Filtering AC line - one of the best ideas in audio engineering

This page is NOT for the people who don't know which end of the soldering iron to hold.
Don't kill yourself with electricity and above all - don't blame me. Leave your will at a lawyers office before embarking on the AC messing project.

Lets discuss the AC filtering for a moment.

I am a strong believer in the idea of  filtering RFI from AC in our wall. RFI means radio frequency interference.  But at the same time I am a NON BELIEVER in AC REGENERATORS and all devices claiming to make "perfect AC sinusoid".  There are many cases of products and their makers who claim to offer "improved AC" or perfect AC, and usually they write about light dimmers and appliances introducing "dirty noises" into the AC and this creates noise in our equipment. Absolute majority of this writing is a pure nonsense and it is only meant to SCARE YOU and make you buy the power box for 1000 dollars or so.

Lets talk about FACTS. The AC powered audio devices in fact DON'T USE AC VOLTAGE AT ALL !!!

They are all DC devices and hence they need clean DC not AC. So creating "pure sine" is a most stupid, utterly heretic waste of time.
The AC, after entering our gear, becomes straight away brutally rectified into DC and that is the end of the road for the sine. Then the big electrolytic capacitors accumulate the electrons in a "bottle". These electrons become a source of energy for the active devices in our gear. They are being released slowly as a DC current.
THESE ELECTRONS HAVE NO MEMORY of their "childhood". They do neither remember nor care if they arrived in a form of perfect sine, cosine, square wave, or chaotic ripples. As long as they arrived, their sins are forgiven in the capacitor and they are purified.  So in other words - the hi fi gear is  indifferent to AC shape.

An absolutely another story is RFI. This frequency is millions times higher than the AC frequency. It behaves like radio-wave and it can penetrate the gear. What it does inside is this:
1. creates fast oscillations in magnetic field of transformers
 2. becomes amplified by transistors, tubes and op-amps
3. mixes with digital signals and creates errors
4. interferes with various clocks and oscillators
5. moves operating points of devices
6. Causes erratic behaviours of integrated chips
7 can be reproduced as sounds above hearing threshold but causing us to be tired and annoyed.

The problem is that the RFI can enter gear DESPITE the galvanic separation by transformer and  despite filtering effect of capacitors in power supply. The RFI becomes the Trojan Horse of AC grid power system  in our gear.

Getting rid of these is a bit tricky. But my advice is - that our intervention is focused on hundreds of kilohertz and not close to 50 Hz AC.  So the intervention is very gentle, very low order, and  nothing to do with our regular current feeding the gear.

The filtering is achieved by two opposite effects: parallel capacitive shorting and series inductive blocking.
The kindergarten explanation for non-engineers is that for high frequency - a capacitor presents a SHORT, and coil - presents a high resistance (impedance).
So a RFI noise rides by the AC cabling to our house very happy, like a Porsche on the autobahn. But when it arrives to the filter - the road becomes blocked by the coil. Oh shit thinks the noise, getting through is gonna be real tough.  But then it notices an alternative route - an easy one - via a cap. It chooses the easy route and ends up in the ground. Dead.  That's our noise trap. That applies of course  to very dumb Porsches only.  This method does not affect the AC performance because the cap is VERY small and the coil has a DC resistance of milli-ohms. Literally - of a foot of thick wire. So the impulse response of the AC source to the amplifier demand is not limited. We say that the supply is NOT CHOKED.

The audible effect depends on many many factors - the circuits in gear, the internal filters commonly built in, the ground quality and arrangement in the house, the type of devices playing together etc. It is IMPOSSIBLE to predict the degree of improvement, but strangely - the result s are very consistently proving in every house that:
1. The change is audible
2. The change is Positive
3. The negative effects do not exist at all (trade-offs)
4. Every gear responds more or less
5. Every owner hears it and accepts it.
6. Noone wants to go back to raw AC

What I hear - is that the musical event becomes more real, instruments better separated, threads easier to follow, like more microphones were used. Timbres become more natural, the woods, the brass, the drum skins, the strings - all sound more "analogue".
The switch back to no filter is perceived like a bad thing,  the filter is very addictive.


The active components in AC line filter cost circa 2-3 Euro at  most. Rest is beautiful case and labour.
The whole construction and part selection is my own. It is basically a copy of the CLC filter from second page of the electrical engineering book grade one.

                   for reference also see the reviews at: REVIEW 1    and REVIEW 2  and last update : REVIEW 3

                            original plans of Jon Risch are at:

                            Latest and best sounding version: schematics (remember to check also the Chopins Piano Festival info on the filter)

                                       CLICK ON THUMBNAILS for larger resolution pictures

  AC filter - corner view
back view 
even more back view 

  mini filter- front view
this is identical to full blown Silk, 
but for one outlet. Total cost is 6 USD
mini filter- rear view 
some components:
two kinds of rings , 
a blue varistor (MOV)
a gas type varistor
toggle switch 
AC inlet socket

The guts - voltage DIN rails
The guts - one ring with wire 
The guts - general view

The guts - side view
The guts - one outlet with a cap and wiring. 
Also visible the earth toggle switch 
Schematics - the last iteration


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SILK AC filter