Casita "Twisters" Ports and Final Bracing

Four years ago, when we purchased our first set of Casitas, we also purchased a set of North Creek "Twisters." These are RLC networks that are used to reduce the impedance bump that occurs around the crossover point, and make the resulting load easier to drive especially for Tube amps. George Short, the North Creek Designer said the following:

"Regarding the "twister" circuit, this is a series RLC filter which is run in parallel with the entire loudspeaker, to reduce the impedance peak that the amplifier sees near the crossover frequency in the absence of this filter. It is called a "twister" because if one does a Nyquist plot of the impedance phase on a slow computer, one sees the phase response twist itself around a couple of times. It looks pretty cool. I have been calling this a type of circuit a "twister" since 1989, when I first measured it with an early MELISSA system, long before North Creek was even conceived of. It is not an uncommon circuit; Adire Audio and Merlin openly employ them as well, and I am sure there are others." (See this comment on the North Creek Music page for the Pegasus speaker in the comments at the end of the page)

Here is a photo of the Casita Twister Board. The second photo shows the labeling that North Creek does. Every crossover board was clearly marked as to what it was on the bottom.

Click for Hi Res Version

Click for Hi Res Version

We decided to install the Twisters in the Left and Right pair of Casitas after we had already installed the other crossover components and glop. It probably would have been better if we done it earlier but better late than never. So we decided to glue the Twisters on top of the glop on the side of the cabinet up toward the top and rotated so that no two inductors in the crossovers were in the same plane so as to minimize cross coupling. In addition, the top corner provided the most separation from the other crossovers. It was a tight squeeze but there is just enough room for the port tube to be installed without touching the Twister circuits.

Click for Hi Res Version

The next step is to glue in the port tubes. This is done with liquid nails, but in the following photo we got too much glue on the port and it flowed outside the port just a touch. We cleaned it up easily with a spot remover solvent. For the rest of the port tubes we used a lot less liquid nails and there was no cleanup needed.

Click for Hi Res Version

After waiting several hours for the port tubes and Twister board glue to dry, we finally came to installing the center brace that we chose to add. The true need for this brace is not proven, and it may be that it isn't needed with these cabinets, but it feels right to add it as the front of the cabinet is much more resonant when rapped with the knuckles that any other part. We glued in the braces with clamps and Titebond III.

This is the Casita Brace. Click for Hi Res Version

This is the Kitty Kat Brace. Click for Hi Res Version

After some drying time the cabinets do seem a bit less resonant than before so perhaps the braces will do some good. We should mention here that we were worried that the braces would fit behind the woofer magnets, and we measured the braces before beginning installation several weeks ago. However, we were concerned so we installed one woofer on each of the two types of fascia and made sure they fit over the woofer crossovers and that the braces would fit behind the woofers as desired. All was well!

Please see the nest article in the series Sanding, Painting, and Finishing the Front Panel Fascia

Copyright © 2010 David L. Bytheway