Mains voltage DC block for the modular high end audio amplifier system

Project presentation

Abstract

This project has an interesting background, although what I present here is mostly trivial. Rod Elliot wrote an excellent article about impact of DC on mains voltage on large toroidal transformers and how to mitigate the problem. I highly recommend his work. A DC voltage on the mains can cause sufficient current flow in low resistance primary windings to saturate the core. Lower primary winding resistance is the reason why large transformers are more likely affected than small ones. In this article I just present a tiny afternoon project designing a PCB for the circuit proposed by Rod. I don't think I have any mains DC issues at home, so the project is not to solve a problem.

Design presentation

I have designed two variants of the DC block module. A small one for just one transformer winding and a bigger one for two transformer windings. As I have dual primary transformer windings throughout the project, the larger one is more suitable for my application.

Schematic

There is nothing special about the schematic. Note connectors N_1 and N_2 for looping through the neutral conductor. This allows to route mains in a tight loop, reducing emissions. Diodes need to be capable of conducting very large currents in case there is a short on the mains so that the circuit breakers will trip. The capacitors are small, but can handle enough ripple current even for powerful amplifiers.

Assembly

Schematic

Assembly

Summary

I don't have any information on how likely DC voltage on the mains may occur. In case it does, the DC block solves the resulting issues with the transformer. The form factor fits the grid system so in case it is required, it can find its place anywhere inside the amplifier chassis. It took me some iterations to shrink the module as much as possible. Transformer power rating should not exceed 500VA when using any module presented here.