DC on the mains

So, I think I need to build myself a DC blocker. There are plenty of circuits available, and even bare PCBs available.

The question is, what kind of current capacity should I allow for? With the soft start circuit, I can run my amp with a fuse of about 5 amps, so if I went for 10A, this should give me enough headroom? 

The fuse rating was calculated using the following:

So if I build a DC blocker with 10A constant current capacity, it should be more than adequate?

I’ll add a little disclaimer - if you don’t know what you are doing with mains electricity, then don’t mess around with it. It can, and if given a good enough chance, will kill you.

This seems to be the Audiolab one - 

Not much in it for £100. A bridge rectifier, and a few caps. Oh, and the certification of a product, and the Audiolab marketing costs.

I think I can build two for less that, based on the ATL HiFi PCBs.

I can assure you that "DC on the mains" is not DC, although it is possible to measure it as such.

Toroidal grumbling is due to the transformer's large inrush current as it has to adjust to the received waveform's sudden changes in shape.

The cause of this change is often found close to home or in the home. It is a "recent phenomenon" due to energy-efficiency of other mains powered equipment.

It is the firing of thyristors in energy-efficient products and one example is the automatic washing machine. If only one half cycle is used, the energy consumed is lower than if the whole cycle is used. It has been noted elsewhere (an electrical engineers forum) that even a half-speed hair dryer is capable of causing a change in mains waveform.

Obviously it also depends on the mains wiring impedance, which must be such to allow the sudden "attenuation" to one side of the waveform. Even where an electrical installation has two separate supplies from the meter tails, this waveform change still happens. As such, it might be seen that the electric meter's "shunt resistance" might be all the impedance required. It should then follow that the source of "interference" must be inboard of the electricity meter.

How this energy-efficiency affects a toroidal transformer is shown by monitoring its magnetising current. During an event, one side of the magnetising current waveform "disappears" while the other side "grows", and after the event, it returns to be equal for each half cycle.

The reason this happens to a toroidal transformer and seldom to any other type, is because toroidal transformers have much lower magnetising current. It can range from units of milliamps, to a few tens of milliamps (about 30mA).

Unless the transformer has primary taps to which it is optimised to the local mains, the manufacturer must be careful to ensure it doesn't reach saturation at the highest envisaged supply voltage.

The universal 230V EU voltage can have a spread between 207V and 254V.

To prevent saturation at 254V the magnetising current at 207V will be only just sufficient, and might be in the order of 5mA or lower.

Therefore it is possible that use on a continental 220V supply might result in a worsening of the noise.

The rectifier "DC filter" might work to reduce or stop the noise, but doesn't always work. Whether the large can electrolytic capacitors have any effect is debatable. As they are supposed to let AC pass unhindered, and the problem is the AC waveform, then all the rectifier does is drop the voltage by a volt.

One large American professional amplifier manufacturer simply used a bridge rectifier without the capacitors. I tried it and it seemed to do much the same. I note that the later models of the amplifiers no longer use them.

You can find more on this subject at https://sound-au.com/articles/xfmr-dc.htm

I have the PCB on the way from Sweden, and the diodes and capacitors on the way from the States.

Finding somewhere with the diodes and capacitors in stock was a challenge for the parts I wanted.

Total cost is about €90 for the parts, but you could do this cheaper. I have gone big on this, probably too big, but this is what the PCB was built for. It has 16 diodes, and 6 capacitors. Apparently, it works well on large toroidal transformers.

It should not take long to solder up, but will take a little longer to fit to the amplifier, as I may have to redo the wiring to fit it in the case.

I will post more when it is complete.