Important news about connecting a DaDa double PSU in a Quad 405

I had some mail conversation with Keith Snook of DC- Daylight about double PSU’s, with or without the active ground and the protection options. Keith mentioned extra noise and 100Hz peaks when using double PSU’s. I did not have a clue, but lucky for us all, Keith solved the mystery, but also had a cure for it.

The 405 transformer is built up as a so called C core. Apparently they built it with two identical sections around the C core. If you look at the connections, you will recognize the symmetry.  So it looks like a secondary is closely coupled to its partnering primary, but more or less loosely coupled to the second primary section. In the original setup, see the 240V AC example in the drawing, all the secondary’s are connected in series and then in parallel, so each section only drawn halve the current from the primary side, and the load is balanced across all the primary’s and the secondary’s.

When you split the two secondary segments in the normal way to build up the dual supply, the load balancing is gone. This unbalance and the ineffective coupling of the first secondary and the second primary give rise to nonlinear behavior with the effects mentioned by Keith as a result.

The cure is simple, Keith find a way to balance the load across the primaries and the secondaries. See the drawing in the 230V AC example. To make maximum use of this balance, also the primary side must be connected in balance with all the AC mains voltage options. So when you have to rewire for a different voltage setting instead of clicking the voltage selector switch setting with the older 405's, look at the diagram for the correct layout.

Joost Plugge
DaDa Electronics