Driver Replacement in Westlake Audio Monitors
Care has been taken in the selection and application of the driver units in the Westlake Audio Monitoring Systems. During production the driver units have been selected to produce left/right pairs (consecutive serial numbers) that have matched amplitude and phase response within their ranges. This contributes greatly to the superior stereo imaging and transient response of the system. To maintain this performance the following points should be considered when replacing damaged transducers.
Consider changing the diaphragm in the H.F. units, rather than complete assemblies whenever possible. This includes all compression driver (M.F. and H.F.) and dome tweeter units with changeable diaphragms. This can be accomplished in all but the smallest units. The reason for this approach is that the mechanical assembly (magnet, gap and phase plug in the case of compression drivers) can account for a large part of the variables in these systems. The diaphragm, while a variable in itself, is only one of several elements in the assembly. Probability tells us that the chances of maintaining close performance lies in limiting the variables. While the obvious seems to be to change all units out with matched pairs, experience shows that this is seldom accomplished in practice (although Westlake will ship matched pairs of components if ordered as such). Changing a diaphragm does call for a little more skill and care than changing the assembly itself but should be able to be accomplished by observing the following guidelines:
A. Perform the maintenance in a clean environment. Care should be taken to keep dirt and particularly magnetic material (metal filings, etc.) out of the assemblies. Dirt can usually be removed from the magnetic gap by using masking tape. Be careful when removing and replacing screws and other hardware. The exposed magnetic field while not harmful to the technician can be fatal to his pocketbook. A diaphragm that has had a loose screw pulled into it by the magnet looks extremely ugly!
B. When in doubt of a unit's performance (i.e. in multiamp systems that have had an "oopsy" like overpowering or amplifier preamp/console malfunction) it probably is best to change out the diaphragm even though it still appears to produce undistorted sound. In the case of soft dome tweeter units, overpowering may cause separation of the windings before complete failure. This can result in an overall output level loss. When changing these units, make sure that all old material (including loose voice coil windings) is removed from the gap before installing the new diaphragm. Old soft domes can also exhibit poor/low sound due to deterioration of the diaphragm, causing holes to show up. This can be observed by holding the diaphragm up to a light source. This can show up in fairly new tweeters due to mechanical abrasion or use of cleaning solvents on the soft dome itself. While a dome free of fibers is necessary for good sound, care should be taken to assure the air seal of the diaphragm is not broken. While it's tempting to use masking tape, if pressed too hard the dome coating can be damaged. A pair of non-magnetic tweezers is more appropriate here, although it can be quite time consuming. Observing the dome using 3 or 5-power (optivisor) magnification is helpful, as is observing the dome (or mid or woofer cones) in a dimly lit room while reflecting a small penlight (not l.e.d. type) above the diaphragm surface. Compressed air in a can will help by causing the fibers to move, so that the eye can pick them up, but in most cases will not free them. They must all be removed for optimum sound quality!
C. Observe wiring polarity (color, etc.) Prior to disassembly so that upon completion you can verify its duplication. This includes internal wiring in the case of most compression drivers.
D. When replacing tweeter diaphragms, please note that many models will have dampening compound that seals the tweeter face plate to the motor assembly. Using an Exacto knife blade, carefully slit the dampening compound 360º, just deep enough to free the face plate once the mounting screws have been removed. At installation of the new diaphragm, run a bead of fresh silicone 360º so the face plate and motor assembly re-seal to each other.
When considering midrange units, be aware that the signals that are sent to these units are band passed. That is to say, the high and low frequencies are rolled off below and above its (the driver's) operating range. As such, if you are a technician looking at the system response or just the midrange response but still connected to the system signal you may think the drivers are matching pretty well when observing them on an analyzer. But due to the fact that human hearing is so acute in the midband area you, the listener, may not feel as good about the sound as the analyzer may indicate. To better evaluate the mids, testing them with a wide band (although limited power) signal may reveal some out–of-band differences that can impair the stereo imaging and general sound definition. 10" (25.4cm) and 12" (30.48cm) drivers are generally reconed rather than replaced due to their high frame and magnet cost. Smaller units are often just replaced although these too might be reconed in some cases. If reconed properly (same kit, etc.) most of the midrange's properties (sensitivity and frequency response) should be retained, especially pass band characteristics. However, out-of-band response should still be observed to confirm that no large (in excess of 1.5dB) differences are occurring. In a studio application, replacement often takes on an "emergency" mode during session. The temptation to just leave the monitor "as is" after such repairs is great and should be resisted. Preparing for such emergencies by having matched mid-drivers on hand can be helpful. If the spare has not been matched to the original mid-unit, it should be replaced by the original unit as soon after reconing as is practical.
