The Rubyk project by Ben “Wolf” Shaffer 5/17/07

Goal-
This project started as an idea for a full-range single-driver speaker, and would further be augmented by the Peerless 830878 3.5” passive radiator. I wanted to get decent bass extension, have a controlled xmax curve, and be able to basically cover the audible frequency spectrum as low and as compactly as possible.

Driver selection-
I proceeded after the initial idea was created, to model most of the available 3” full-range drivers in an enclosure with the 3.5” Peerless. I was not greeted with very many results that suited the project until the HiVi A3N buyout driver was placed on the Parts Express website. It was supposed to have extension to 20 kHz, and looked like a good candidate. Specs were decent, and they were cheaply sold in pairs.

Enclosure design-
As stated, I modeled the A3N, and produced an enclosure of 5 liters, with an F3/F10 of 104/52 Hz. In a lesser size, I lost 4 Hz on the F3 (108 Hz), and retained the F10 for 52 Hz. The finalized size is a 5.5” internal dimensioned cube, for a net volume of 2.7 liters. The 3.5” PR helped mimic a sealed type enclosure, and is essentially an EBS design. This is the reason why the xmax is well controlled. Heavy fill was selected as damping.

Enclosure Construction-
I decided to use 1/2” MDF as cabinet material, and due to the small size, bracing really was not an issue. I chamfered all the edges with a 45 degree bevel router bit, leaving the corners untouched. The A3N and Peerless are surface mounted. I scalloped around the inner side of the cutouts to allow driver breathing room. The A3N is on the front in the centered position, and the Peerless is on the adjacent side in the centered position. I found the Peerless worked better on the side farthest from the other speaker, or toward the sidewalls of the room.

Crossover design-
Wait! There isn’t more than one driver here! There wasn’t…. I decided after looking at the manufacturer’s frequency response plots, and doing a little listening, that the A3N needed a “helper” in the top octave. I chose the Dayton ND16 5/8” tweeter to coincide with the A3N.

For position, it is offset towards an upper corner on the baffle, with the edge of the A3N being 1/16” inside of the same edge of the ND16 in the horizontal dimension. It is also offset towards the center of the room. I used a forstner bit just smaller than the diameter of the tweeter, and then sanded it out slightly with a Dremel sander for a snug fit. During installation, I used a rubber mallet to tap the tweeter in place, as it is a press-fit design.

Back to the crossover, I decided to bring in the ND16 at about 10 kHz. This is not optimal center to center spacing, but I don’t seem to notice the dilemma in this case. The A3N looked to have a rise in the midrange response, so I shelved it down a couple dB above 2 kHz. The 2 ohm and .5 mH coil accomplished this contour. I used a standard l-pad to attenuate the tweeter, but shot for a nominal impedance of 11 ohms. This allowed using a single 1.5 uF cap to engage the ND16 at the desired frequency. This circuit and tweeter were placed after the shelving filter, to take advantage of the present attenuation. The A3N also had a considerable impedance rise due to the inductance of the unit, so I compensated the A3N and ND16 together with a zobel circuit across the A3N. The values are 6.8 uF and 10 ohms. The resulting impedance/phase graph shows decently flat impedance and phase plots. The minimum impedance was 6.7 ohms, and the phase is within 5 degrees from 450 Hz to 20 kHz. Please see pictures for clarification.

Conclusion-
I feel that this project is a great bargain, and with the ND16 helping the treble, it does what I intended it to do. I did not complete the project as a single-driver with PR, but instead, as a single-driver with a little help at both ends. I think the vocals are very clear, the top-end is nice and airy, and the bass is decently extended. If you place these near a wall when set up, the bass will fill in much nicer. For $54 in drivers, again, these are a fantastic bargain. Oh- The name resulted from my wacky paint scheme, and you are welcome to finish them as you wish.