Mains DC block filter

After a number of listning sessions at my speakers I found out that sometimes there was a hum present during pauses and at very low audio levels.
Putting my ear in the bass and midrange speakers proved that was not the source.
After taking an amplifier module out of its speaker cabinet I identified that the power amplifier torroid transformers were the source of the humming sound, something I did not experience during the build and test phase of the project.

I was under the impression that torroids don't hum!

Each power amplifier module holds a 50VA, a 225VA and a 300VA torroid for the auxiliary supply, the mid and high power amps and the bass power amp.

All four power amp torroids were humming at certain times so no accidently badly wounded one.
The torroids in my amplifiers are mounted with rubber pads so mechanical resonance was not my initial thought.
What I also found strange is that only the high power torroids appeared to generate the hum.

Asking around and Googling the web I read more and more about a DC component on the mains that supposedly was the cause of this intermittend hum from the transformers. It was also commonly accepted that this only appeared with higher power torroids, like what I experienced.
Needless to say that I was not an instant beleiver of this mains DC story but when also a good friend, who has built speaker systems and amplifiers for many years confirmed this DC bias story I decided to put the statements to the test.

First I tried to measure the DC level on my home mains lines with my available multimeters.
One digital multimeter indicated almost 2 Volts of DC and another multimeter indicated absolutely zero. So much for reliable measurement results.
Browsing the web again I ran into an article that gave me the impression Mains DC is not all humbug. (

When you think about it it sort of makes sense. Higher power Torroids have such low DC resistance (thick primary windings and not a lot of them) that any DC imbalance puts them quickly into saturation. That will make them hummmm!

The thing I was still sceptical about was DC on the mains. But if you think that many loads pollute the mains, like hair dryers that switch for half power a single diode in the mains, causing a DC offset I began to buy into the "DC on the mains" concept.

I decided to build a simmilar Mains DC Blocking circuit that was described in the article.

There are many ways to implement a DC blocking mains filter. The common idea is to have a big capacitor in series to block any DC flowing through the transformer primary.
Obviously you want as little power dissipated in the DC blocking capacitors as possible so the voltage drop over them should stay low.

I took as starting point my 500VA transformer where the primary current at nominal power is 500VA/230V= 2.2Amps. Take a maximum voltage drop over the capacitor of 1Volt you end up with an impedance at 50Hz of ~2 Ohms.
The capacitor value therefore needs to be higher than C=1/(2*3.14*50*2)=1600uF.
Such capacitor values come only in polarized electrolythic versions.
You can place two in series with the pluses or minuses connected, halving the capacitor value.
With 10-20% capacitor tolerances you have no control which cap takes the highest voltage drop.
You also want to limit the maximum current through the capacitors. That is where the diodes come into play.
Asuming that worse case DC levels on the mains stay within two diode drops (~1.5V) two diodes in series will not conduct any DC current and the capacitors in parallel with them also block DC.
When the momentary primary current drawn by the transformer gets to a level so that the voltage drop over two capacitors in series exceeds the forward voltage drop of the two diodes in series these diodes also start conducting current thereby limiting the current through the capacitors.

Electrolythical capacitors allow for a small negative voltage so putting two of them in series with diodes in parallel to limit the voltage drop per capacitor to 0.75V and putting two of such configuration anti parallel will block any DC voltages up to 1.5V, doubles the current rating through the capacitors and keep currents through the capacitors limited.
The reason I choose for 4x 10.000uF/16V comes from their maximum current rating. The higher the voltage rating and the higher the capacitor value the higher the current rating of an electrolythic capacitor. The 10.000uF/16V types I choose are rated for 2.5A continuous current each and are a good compromise for size and cost.
Higher currents will happen since amplifiers need DC power to work which means a rectifier and buffer caps at the secundary side of the transformer.
The current through the secundary windings and thus the primary will only flow when the momentary recified secundary AC voltage exceeds the remaining voltage in the buffer caps. That peak current will also flow in the primary that can be substancially higher than the 2.2Ams we spoke of earlier.

First I connected a 500VA torroid directly to the mains without loading the transformer.
Switching it on and off a number of times reminded me why I have inrush current provisions in my amplifiers. The 16 Amp mains fuse kicked in due to the excessive inrush current of the torroid transformer.
A clearly audible hum came from the torroid, also when taking the transformer in my hand to avoid any mechanical resonances with the lab table.

Then I connected the filter between the mains and the torroid. To my expectation and some surprise the torroid was absolutely..... SILENT......
I hooked up a smaller 225VA Torroid transformer to the mains without the filter for comparison, it hummed all right.
I kept both torroids powered for several days and at different times I went in my lab to if check if the transformer with the filter in front was still silent, which it was. The non filtered transformer exhibited different hum levels over time.

That was enough to build two of these filters in an isolated enclosure to connect to my speakers for further testing.
I took simple Veroboards, mounted the power diodes on small heatsinks and the 10.000uF/16V electrolitic capacitors and soldered the connections at the backside of the Veroboard.
Connections to the outside were made using a powercord with molded male/female mains connectors where I cut of both ends and soldered them to the board.


Up to now I haven't heared any hum coming from my speakers again so you might say that I became a beleiver of mains DC blocking filters to avoid humming torroid transformers.