Replacing a Speaker Driver

Replacing a speaker driver is straightforward once the replacement is right. Most failed swaps are decided before the old driver comes out: wrong cut-out or depth, a different impedance on a shared channel, or T/S parameters that don't suit the box. The steps below are for a front-mounted 18" sub and apply to most PA cabs.

Match the replacement

• Cut-out, bolt circle, hole count. Measure the cab, not a datasheet for the old driver. The WM1850/8, for example, needs a 392 mm cut-out.
• Depth. The motor must clear bracing and the rear panel with room for air to move round it. WM1850/8 depth is 202 mm. See will it fit: driver swap.
• Impedance. Match the original, usually 8 Ω or 4 Ω. A different value changes the load on the amp channel, and in a multi-driver cab or parallel stack it unbalances power between drivers. See 4 ohm vs 8 ohm.
• Enclosure fit. Fs, Qts and Vas determine how a driver behaves in a given box; a scoop or folded horn driver may not suit a reflex cab, and the reverse. See Thiele/Small parameters.
• Power. Compare continuous ratings (AES or RMS) against the amp you run. See power ratings explained.

In multi-driver cabs, replace with the same model as the others, or replace the set. Mixed drivers in one box load each other unevenly and sum unpredictably.

Tools

• Screwdrivers or hex keys for your bolts (no impact driver).
• Multimeter.
• Soldering iron or correct crimp terminals if the leads are not push-on.
• Foam gasket tape if the old seal is damaged.
• A soft, clean surface to lay the driver on, cone up.

Step by step

1. Isolate. Amp off, cab unplugged.
2. Cab on its back. Two people, lifting with legs. Face up means a loose driver cannot fall out.
3. Grille off if fitted.
4. Bolts out a little at a time in a cross pattern. Keep them together.
5. Lift the old driver by the basket and motor, never the cone. If it sticks to the gasket, ease the frame with a flat plastic tool. A WM1850/8 weighs 20 kg; get a grip before you take the weight.
6. Record the wiring. Photograph which lead goes to + and − before disconnecting.
7. Inspect the cab. Loose bracing, spun T-nuts, perished wadding, air leaks. Fix them now.
8. Check the seal. Front-mounted drivers seal on the rear of the flange against the baffle. A torn or missing gasket leaks air and buzzes; add foam tape if needed.
9. Connect. + to + (usually marked or red), − to −, matching every other driver in the system.
10. Meter it. Measure at the cab input before fitting; the reading should match the wiring (DC resistance reads below nominal). See testing a driver with a multimeter.
11. Seat the driver square in the cut-out, leads clear of the cone and spider.
12. Bolts in by hand first, all of them, then tighten progressively in a star pattern.
13. Grille on, and stand the cab up with help.

Bolt tension

Even, firm compression of the gasket, nothing more. Over-tightening cracks cast baskets, bows pressed-steel frames and strips T-nuts; under-tightening leaks and buzzes. Hand tools only, star pattern, and the maker's torque figure where one is given.

Polarity

A reversed driver moves in while the rest move out, so the bass cancels around that cab and the whole stack thins out. Check every cab after a repair. See phase and polarity and Speakon wiring.

Test before the gig

Low level first, with a sine sweep through the cab's range: listen for rubbing, rattles and air leaks, and feel round the flange for escaping air. Then bring it up gradually with your normal high-pass and limiter settings in place. Fuller routine: testing a new cabinet.

Replacement 18s

Check fit against your cab with the replacement guide. Both drivers can be heard in our listening room by appointment.

Sources: Wavemark WM1850/8 spec sheet and WM18X451 product data.