McIntosh XL-1W Bass Speaker
A History

Copyright 1996-2003 by Roger Russell
All rights reserved
No portion of this site may be reproduced in whole or in part
without written permission of the author.

I refrained from calling the XL-1W a subwoofer. It was designed as a supplementary bass speaker for the XL-1's. It converted the XL-1, which was a two-way system, to a three-way system for deeper bass. The crossover frequency was at 90 Hz to complement the normal rolloff of the XL-1's. It could be used as a common bass speaker and the XL-1's as stereo satellites. Alternately, two Xl-1W's could be used to make two complete three-way systems.

Carl designed a special dual voice coil woofer for this system. There were two windings on the same voice coil, each having 8 ohms impedance. One coil could be connected to the left channel and the other to the right. The bass was essentially combined into one woofer. Each channel had a separate crossover incorporating two low distortion laminated core coils.

I decided against a downward facing woofer. Woofers having long excursion and high compliance can easily sag from the pull of gravity over a period of time. As a consequence, the voice coil gradually drifts more and more out of the magnetic gap. The suspension is then biased outward and distortion increases dramatically. A temporary remedy is to build the speaker with the center of the voice coil winding located back slightly to compensate for the sag when the woofer is faced down. The voice coil will then sag into a centered position. Eventually, though, it will continue to sag and distortion will increase.

In the 1960's Hartley made a woofer that had a magnetic suspension. A layer of powdered iron was located under the center of the voice coil winding. The voice coil was pulled back into the gap no matter what the woofer position was. Unfortunately, the distortion didn't decrease. I made a current versus displacement curve and found a double S shaped behavior. The smaller S shape was located where a woofer is normally linear. It was due to the magnetic powder. The second S shape was due to the mechanical suspension. In normal woofers, a single S shape curve occurs when the displacement no longer increases in equal steps with equal steps of input current. The behavior is no longer linear. The suspension begins to allow less and less displacement for equal values of input current. By applying the current in both directions, the complete S curve can be measured.

It can be argued that combining the bass from two channels could cause a reduction in overall bass output from a single bass speaker. This is due to low frequency phase differences in the program material between the channels. It depends, of course, on how the recordings were made. For example: some stereo recordings are made with two microphones at the same place, but pointing in different directions (coincident microphones) and there are no low frequency phase differences.

Other recordings are made with microphones as much as 20 or 30 feet apart and there are significant low frequency phase differences. If these channels are mixed to mono, some of the low frequency information will be lost and less bass will be heard. It's almost impossible to know how each recording is mixed.

There are several ways to observe low frequency phase differences. Most McIntosh preamplifiers have a mode control that can be set to seven different positions including stereo and mono. By switching from stereo to mono sometimes a loss in deep bass can be heard. In that case phase cancellation of the program material is taking place. A single bass speaker or subwoofer is inadequate for that program material.

Another method is to use a McIntosh MPI4 or an oscilloscope to display Lissajous figures. An adjustable electronic crossover having a rolloff of 24dB/octave and set to an 80Hz crossover frequency can be connected between the preamp output and an MPI4 or oscilloscope. This will eliminate much of the higher frequencies and make the deep bass easier to see. For the MPI4, push the Stereo button. For the oscilloscope, connect one stereo channel to the horizontal input and the other channel to the vertical input. With the preamp in the mono mode adjust the MPI4 or oscilloscope gain to display a 45-degree straight line.

Switch the preamp to the stereo mode. Any phase differences between channels will depart from a straight line and have a circular motion. When deep bass is heard, a corresponding pattern may be seen. If there are no phase differences, the pattern will remain a straight line. Different recordings will show a wide variety of displays.

If you are concerned about accuracy for all types of program material, two XL-1W's should be used, one for each channel. I recommended facing the XL-1W out toward the listener instead of towards the wall. An XL-1 can be placed on each bass cabinet and set back from the front edge for improved arrival time. This arrangement provides another option where the XL-1's can be rotated toward the listener, if desired.

About Subwoofers

Some people, who are not burdened with knowledge, maintain that bass is non-directional and therefore bass speakers and subwoofers can be located anywhere. What constitutes a bass frequency? In truth, the crossover frequency determines if this can be significant. Perception of separate sound sources occurs when the distance between the speakers is greater than 1 meter. A few people are able to perceive separate sound sources as low as 70Hz. By 150Hz most people can distinguish this. If crossover frequencies above 70-90Hz are used for subwoofers, stereo imaging accuracy can decrease. In that case two subwoofers are needed and should be kept close to the main speakers.

Some people maintain that subwoofers, or even woofers for that matter, can be faced against the wall and crossed over at 250Hz or higher. The same reason is given that bass is not directional. The truth is that directional cues are then spread out over a large area smearing the stereo image. In addition, arrival times from the mid and tweeter are advanced relative to the woofer instead of being delayed, causing further loss in accuracy.

XL-1W Specifications

Response: 20Hz to 150Hz
Power rating: 250 watts peak
Output sensitivity: 85dB/watt/meter re: 8 ohms
Impedance: 8 ohms
Woofers: 12" McIntosh dual voice coil
Crossover frequencies: 90Hz each channel
Overload protection: one fuse for each channel
Finish: walnut
Dimensions: 27-7/32" high, 17-1/2" wide and 11-3/4" deep
Shipping weight: 62 lb.
Sold from 1985 to 1991
Last retail price: $549.00 /pair

One Sub or Two?

"When an orchestra or pipe organ is recorded with widely spaced mikes the bass is not monophonic, and stereo woofers increase the realism of the playback at even the lowest frequencies. In the realm of subtle rather than spectacular pleasures, the reverberant ambience of a concert hall or cathedral contains a large random phase component of low frequency energy that, if recorded and reproduced with two or four channels, genuinely enhances the illusion of hearing an acoustical event in a real environment--imparting a quality of you-are-there realism that is difficult to appreciate until you experience it."

Stereo Review July 1992 "All about Subwoofers"
by Peter W. Mitchell

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Created by Roger Russell
All contents are copyright 1996-2003
by Roger Russell All rights reserved