Doh!

You may have noticed that the mid range and the tweeter are too far apart. I completely ignored interference fringing around the crossover frequency. The units need to be separated by less than the wavelength of the crossover frequency - and they aren't. So as you move vertically (perpendicular to the axis), the waves from the two units interfere constructively and then destructively. Doh! This is something I really should have thought about. So I will be building a new baffle with the two units mounted much nearer together. Good job it's only screwed (sic) rather than glued. Oh, and I need to get a new amp or repair my old SUV6 which is cutting in and out on the left channel. A cheap valve amp would be nice but probably out of reach. 

Winding the coils

Since the exact values for the inductors I needed weren't available, I decided to buy some 16 gauge magnet wire and wind my own. After looking at a number of U-tube videos, I built a simple winding jig:  It wasn't quite as easy as the video made it look but eventually I had four coils. One was about 20 turns light because I ran out of wire. I think I was short changed on the spool I bough on Amazon - I won't be  buying from TechFix again. The online calculator seemed way off for two of them. Instead of 2.25 mH, the large coil was 1.6mH; not even close. So it was back to the shed with a hack saw and some nails. The addition of a steel core (should be iron but steel was the best I could improvise) boosted the impedance to 6.8mH. That meant I could unwind more than half the turns on both coils. The two home made cores are on the right - on the 2.25mH coil you can just see the nails under the blue insulating tape.  On the right is the network schematic. 

While I stated with a set of theoretical values, must succumbed to trial-and-error fine tuning in Speaker Workshop as I matched the network to the particular measured characteristics of the mounted drivers.

My wife's comment when I showed her the finished crossover was "it looks like something a 5-year old would have made". I was pleased; that something that looks so rough produces such nice results evoked the same feeling I had when I first visited the 8th floor of the Blackett laboratory. In display cabinets in the elevator lobby were devices that had been instrumental in a number of major experimental discoveries in particle physics. They too looked like "something a 5-year old would have made". It doesn't need to look good to work well, something Prof. Ken Bignell also taught me

Phase IV - measurements

Here are the far field frequency response graphs for my Phase IV prototype and my old B&W DM6s. Both have a trough at 110 Hz which I am assuming is a function of reflection in the room. The Phase IV has a trough at 215 which I think is internal reflection in the cabinet which currently has no damping whatsoever - so I need to buy the SAE 13 felt but at $80 I had been holding off. I  don't imagine that it will make a huge difference when I put it in but I may have to design the crossovers. From 700 Hz and up the response is very flat and looks rather better than the DM6 which has a biggish hole from about 550 to 1.5k. I'm pretty much done with this experiment now and in March will be beginning a new design based on a similar approach to the cabinet but with better quality bass and high frequency drivers.

Three goals were accomplished with Phase IV: the first was to test the general approach to the design of the cabinet; the second was to test the design process from beginning to end, in particular the design of the crossover based on the measured 'in-situ' response of the drivers; the third was to build a speaker comparable in quality to my DM6.