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KEF Subwoofer

INTRODUCTION

I open this project because for the second time a my friend brought me a KEF subwoofer to repair.
Founded in 1961, KEF is a renowned British manufacturer of audio products with a high reputation but in this case I found that these products were designed with problems.

All the KEF Subwoofer models PSW2000 (Aug 2002), PSW2010 (Jun 2003) and PSW2500 (Jun 2003) have the same problems, only starting from the PSW3000 model the design has changed.
Both the problems are in the power supply section of the subwoofer, the first concerns the absence of switching off the amplifier when it is in stand-by and the second one is an excessive heating of some resistances.

 

FIRST PROBLEM

During the standby all the amplifier parts is still powered and only the loudspeaker is disconnected from the amplifier.
This problem cannot be solved,  also adding a second transformer for the driver stage and other services many parts of the amplifier are always powered during the standby so the solution
is remember to turn off the switch after use if you do not want to risk your house can go fire.
In any case here an interesting article in which this change is suggested KEF PSW2000/2010.

 

SECOND PROBLEM

The main transformer with about 35V-0-35V AC is used to create the 50V-0-50V DC to power the final section of the amplifier and for the driver stage and other service a lower voltage is obtained by linear regulators like 7815  and 7915.
These linear regulators cannot accept an input voltage greater than 35V so the KEF designer have created a resistive divider to reduce the 50V.
A resistive divider create much heat and the 2W resistors used cannot support this for long time H24.

The KEF service in some case replace the 2k2 with 1K8 to reduce the input voltage for the voltage regulators and this avoids the breaking of these.
Both the resistors create much heat that after some years goes to destroy the capacitor 47-100uF near the big 8200uF 50V so is necessary replace it if the standby led blink and you ear click on woofer (output relay).

The solution is repacle the 4 resistors in the divider with 4 x 5W and fix all on the ittle heatsink of final stage.

WOOFER REPLACE

In same case it is necessary replace the woofer probably because the original (the 10" have the cod. 8900-6140-0) cannot support all the power of the amplifier 27Vrms on 4ohm with 0.5% THD so 182W.

I have searched on the market a valid substitute because the original is no longer available as service part after the acquisition of the brand by the Chinese.

The best solution is use a Car Audio loudspeaker designed  to work in low volume box like this and able to support high power.

My choice was the CXX10 by BOSS System found on Amazon Warehouse at low cost, it is able to support without problems a power of 400W.

PHOTO

MEASUREMENT

Follow the plot of frequency response with minimun cut off.

Follow the plot of frequency response with maximun cut off.