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Dear Dick, LF Group,
Thinking of cheap sources of ferrite, I have used
the cores from old computer monitor line output transformers for some time for
supression purposes - but the type of ferrite they are made from is very similar
to the 3C8/3C85/3C90 grades often used on LF PA circuits.
So I tried one as a power transformer - after some
rough calculations I made a 1:1 transformer with 2 x 20turn windings of 1mm dia.
enamelled copper wire. Feeding 500W into this made it only warm to the touch. I
then tried it at 1kW continuous for several minutes, after which it was quite
hot, about 60degreesC, so still OK really - I would be quite confident to use
this for a PA producing several hundred watts.
The core I used was typical of several I have
extracted from scap VGA monitors - it consists of 2 "C" shaped halves held
together with a metal spring clip, and measures 63 x 39 x 14mm overall. They are
usually glued into the LOPT transformer assembly, which makes them a bit hard to
remove without breaking. The best way to do it I find is to cut radial
slots through the moulded EHT winding with a hacksaw, and carefully break away
the windings from the core. After that, seperate the two halves of the core, and
remove the pieces of plastic film that maintain a gap between the mating
surfaces of the core halves (important!). Then carefully clean up the mating
surfaces so they fit together perfectly without a gap - any gap will greatly
reduce the inductance, which you don't want to happen for a PA transformer.
The way I wound the transformer was to assemble the core, then cover the centre
leg with 3 layers polyester tape for insulation. Then I wound the first 20 turns
onto the round centre leg as if winding a toroid, covered this with another 3
layers of tape, then the second 20 turns on top. The 20 turn winding has XL ~
900ohms, so more than enough for a 50ohm winding. Obviously you would use fewer
turns of thicker wire, or multiple parallel strands of wire , for the low
impedance windings of a PA transformer.
Cheers, Jim Moritz
73 de M0BMU
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