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Illuminator-71, revisited
Copyright 2022 © Troels Gravesen




Some 60 years ago a copy of Popular Mechanics came into my hands, in Denmark Populær RADIO Mekanik. These were mostly translated versions of the American issues, which often meant that all the wonderful things that could be made couldn't be done due to unavailability of components. In the US you had postorder catalogues, something that was still in its infancy here in Denmark.
From the issue I had there were 10 watt EL84 PP tube amplifiers from ready made boards - highly unavailable. There were large speakers with drivers totally unavailable. And even if available, the price would be way above a young boy's pocket money.
Anyway, these publications fueled our imagination and led to countless projects in the pursuit of better music reproduction. My older sister would listen to Radio Luxemburg all the time - and I wanted that too - in my own room.

Eventually my parents got a transistor radio and I had their old tube radio and dismantled the whole thing and had the chassis with all its glowing valves standing on the floor and the speaker driver hanging from my bedpost. Good fortune saved me from being electrocuted having high-voltage next to my bed.
What was immediately learned was that the sound coming from a free hanging driver wasn't as good as when it was in its radio cabinet. Hmm.. How could that be?
This was where Popular Mechanics came into the picture, because one chapter was dedicated to a very simple speaker construction shown in the images above. Some years later, having bought the book Loudspeakers by Gilbert A. Briggs, I would understand this was a short transmission line, supposed to enhance bass performance and not least taking advantage of room-gain.
So, I went to the local carpenter and had a board made, one board - mono times - some 25 x 100 cm and drove it home on my bike and made the shown "cabinet".
This was for sure a game changer and lead to further reading of the publication on a corner speaker with double Peerless full range drivers - at least these were available being made in Denmark. Also the famous Karlsons speaker cabinet was shown, albeit way out of reach using a suggested 12" coaxial - or even triaxial speaker driver - as though that would be available anywhere near!
 

When I finished my Illuminator-71 speaker, I was really excited about the sound and thought this was the best two-way I had ever made. Eventually the guy who had bought my SBA-761 and later Discovery-861 sent a mail and told he had a friend that would be interested in the Ill-71s. Well, a quick sale doesn't hurt and the speakers were gone. Only bad thing was that after some time I found out I missed them. I missed their transparent midrange, their - for the size - astonishing bass and I missed the smooth treble.
Only thing to do was build another pair! I want this speaker to be one on my few stock speakers as an example of what can be achieved from a simple 2-way.
Cabinets are exactly the same, 21 mm Baltic birch, bitumen pads all over, same damping, etc. Only difference is a beer can for treble as can be seen below, and crossover placed at bottom due to size of the copper foil capacitor. This copper cap costs as much as a pair of 6600 tweeters, but I had them surplus, so why not? I love the sound of the 6600 tweeter, so much I'll treat it with the best available.

 

So, the whole point of this scribbling is that after having made more than two hundred speaker constructions, would I be as excited setting up the new Ill-71 as I was some sixty years ago when I placed a small ~6" driver in a transmission line?
The answer is yes! Call it an obsession, call it crazy, I don't care. For sure it's a bug that never goes away, so be warned before engaging in DIY speaker building.

Looking at the 18W/4741T00 Illuminator driver we have come a long way since then! Underhung voice coil providing linear excursion, SD3 symmetric drive ensuring low voice coil inductance, non-reflective motor system, sophisticated sandwich cone, titanium voice coil and way better power handling, etc., etc. All ensuring lower distortion and subsequent midrange clarity*. The tweeter dome was hardly invented back then and today we have an abundance of excellent tweeters at our disposal. We have much better crossover components and we have measuring equipment to get the basics right. Only thing left is the choice of crossover topology and adding our personal taste to overall voicing of the speaker system.

*
For the record, some of these technologies were at its infancy back then, but never combined and brought to full fruition as seen in the Illuminator driver.

 


My new Illuminator-71s in my workshop.