Articles, Brochures & Reviews, oh my!

A big part of this project has been trying to learn all I can about the original RSTL and RSTL-M. Through the generosity of others, most especially Nick Mellor from the UK, Allegro Sound, Falcon Acoustics and others (both documented and not), I’ve been able to amass quite a collection of literature on the RSTLs. Here is a summary:

  • DIY Articles & papers
    • Klang & Ton / Dec 87-Jan 88 / German
    • Klang & Ton / ?? / Hifi Boxenheft Spezial Nr. 2 / German
    • ?? / ?? / Norwegian? / forum
    • A+O Stuffing Instructions / ?? / German / private
  • Reviews
    • Gramophone / Dec 86 / English (UK)
    • Hi-Fi News & Record Review / Jul 88 / English (UK)
    • Stereophile / Dec 89 / English (USA)
    • Gramophone / Apr 96 / English (UK) – RSTL-M
  • White Papers
    • A Monitoring Approach to Loudspeaker Design / May 87 / English (UK)
  • Brochures
    • TDL Brochure (6 pages) / 1988 / English (UK)
    • TDL Reference Series ‘M’ / 1995-6? / English (UK)

The Holy Grail found?

Through DIY Audio member Duck-Twacy’s generosity and member Sark’s vision, I finally have a copy of the Klang+Ton article on the DIY version of the RSTL from the December 1987/January 1988 issue. That was the first thing I learned – this article was in a single issue. I’d just assumed Klang+Ton was a monthly and it was a two-part article. Way back in 2003 Duck-Twacy posted on the DIY Audio forum that he had the article and was willing to scan for interested parties. This sounded promising, but this post was over 9 years old. Would he still be reachable on DIY Audio? Would he still have this now 24+-year-old magazine?

The answer to the first question is yes, Duck-Twacy still is reachable via the DIY Audio site some 9 years later. Amazingly, he responded overnight to my request for the scan! Did he actually still have the magazine? It didn’t sound like it, but his colleague Sark on the forum had the foresight and vision to scan all of his Klang+Ton issues, presumably just for circumstances like this. They made the entire issue available to me.

Gentlemen, thank you both. I’ve been searching for this article for a long time, so I can’t tell you how much I appreciate you making it available to me!

Norwegian article about DIY kit

In this 19 page thread on the Norwegian site HIFISENTRALEN:,17539.0.html

I found an article about the kit version of the original RSTL. The good news is that it is easily readable. The bad news is it clearly isn’t as detailed as the Klang+Ton article. Still, I’m really looking forward to translating it. The pictures are way cool too! I’ve attached the scans without permission below.

Cutaway shot of speaker

Page 1 (click for full size)


Frequency response, picture, text

Page 2 (click for full size)


Page 3 (click for full size)


Klang + Ton RSTL DIY Project

The original RSTL design was published as a kit in the December 1987 and January 1988 issues of the German DIY audio magazine Klang + Ton. I’ve searched all over for the entire article, but so far I’ve only found a link to a photo of one page from this topic on a forum for classic KEF speakers:

I’ve copied the picture below to make sure this priceless heirloom isn’t lost. I’ll remove it at the request of the original owner. As you can see though it really isn’t clear enough to make out much of the required information, though after pouring over it for some time, I have gleaned a great deal of information from it:

Fact sheet from RSTL plans in Klang + Ton

The "Fact Sheet" from the Klang + Ton plans for the RSTL

My German is not what it used to be – actually I don’t know German at all – so I sat down with Google Translate and came up with this translation of the text:

Fact Sheet

Kit Name





TDL/John Wright

Enclosure Type

Transmission Line

Net Internal Volume

Not important for TL

Dimensions in mm

WxHxD 540x1209x492

Number of Drivers



Ovalbaß (oval)


