Having just published the new section on this site for Quiescent, I though I'd write a blog about balanced mains power. Quiescent manufacture two excellent balanced supplies, the 300VA Balanced Mains Block and the 500VA T-Series Balanced Power Supply.
In the UK, we have a mains system that provides 230V RMS on the live, with a 0V return line (neutral), and a ground line. Power is derived by the live line swinging that voltage as a sine wave going+/- 325V peak to peak at 50Hz. This is connected to one side of the power supply in each component, with the other side connected to the neutral at 0V. But of course, under this normal configuration, that power supply also sees lots of noise coming in on both the live and the neutral lines. We can use many ways of filtering that noise, but what if we could disconnect those feeds completely, to stop the noise, but still get power through? That would be cool. Well actually, we can go one step further than that and create a circuit that is disconnected, but is naturally noise cancelling too. Enter the transformer-based balanced mains power supply.
The balanced transformer is wound with an equal number of turns on its input and output sides. On the input winding one end is connected to live and the other end to neutral. But the output winding has a centre tap that is connected to ground and this makes the two ends of the output winding produce half the voltage, and in reverse polarity. Each output swings at 50Hz from a peak of about +162V to -162V, and when one output is at it's positive peak the other is at its negative peak (hence making the same effective 230V RMS). But how does this configuration stop noise being 'seen' by the power supplies in your components?
The first reason of course is that the power is 'transformed' by a magnetic field, so there is no direct electrical connection. This acts as a natural barrier to higher frequency
noise as mains transformers are by their physical design effective only around 50 Hz or so. You will have seen that valve amp audio transformers, that can work up to about 20kHz, are very different in their design (and much more difficult to make).
And then there is common mode noise rejection. Any noise that might now be picked up downstream of the transformer, by any high frequency radiated EM signals that interact with any of the downstream mains leads, would be picked up equally and in the same phase by both of the balanced power lines. Let's say it's a spike that goes up to 0.1V then down to -0.1 V then back to zero. This spike would be common to both feeds and because the whole circuit from the balanced transformer, into the component power supplies, is now totally symmetrical, the spike will be fed to either side of the power supplies in your system at exactly the same time with exactly the same waveform shape and voltage. So there is no net difference in the voltage either side of the supply input caused by that spike. That noise is common and it is naturally rejected because of the zero net difference.
So there it is. Balanced mains – it's very cool.
