

Higher sensitivity certainly makes life easier, but my main consideration is SQ and keeping well below limits so that peaks and fast attacks are handled with aplomb. The same goes with amplifiers.

The golden rule is never get even close to maximum specifications.

are you talking about me? Because if it's so, the discussion ends here.

I shouldn't even have started it. The point of abuse has been reached soon..
And as far as you are concerned, the late Maurice Chevalier is reported to have said: Men choose a woman in such a dim light they'd never buy a dress in. I suppose this applies to you.
Well I prefer to know what I buy. Thanks to all, I am leaving the thread.
All other things being equal (eg, ignoring things like actual level of sensitivity levels and type of amp, damping etc), I’d prefer a speaker with reasonably flat impedance and phase curves.

Goes to things like imaging and other issues of import. Get the first principles correct and you are sprinting off first base.

What is Asperger’s-level MDS and how is that relevant to this fairly focused discussion to a well considered question by the OP?
@martinguitars              Hope you're back on thread.  Don't feel picked on by miller, he can't help it, he does it to everyone.  He will never reform, he thinks too much of himself.   Best to just ignore him like the rest of us do.

In almost all affected speakers, low impedances occur only across short frequency ranges.
Low impedance points  at bass frequencies causes amplifiers the most problems.

The phase angle at low impedance points should also be examined.  Where this is low, it should not cause the amp too much difficulty.  But a combination of low impedance and high phase angle at any point will mandate a more muscled amp.

Some of us remember the Apogee Scintilla that had an average impedance of pretty well 1 ohm.  In those times only big Krells could drive them.  Things are better now.
Although all the technicalities and electronic measurements can be interesting, with an amp + speakers combination there's only one determining factor ... your ears. No matter what weird variations in impedance they may have, in the end it's how they sound that matters. By the way, room acoustics will probably contribute a great deal to the overall sound.

thanks, those are the criteria I am currently considering of importance; as for the Asperger affair, I am always amazed at how people feel entitled to throw judgments and epithets into the discussion because they are safe this side of the Internet... But I am no newbie to forums.


if the only determining factor were one's ears, audiophiles wouldn't spend a lot of time, money and mental energy changing and listening.. I find it funny that most of them buy a car, a fridge, a pair of shoes with the outmost care then rely on absolute lack of science and on hearsay to assemble a music system that is supposed to rely on science to do its job properly. Nothing wrong with some objectivity I think. Thanks for your opinion.

@Max Yes, doing our research, our reading, and maybe even some measurements, is big part of the fun of the hobby, as is exchanging knowledge, ideas and opinions on a forum like this. But while I might buy a new photo camera solely based on reviews ... amps, speakers and headphones are the kind of products I would never buy without having heard them myself first, no matter what raving professional- or user reviews or semi-scientific analysis may say.
@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:


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


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.

Actually Erik,  no, I am not mixing up a driver vs a finished system.  I didn't say ALL speakers rise with frequency, Many drivers have a fairly smooth curve.   Here is a simple 2 way,  they show driver impedance as well as system impedance and add zobel.  Impedance compensation is the only thing that fixed these curves. You can see the curves.  
I'm sorry, but I've read through that blog that you reference, it give very basic info and doesn't show detail.  What you are saying about a driver vs that same driver within a system has minor merit, good compensation will affect the curve, but to say that a drivers individual impedance curve does not effect or control the system in the frequencies that it operates in wrong. This is not opinion, but fact.  I've done this hundreds of times. Too bad that we live so far apart,  I have an entire side of my basement shelved with drivers. We could do measurements together as well as do some system designs.  Its easy enough to show. I think that the miscommunication or (mixing up) is what a drivers curve looks like in free air vs what it looks like in its enclosure.  The curve can change quite a bit in an enclosure.  Again,  my goal here is not to argue, but to give the op some information on his inquirey.  I believe that I know of a good example.  I'll look around and if I find it,  I'll post. 
BR-1 manual.indd (daytonaudio.com)
Here you go op,  this should help.  Normally when I am building a speaker.  In design phase, I keep the crossovers external with the drivers in the box, this helps give me very accurate impedance and phase all along the design process.  
This person has done a pretty good job explaining as well as several examples that will help you get a good handle understanding. 
Speaker impedance curve explained with examples - Audio Judgement
You asked a very reasonable question and have received some excellent and thoughtful replies (@audiokineisis as one fine example). Don't become discouraged and just ignore the immature and non useful comments.  I have no doubt that you'll find an excellent speaker match for your Primaluna amplifiers. 
One other thing needs to be stated: don't match/buy a speaker for a certain amplifier based solely on the published specs that come from the manufacturer.  This applies to nominal impedance (a real moving target), sensitivity, whatever. Notice, too, that sensitivity can be expressed in different ways.  When magazines with the technical wherewithal measure speakers, the results are very frequently different, and when they are different they are almost always less favorable to the speaker (i.e. it will be harder to drive for one or other reason).  
Learning more about technical specs is always helpful, but since what matters is they sound good to your ears. I'd put emphasis on simply trying different speakers and not get caught up in tech talk. Graphs won't guarantee them pleasing your ears. That amp is small enough to take to a dealer for  demo.

