The DC-Filter

Development and realization by F. Wilker

This project description will continue to be supplemented!

last change: 12.01.2021


Another filter?

Again a customer asked for help. His EL34 tube amplifier was humming. And it was obviously mechanical.


Mains transformer


So I unpacked the thick tool and loosened the power transformer. By shifting and underlaying plastic disks the problem could not be the problem could not be solved. So the transformer is defective? You know the result: A newly wound transformer brought no improvement. improvement. In fact, the device hummed only very quietly to not at all at my power supply. At the customer's it hummed again unpleasantly and clearly louder.

So I did some research on the net and while reading the project on the snubber network Quasimodo I became aware of the oh so unpleasant DC components in the power grid.

Another search quickly brought me to the keyword DC filtering. Could this really be? A few volts of direct current cause that a power transformer starts to hum?

The very clear answer: Yes!

technical explanation

We now take a short leap into tube amplifier technology; namely to the SE amplifiers. There the output transformer is biased with a DC voltage. This power is usually at least in the two-digit watt range. If I would do this with a standard transformer, it would be saturated and overloaded after a few watts. For this reason, these transformers are given an air gap in the core. So the transformer does not saturate too early. saturation too early. Unfortunately, our power transformers do not have an air gap and are already loaded by their main work. Therefore, a DC load quickly leads to overload and thus to hum.

This circuit provides a remedy:



a high pass

A capacitor in the power line blocks the DC voltage. From a technical point of view, this is a high-pass filter which allows the mains voltage to pass through unhindered. Quite simply, that's it.

To protect the electrolytic capacitors, the capacitors are bridged with diodes. This prevents overvoltage at the capacitors. Polarized capacitors which are operated on an AC voltage must not be loaded with more than max. 20% of the nominal voltage. of the nominal voltage. A 25V type therefore with not more than 5V voltage drop at the electrolytic capacitor. With appropriate dimensioning of the capacitors and diodes, this case will never occur, however.


Here is the result of my efforts: A circuit board routed. Fuse, two capacitors, diodes and screw connectors formed my first DC filter.

The result speaks for itself: filter on - hum gone; WOW. Filter off - hum. I had not really expected this effect. However, not because I did not believe in the principle. But more, because I did not believe that already that these small DC voltage components in the AC voltage could trigger such an interference effect.

many built filters later, I know that this is probably the case.

The next boards then received an upgrade:

  • variable grid dimension for different electrolytic capacitors
  • more space for more electrolytic capacitors, for different powers
  • Screw connector for easy installation
  • Varistor as overvoltage protection (<25ns/12000A/12kA)
  • clear labeling now facilitates correct installation for the DIYer.
  • increased number of UF diodes, for better protection of electrolytic capacitors

As already mentioned, it is necessary to adjust this filter to the power taken. This is the only way to ensure that the electrolytic capacitors can do their job properly.

With the DC filter boards now available, power ratings of up to 2000 VA can be realized. Here additionally still another combination of DC and mains filter.



unmounted boards








I have already written it: The filter removes DC voltage components very reliably. For this statement I would not have needed another subitem. I am not a click addict.

In recent months, there have been frequent inquiries from customers who wanted to upgrade the sound of their system with a DC filter. In most cases there was no indication of a DC problem. My recommendation to rather use a mains filter led to a lack of understanding, respectively to the potential customer leaving. Fortunately, some customers were not fobbed off and ordered one or more DC filter modules.

I was surprised. The feedback clearly showed that DC filtering can improve the sound image.

A colleague at work even had a DC filter built from high-end components. Before that, I supplied him with filters of various power ratings, which he used to test which filter delivered an improved sound.





Jantzen Film Cap




Ultrafast diodes

Housing construction

div Gehäuse

diverse Gehäuse

Company Hammond

Again, I rely on the aluminum housings of the Hammond company. I have to say that requests for a complete unit are rare. complete unit are rare. Most DIYers want a module that can be built into existing housings (whether amplifiers or filters). or filter). I can fulfill this wish with my circuit boards.

Kunden, die ein "personalisiertes" Netzfilter ordern, wählen mittlerweile häufig die Option ein DC-Flter zusätzlich einbauen zu lassen.


A customer had a Dynavox power strip (X4100) converted. This strip with mains filter offers a total of eight sockets, four of which are wired as unfiltered. This strip is very well manufactured for its price and is excellent for excellent to retrofit a DC filter.

Dynavox 4100

The finished device

Customer satisfied

The following modification was made by me:

  • DC filter in an external housing
  • fixed wiring with the Ölflex CY110 in 2,5mm²
  • additional bushing in the Dynavox to accommodate the DC filter cabling
  • Conversion of the four unfiltered sockets to DC filtering
  • Power cable with rhodium components for the Dynavox power strip

The original statement of the customer:

"Think I'm a nerd - but the difference between this and a "normal" power strip, like the one I'm using right now using again, is actually quite audible."

"I have some reference tracks that I can clearly pinpoint such differences on - and am now slowly "re-working" my "rebuilding" my music collection (which I'm very excited about...)."

Another customer ordered several mains filters with two sockets each. On my advice, he had one socket fitted with a DC/mains DC/mains filter, whereby the DC filter could be switched off by a switch.



Comp. Hammond



both sockets with 900VA

Das Netzkabel

power cable


His special request was for a particularly high-quality power cable: the NEOTECH NEP-3003 III Silver Plated UP-OCC in 2.6mm².

power cable


Without words




ready unit

The final product


A combination of DC and mains filter is now also available in 500VA and 900VA power ratings. Because it is a combination of the described filters, I did not create a separate project page for it.

DC-/Netz-Filter 500VA



DC-/Netz-Filter 900VA



Click here to go to the project page: Mainsfilter


Frank Wilker

Thanks to all those who helped with tips and suggestions to improve these filters.

Leave me a message about this project

Nützliche Links

  • Audio Regler Fachlich fundierte Infos verständlich rüber gebracht
  • Bohne Audio Teure Netzfilter - Aber einige Wahrheiten über die Nutzung von Netzfiltern
  • Auktion DC-Filter
  • AAA Diskussion im Analog Forum / daher subjektiv
  • MPaudio Snubber Netzwerk - wenn nichts mehr hilft