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FAQ - Upgrades and Tweaks Circuit and Component tweaks

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Old 6th January 2006, 10:28 PM
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Default Simple Mains Filter and Spike Suppressor

This little circuit can be added to the back of the IEC inlet inside your amplifier to help to suppress mains interference and spikes;

2 mains suppression capacitors; X class 0.047uF / 47nF between L & N, and Y class 0.047uF / 47nF between N & E
X cap RS 210487 and Y cap RS 210617

1 VDR (Varistor or Voltage Dependent Resistor) 275V spike suppressor soldered between L & N
Varistor RS 229056

Slide insulating tube over the legs of the components to prevent short circuits.

Note that capacitors designed to go across the mains from Live to Neutral or from either line to Earth must be of a special construction so as not to catch fire or cause a circuit to earth in the event of failure and must be of the correct X or Y rating.


A similar mains filter can be made using a commercially available Delta Capacitor and Varistor.

Rifa Delta cap RS 239668 PZB300MC13 100nF X2 with 2x4n7 Y2 (this is 3 capacitors in one case)


Varistor 275V RS 229056

If you first piggy back the varistor onto the delta cap, by twisting and soldering its leads to the 2 outer leads of the cap, you're then left with just 3 leads from the delta cap to solder to the back of the IEC socket. Take care to sleeve the leads with ptfe or similar tube where needed.


Mains voltages can kill. The reader is responsible for his/her own safety. If there is any doubt the reader should seek professional advise or not undertake any work of this nature!

WD and the contributors to the FAQ of this BB will not be held responsible for any injury or damage.

Before you start make sure the equipment is unplugged from the mains! Use the illustrations as a guide only.

Attached Images
File Type: gif mainsfilterdelta.gif (12.3 KB, 257 views)

Last edited by Richard; 13th October 2016 at 10:51 PM. Reason: attach pics

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