Upping voltage on a toroid

[carlmart]

I'm updating and upgrading a power amp I have. This time I'm willing to use a toroid instead of the EI transformer I have now.

At the same time I was thinking of trying the trick of adding some more spires on the outside of the wired toroid I will be buying from the store.

Apparently that would allow me getting something like 10v more on each side so I can feed & regulate the first/lower current stages with a higher voltage than the output stage.

Has anyone tried this? Any special care I should have?



Carlos

[djk]

Very easy to do.

Make a test winding of about four turns and determine the number of volts per turn. Figure out how many turns you need (won't be very many).

Use an appropriate wire gauge for your load, very small will be OK here. I use a pair of 24ga? telephone wire stripped out of a 50 pr scrap piece. The twisted pair assures that both secondaries you add will be the same length. Tape it down when done.

Do a test to determine phase, connect one end to the main winding, put a light load on the whole winding. The voltage will either add or subtract. Test with a lightbulb in series in case you really screw something up, 75W is good for 120V (will light up if you have a shorted turn).

[AndrewT]

Hi,
I'm going to suggest a slightly different technique.
Wind on 0.6mm diameter enamelled copper wire to the required number of turns.
Take both ends to a terminal strip. measure the voltage.
Adjust if necessaery.
Repeat for the second winding.
It is better to solder on flexible leads, about 0.5sqmm or 0.75sqmm will do. Put/stick two extra layers of insulation under AND above the soldered joint to help prevent rubbing through other insulation/enamel.

Insulate around all the extra windings. For these low current supplies, I have found they perform perfectly well without removing the existing outer layer of insulation.
These 0.6mm diameter windings will give a maximum continuous DC current of about 400mA. This is way over the requirement for two channels of front end supply. 0.4mm diam <=190mAdc

You now have four extra terminal connections. These are added to the AC tappings from the ordinary thick gauge windings to create new higher voltage AC tappings.
Run a separate low current bridge for each HV tapping.
Finally connect all four zero volts from the smoothing caps to the main power ground. Then run a short, thick wire from the power ground to the audio ground.
I do not believe that bi-fillar winding the extra turns is important,

[carlmart]

Originally posted by djk
Make a test winding of about four turns and determine the number of volts per turn. Figure out how many turns you need (won't be very many).

Yes, that's what I was thinking of doing.

Use an appropriate wire gauge for your load, very small will be OK here. I use a pair of 24ga? telephone wire stripped out of a 50 pr scrap piece. The twisted pair assures that both secondaries you add will be the same length. Tape it down when done.

I was thinking of using some enameled magnetic wire I had bought in Radio Schack years ago. Do you think Cat-5 wires would do the job too? Aren't they too thick?

Do a test to determine phase

Why should I have phase problems here? These wires are just an elongation of what's already on the transformer.

In my head, phase is a problem when paralleling wires from different transformers, which is not the case. Am I missing something?

Thinking again I think I know what you mean. I have four separate wires on the secondaries. The wires will come written down as this: Red-Yellow/Orange-white. I will check with manufacturer, but it's likely Red and White ends are in the same phase.

connect one end to the main winding, put a light load on the whole winding. The voltage will either add or subtract.

Test with a lightbulb in series in case you really screw something up, 75W is good for 120V (will light up if you have a shorted turn).

You mean a shorted turn because the isolation failed?

Thanks for the suggestions.

[AndrewT]

Hi Carl,
this counts as a modification.
use a mains light bulb tester in the mains feed.
If there is anything wrong with your wiring up, the bulb protects you and your equipment.

If nothing is seriously wrong then just try interconnecting two of the secondary terminals and measure voltage. You'll soon find which ones add and which don't. The don't = out of phase.
I would take all 8 leads to separate terminals until you KNOW which are correct, then you can start combining leads to save space if needed.

[carlmart]

The wires will come written down as this: Red-Yellow/Orange-white. I will check with manufacturer, but it's likely Red and White ends are in the same phase.

I just checked at the manufacturer's site and their diagram does confirm my hunch. Yellow + Orange are the ones which should be wired together for the ground, so I should add spires to Red and white.

[AndrewT]

Hi,
if you convert the transformer to centre tapped, then that severely limits your options for regulating the high voltage, low current supplies. Dual regulators for each dual polarity supply keeps more options open.
It may also introduce hum loop problems due to the common zero volts connections for all the amplifiers.

[carlmart]

Originally posted by AndrewT
if you convert the transformer to centre tapped, then that severely limits your options for regulating the high voltage, low current supplies. Dual regulators for each dual polarity supply keeps more options open.
It may also introduce hum loop problems due to the common zero volts connections for all the amplifiers.

Yes, I am considering that trick, and this transformer would allow that. But the secondaries, which still are two, would still be shared by the high and low current stages.

What I was going to do was split the supplies with separate rectifiers for each channel.

What I am looking for is for some way to improve on the low current regulators I now use on the low current stage, which are 3X7 types.