Quote:
Originally Posted by Amraduk
Probably, I hadn't given a great deal of thought to that yet but there are three low voltage secondary windings from which I can choose: 24V, 22V and 12.6V. I think it will probably have to be the rectified 12.6V, the current drain isn't going to be very high so it shouldn't affect it adversely.
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There is 6.3V as well - look to the top-left of the transformer.
But thanks for making me look more closely; I hadn't spotted that it uses a DC supply for the preamp valves. Very interesting
Either would be fine; the 12VAC would mean the voltage regulator runs a bit warm, but there's no shortage of metal to bolt it to. The 6V3 might be a little on the low side, but the PIC is happy down to quite low voltages, so you'll probably get away with it. Suck it and see
For the PSU and power-fail detect, the circuit used for my A4 amp is a good starting point (ignoring the capacitive dropper, of course, and drop the 10V Zener too). The IC would need to be a bit beefier than the 78L05, and you might want a larger smoothing capacitor. But that can be thought about later...
EDIT: Just looked again; the AC windings are 20-something volts, so disregard my mention of the 12VAC. And I'd be disinclined to use the 12V DC rail, because that is only 12V because 24V is being divided down by the valve heaters. The load added would upset the balance of the heater voltages (and possibly induce noise into the preamp). Looks like we should use the 6V3 AC winding