Kef subwoofer cuts out at high volumes - KEF Audio PSW 2500 Subwoofer

I have experienced the problem of the output cutting out in a KEF PSW2500. I have also previously replaced C52 which is mentioned in previous posts and is the storage capacitor to power U13 that protects the speaker from direct current in case of fault. The problem of cut out at high volume levels occurs on my unit as follows: When the input level is high in bass content, regardless of the input level (VR1) setting on the back of the unit, the output has a loud pop and stops until power is recycled, or just pops loudly on peaks but will recover. If I remove one channel of the input the problem is much less apparent.

The previous C52 fix corrected the buzzing noise caused by the protection relay coming on and off when the capacitor was dry, but the new problem is slightly related as follows: The audio operational amplifiers (opamps) make up the filter and gain stages and are powered by a +/-15V supply composed of U14 and U15. If you measure the -15V output (this can be done without the speaker connected and using a VOM) you may see that its output will go to 0V during peaks or may lock at 0v while the +15V side is stable. If you do measure this, the result is that the output of the opamps will have DC on them instead of 0V, causing U13 to detect this and shut down the output to the speaker to avoid damage.

Cause: The cause of this problem is due to the lack of input capacitors in the original design, between the Vin pin and ground of the voltage regulator IC's which in some cases can lead to latch up of one of the regulator IC's; especially the negative regulator (U15, LM7915). This part is not defective, but just latching up due to instability in the design.

Fix: To fix this problem, you need to add some components as follows: For simplicity sake, I used the same 47uF/63V parts that are used in the C52 repair. You need to solder 2 pieces in parallel with Vin and ground of U15, making sure the minus side of the capacitor is to the input and the plus side is to ground. Secondly, you need to solder 1 piece to the Vin and ground of U14, where in this case the minus side will go to the ground and the plus side to the input. Note: If you get the polarity wrong, the capacitor will blow up due to overheating. The reason that you unbalance the capacitors, is so that you make sure the positive regulator comes on first and then the negative regulator second. This is well documented by the manufacturers of the parts so it is not new.

Since this instability may be due to parts ageing, you may also want to add 1 piece of capacitor at each side of the +/-15V connector on the preamp board side, which is labeled CN2. This will make sure the audio circuits still have a healthy filter on the supply to keep the audio nice and clean. Note that the ground is common between the supplies so CN2 is only 3 pins.

You can test the unit without the speaker connected first with some high level content while re-checking the 15V supplies. Both should be stable and the protection relay should only come on when powered up and stay engaged anytime there is music content. If that checks out, re-assemble the unit (remembering to connect the red and black speaker wires) and enjoy your new sounding sub-woofer.