speakers with a "smooth" impedance curve

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:

http://www.audiocircle.com/index.php?topic=56415.0

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.

Duke
dealer/manufacturer

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.

Duke

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.