Power Filter

RD EMI Neutralizer

is a complex power filter with integrated surge protection. Filter omnipresent high frequency interference effectively and supply your sensitive audio components with clear power! The filter increases the distance of noise in the background from useful signal; making the sound more crystal-like, with more details.

The voltmeter on the front panel shows the voltage in the power grid. Six power sockets placed on the rear panel are divided into independent power circuits. Each circuit has its own isolation transformer. F4SX combined filter of our own construction is responsible for filtering the high frequency component from the power grid and for surge protection.

General Parameters

  • Filtration of both symmetrical and unsymmetrical interference
  • Filtration of DC direct current component in the grid

Surge Protection

  • Active even in off state
  • Lightning arresters 8/20 µS – short-circuit current 3x 20000 A
  • SURGE overvoltage protection >4500 V
  • SURGE short-circuit current 3x 8000 A
  • BURST overvoltage protection >4500 V
  • BURST short-circuit current 3x 6000 A

Galvanically Isolated Power Circuits

  • 1x power circuit 1000 W
  • 4x power circuits 100 W
  • Isolation between circuits 20000 V
  • Isolation from the power grid 10000 V

Other Parameters

  • Circuit regulating the peak of current during power up
  • Dimensions: 495 x 425 x 145 mm
  • Weight: 37 kg
  • Ambient temperature: -10 to +50°C
  • Sockets type: 6x European Schuko
  • Accuracy of the voltmeter: +-0.5 %
  • Protection rating: IP20
  • Finish: Silver. Price: €3 960.
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Combining Passion and Technical Perfection

The concept of the functional parts of EMI Neutralizer reflects the combination of the most rigorous safety requirements, overvoltage protection and first and foremost suppression of the omnipresent interference in the power grid. We based the construction on our own 20-year experience in the field of EMC (electromagnetic compatibility). We know what kind of interference occurs in the power grid and in what way it degrades the pleasure of listening to music. We know what we want—to hear details. Technically speaking, this means to suppress the power grid interference as much as possible in order to increase the distance of noise from useful signal. We reached this goal by dividing the filter into five separate circuits using T-ULN transformers complemented by an efficient input F4SX filter. The functionality of the final construction has exceeded our expectations. Results of measurements, videos, reviews and reactions of our customers will be continuously updated.

The filter is assembled and all its parts are made in the Czech Republic and it is tested by EU SURGE and BURST overvoltage standards tests at a laboratory of the Military Technical Institute in Vyskov, Czech Republic.


F4SX Filter

Our own unique audio filter with integrated three-stage overvoltage protection. The total of four filtration stages operate in the range of 9 kHz to 4 GHz with the efficiency of -23 dB to -100 dB. The filtration stages are designed for a very wide frequency range. The filter is most effective in the operating range of switched-mode power supplies which are widely used today. F4SX is designed for filtering of both symmetrical and unsymmetrical power supply interference.

Another important filtration element is an integrated HF ferrite core inductor on the PE potential. The PE ground of the sockets is therefore also separated from the PE of the power grid.

The three overvoltage protection sections are as follows: The first section consists of three lightning arresters absorbs high pulse energy (lightning strikes). The second section by three metal-oxide varistors for absorption of fast pulse overvoltage. The third section is formed by three fast varistors which eliminate any remaining high frequency overvoltage.

Five Independent Galvanically Isolated Power Circuits

Each audio component deserves only the best—its own power circuit. This thought is the basis of the EMI Neutralizer topology, which offers five independent circuits for each audio component. The components therefore cannot influence each other through the power source. Each power circuit is galvanically isolated from the power grid using a low noise T-ULN transformer, the construction of which also reduces HF interference from the grid and between the components.

One 1000W circuit provides power for an amplifier (two sockets are available) or monoblocks. Four 100W circuits are meant for signal sources (CD, DAC, record player, computer and others). (100 W is entirely sufficient for signal sources which usually have lower than 30W continuous power input.)

Low Noise T-ULN Transformers

The T-ULN transformers, again of our own construction, are designed specifically for audio. They remove harmonic components and DC direct current component from the power supply voltage. Their primary and secondary windings are separated by a special isolation layer in order to minimise unwanted capacities between them. Very low capacity between the windings eliminates the transfer of HF interference from the power supply. The transformer windings are shielded, vacuum impregnated and sealed to prevent power hum of the windings (50/100 Hz).

Each of the transformers is protected against permanent overload using a resettable thermal fuse which disconnects the overloaded circuit if the temperature of the transformer winding exceeds 100°C.

Attenuation Characteristics

in the frequency range of 10 kHz to 30 MHz and 10 kHz to 4 GHZ:



Questions and Answers

Why filter the power supply?

Electromagnetic interference is all around us. Be it a large metropolis or a small village, we intercept mobile signals of multiple providers, we tune in radio or television. Electromagnetic waves are transmitted by air and the power grid. Transmitters operate in large frequency range from 1.5 MHz to hundreds of GHz. Lower frequencies are better transmitted over the grid but we can find high frequencies even here.

