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.

Running in

Astoundingly, the bass on my new speakers is opening up. The two 7" woofers don't have a great deal of linear excursion and I thought that this was limiting the low end (< 80Hz). However, today for some reason the base is as clear as a bell; perhaps a little too 'in your face'. What surprised me (but on reflection shouldn't have) is that with playing, the rubber suspension will soften a little and the impose slightly less damping. Perhaps running a 20Hz sine though them at fairly high power yesterday had something to do with it. It may not be an accurate sound, but as a bass player, hearing the bass lines so well defined is nice. 

So now I'll have to repeat all my measurements for the empirically-based crossovers (the ones I'm using at the moment were cobbled together from some components I bought based on the published performance of the drivers). The good news is that I didn't spend $80 mainly on coils I may not need. Coils are really pricey. I'm probably going to buy some enamel coated wire and wind my own.

Aja

This was one of the about three of four (audio) cassettes I took with me when I started working for IBM in 1983. I rented a room in Linstead Hall for about 6 weeks before the Hamlet Gardens house was up and running. I had two single-breasted pinstripe suits, one blue, one grey, a red tie, some M&S white shirts. Strange how vividly the music brings back some memories. The carpeting was a dark viridian; the room had a view over Princes Gardens. I remember getting up in the dark but I can't think why since I began work in late July. If have a feeling the room was #622, but that might have been my last year in college. The Linstead Hall I knew, built in 1968, is gone now, as is the house I grew up in. Such is life...

Phase IV (ants), Huey, Dewey and Louie.

In 1974, I went to see Phase IV. The B-feature at that showing, Silent Running, left a much more profound impression than the main feature. It has become something of a cult classic.

Phase IV complete

This post has nothing to do with ants. Twenty five years ago Graham kindly sold me at cost a pair of really nice bass drive units which I built into a pair of speakers. The only calculations I did were some fairly rough estimates of air volumes for the cabinets. I built a new design using the same drivers about 2 years later and the result was a slightly worse sounding speaker. I left them in my father's attic when I moved to France in 1990 and he, in turn, left them for the dustmen in one of his major clean-outs. So my 801 bass units are, regrettably, gone.

So when I set out to build another pair this August, I decided to use the cheapest drive units I could find. This time, however, I had some really remarkable design software thanks to Mark Zachmann, a computer, and some measuring equipment. The design was sketched in August I started cutting wood in September. The speaker (I decided to build only one prototype) saw its first activity this week.

Here it is next to one of the B&W DM6s I bought in 1979 and which has been my principle pair of speakers ever since. The crossover is sitting on the top of the DM6; I wanted to be able to adjust the component values without having to take the cabinet apart.  

I can't say for certain yet, since I'm listening to a B&W in one channel and my latest attempt on the other, but I think this time I've got something that sounds at least as good or even a tiny bit better than the B&Ws.

I've tried to do a farfield measurement and the only problem seems to be a bit (-15dB!) of a hole between 1-2k, but I'm not sure whether that's something to do with the room since the B&Ws exhibit a simialr problem when I measure their response. I've been listening to them almost continuously for three days and the sound is not in the least bit fatiguing, a good sign I think.  

So over the break I will be getting some more wood and building its companion. I know it's ugly, but I have yet to learn how to apply veneer. The drivers are Vifa D27TG45 (unfortunately no longer made but Scan-Speak makes some pretty similar ones), Scan-Speak Discovery 15M-4624G and a pair of Silver Flute W20RC38-08s. The Silver Flutes are made by a Chinese company, Vifa is a merger of a Chinese manufacturing firm and Danish designer.

I'm also thinking about building (Phase V) a two driver design using an Aurum Cantus  250F1 and an SB-Acoustics SB29RDC-C000-4 which I will probably cross over at about 700Hz.  Too much to do, too little time...