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Sunday, March 24, 2013

MC step up transformer with selectable gain

Hi!

Often people use different cartridges with very different output voltages and impedances. For such situations it is desirable to have a MC step up transformer which can be adapted by switching between different step up ratios.




This is easily doable with transformers with split windings. Lundahl winds all his transformers with dual coils, so both the primary and secondary are composed of two individual windings. The datasheet of the LL1943 suggests two ratios which can be achieved by either wiring the primaries in parallel or in series. Thus allowing either a 1:16 or 1:32 step up ratio.

This can be easily made selectable. All that is needed are two selector switches with 2 poles. One for each channel. Or a single 4 pole switch which would allow to switch two channels at once. The schematic below shows how this is done:




In the schematic the switches are in the position which puts the primaries in series. Thus this represents the lower step up ratio.

A minor drawback of his schematic is the fact that the connection between the two windings goes through two switch contacts in case of the serial position. The next schematic shows how to avoid this:




It is quite easy to assemble such a MC step up. I will show this in a few photos. The RCA connectors, switch, ground connector and transformers are all wired on a metal plate which serves as the back panel:





The second schematic is a bit more difficult to wire but doable. The photo below shows the switch prepared for this:




I cushioned the MC transformers so that the mu metal casing doesn't get bent which would impact it's permeability. A close up showing the mounting of the transformers:




The back panel completely wired up:




A close up showing the wiring to the output jacks:




And a close up of one of the switches at the input side:




Now everything can be assembled into a wooden housing. The front panel holds the bolts to which the back is attached later on. It also gives some weight to the transformer, so it is not too easily pulled from the rack by interconnects:




The finished MC step up:




The back panel:





Best regards

Thomas

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