speakers with a "smooth" impedance curve

I had started a thread asking about speakers that are well designed for tube amps (I am currently using a Ming Da MC 34AB with 8 EL34 power tubes 75 wt/ch ultralinear; 40 wt/ch Class A).
There has been a consistent recommendation for efficient speakers with a "smooth impedance curve".

Any recommendations out there for some tower speakers in the less than $5000 price range with smooth impedance curves that are "tube friendly"?
If your speakers impedance doesn't swing too wildly and/or deeply below 4 ohms, then you may not need to worry unless you have a tube amp without 4 ohm taps.

But it all depends on how loudly you like to listen.

Now if you're talking about SET amp, that's a different story. SET tube amps have limited power & like smooth impedance curves above 8 ohms for the most part.

Read this article by Welbourne Labs (http://www.welbornelabs.com/recomendspeaks.htm). Be sure to scroll down & take particular notice of article "Some Insight into Proper Speaker Selection."
To Beavis

My current speakers have a nominal impedance rating at 6 ohms. I can't find an impedance curve on these speakers, but the manufacturer recommended the 8 ohm taps on my amp. I think the system sounds much better using the 4 ohm taps. The question is, if the impedance curve does drop below 4 ohms, does that strain the amp and risk damage to the amplifier?
>tower speakers in the less than $5000 price range

That was my price range 7 years ago. I bought Coincident Super Eclipse IIs in black. I have Welborne Lab 300B SETs. I love these speakers.

According to the manufacturer the load does not drop below 10 Ohms, and the sensitivity is around 93 dB/w/m. Your 40 watters would make the Super Eclipse sit up and beg.

DeVore Fiidelity. I am running Gibbon Nines right now
w/ 75 watts. It is plenty of power.
In addition to those mentioned above:

Joseph Audio RM25XL

AudioKinesis Jazz Modules

Merlin VSM-Mxe (used)
I'm not really sure any of the above loudspeakers have a "smooth" impedance curve.

What's more, I'm less an adherent of this mantra than most. Tube amplifiers prefer higher impedances, and most tube friendly loudspeakers have far more wild impedance curves than the conventional wisdom of smooth impedance curve thinking would ever lead a person to believe.

Why? With most loudspeakers (multidriver and parallel crossover network), the requisites (a bevy of crossover correction/compensation circuits) to produce this desired smooth impedance curve do a lot more harm than good in terms of making the loudspeaker more friendly to a tube amplifier. Put it this way, in order to attain this smooth impedance curve, more crossover parts are required, and crossover parts act as speed bumps in terms of a tube amplifier putting power into a loudspeaker. Beyond that, such "correction" circuits rob the music of life, vibrancy, and immediacy, even if the partnering amplifier seems up to the challenge of blowing through those speed bumps.

As an alternative to seeking a smooth impedance curve, look toward loudspeakers that offer as simple a crossover topology as one can find, and several of the above loudspeakers mentioned above fit that paradigm.
Totem Forests are a possibility if you find their sound to your liking. I believe they don't dip below six ohms and are priced well under your $5000. mark. This will leave alot of dollars to cater to the system with cables, etc. I am getting beautiful, magical sound from the 8 ohm taps of 15 watt mono amps. And yes, the system will play loud and rock when needed.
So now your mantra becomes "a smooth impedance curve above 8 ohms with a first order crossover ."

Good luck.
Most Tannoy's are very easy to drive.
Having used a particularly load-sensitive tube amplifier in designing fairly high-efficiency loudspeakers, in my experience smoothing the impedance curve via additional crossover componentry does not have a downside. Electrically, the amplifier sees the net impedance curve (including its phase), rather than seeing numerous individual components. Smoothing the curve (and reducing its phase angle) is beneficial.

Let me give an example. A friend of mine has speakers whose impedance curve wouldn't work well with an OTL tube amp, so I designed an outboard filter to smoothe the impedance curve. Not only did this work with the OTL amp, but he also reported a significant improvement even with his solid state amp - which is what his speakers were originally designed for.

