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!

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!

TDL Reference Standard Loudspeaker – Stereophile Review – December, 1989

In the December, 1989 (Vol. 12, No. 12) Stereophile, Robert Harley reviewed the original TDL Reference Standard (not the ‘M’). For the most part it was not favorable. Here are some excerpts:

Indeed, I immediately noticed an unnatural huskiness in her voice that could clearly be heard to resonate in her lower registers. This peak was also audible on piano as a “wooof” [sic] sound in the left hand. Further listening with a variety of music confirmed these impressions.

…In addition, I detected another coloration, a peak in the upper-midrange/lower-treble area that added an edge to sibilants. This peak was lower in frequency than the common 10kHz – 13kHz range where many speakers are bright. The coloration added more of a nasality and stridency rather than the spitty sound produced by other speakers with elevated top ends. Cymbals also took on a forward, splashy character without being overly tizzy in the extreme top end.

…The RSes’ imaging was somewhat better, but not convincing. …it tended to cling to the sides around the speakers. Throughout the listening sessions, I never had the impression that the speakers disappeared into the music. …just didn’t have the three-dimensionality I have come to expect from speakers.

Not exactly the speakers dreams are made of, would you say? 😉 It was not all a total loss though. After all, this is one of the most ambitious transmission line designs of all time, so hopefully Harley had some nice things to say about the bass? You betcha:

Bass reproduction through the TDL RS was easily the deepest, most effortless, and most enjoyable I have heard through any system. They present low frequencies with power and impact only hinted at by other speakers.

…In addition to this remarkable LF extension, the tonal character of the lower octaves was very neutral, without a trace of tubbiness or boom. Bass was fast, detailed and articulate. …I found myself playing recordings that featured my favorite bass players, reveling in the satisfying combination of seemingly bottomless depth and tight, punchy attack.

In Harley’s view then, the RSTL was something of a one-trick pony. This is the harshest evaluation I’ve encountered, but it is also the only one so far from a professional reviewer under controlled conditions. Next I’ll publish excerpts from the Gramophone review of the RSTL “M”. It is considerably more favorable, but also not nearly as in-depth.


Light shed on a mystery

Since undertaking this project, one of my big questions has been why the RSTL and RSTLM are designed to be angled such that they cross in front of the listening position. I finally found the answer in the 1989 Stereophile review of the RSTL:

When properly positioned, the front baffle, with its mid- and high-frequency drivers, is 45° off-axis from the listener. This scheme follows Blumlein’s recommendations for stereo reproduction. To increase dispersion, the supertweeter augments high-frequency output off-axis. Consequently, when measured on-0axis, the RS has has a rise in high-frequency response which the manufacturer says disappears at the listening position.

Interesting. I’d imagine this would be stunning when playing back a recording made with a Blumlein Pair, but it is not obvious to me that it would have technical advantages otherwise.