13cm, plastic cone


25 mm, Plastic dome

Super tweeter

19 mm, Plastic dome


6 Ω

Efficiency, 1W/1m

Not available

(Power) Rating

Not specified

Recommended amplifier power

Not specified


Finished crossover RSTL

Kit price/pair

About 3000 DM


A. Oberage
Postfach 1562
8130 Starnberg

Bill of Materials

Loudspeaker Drivers TDL
  • 1 woofer 128/25, 16 Ohm
  • 1 woofer 128/20, 8 Ohm
  • 2 mids 130 NS, 11.1 Ohm
  • 2 tweeters 25 DT, 16 Ohm
  • 1 super tweeter 19 DT 06, 8 Ohm
  • L1, L2 = 8 mH Iron core
  • L3 = 1.1 mH Air core
  • L4 = 1.7 mH Iron core
  • L5 = 0.25 mH Iron core
  • C1 = 100 μF Electrolytic
  • C2 = 30 μF Electrolytic
  • C3 = 10 μF Film
  • C4, C5 = 4.7 μF Film
  • C6 = 3.3 μF Film
  • C7, C8 = 1 μF Film
  • C9 = 0.47 μF Film
  • C10 = 10 μF Film
  • R1, R2 = 15 Ω
  • R3 = 22 Ω, ceramic, 11W

Cabinet Parts
refer to drawing and text

Exterior walls, 20mm MDF, all dimensions in mm, gross
  • 1 front wall 1209.5×170
  • 2 side walls 1209.5×262
  • 2 side walls 1209.5×306.5
  • 1 back wall 1169.5×500
Interior walls, 12mm MDF, all dimensions in mm, gross
  • 2 MT-Chamber 541×303
  • 1 MT-Chamber 500×303
  • 1 Divider 500×492
  • 1 Divider 500×498
  • 1 MT-Channel 314.5×104
  • 2 MT-Channel above 553×150
  • 2 MT-Channel center 490.5×150
  • 2 MT-Channel below 452×125
Use hardwood 15x15mm, all dimensions in mm
  • 2 616.5
Back wall assembly
  • 2 1169.5
  • 2 184
  • 2 80
Damping Material
  • 5m Rockwool 38cm wide
  • 5 BAF-Wadding, 50×35 cm
  • 100g pure lamb’s wool
  • 16 Allen screws, M5x50mm
  • 16 T-nuts, M5
  • 12 Screws, black, 4x20mm
  • 75 Screws, 3.5x35mm
  • 5m wire, 2.5mm², for wiring
  • Putty to seal the enclosure openings
  • 1 pr gold binding posts

IMF woofer closeups

Thanks to Audiogon member Dipolemann, I have a few closeups of oval woofer similar to the ones used in the RSTL. These are actually of the IMF 128/20, which are used in some of the later IMF models, but they are visually very similar to the 3021GT 01 and 3021GT 02 used in the RSTL.

IMF 128/20 front view

IMF 300mm x 210mm woofer - front view

IMF 128/20 - rear view

IMF 300mm x 210mm woofer - rear view

Technical data gleaned from the 1989 Stereophile review

In the 1989 Stereophile review of the RSTL, there was some technical data I hadn’t run across before:

Midrange line is fully stuffed

The midrange line is fully stuffed, unlike the woofer lines

  1. The midrange line is not intended to increase output at all, but rather completely absorb the rear wave. Note the midrange line is completely stuffed, whereas the woofer lines are not
  2. The output of the midrange line is claimed to be 40dB down from the main output
  3. The midrange transmission line does lower the resonance frequency of the driver (to 40Hz), moving it away from the crossover point and hence (potentially) simplifying the crossover
  4. The crossover slopes are 3rd order. It is not clear to me if this is stated in the literature or observed in the measurements. It is also not clear if this is electrical, acoustic, or both
  5. The design mounts the two 6″ midrange units on a 5″ wide front baffle 😉

Point #1 is not surprising given the mids are crossed over at 200Hz in the RSTL, so there is no need for any bass augmentation from the mids.

Point #5 shows just how creative John Wright is!