Mains filters

Mains supply is often considered as granted but carries many noise, transients voltages and harmonics content. An effective filter is needed to prevent these noise from affecting the amplifier.

A word on mains electricity

Mains is the general purpose alternating current supply, his two main properties are frequency and voltage, which may differs from regions. Some are supplied in 120V, others with 230V. There are also two main frequencies : 50 Hz and 60 Hz. The risk of connecting a device to a incompatible voltage is reduced by the use of different and incompatible plugs and sockets. The waveform is assumed to be a pure sinusoid in each cases, but the reality is quite different.


DC voltage on mains

Some appliances can inject a constant voltage on the mains, called DC voltage, while they are working. This is unpredictable, inevitable and can interfere with other appliances.

The growing presence of this DC voltage on mains forced new electrical protection circuits to be created. These new types of residual-current devices, also known as RCD, are fitted in every houses, and protects against the presence of DC leakage.

The following curves shows the waveforms of a clean mains and then that of a mains affected by DC voltage :


Mains no DC legend


Mains with DC legend


Practically, the DC voltage is rarely steady, and is constantly changing over time. The following illustration shows the evolution of DC voltage over a longer period of time:

DC mains pulsating


Spikes, harmonics and other perturbations

Many other perturbations can affect the voltage on mains. Following is an example of voltage drop for a small period of time, this occurs when a high-power load is connected to the electrical network:

Mains CreuxTension


Another perturbation could be spikes which are high voltage and short term pulses, mainly produced by inductive loads switching:

Mains Spikes


These spikes could cause electrical damage to all appliances connected to the electrical network.

High frequency signals can also be carried through the mains electricity :

Mains HF



At his worse, the mains voltage could probably looks like this, far from ideal waveforms:

Mains all


The sources of mains pollution

There are many sources of electricity pollutions.

Power lines stretch vast distance from the generator before reaching populated areas. These lines are long and constitute a very effective antenna. Each electro-magnetic signal is then conduced by the wires. Radar and many other electro-magnetic signals are then presents on power lines and power transformers doesn't stop much more noise.

Power lines are protected from lightning by arresters, which shunt abnormally high voltage, but some of the energy will still be carried by the line through the customer in the form of short over voltage.

Others perturbations of electricity network includes short term drop of voltage, when high power load is connected, or harmonics distortions.

There are also many sources of pollution in-house. Refrigerator, for example, creates spikes when compressor turns on or off. Microwave oven or hair dryers creates over and under voltages, as well as DC voltage. Because all plugs are linked together on the same electrical network, each of them will be affected.


Effects of mains pollution

The first signs of a DC voltage on mains is the supply transformer making a hum noise. The positive peak mains voltage value could be higher and the negative mains voltage value could be lower. A transformer supplied with this waveform will endure a higher voltage and then will heat much more, compromising his life expectancy. Moreover, the DC current resulting from the DC voltage will magnetize the transformer's magnetic core and this could lead to saturation. When saturation occurs, the power consumption will rise, the transformer will overheat, and the output waveform will be compromised.

 Other signs are degraded performance of the amplifier, since all noise and spikes will make their way through sensitive electronics if no EMC filters are installed.

Lastly, in case of huge spikes, the amplifier may be damaged due to over voltage.


Protections against pollution

Power surges and spikes are to be threated with surge protections, which are components capable of withstanding high current values while maintaining the voltage at an acceptable value.

Noises are arrested by EMC filters which are circuits that only let pass the mains voltage. The choice of a correct filter is a complex task since many factors must be taken into account.


Final words

Filters and surge protection against spikes, noises and DC voltages are mandatory to ensure best performances and reliability for your amplifier.

All Monange amplifiers integrates both of them, as they couldn't be an option.