Replacement woofers are the least sensitive to unit to unit variables due to the long wavelengths involved, and the physical size of the parts. They do, however, have their own idiosyncrasies that need to be considered. Most of the Westlake monitors have two transducers in a common air volume. When new, the two drivers had similar resonant frequencies and dynamic characteristics. This is particularly important under high level operating conditions. Unmatched woofers in a common air volume will produce an offsetting effect (one woofer offsets inward and the other outward, although they are in electrical polarity) at high level that can limit maximum SPL and damage a driver’s voice coil prematurely. If a monitor system has been in service for some time (and driven at high level) it is best to replace both woofers with ones of similar age (new) and dynamic operating characteristics. If unknown, a fairly simple test can be performed to determine comparative resonance and Q factor of two or more woofers. The test requires that an audio oscillator be swept through the resonance (usually in the 15Hz-100Hz range) frequency while the driver is fed from a known source of 100 to 1K ohm. This test should be performed with the woofer(s) removed from the cabinet and held in free air. Care should be taken not to block air flow from the rear vent (if any). Do not lay the woofer face down on a flat surface. The voltage across the driver is observed for amplitude and frequency at which the amplitude is greatest (the peak). A comparison of the measurements of two or more drivers should be made to assure close match of the drivers. The chosen units should be within 5% if possible, although a limited number of units sampled may only allow a 10% match. A greater than 10% difference should be avoided if possible and units more than 20% different should only be used in an emergency until other units can be sourced.
4. When physically replacing a driver in a Westlake Audio monitor, be aware that a shock absorbing, self-adhesive, foam rubber gasket material is used to seal the driver in the cabinet. When removing a driver it may be necessary to apply a constant force to the driver for up to one full minute to break the bond between the gasket and the speaker frame. Some mechanical aid such as a screw driver or mechanical hook may be necessary to exert an outward force. However, care should be used not to cosmetically damage the cabinet or driver itself. A word of caution: the driver will often stay in place after removal of the mounting screws due to the adhesive characteristics of the gasket material itself. Left unattended though, the driver will eventually fall out on its own, potentially damaging the cabinet face, or nearby equipment. When replacing the driver, only tighten the mounting screws enough to guarantee an airtight mounting. Do not compress the gasket completely as this defeats its shock absorbing characteristics and will likely damage the isolation grommets or screw mounting hardware. Normally only a 10 to 25% compression of the gasket is all that is required to seal the cabinet. If prior service (or age) has left the gasket permanently compressed from its normal 1/8" (3mm) thickness (some units use 1/16") then new gasket material should be fitted before re-installing the drivers.
5. Upon completion a system test should be performed to assure there have been no wiring errors and that the system is functioning normally. While several techniques can be used (red/green polarity testers, swept response, triggered scope, etc.) the easiest, is to just listen to a pair of speakers. If you are the user, in a damped environment, you will probably notice immediately if one of the drivers is out of phase. It is not as noticeable if both drivers (I.E. tweeters) in a stereo pair are out of phase. A bit of extended listening though, should reveal a lack of definition at the crossover points. On axis a swept and warbled sine wave (or pink noise) will show a distinct droop at the crossover frequency when miked in the close field (approximately the largest dimension of the cabinet) if the drivers are out of phase. When listening to music, be sure to have the speakers equi-distant* to the listening position and that you have no system wiring errors, particularly if you have biwirable or biampable systems.
*See separate Application Note
Revised 9/1/12 WESTLAKE AUDIO, INC.