You can read his account here:


If the additional circuitry was somehow detrimental, but just happened to synergize well with the tube amp, then surely that detriment would have been revealed by my friend's high-resolution class-A solid state amp. What he said:

"...also mentioned even my current SS amp's performance might improve w/ this filter. Sound quality was indeed improved: smoother, more ease, less tension, less grain, tighter focus + increased musicality, image/stage improved a lot...etc. The difference can be heard even in the next room."

I think the effect he describes comes from the reduced phase angle of the impedance curve; at any rate, there was no detriment reported.

Well, I guess we cannot agree all the time, Duke...

Anyway, I probably understand where you are coming from, even if I disagree as far as the benefits go. I just would like to clarify a bit more so that I can at least say that we are more or less on the same page of what is at work here, even if our opinions diverge.

Along those lines, a few questions, if I may:
1) From your statement, I infer most of the direct experience you have with the network you mentioned implementing is in partnership with OTL amplification.

Is this correct?

2) The solid state amplification experience is indirect, through the customer.

Is this correct?

3) Normally, large impedance swings are "smoothed" by networks that flatten their peaks (though I know of one that works the other way), which, if we are interested in partnering with OTL amplification would kind of run counter to that. Though, without making an already long post even longer, I can understand the argument against the statement I just made, and in some instances, I even support it.

Can you provide a high-level description of this network you are referring to, please? Of course, I respect the fact that some folks are more sensitive to discussing things than others such as myself normally are. But, if it's a well-known, previously illuminated topology, as opposed to something truly unique, novel, and patentable that you would feel uncomfortable in discussing, please describe it.

Finally, again, I understand what most designers are aiming for with these impedance compensation networks. In fact, I accept that I'm likely representing the minority opinion here, as without naming names, what I consider to be the flagship loudspeaker of the current high-end audio marketplace implements them, as well as a highly successful company that offers tweaks as an add on to one's existing pair of loudspeakers. The benefits are more or less exactly as outlined in the words of the customer you quoted. Nevertheless, I disagree with their implementation in most instances, as I feel the detriments they add in terms of robbing the music of immediacy, openness, and dynamism seem incredibly difficult to overcome elsewhere in the audio chain.
as usual I agree with Duke. I'd just add that when a crossover is designed for a speaker to have a smooth impedance curve this can be done without using any filters or extra components, it just takes a lot more time etc., some companies do this some don't. Mine are built and tested using solid state amps, but smooth is smooth and the real difference between the SS and tube amps is what impedance range they like to see, both work best with a smooth load.
Hi Trelja,

Yeah we finally disagree. It's about time, don't you think??

The circuit I used is nothing you won't find in the Loudspeaker Design Cookbook, and it flattens an impedance peak somewhere around 2 kHz (the specifics are foggy now). I've used that and other impedance-smoothing techniques in speakers that have been hooked up to a wide variety of amps, but only used that one as an external add-on that one time... that I can remember. That case was interesting because it gave a before and after comparison. If you shoot me an e-mail I'll give you the guy's e-mail and you can contact him directly. There's a small possibility that you know him.

I've heard his speakers with and without the external impedance filter on his solid state amps, but his assessment is better qualified than mine and besides a designer patting himself on the back isn't credible, even if it's me! I prefer to let other people pat me on the back... or kick me in the butt, as the case may be.

Now some impedance-smoothing techniques are detrimental to clarity, as are some equalization techniques, in my opinion.

If you shoot me an e-mail I'll tell you what the basic circuit is (I don't have a record of the values that I used), but I'd rather not post it here. You've probably already figured it out. If you have a speaker in mind that you'd like to try it on, if you can get me a look at the impedance curve I'll see if it's feasible and if so make some suggestions as to values you might try. A candidate for this type of circuit would be a speaker that was designed for a solid state amp, which has a single peak in the impedance curve somewhere above the bass region, and you want to drive this speaker with a high output impedance tube amp like an SET or OTL.

If I can clarify something here-

If the speaker was voiced to work with transistors, the smoother impedance curve is going to be a real boon if you are trying to use tubes.

