Go deep!

In my previous post, I mentioned a discussion about this project on the diyAudio site. One comment referenced an analysis by Martin J. King (http://www.quarter-wave.com/), widely regarded to have the best transmission line modeling software available. He stated that Dr. King had found “Dalines” (what G. L. Augspurger calls “Pipe with Coupling Chamber” ?) inferior to designs. I did find this reference on diyAudio that supports his statement.

Without further ado, here is my model of the Tang Band W8Q-1071F in a RSTL line. Could there be something better? Maybe. Does this strike me as a configuration that will have deep bass? Yes. Am I worried about the dip at 100Hz? Not really – right now I expect I’ll have an active crossover at or below 100Hz, so I’m not at all concerned. Should I go full passive, I’ll have to review my stuffing plans.

Simulation of TangBand behavior in the RSTL line

The prototype drivers are here!

Since my second article back in February, I’ve done some re-thinking on drivers. I got a sample of the Vifa, but it my mind it was just too small. And too expensive for what it was. After finding and pouring over plans, I concluded that I could move to a 5″ class driver. After hours studying the Zaph Audio 5.5″ Driver Comparison page along with countless other resources, I decided try the SB Acoustics SB15NRXC30-8-UC for my prototype. For about $55, the distortion measurements are very close to the $200-$300 drivers he surveyed. On paper, it looks like the best 5″ value around. We’ll see how it sounds.

Option 1: center panel layout using a single Seas 27TBCD/GB-DXT

For the tweeter, I’ve had two ideas. One is to abandon the Tweeter-Super Tweeter-Tweeter arrangement completely and use a single tweeter with exceptional off-axis response instead. That is the reason Wright really went to the T-ST-T arrangement from what I can tell, to get good off-axis (the way the speaker was intended to be used) high frequency extension, not to extend on-axis response to 35K. The tweeter capable of crossing over at around 2K and yet having flat response at 20K @ 30 degrees off axis? Why the Seas SEAS 27TBCD/GB-DXT of course. The published data sheet shows the response 30 degrees off axis is only 1-2 dB down from the on-axis response. Wow!

This would be a major break with tradition though, which is disturbing to me. The center column then becomes a “regular” M-T-M. I wonder if Wright were alive today though if he would not do the same thing.

Driver layout using the SB Acoustics SB29RDCN-C000-4 and Vifa NE19VTC-04

To take a more traditional approach, I’ve decided to change from the Scanspeak D2608/9130 that I had originally had my eye on to the SB Acoustics SB29RDCN-C000-4. This will allow me to keep the original driver spacing. I’d originally shied away from it due to the 4 ohm impedance, but with 94 dB sensitivity, I’ll have all sorts of room to manipulate the impedance in the treble. The M-T-ST-T-M configuration (I’m still using the NE19VTC-04 for the supertweeter) will look something like the picture to the left.

My current thinking on drivers

I’ve spent a long time thinking about what drivers would be suitable build my TDL RSTL M. While it changes frequently, here is my current thinking as of Feb 2012.

Overview

A real challenge has been finding high impedance drivers. We live in a 4 ohm world. That is fine when you use a single driver per speaker, but the RSTL uses dual woofers, mids and tweeters. Wiring in series is an option, but that comes at a high price in efficiency. A nominal 2 ohm load seems too bruising to me, especially since the crossover is likely to make it even lower at certain frequencies. For the woofers I had no choice, but for the others I went with 8 ohm drivers.

I also agonized over insisting on metal mids and tweeters to match the “M” version of the RSTL. That is what the “M” stands for – metal. So far I’ve come to the conclusion I’m going to use the best drivers I can find for the application, regardless of material. I think that is what Wright would do. He used metal drivers for the mids and tweeters because those were the best available to him at the time. If there were something better, he would have used them.

Woofer

Tang Band W8Q-1071F – a serious sub

There is only one approximately 300 mm x 210 mm format woofer I could find – the Tang Band W8Q-1071F. There are several problems with this woofer – first it is not being imported to the US anymore. In fact, I only have three currently. Does anyone have one they’d like to sell me? Perhaps it is not even being made? Second, it is a 4 ohm woofer. I’m not sure yet if I’ll wire it in series or decide the amp will need to drive a 2(?) ohm load. We’ll just have to see.

A brilliant design move on the part of Wright was to use two different woofers. While they looked identical from the outside, one was a 16 ohm driver, the other 8 ohm. That allowed them to work in different ranges while not adding complexity to the crossover. It is not clear yet if or how I might achieve a similar result with the Tang Band.

Another BIG problem I’m facing is the depth of the W8Q. The originals were about 110 mm deep; the W8Q is a whopping 279. I’ll clearly need to rearrange the internal baffling to make room for the extra depth.

Other drivers considered

•  NOS originals if I could find them

Midrange

The NE123W-08, a new 4″ “full range” from Vifa

The midrange I’m really struggling with at the moment. I keep going back and forth, up and down. Because of the narrowness of the front section of the cabinet, I’m pretty much limited to 5″ drivers (the original calls for a 13 cm driver – almost 5-1/8″  external diameter for us Yanks). The cutout diameter needs to stay under 4″. The new Vifa NE range blows everything else out of the water at the price, at least on paper. The NE123W-08 is the right size and 8 ohms.

Other drivers considered

• Dayton RS125-8 (cheap, metal and low distortion)
• Peerless – several 4″ models
• SB Acoustics (where did the 8 ohm 4″ go?)
• Scanspeak Discovery 4.5″
• Seas L12
• Tang Band Titanium 4″ (Neo and regular)

Tweeter

Scanspeak D2608/9130 8 ohm tweeter

As I mentioned, the TDL RSTL uses two tweeters in parallel, so I focused on 8 ohm tweeters. I thought about find high-efficiency 4 ohm tweeters in series, but that did not seem as satisfying. While the Scanspeak D2608/9130 is certainly not cheap, Zaph|Audio says, “Peerless finally creates a world class tweeter to compete with the best from Seas and Scan-Speak.” In case you were wondering why a have a quote about a Peerless tweeter in a discussion about a Scanspeak, the D2608/9130 was formerly known as the Peerless HDS 810921.

Other drivers considered

•  Dayton Reference RS28A-4 (Aluminum) and RS28-F (Silk) – both 4 ohm
• SB Acoustics SB29RDC and SB25AC – 4ohm
• Seas – several models – almost all are 6 ohm
• Usher 9950-20 – my second choice?

Supertweeter

Vifa NE19VTC-04 3/4″ Deep Anodized Aluminum supertweeter

The TDL RSTL uses only a single supertweeter, so here I feel pretty comfortable (finally!) with a 4 ohm driver. The TDL RSTL is designed to be angled such that the speakers cross in front of the listener. So the job of the supertweeter is to provide excellent off-axis response. Most any top-quality, modern tweeter is sufficient on-axis, but for the supertweeter I needed something with excellent response at 30 degrees off-axis as well. That pretty much dictates a 3/4″ tweeter. After pouring over every factory response plot of a 3/4″ tweeter with a small flange (that ruled out great tweeters like the Hiquphon and Vifa XT19 ring radiator), I decided on the Vifa NE19VTC-04. Vifa has a whole series of tweeters with that frame – Silk, Aluminum, Titanium and “Deep Anodized Aluminum”. The NE19VTC-04 is made of this latter material. The supertweeter is only used from about 10K up and this was the flattest off-axis in this range.

Other options considered

• SB Acoustics SB29RDCN ring radiator
• Scanspeak Illuminator R3004 – price!
• Vifa – the other NE series 3/4″ models