Variable transformer

Any one know how to build a variable transformer 0-120 vdc,or even where to buy one.Want to try a pair of Atelier Rullit field coils.

You don’t need a transformer you need a linear power supply. It looks like they need about 135V, although that is a matter of preference.

A quick search brings up something like this:

Although you might want a different voltage range.

I guess they require DC, which means a complete power supply not just a transformer (transformers only handle AC). The supply has to be happy driving an inductive load and, assuming the electronic noise spec doesn’t have to be super-tight, the remaining questions are ‘How much current does it have to supply ?’ and ‘How stiff does it have to be ?’. The latter question arises because the music signal in the voicecoil will try to perturb the magnetism provided by the field coil, and the power supply’s stiffness will determine how good the field coil is at resisting that. It boils down to the supply’s output impedance and is very much the same sort of consideration as ‘damping factor’ in a traditional amp.

As Adam says you will almost certainly get a better value result more quickly by buying a commercial unit rather than building one. I don’t know if you want to tune the field coil strengths on the two drivers independently, in which case you’d need two supplies (dual supplies are available, although maybe not at such a high voltage). Consider buying second-hand. As always you get more for your money that way.

Given the relatively high voltage you could consider doing it with a big valve.


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Further searching shows this:

There’s a monster valve power supply, so clearly a rabbit hole can be descended. I think I’d be getting a modern variable one first before committing a lot of money on a fixed voltage one, to find the right voltage for my usage.

Looks a fun project!

The 393A is a mercury thyratron What’s that about ?


You can use them as rectifiers - just don’t connect the grid.

Yes i saw these. Oleg Rullit is selling some as well without power supply.It was him that said the variable transformer with aeg rectifiers and pio caps was the way to go for a start.

Yes, thinking about it I wondered if that was what it was.

Stewart of Reading say they’ve got an HP6655A: 0-120V, 4A which looks nice. Might be overkill though.


This one is cheaper and 5A should be plenty

Triacs though. How noisy ?


The hum from the speakers will drown them out.

What is the intended application? Looks like the manufacturer suggest front-loaded horn, while a reviewer used them open baffle.

Will be used in open baffle,i use Voxativ 1.6 at the moment but fancy trying some field coils.

In theory noise isn’t an issue for field coils, as long as you are exceeding B_sat.

Some of the old jukebox/organ cabinets that used Jensen speaker drivers used one of the driver field coils as an HT choke. The drivers tend to come in pairs with one low voltage/high current coil and one high voltage/low current coil.