Chris Roger's DIY Transmission Lines

[Donniboi]

In the mid 1970's, I bought a pair of IMF TLS 50 transmission line speakers. A friend, who was an organ freak, liked what he heard and built a pair to the Hifi Answers Rogers design.

When he asked my opinion, I told him that they weren't great (nearly as much sound coming out of the sides as from the front, splashy treble...). He didn't speak to me for months!

Eventually he said "what are we going to do about it?". My suggestion was to double-wall the side panels behind the bass driver and fill the gaps with sand (not my own idea - I had read it in a Gilbert Briggs speaker book). Big improvement! This led to him "isobariking" the bass drivers, with advice from Falcon on crossover mods. The result was amazingly deep and tight bass. The best I had ever heard. Of course, the rest was still pretty ****. Next suggestion was to surround the hf drivers with thin felt (Russ Andrews Accessories sells something similar, but ready to use out of the box).

In 1978, I built myself a pair of closed-box speakers based on Radford T90 kits from Wilmslow Audio. These were of the double wall sand-filled construction. I still found it difficult to completely kill panel resonance until I rebuilt them in 1985 with plywood bracing filled with high-strength expanded polystyrene packing I got with a new oven (imagine Russ Andrews Torlite with the gaps filled with odd-sized white plastic polystyrene blocks). I experimented with various types of polystyrene, compression testing them till I found one that I could stand on without compressing it (hence the oven packaging). Inside the double-walling, the bracing was closely-spaced with no two spaces the same size. I also reversed the bracing back-to-front and top-to-bottom on opposing panels. The improvement to the resonance behaviour was dramatic.

I tackled the front panel splash issue in the first Radford build by using felt-lined conical waveguides (see ATC and many others who also use similar approaches).

If anybody is considering building the Rogers transmission lines, i would suggest you go straight to double-walled, braced and polystyrene filled panels - and two bass drivers per cabinet in isobarik formation, as well as tackling the front panel splash/directivity issue with lined waveguides.

Obviously you will have to either modify the design for modern drive units or trawl for originals from eBay or the like, but it will definitely be worth the effort.