Multi shielded 1000VA balanced iso transformer


This is an independent hobbyst project and non-commercial site: no MANUFACTURER has any association with this work.

The information here presented is believed to be technically correct and everything presented on this site is done so in good faith. Anyhow you (the reader) are responsible for anything that you might do as a result of reading here.

Gianluca G., Italy 2019

This is a custom isolation transformer built according to my exact specifications by a manufacturer from the north of Italy, specialized (for more than 30 years) in the construction of small and medium sized electrical transformers.

The transformer has a nominal capacity of 1000VA, which is more than four times the load to be fed (network player, DAC and power amplifier) and it has triple electrostatic shields (between the primary and secondary windings and between the secondary windings and the core and covering the external winding) that prevent electrostatic coupling of line noise into the secondary. The transformer also has a centre-tapped secondary winding to provide a balanced power, with the midpoint connected to Ground (Neutral in my circuit), which helps to eliminate some additional harmonics.

It also has an external copper flux band applied around the entire transformer boundary to provide a short-circuited turn to the remaining stray fields that could cause electromagnetic interference to surrounding devices and wiring.

In the photo I put a 1 euro coin to attempt to show you the amazing size of this custom transformer which weighs 15Kg itself (33 Pounds)


HOW TO SAFELY CONNECT A BALANCED TRANSFORMER: if the the center tap of the transformer secondary is connected to Neutral, the house RCD will trip in case of equipment enclosure fault. Since Neutral N is bonded to Safety Ground G (EARTH) in the electrical box, it still provides adequate low impedance path to attenuate noise.


UNSAFE CONNECTION OF A BALANCED TRANSFORMER: if the the center tap is connected to Safety Ground, the house RCD won't trip in case of equipment enclosure fault because the balance of the currents of the the supply L (Line) and return conductor N (Neutral) is always zero. In the shown simulation the fault generates enough leakage current (few dozens of milliamperes at 115V) to kill a person. Both the fuses on the secondary will not blow, as sized for much higher load currents.

In the photo is visible the R-C snubber network on the secondary side of the transformer, formed by the Russian orange (unclimactic) K40Y-9B 220nF 1000V paper-in-oil capacitor and a 51ohm 3W resistor.

Snubber networks can be placed on the primary or on the secondary side of the transformer due to the fact that the two windings are strongly coupled; or, better, on both sides as here: the other RC network, on the primary side, is made by the blue 1.5µF X1 film capacitor plus a 15ohm resistance. RC snubbers act as a lossy tank and dampen the resonance created due to parasitic capacitance and leakage inductance of the transformer, wiring and components of the load. These resonances can have audible negative impact on listening. In addition, snubber circuits prevent arcing on the relay’s contacts by transfer power dissipation from the switch to the resistor and contribute in reduce EMI coming from the mains by damping voltage/current spikes and ringing. RC values have been selected "by ear" during listening sessions: usually R between 10 and 100ohm and C between 100nF and 2.2uF are ok.

The isolation transformer inside the "Anniversary Balanced Power" suppresses Common Mode Noise by introducing grounded electrostatic shields among its primary and secondary windings and the steel core. The grounded shields provide a low impedance path to ground by capacitive coupling, which prevents unwanted high frequency signals contained in the source voltage from reaching the transformer secondary.

LBAL,  Longitudinal Balance or Common Mode test circuit. This test is used by some manufacturers to check the ability of the transformer to refuse any unwanted signal which is common to both inputs, in reference to the output circuit. Two measurements are made each by applying a voltage to the transformer and measuring the resulting voltage.  The result is the ratio between the two, expressed in dB. The test circuit uses two 300 ohm resistors to provide a balanced source. This source can have few volts of Earth noise; in this case, the Earth noise is 230 volts 50Hz AC. The load is a 10K ohm resistor in parallel with a millivolt meter. The meter measures 0.450mV when the test voltage is applied as common mode signal and measures 163V when applied as a normal input voltage: that equates to a Common Mode Rejection Ratio (CMRR) of -111dB. This means that the transformer will reduce noise by 111dB at 50 Hz. In the real world, Earth noise is usually much lower than 230V, so we can expect any noise due to earth voltages to be blended into the system's noise floor.

MEASURE ONE: The common mode rejection ratio test is used to measure the transformer's ability to reject noise, mainly thanks to a particular configuration and connections of the electrostatic screens between the windings and the steel core. A CMRR of 111dB means that this transformer will reduce noise by 111dB at 50 Hz.

MEASURE TWO: The leakage RMS current to Earth from the Line measured at the input of the "Anniversary Balanced Power" is less than 3mA during normal operation: the relay between Line and Earth is activated, the red LED indicator is blank, which means that the Line and Neutral are not reversed at the socket and the Earth path is not open. In case one or both conditions are not met, the relay will prevent the operator from turning on the machine.

The maximum value of Line to Earth current is limited by the leakage requirements imposed by various safety agencies. Excessive leakage current to Earth is considered a shock hazard and, therefore, is regulated. The leakage requirements worldwide vary from 0.5mA to 5mA depending on the application, the safety agency and the country.

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