speakers with a "smooth" impedance curve

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.

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.

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.