The drivers are four ohm impedance and are wired in series for an eight ohm nominal load. Unfortunately their sensitivity is relatively low, and they have a rising response curve much like the Fostex Fe-167E that requires a network for any semblance of balanced sound. Consequently the sensitivity is about the same as I recall from a pair of Magnepans I used to own. Low power SET tube amps are not recommended. My 200w/ch Krell drives them just fine. For the network I used a 2mH air core inductor with a five ohm resistor because that's what I had in the parts bin, but I believe a 1.5mH with a four ohm resistor might be adequate.

I auditioned these speakers for the most critical listener I know, my wife, and she said "they sound good". No audiogeek-speak from this chick, either they do it for her or they don't. These do. My impression is that the bass is very good for such small drivers and the treble is clear and extended enough that you don't sense any absence of air. The big plus feature of these speakes is the open airy soundstage - not as precise as with a monopole speaker, but more like live music actually sounds in a music venue.

The enclosures are built of 3/4" birch plywood. This is a fun project that can be built for less than $150.

• MLTL Bipoles

Bottom line is if I auditioned these two speakers blindfolded I could instantly tell you the Zigma is a small driver full range. I would not be able to identify the bipole as a single driver or as having a small driver. If I had to choose only one pair to live with the rest of my life I would choose the bipoles.

Yup, of the various speakers I've made (probably somewhere around eighty different designs, going back to 1979), the bipolar format is what I'd choose for my "last pair".

Fred, if you ever want to BS about bipoles, let me know and we can spend some time on the phone.

Duke