I've used a PL Prlogue, which is the forerunner to your EVO 100. Most of the popular brands are 87-89 db, and will work fine-for "normal listening."  As the manual suggests, try both 4/8 ohm taps.

I used a sealed cabinet, 89 db speaker with good results. 


Hi all,

thanks very much for the several interesting and useful replies. And for the kind welcome, I am new to this forum.
I've bookmarked a few links and will read the pages carefully; on the other hand, I am well aware of what some, like tablejockey, have said - it's not a good idea to rely entirely on specs, and that's what I am not going to do. My starting point with this thread was to take advantage of the solid background of many to bypass, for once, the usual criterium of just listening and to try and find a bunch of speaker systems that match well my amp on an electrical basis. To do that, I needed to learn how to correctly interpret an impedance graph.

I think that with an amp like mine - which I am keeping because I like it, sonically and visually - it may be better to use a speaker system with not necessarily an extremely high sensitivity - which in my experience is often paired with low impedance, at least in critical points on the FR - but with a regular, highish impedance pattern. I too think that 87/88dB or so and a minimum 6ohms should be good companions for my tubes.

Earlier this afternoon I listened again to a pair of Naim n-Sats, which I used to have; they are not at all of anyone's taste, but have some nice qualities. They are a sealed box and the 'woofer' is about six inches. I am not saying that I like them more than anything else, but they surely would be a sensible choice. Unfortunately, they have an idiosyncratic voicing and can only satisfy certain room acoustics. But I got a better impression of them from a more aware listening.

Thanks to the careful reading of a number of papers I didn't know, I now have a clearer idea of what to look for; one year ago I would have based my choice on brand, looks, number of drivers... Now I know better and I can demo speaker systems with a clearer idea of what can be ruled out soon and what can be tried at home.

Thanks for now for the precious help,
greetings from rainy Italy.

We are clearly talking past each other and it's unnecessary so I am stopping now. 
I've been looking at both impedance and phase angles, in certain instances speakers employing  
mid bass drivers with low impedance dips and 45 deg
or higher phase angle swings will overheat and stress
some amps.
In one of the previous links the example of a 100 wpc amp with 
 a 45 deg phase angle will produce 400 watt's heat. Unclear
if that incl a  matching dip in impedance.
As phase angle increases beyond 45 deg things can get serious.
depending naturally on the amplifier design.
thanks. I'm reaching the point where knowledge is not enough to solve problems but is enough to create some...

I can add a pair of Klipsch Heresy IIIs to the system and I think that'll be the answer. Conceptually contemporary. From this point on, it's a matter of amp and speaker dealing with it. So to speak.
thanks. I'm reaching the point where knowledge is not enough to solve problems but is enough to create some...


Now please scroll up. Read again what russ69 said. You are beginning to see the wisdom now? 
Not that it makes any difference in this discussion, but power amplifiers are primarily current amplifiers not voltage amplifiers.  Preamplifiers are voltage amplifiers and the first stage of a power amplifier is also often a voltage amplifier, but it takes current, not voltage to drive speakers.  Which is why you don't get a shock from your speaker cables, low voltage, high current.