Balanced Power Supply Technical Notes

Over the last couple of decades balanced power supplies have been widely adopted by professional sound & vision companies and audio enthusiasts for the following reasons:

  • Attenuated power line noise
  • Earth loop elimination
  • Reduction of radiated fields from power cords
  • Reduced interference between equipment
  • Elimination of reactive current in the earth wire
  • Varnish Inpregnated transformers to ensure very quite operattion
  • Case temperture at full constant load less than 35degC

All of this leads to a lowered noise floor, dramatically allowing high-end audio & video equipment to provide the cleanest sound & vision

Balanced powers supply transformer


Why is a Balanced Power Supply Required?

The mains supply consists of: the live wire which carries mains current (in the UK the mains supply is nominally 230V +10%, -5%), the neutral wire which also carries the mains current and is earthed at source; it is usually at a potential just above 0V & the locally connected earth wire which is at a potential of 0V

Unfortunately the majority of mains supplies are far from being clean sinusoidal waveforms, all sorts of ‘electrical pollution’ caused by lighting equipment, motors, motor controllers etc is present. This unwanted noise not only appears on the live wire but is also present on the neutral line which is remotely connected to earth.

Most mains powered audio equipment incorporate power supplies to convert the mains AC supply to DC voltages. Unfortunately these are not 100% efficient and some of the wasted power is repelled back into the mains supply as reactive power. Some of these reactive components end up on the earth line & ultimately the earthing points, earthed cable shields & the equipment chassis. This means that these earth points are no longer truly at 0V potential but have a superimposed electrical noise which is highly undesirable as most audio & video signals are referenced to earth.
Balanced power supplies manufactured by Airlink Transformers use a star earth point, thus avoiding earth loops.

The output from a balanced power supply consists of two anti-phase 115V voltages which have been found to reduce radiated field from power cords.

What is a Balanced Power Supply?

What is a balanced powers supply

Isolation transformers are very effective in filtering out mains-borne interference.


By using a large isolation transformer, a low impedance, floating output of 230V is produced. Care has to be taken with earthing as there is a chance of high voltages appearing between isolated & non-isolated equipment.

A balanced power supply goes one stage further.

At the heart of an Airlink Transformers balanced power supply there is a large toroidal isolating transformer with an earthed centre-tapped secondary.

The transformer core is run at reduced flux density & is impregnated with varnish to reduce mechanical buzz.

Inrush current & overload protection are provided.

Inrush current & overload protection are provided

This balanced configuration produces a 230V output voltage consisting of a symmetrical pair of 115V out-of-phase voltages, i.e. the two ‘live’ wires are inversely phased, centre-tapped to earth

balanced configuration produces a 230V output voltage

Any (common-mode) reactive currents are likewise inversely phased. At the central earth point these equal & opposite currents meet and cancel each other out, they are not superimposed on the earth. This is known as common-mode rejection.

What is a Conditioned Balanced Power Supply?

What is Conditioning Balanced powers supply

Airlink Transformers also produce a range of conditioning balanced power supplies.

These power supplies have an additional high quality EMI/RFI filter on the mains input to provide an even cleaner balanced output

high quality EMI/RFI filter

Like the standard balanced power supplies the transformer core is run at reduced flux density & is impregnated with varnish to reduce mechanical buzz.

Inrush current & overload protection are provided.

Installation Notes

For best results a dedicated spur circuit is recommended. This will avoid many problems with mains pollution caused by various equipment –hair dryers are a typical source.

The power supply must not be covered. A loaded power supply will generate some heat which must be allowed to dissipate.
All of the Airlink Transformers range of balanced power supplies comes with soft start devices. To avoid nuisance tripping of circuit breakers etc allow a minute or so after switching off before the unit is powered up again. This will allow time for the inrush current limiters to reset.
Never replace fuses & circuit breakers with wire links!