PCB for Combined DC Blocker (trap) & EMI/RFI filter

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PCB for Combined module DC Trap Blocker Filter for toroidal transformers (toroids) & Common Mode filter

In stock (can be backordered)


Nowadays we use an enormous amount of Switched Mode Power Supplies (SMPS) equipped domestic appliances, industrial electrical consumers etc. in our everyday life. These appliances can cause not only a DC shift in the AC Mains which leads to buzzy toroidal transformers (especially the power ones) but they also throw a great deal of RF hash which sneaks and disturbs the precious analogous networks of our audio equipment.

The unit proposed here is comprised by two sections – a DC Blocker/Trap/Filter and a Common Mode Filter (CMF).
Schematic of the unit can be seen below:

Combined module DC Blocker & Common Mode Filter
Combined module DC Blocker & Common Mode Filter

The DC trap section will remove or at least lower the toroidal transformer buzzing.
Schematic is similar to this one used in well known Vladimir Shushurin’s Lamm-1 amplifier. An article with in-dept explanation of how DC blockers work can be found on Rod Elliot’s site (

The Common Mode Filter will deal with the high frequency pollution in the AC Mains Network.
It was inspired by the very successful “Felix project” (

In-depth explanation and examples of Common Mode Filters can be found here:

In the process of CMF simulation the following results were outlined:

– The damping resistors lower the spike in frequency response with approx. 20 dB

CMF without damping resistors
CMF without damping resistors
CMF with damping resistors

– Removing the load-side shunt capacitors (as supposed in the Fo-Felix version of the filter) will give us a smoother frequency response but the corner frequency is shifted to the higher frequencies approx. 25 times

CMF without load-side shunt capacitor

Board specifications:

Dimensions of PCB: 100×115 mm.
PCB type: FR4 1.6 mm. Copper thickness: 70 um (2oz). Surface finish – Nickel Gold (ENIG).

The board is TWO-layer design which allows the PCB traces to be doubled on the TOP and BOTTOM layers thus doubling the current handling capabilities of the board. All current conducting traces are widened as much as possible which additional raise the max. handled current.
PCB has solder mask coating on top and bottom layer for best durability.
Silk screen with parts layout is placed on the top of the PCB.

PCB has following abilities:

DC Blocker Section
– Possibility for mounting 6 discrete diodes which increases the max. blocked DC voltage.
– Diodes with 2 different bodies can be mounted on it – DO-201 or TO-220.
– Electrolytic capacitors C1, C2 are snap-in with lead spacing 10 mm. and max. outer diameter 40 mm.
– Resistor R1 has lead spacing of 53 mm. which allows various types to be used.
– Bypass capacitor C3 is WIMA MKS4 400VAC with lead spacing 15 mm.

Common Mode Filter Section
– Shunt capacitors C4 – C7 are X2 type safety polypropylene capacitors
– C8, C9 – Y2 type safety polypropylene capacitors
– Common Mode Choke can be up to 20A rated one
– AC input/output connectors can be of the following types:
a) MOLEX 3- or 5-pins terminal blocks with lead spacing 5 mm.
b) 3-pins Terminal Blocks with lead spacing 7.62 mm.

All parts used are easily obtainable from big supplier stores (Mouser, Farnell, Digi-Key etc.) or via e-bay.

Note: the listing is for PCB only – pictures of assembled device is for information purposes only!

Schematic and parts list will be sent to the buyer’s PayPal address after purchase. If for some reason a different e-mail address must be used, note it in the process of checkout.

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PCB for Combined module DC Trap Blocker Filter for toroidal transformers (toroids) & Common Mode filter
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Additional information

Weight 0.065 kg