Hope someone can help, building 2 x h140 clones as mono blocks but need 12vdc for speaker protection circuits. Do I go for a Toroid with a 25v (for power and 12v for protect) or can I take 12vdc somewhere off the main board.
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It really narrows your choice if you want to get both voltages from an off-the-shelf Toroid and it would be a shame to restrict your choice of main rail voltage. Many commercial amplifiers and HT receivers have transformers with additional windings that may suit but this depends on what you can find in the junk.
One option is to derive your 12V supply from the main power rail with a simple dropping regulator. A modern switching regulator chip can be inexpensive and efficient but likely you don't need a lot of current and a pair of the regular LM series regulators may do just fine. I assume you need dual rails, the dc-protection circuit needs to have + and - in order to detect a dc-error that could be either polarity.
One option is to derive your 12V supply from the main power rail with a simple dropping regulator. A modern switching regulator chip can be inexpensive and efficient but likely you don't need a lot of current and a pair of the regular LM series regulators may do just fine. I assume you need dual rails, the dc-protection circuit needs to have + and - in order to detect a dc-error that could be either polarity.
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You could also just add an extra transformer - you don't need a lot of power for a protection circuit. A 3VA transformer would be plenty for example. You could probably get one by smashing open an old linear wallwart.
There's another interesting option, since we are diy'ers. If the main transformer is a toroid, it's not too hard to add a few more windings to get the low voltage.
Here's a link that describes that process for the toroid used in the GT-101 power amp...just look down the page to the heading for "Add Some Signal Processing"
Extending the GT-101
Here's a link that describes that process for the toroid used in the GT-101 power amp...just look down the page to the heading for "Add Some Signal Processing"
Extending the GT-101
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