Actually, no load in small power transformer can be higher than a load power itself. And that also need to to add diode bridge, pair bigher capacitors , voltage regulators... Smallest trafo i tested had 2,4VA rated power and 8ma consumption with only primary connected, used small old phone charger instead, less than 1ma consumption. No heatsinks, no hot resistors, and more chance to work longer. Hot parts fails more frequently , from my practice.
When on digikey I searched "power transformers" the selector table went to zero items when I selected "toroid". If digikey wants to sell talema they need to list them.
I had a collection of used wall transformers in my attic, bought for $1-3 before every product became a howling switcher. My local unemployed housepainter hauled them off to the steel scrap yard for me. I miss more the TO3 On Semi transistors I paid $2 to $5 for. Also the MPS8099 at $8 a hundred. Perp thought he was going to get copper prices for electronic "scrap". Magnet test puts MJ21193/4 in the steel bin at $180/ton. At least the perp did time on home incarceration.
I had a collection of used wall transformers in my attic, bought for $1-3 before every product became a howling switcher. My local unemployed housepainter hauled them off to the steel scrap yard for me. I miss more the TO3 On Semi transistors I paid $2 to $5 for. Also the MPS8099 at $8 a hundred. Perp thought he was going to get copper prices for electronic "scrap". Magnet test puts MJ21193/4 in the steel bin at $180/ton. At least the perp did time on home incarceration.
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You don’t need to un-wind to add voltage taps. Add another winding over the whole works, using 22ga. solid insulated wire. Without taking anything apart. The added winding does not need to be as space-efficient as the main one, so magnet wire isn’t needed and it solves your insulation problem. If you DO use magnet wire, over-wrap the whole thing in a tape suitable for transformer windings. Fuse the extra secondary appropriately, because if a short does develop (even at the load) it might NOT blow the mains fuse and get quite hot. If you need +/-V, do a bifilar wind to make it easy on yourself.
Wowee.. I didn't expect this thread to start such a debate. I guess there are plenty of valid options and it's a matter of working out the best balance of simplicity, performance and price. I won't be spending much on the solution whatever I do - this subwoofer is already going to be overkill for its purpose. This project is probably more for my own enjoyment than for the outcome.
I just need to find some time to start on testing and working out some "actual" currents and voltages.
Also, for the record, I'm not in America or Europe; I'm in Australia. So that should give me around 230v from the wall, though I based my transformer output on 240v just to be conservative with maximum voltage, as that's the main constraint. The toroid is supposedly 250v -> 2x39v, so 240v should give me about 37.4v and 230v would give me 35.9v if I worked it out properly.
I'm already enjoying working my way through the options here and learning more about how to apply them. So thanks everyone for the input!
I just need to find some time to start on testing and working out some "actual" currents and voltages.
Also, for the record, I'm not in America or Europe; I'm in Australia. So that should give me around 230v from the wall, though I based my transformer output on 240v just to be conservative with maximum voltage, as that's the main constraint. The toroid is supposedly 250v -> 2x39v, so 240v should give me about 37.4v and 230v would give me 35.9v if I worked it out properly.
I'm already enjoying working my way through the options here and learning more about how to apply them. So thanks everyone for the input!
It should be noted that most buck converters can be used as inverters to generate negative voltages (from a positive source).
https://www.ti.com/lit/an/snva856b/...91648&ref_url=https%3A%2F%2Fwww.google.com%2F
https://www.ti.com/lit/an/snva856b/...91648&ref_url=https%3A%2F%2Fwww.google.com%2F
My exact situation was solved by this - (below). +/- 55V rail subamp , with a +/- 12V LPF "tap".
"PreReg" can be a 24V zener. last stage = LM317 +"denoiser" will give -140db PSRR , even under the worst amp duty.
I just use the 317 alone for my 400W Hypex amp.
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Elegance is not needing to drop voltage. To transform voltages to the right voltages at the right places just like the grid works. That is simply a choice of designing nicely versus cost cutting. Many older good quality audio electronics had a transformer with sometimes 4 different voltages so all could be done nicely. If power distribution would work with voltage dropping the world would be hot and we would not have enough power 🙂
Except cost there is not much against it.
Except cost there is not much against it.
I did this with simple insulated wire to get a small single 8 V AC voltage for the protection circuit. The single AC is then rectified to a dual (symmetric) DC supply with increased smoothing caps.
Please don't use paper masking tape as I did!
(And let me know if I did something else terribly wrong ...)
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The turns need should be evenly spaced around 360 degrees of the toroid ideally... For low power winding its not crucial though.
The OP needs 4 times as many turns (to get 2X16 VAC, before rectification and a pair of 7815/7915). That does not look like a stretch at all, and by the time it’s all on there, can be made symmetrical. Radiated field won’t be much at preamp current draw, even if unsymmetrical.
My experience is that there is enough winding window in there to DOUBLE the output voltage at the full VA capability of most trafos if regular magnet wire sized for half the original current is used. That’s even using a proper winding shuttle. A 2x16 winding with bell wire will fit.
It’s either that or buy a little 50 VA Antek. It’s their most cost effective size - the 25’s and 10’s aren’t cheaper enough to justify unless space was a factor.
My experience is that there is enough winding window in there to DOUBLE the output voltage at the full VA capability of most trafos if regular magnet wire sized for half the original current is used. That’s even using a proper winding shuttle. A 2x16 winding with bell wire will fit.
It’s either that or buy a little 50 VA Antek. It’s their most cost effective size - the 25’s and 10’s aren’t cheaper enough to justify unless space was a factor.