My goal was not to make a proper crossover, but rather to convince myself a crossover existed that would reasonably blend the upper three drivers (I’ve already decided it will be at least bi-amped) and have an acceptable impedance curve. I’ve been worried about the latter given that most drivers these days are 4 ohm and it is almost impossible to find any drivers over 8 ohms. Here is what I ended up with – a 4th order LR @ 2.2kHz and the super tweeter coming in at about 13kHz, 1st order.
CALSOD model of frequency response for a proof of concept crossover
CALSOD model of impedance for a proof of concept crossover
The frequency response is quite acceptable, with the only major glitch being the notch at about 1.4kHz. That is in the woofer response and not a crossover anomaly. I’d have to look at a lower crossover point and/or a gentler slope to allow the tweeters to help fill that in. We’ll see. The impedance stays above 5 ohms until about 13kHz, so it shouldn’t be a difficult a load.
So I’d say mission accomplished.
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.
The same thread I referenced in my post also contained pictures of a finished kit. Very minimalist. Very modern. I love the white finish. HIFISENTRALEN member 2-mas clearly did a beautiful job on them.
Front view of finished RSTL kit
3/4 view of finished RSTL kit
Finished pair. Note blocks underneath
In this 19 page thread on the Norwegian site HIFISENTRALEN:
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.
Page 1 (click for full size)
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