Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-PN-SPAMFiltered: yes X-Spam-Rating: 2 Content-transfer-encoding: 8bit

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