#264-555 Vifa XT25TG30-04 Ring Radiator Tweeter

#297-324 Vifa PL18WO-09-08 7" Woofer

Discussion

The Nebula system was born in the Spring of 2002 out of a discussion on the Madisound Audio Discussion Board. Peter Thomsen of Acoustic Reality, had presented a new series crossover he'd developed for use with the Vifa XT25 ring tweeter. I had been working with this tweeter for several months using simple series crossovers with no luck and was about to start trying higher order parallel topologies or give up on it all together. In the long discussion that developed, he suggested some parts values and a schematic for me to try and I built the circuit. It was the best implementation of the XT25 I had tried to date.

There was upon extended listening, however, a bit of high frequency breakup at 5khz characterized by a glare to the sound caused by the Vifa PL18 woofer rolling off too shallow. John Kreskovsky, the resident crossover guru at the MAD board, sent me an alternative circuit to try. It added a capacitor in series with the tweeter and a damping RC shunt across the woofer, resulting in what amounts to a second order series crossover. It yielded a second order rolloff for the tweeter and a damped second order rolloff (or third order if you think about it) for the woofer. It also allowed a range of adjustment to taste in the level of the tweeter and worked well with slight errors in acoustic offset of the drivers, allowing for some leeway in tweeter mounting. I built John's suggested crossover and made adjustments by ear to C2, C3, R1 and R2 to come up with the final crossover after several days of tweaking. The actual crossover frequency is 2.3khz, which is about the limit of the XT25 unless higher order slopes are used.

The enclosure is the simple to build Parts Express .55 ft^3 MDF box kit glued together with Titebond. The finish is two coats of Duraliner truckbed liner, rolled on with a 3" trim roller. The box is tuned with a 2"x7" PVC duct mounted on the back, 1/3 of the way down from the top. It's lined with a compressed layer of 2" thick pink fiberglass, available in 16"x48" packages from Home Depot. Eggcrate foam can be used as well.

The PL18 woofer will work in a variety of enclosures up to about 1 ft^3 but tuning should be adjusted if the box volume is altered. Don't make the baffle too wide or you'll throw off the baffle step compensation built into the circuit and will need to adjust L2. The crossover will otherwise work fine but it's recommended that the resistor R1 be adjusted to your ear, and can be adjusted over a range of 5 ohms to 7.5 ohms. A lower value will raise the level of the treble balance and a higher value will reduce the treble and you can feel free to adjust this to taste or use it as is.

Warning: If the value drops below 5 ohms the overall impedance of the system will drop to dangerous levels so use caution when adjusting this resistor.

Keep in mind that this is a series crossover and everything is in the actual signal path from positive in to amp return. Because of this, all parts should be of excellent quality and all solder joints must be tight. It is also highly recommended that all caps be bypassed with a good quality bypass cap such as the Audiocap Theta. There should also be no substitutes for the chokes unless EXACT values for inductance and DCR can be found. This is not an inexpensive or budget system but a pair can be built for around $500. All parts are available from Parts Express.

Nebula Crossover Schematic

Assembled Crossover

Materials List (For One Speaker)

Crossover Notes:

Measurements:

Software Modelled Frequency Response

Software Modeled Impedance Profile

Measurement as taken at DIY2002 Atlanta

Special Thanks To:

Designed and Built by Ken Perkins