Today, almost all electrical devices use switched-mode power supplies. These and many more parts pollute the electrical distribution network. You share the power grid with your neighbours. Switched power supplies interference (take into account heat pumps, solar and wind power station converters, chargers, TVs, computers, Wi-Fi and other appliances like hairdryers or microwaves) is transmitted through the power grid to your sensitive audio components. All appliances should comply with the limits of interference they emit, as defined in the EN-61000-6-3 standard. However, interference adds up from all the devices connected to the grid.

Sensitivity of the Human Ear

Although the human ear cannot beat that of a bat, dog or a cat, it is nevertheless very sensitive. We hear the range of 20 to 20000 Hz, with the sensitivity of about 0 dB (threshold of hearing) to 130 dB (threshold of pain). dB is a logarithmic unit. The finest audible detail has an acoustic intensity of 10−12, or one billionth, of W/m2 (watt per square meter), in comparison to a loud 120 dB disco, which has the acoustic intensity of 1 W/m2.

Billion is the range we are able to hear. If we put a million coins onto a pile and had a million of these piles and put them all onto one pile, if we then added or took away just one coin, it would be the equivalent of the difference that the human ear can distinguish. Even a small increase in interference, amplified by an amplifier along with the useful signal, will be recognisable.

Why should we care about high frequencies?

Human hearing is limited to 20 kHz, we cannot hear higher. When young, we hear higher frequencies, as we age, the hearable frequency decreases. We cannot hear 1 MHz, for example, but we can and do hear low frequencies modulated on the high ones. Radio uses this principle called amplitude modulation: we hear low frequencies modulated on high frequencies transmitted through air—AM broadcasting. Just as through air, high frequencies are transmitted by power lines.

Background Noise

Hum, buzz, noise is omnipresent. In electrical engineering, it is a signal corresponding to acoustic noise or visual noise (large grain in digital photos or videos). EMI Neutralizer significantly reduces electromagnetic interference, or noise, coming from the grid. The tiniest details in music, otherwise hidden in the noise area, come forward.


Isn’t the wiring at a house supposed to stop any possible overvoltage?

Lightning strike accounts for most damage and destruction of electronic devices. The transients in question are too fast for a breaker or a tube fuse to take any action. A burning thread in the tube fuse forms plasma, which is conductive, and the pulse current in the order of thousands of amperes gets to the device.
There’s a norm for overvoltage protection (EN 61643-11:2012). If the wiring at a house is done in accordance with it, you should be protected against most types of overvoltage. The norm says that there have to be three stages of overvoltage protection. There should be at least at least 10 m of cable between each of them. That’s because with the jump in current (the upsurge in voltage is up to 1000 V/μs.), you need serial resistance and induction, so that the energy of the strike is distributed in such a way that each step of the protection functions with the kind of discharge it is constructed for. Were all the three stages put into one distribution board, for example, they wouldn’t work. Our filter has all the three stages separated by inductors, so they function in one component.

Harmonic Components: Distinction

When explaining the technical side of our filter to our customers, when adding that it filters the harmonic components from the grid, with customers, we often hear “But I don’t want to filter harmonics.” Truly, there needs to be made a distinction: harmonic components in music (in useful signal) do not equal unwanted harmonic components of the current in the power grid. We obviously do not filter the former.

Harmonic component means a multiple of a base frequency. With music, it is a part of the useful signal. (See our blog post where we describe the phenomenon.) In the power grid, we talk of harmonic components caused by switching (again frequency multiples of the base frequency; with converters of 16kHz switching frequency, this can be up to hundreds of MHZ) and of deformation of current against voltage (from the frequency of 50 Hz up to 9 kHz), which is caused by rectifiers. The total distortion is described by the THD parameter.

What is the difference between RD Power Filter and RD EMI Neutralizer?

RD Power Filter provides complex surge protection and includes a single-stage filter. Its maximal transmitted power is 500 W and it is meant to power a single component. RD EMI Neutralizer provides three more filtration stages (four in total) including separate galvanic isolation for every audio component.

Chosen Posts From Our Blog

What Is an Audio Power Filter for?

Basic information on the principle and function of an audio filter. Sources of electromagnetic interference.

23 JANUARY 2018


The history and present of electromagnetic compatibility.

22 JANUARY 2018

Image: Grethe Spongsveen [CC BY-SA 4.0 ],
via Wikimedia Commons

Measuring Power Grid Interference in Practice

23 NOVEMBER 2019

What Is an Audio Power Filter for?

Basic information on the principle and function of an audio filter. Sources of electromagnetic interference.

23 JANUARY 2018

Grethe Spongsveen
[CC BY-SA 4.0 ],

via Wikimedia Commons


The history and present of electromagnetic compatibility.

22 JANUARY 2018

Measuring Power Grid Interference in Practice

23 NOVEMBER 2018