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wuffwaff 25th January 2002 01:36 PM

dc filter

Ive got a question concerning a "DC-Filter" offered by LC-Audio. Its supposed to remove dc from the mains and reduce transformer noise.

Does anybody know how this works?


Nelson Pass 25th January 2002 06:31 PM

If you are experiencing mechanical hum from your
transformer, it is often caused by the presence of
DC on the line. Usually this comes from some appliance
using current asymmetrically, such as a lamp dimmer.

The hum comes usually from toroidal transformers, which
saturate easily with DC, and when they recover, they
draw an extra pulse of current, causing the noise.

You can put a pair of back-to-back electrolytics in series
with the AC power line to block this, and it works fine.
Makes sure the current rating of the electrolytics is
high enough, and the they are joined at a like polarity,
such as + to +.

leroy 25th January 2002 06:45 PM

I'm wondering how the back-to-back electrolytic capacitors will behave during a fault condition such as a short circuit. What I'm trying to say is that the capacitors could limit the current and thus prevent the fuse to trip which may lead to dangerous situations.

GRollins 25th January 2002 08:09 PM

Note also that transformers can hum if they get heavily loaded.
I've got a fairly large EI transformer that I picked up cheap once upon a time. It hums. Turned out that one of the laminations had a tab sticking off to one side that wasn't held firmly against the previous lamination (right in the middle of one side, away from the screws that hold the thing together). The flux made it vibrate against the rest of the transformer. I slid a piece of paper between the laminations and the hum stopped completely.
One of these days I'll get out some shellac and do a proper job of fastening that little fella down...


wuffwaff 25th January 2002 09:11 PM

thanks for the replies!

In my case I think it is DC causing the hum. Its not always the same but about a month or two ago it has become louder as before and can be heard when all is silent. (and gets very very loud when somebody uses a hairdryer)

Any suggestions for a specific type of capacitor? I assume they need a 400V rating (230v mains) but Im not sure what capacity will be sufficient.


wuffwaff 25th January 2002 09:52 PM

oh and I almost forgot,

the LC audio filter seems to have only one 22000microF /25V cap, a diode and a fuse.

must be another way of blocking or removing a dc component?


Mohan Varkey 25th January 2002 11:27 PM


In Australia, we have three phase powerlines. Every third house is connected to the same phase to preserve a balanced power drain. You could request your power supply company or utility to change the phase to your house.

I use electrostatic speakers and valve amps at home. I had similar problems and I suspected DC on the powerlines. The power utility company changed the phase to my house and the hum disppeared.

I would imagine that the density of appliances connected to each phase would be a great deal higher in Germany than in Australia and so perhaps this suggestion may not be applicable. Nelson's suggestion is a good one.


subwo1 25th January 2002 11:30 PM

It seems like they put the diode across the capacitor so that it can only charge in one direction. That would be the direction from which the dc is trying to get through. So, if it doesn't solve the problem hooked up one way, my guess is that you would have to reverse it.

The 25 volt rating assumes that the impedance of the load, say a transformer is much less at 60hz than the load. Xsubc = 1 / 2*pi*f*c. Xsubc, the impedance of the capacitor comes out to be .12ohm. I guess this process is what is going on.

R. McAnally 26th January 2002 01:49 AM

I had that problem with a large toroid at one time (intermittent humming)... you might want to try reversing the polarity of the primary winding -- this seemed to fix my problem completely. Also, unbalanced load on the secondary can cause hum on some toroids. This could occur if your protection relays or other circuits are powered only from the + or - rail and not both rails evenly. Hope this helps!!

subwo1 26th January 2002 02:08 AM

"The 25 volt rating assumes that the impedance of the load, say a transformer is much less at 60hz than the load."

I meant to say the 25 volt rating assumes that the impedance of the load, say a transformer is much more at 60hz than the capacitor.

Sorry for any confusion. Take care all.

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