If the speaker is designed for tubes, the impedance curve likely will not matter too much. So both Duke and Trelja are correct as this is an issue that is very use-specific.

for more information see:
Daedalus, what you describe is what I do in my speakers, instead of using an outboard impedance equalization filter. And I would estimate that designing for a smooth impedance curve as well as a smooth frequency response curve roughly quadruples the workload it takes to get the crossover right. But then as you said smooth is smooth, and amps like that.

And by the way, your crossovers are particularly well done; I can tell that attention was paid to the power response, not just the on-axis response.
Thanks for laying things out so well, Duke. I will fire off an e-mail in the near term. Anyway, we're way past due on sharing an e-mail or three.

Lou, I think your statement about being able to design a smooth impedance into the loudspeaker is a very good point, and fits into what I've been espousing perfectly. No crossover parts needed to accomplish it, simply good, well-thought out design. That makes the speaker a friendly partner for the type of low-moderately powered transformer coupled tube amplifier I happen to prefer.

I will say that from afar, your loudspeaker designs inherently make that easier than most on the market these days. I think ala your using more drivers. So, you are taking advantage of handing off to the next driver up the ladder before the rising impedance due to voice coil inductance rears its head.

As most loudspeakers today use something like a 6.5" midrange/midwoofer handing off to a 1" dome tweeter somewhere in the 2000 - 2500 Hz region, that impedance rise can become a problem.

The cookbook solution is a Zobel network, which flattens the rise, but I feel they do far more harm than good in the ways I've previously mentioned. Again, I accept I might not be in the majority here, as a buddy of mine and I have the EXACT same loudspeakers, which were an outgrowth of the pinnacle of Bud Fried's lifework (Valhalla System, though with what we've been able to move forward with in terms of crossover design), even to the point of independently arriving at more or less the same Zeta value of somewhere in the 1.0 - 1.1 (Bud's commercial designs, as well as his personal pair which I own come in around 0.6 - 0.7, personally, I find that value a bit too forward) range. The only difference between our speakers is the fact that he feels the Zobel makes the speaker, and I feel it breaks it. As I like to say, that's why they make vanilla AND chocolate.

This impedance rise is what I blame for so many of today's systems sounding overly forward and bright, as there is simply too much energy in the presence region. Especially, when driven by a tube amplifier, which is putting its power better into these higher impedances, and manifests itself in terms of the hard, glassy presentation that people tear their hair out trying to ameliorate with room treatments, cabling, and trying to put even more tube equipment (preamplifier, CD player, tube buffer, etc.)into their system.

I find a lot of the more creative and forward thinking designers these days are shifting the crossover point upward, in the effort to stay far away from the resonance frequency of the tweeter, as they feel that is the cause of the brightness. I would suggest the better way to go is do what is required to move downward to avoid the impedance rise of the midrange/midwoofer driver altogether.
This impedance rise is what I blame for so many of today's systems sounding overly forward and bright, as there is simply too much energy in the presence region. Especially, when driven by a tube amplifier, which is putting its power better into these higher impedances, and manifests itself in terms of the hard, glassy presentation that people tear their hair out trying to ameliorate with room treatments, cabling, and trying to put even more tube equipment (preamplifier, CD player, tube buffer, etc.)into their system.

Trelja (Threads | Answers)

Amen, Trelja.

I tend to agree with your point regarding excess energy in the presence region - sometimes it seems to me that it's almost ubiquitous in high end loudspeaker designs. But... I've never particularly associated this with models that cross to the tweeter at any particular frequency. It's always seemed more universal.

I'm not familiar with some of the technical issues at work here and therefore don't follow your point re: impedence rise vs. crossover frequency. Is this rise associated with the crossover network itself or do you refer to some other phenomenon?

Thanks in advance.

Duke, I wasn't implying that your systems use filters, your systems sound great and I know you pay a LOT of attention to all the details.

Thanks for the compliment on my crossovers, Guy who does the final crossover work is a true genius and I'm fortunate to have worked with him for all these years.
see you at RMAF