The primary is normally wound under the secondary and inside substantial insulation! Sometimes there is even a foil noise shield. Adding additional primary windings over the secondary even with additional insulation would still increase power line noise coupling into the secondaries!
this is another good plan.......just one additional coil to wind.....you can use cotton tapes for insulation.....
If you are familiar with winding transformers, linevoltages and safetystandards you could turn your transformer into one that gives you all the advantages that lower current density in the primary and lower Bmax (with the required more windings per volt) can giv you.
The VA rating will drop but your output current max should stay at least the same.
Regulation will propable also change a bit but by how much depends on the current densities of your original primary and secondary windings.
To drop the secondary voltage and at the same time get for audio use a "better" transformer you have to EXTEND THE PRIMARY by putting a winding on top of the transformer. Check first the original outter isolation if it is up to handle the highest primary voltage at the required safetystandard (it usually is but just to make sure check it and correct if necessary). Estimate how much more windings your primary needs to get your required secondary voltage. Add a few widings on top your estimation and wind your additional primary (at least same copperarea than original primary). To minimize magnetic strays the full circumference of a toroid should be covered with copper evenly spread and with as small as possible gaps between windings (paralleing wires can help). Connect the original primary and your added winding in serial and feed from a low ac voltage source. Measure your secondary voltage. Flip over beginning/end of either original or the added winding and use wichever givs lower output. Now test with full primary voltage and adjust to the voltage you want(under load or no load, wichever you find more important) by unwinding for correction. Readjust to evenly spacing. After all requirements are met put on the outtest isolation (again according to mainvoltage and safetystandards).
The VA rating will drop but your output current max should stay at least the same.
Regulation will propable also change a bit but by how much depends on the current densities of your original primary and secondary windings.
To drop the secondary voltage and at the same time get for audio use a "better" transformer you have to EXTEND THE PRIMARY by putting a winding on top of the transformer. Check first the original outter isolation if it is up to handle the highest primary voltage at the required safetystandard (it usually is but just to make sure check it and correct if necessary). Estimate how much more windings your primary needs to get your required secondary voltage. Add a few widings on top your estimation and wind your additional primary (at least same copperarea than original primary). To minimize magnetic strays the full circumference of a toroid should be covered with copper evenly spread and with as small as possible gaps between windings (paralleing wires can help). Connect the original primary and your added winding in serial and feed from a low ac voltage source. Measure your secondary voltage. Flip over beginning/end of either original or the added winding and use wichever givs lower output. Now test with full primary voltage and adjust to the voltage you want(under load or no load, wichever you find more important) by unwinding for correction. Readjust to evenly spacing. After all requirements are met put on the outtest isolation (again according to mainvoltage and safetystandards).
Just noticed RJM1 suggested this allready.
Nevertheless, I thinck that increasing the primary has (disregarding cost effectiveness and increased weigth) many advantages. I purchase sometimes purposely cheap maxed out toroids (with higher than needed secondary voltages) to make them "quieter" and/or better adaptable to various amplifier loads, line and cap voltages a.s.o. I wind sometimes even more than one additional +- connectable primary extensions on it.
Inrush becomes also less of a prob and cheap toroids can be made as quiet as otherwise only a custom designed at fare less money. Wisely done, toroids used as cheap OPTs will also benefit from similar "sandwich" construction.
Nevertheless, I thinck that increasing the primary has (disregarding cost effectiveness and increased weigth) many advantages. I purchase sometimes purposely cheap maxed out toroids (with higher than needed secondary voltages) to make them "quieter" and/or better adaptable to various amplifier loads, line and cap voltages a.s.o. I wind sometimes even more than one additional +- connectable primary extensions on it.
Inrush becomes also less of a prob and cheap toroids can be made as quiet as otherwise only a custom designed at fare less money. Wisely done, toroids used as cheap OPTs will also benefit from similar "sandwich" construction.
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