Music and Design NAO speakers

Music & Design NAO open baffle speakers

These open baffle speakers were designed by John Kreskovsky of, and built by me several years ago, with lots of advice and guidance from John.   The drivers are the Seas Excel T25CF-002 Millenium tweeter, dual Scan-Speak 18W/8545 midranges in a D’Apollito configuration, and Peerless XLS 830500 12” woofers in a U-frame cabinet.  The rear firing tweeter is a Vifa D27TG-45-06.  The crossover between the mids and tweeters is passive, built using Sonicaps, Solens, and custom Cello polypropylene capacitors and air core coils.  The caps were selected and matched to within 0.1% for channel to channel balance.  An electronic crossover, shown below, divides the mid/tweeter drivers from the woofer, and provides the required dipole equalization for this setup.  

The cabinets are built with baltic birch ply laminated to MDF using the techniques suggested by George Short of North Creek Music Systems.  The base is veneered in cherry, and the mid-tweeter panels are veneered with bird’s eye maple, wenge trim, and solid cherry around the perimeter.  Grilles for both the mid/tweeter panels and for the woofers areincluded, plus there are small round grills over the rear tweeter.

This speaker design is a direct competitor to the Siegfried Linkwitz Orion speakers, but for me, the key differences were: a) this design uses only 2 stereo amps (one of which I built into the active crossover) versus the 6 or 8 amps required for the Orions, and b) the Orions put dozens more ICs in the signal path, which was not attractive to me as I prefer a simpler signal path.  The crossover I built is also fully balanced, meaning four of everything (inverted and non-inverted for both left and right).  I built the crossover into a large chassis with a 125watt/channel Class D Audio SDS-254 stereo amplifier (which also runs fully balanced), and used a re-purposed Audio Research faceplate for a very professional look.  That amp/crossover combo means that you only need a single stereo amp for these bi-amped speakers.

Above: NAO speakers, front view with grills.

Electronic crossover with integrated bass amp (more photos of internals below).

Front, with grills removed.  The upper panels are veneered in bird's eye maple, with wenge trim, and framed in cherry.  The entire woofer sections are cherry veneer.  The panel between the mid/tweeter panel and the woofer cabinet was added after the initial construction, and is veneered in wenge.

Rear view, showing the rear-firing tweeter with its grill.  The woofer connections are made at the bottom, and the section atop the woofer cabinet contains the passive crossover, where the inputs for the mid/high panel are made.  The binding posts are Vampire BP-Hex gold.

Closeup of the mid/high panel, showing the Scan-Speak 18W/8545 and Seas Millenium tweeter.  The drivers are typically covered with, and protected by, their grilles.

Closeup of the rear of the mid/high panel.

Detail of one mid driver.  Note the beveled edge around the mid to maximize airflow.

The passive crossover, showing some marks where components have been updated over the years.

The inside of the active crossover/EQ and bass amp (more below).

Rear view.  Bass amp outputs on the left, XLR inputs and high pass outputs (to an external mid/tweeter amp) in the center, and woofer level controls on the right.

Crossover/EQ.  125 watt Class D Audio power amp for the bass on the right (with its power supply toward the front), active crossover/EQ boards in the center rear (separate boards for left inverting, left non-inverting, right, inverting, and right non-inverting).  In the rear on the left are separate regulated power supplies for each channel for the crossover/EQ boards, and the soft-start circuit just visible on the front left.  The Class D Audio amp includes larger than stock power supply caps, and fixed gain.  The crossover/EQ boards are build using Roederstein Resista resistors, and MIT PPFXS polypropylene film & foil caps in the mid & high pass sections.  The regulators use MUR860 ultrafast TO-220 diodes, panasonic 105 degree caps, and heatsinked LT1033 and LT1086 regulators.

Detail of the DIYAudio soft-start circuit for the the amplifier, mounted on the front wall.  The 12v remote trigger relay is on the small board just below.