Filtering AC line -
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
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
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
the strings - all sound more "analogue".
The switch back to no filter is perceived like a bad thing, the
is very addictive.
THE DIY FILTER FOR AC RFI
The active components in AC line filter cost circa 2-3 Euro at
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
and last update : REVIEW
original plans of Jon Risch are at: http://members.xoom.com/Jon_Risch/surge.htm
Latest and best sounding version: schematics
to check also the Chopins
Piano Festival info on the filter)
AC filter - corner 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
two kinds of rings ,
a blue varistor (MOV)
a gas type varistor
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