Response is DFFT of gated MLS (CLIO, v. 7.13) over a range of output levels
The 290-16L is a very, very large driver. Weighing in at 32 lbs the 290-16L has one of the largest motor structures found on a compression driver. A 6.78 lb ferrite magnet produces a flux density of an astounding 1.9 Tesla across a 2.8" diameter edge-wound copper voice coil. Sensitivity of the 290-16L coupled to the 311-90 is estimated to be 108dB/W/m. Photo below is the driver connected to attenuation autotransformer (Universal Transformer #3618, see crossover network section for details).
THE CROSSOVER NETWORK
A schematic describing a relatively simple, low cost crossover for this system that
provides the correct distribution of low, mid and high frequencies
to the three separate drivers is provided in the thumbnail below. The bass filter is a first order
low-pass, the midrange filter is a first order bandpass and the
tweeter filter is a third order hi-pass. Impedance correction in the
tweeter is achieved by using a series notch-filter (LCR filter) in
parallel with the tweeter. The notch-filter significantly reduces
the large reactive peak that exist in the tweeter impedance plot Z
near the crossover frequency thus enabling the filter circuit to
achieve the desired crossover frequency characteristic. Also, the
Altec midrange has higher sensitivity than either the JBL tweeter
and the Klipschorn bass unit. In other words, it produces more sound
for a given input power. To attenuate the midrange and thus
"balance" the output such that all frequencies are reproduced with
equal loudness, the midrange is connected to an attenuation
autotransformer which supplies the necessary attenuation across the
entire operating range of the midrange horn. The technique is the
same as the one employed by Paul Klipsch on all of his Klipschorn
networks.
Photo far right is loudspeaker system front, note location of tweeter.
For home builders, the SPICE schematic and magnitude predictions are provided below (see thumbnails). The woofer inductor is a simple 2.4mH air-core (or ferrite core) choke in series with the Klipschorn woofer. The autotransformer, used to attenuate the mid-horn output, can be purchased from Universal Transformer (model #3618), PO Box 472, Farmersville, TX 75442 (ph: 978 784 7700). Note that the autotransformer is modeled as a series of inductors. Output taps are labeled 1-5. Note also, the polarity of the tweeter is wired out-of-phase with respect to the bass and mid units.
THE KLIPSCHORN BASS + ALTEC MID + JBL TWEETER
A top-section must be fabricated to house both
the horn and network. The dimensions of the Altec horn are such that
an enclosure, similar to the one used by Klipsch to house the stock
midrange horn, can be fabricated and positioned on the top of the
stock Klipschorn bass unit. As is shown in the photos below, the new
top-section is designed to fit snug into the room corner. If you own
a pair of 311-90s and would like information on constructing the
top-section shown in the photos below we may be able to provide
assistance (see contact info below).
Note under-hung placement of JBL 2404H tweeter.
ALTEC 311-90 HORN : 290-16L COMPRESSION MID DRIVER : JBL 2404H BI-RADIAL HORN TWEETER
One of the better sounding midrange horn reproducers we've heard is the Altec Lansing 311-90 sectoral horn coupled to the massive Altec Lansing 290-16L phenolic diaphragm compression driver. This assembly is capable of producing useful, high fidelity output down to 300Hz. Used in conjunction with the JBL 2404H Bi-radial horn tweeter and Klipschorn folded bass unit, a fully horn-loaded three-way system capable of outstanding sound reproduction is realized. Please note that the Altec 311-90, 290-16L and JBL 2404H are NLA. The information is here for enthusiasts that either have the parts on-hand or willing to obtain them used.
A very simple to construct, three-way crossover network, developed specifically for this system is also described and a schematic is provided below.
ALTEC 311-90 HORNThe 311-90 is a cast aluminum sectoral horn with 1.4 inch diameter throat. The horn cut-off frequency is 220Hz and, when coupled to the 290-16L compression driver, a useful bandwidth of 300Hz to 7kHz is realized with a coverage pattern of 40° vertical x 90° horizontal. Photo below is the mouth of the horn prior to paint.