Sassy/T

This is the initial iteration of a series of experiments in 3-ways with side-firing woofers. The design concepts can be read on the PSS page. Although the early designs were not good enough to post, I took a few days to re-work it, and now find it very listenable.

The drivers used in the design are the Seas L22RN4X/P (H1208) 8" aluminum cone woofer, the Seas L15RLY/P (H1141) midrange, and the Morel MDT-20 tweeter. Average quality crossover component selection will bring the cost if a pair of these speakers at just under $500.

The cabinet is tall and narrow: 37.75" x 7.75" x 12" (HxWxD). For the test box, I put a 3/4" roundover on the vertical edges of the baffle. All walls are 3/4" thick MDF. The upper compartment should be well stuffed with Poly-Fil (1/2 bag or so). The bottom compartment needs no stuffing. The two lower braces (horizontal in the drawing) need to have holes cut through them to allow the woofer to see the entire volume of the box. The upper brace (slanted) is solid to separate the woofer from the mid. Both the tweeter and mid are centered on the baffle. The woofer is centered on the side of the box.

Here is a drawing of the box.

The woofer crosses to the midrange at about 115Hz with 2nd-order LR response. The mid crosses to the tweeter at about 2.6kHz with 4th order LR response. There is an LRC network on both the mid and the woof to provide impedance compensation. These are matched to the box volume, and must be changed if the volume of either cavity is changed. There is also a LRC trap on the woofer to knock down the aluminum cone's natural resonance.

The woofer low-pass (Seas L22RN4X/P H1141)...



If desired, bass level can be tuned by altering R1091. I use 3.3 Ohms, and the value can be varied between 2 and 4 Ohms to suite your taste and your room. R1061 carries a lot of current, and will probably get hot. I recommend using two 20-Ohm resistors in parallel. All coils on the woofer are Erse 16ga iron-core.

The midrange band-pass (Seas L15RLY/P H1141)...



L2101 is an Erse 16ga iron-core coil. R2101 is sum of the coil's DCR and another resistor. Subtract your coil's DCR from 8.5, and select the closest value resistor (i.e., if the coil has a DCR of 0.5 Ohms, use an 8 Ohm resistor). All other coils are 16ga air-core. If desired, the air-core coils can be either 14ga or 18ga.



All coils are 18ga air core. Tweeter level can be adjusted by altering R3091. If the speaker is too "hot for you, try changing R3091 to 8 or 10 Ohms.

Where iron-core coils are specified, do not substitute air-core coils. Air-core coils in these values have very high DCR, which will cause problems. The small resistors in series with the coils indicate the DCR of the coil (i.e., the resistance of the coil itself, not actually a resistor). It is not important that you exactly match the DCR of the coils.

From the simulations, the speaker would be classified an 8-Ohm load. It's lowest impedance is 5.3 Ohms at about 25Hz. Phase is reasonably well-behaved. There is one sharp knee in the phase response, but the impedance at that point is about 15 Ohms, so it shouldn't be a problem. The efficiency is fairly low compared to many commercial speakers. Although my 60W Bryston gets good output, I would recommend that you bring at least 100W of high-quality power to the table.

For use in home theater, a subwoofer is still required, although you should be able to cross as low as 40Hz. If your receiver/pre-pro has a fixed crossover at 80Hz, you would be better off building Sassy.

These measurements are of the 4th-order design, and were taken at about 0.5m with a 4ms gate. Nearfield midrange was then spliced in, and nearfield woofer was added. Response is just over +/-1dB through the passband, which extends out to 20kHz. Woofer extension measures to an F3 of 45Hz. The 4 Ohm series resistor was used on the woofer (R1091), with the 6 Ohm series resistor on the tweeter (R3091). Please note that there are only 2dB between vertical division.