Leaving X-former powered up

[Wynand]

Does anyone see a problem leaving a transformer permanently on and using a "soft-on" feature to disconnect the psu after (or before) the supply caps?

[pinkmouse]

The problem with that is you're switching high current DC, and that's a killer for relays. I'd just use a cheapo traffo to control your soft start, you only need a 100mA or so, so and old wallwart or similar will do.

[Wynand]

The reason for leaving it on and using relays is actually to switch the amp on with a microprosessor.

Would 10A relays not be sufficient?

[CBS240]

find an old, cheap, crapy clock radio and use the transfo out of it to drive the processor. They are usually quite small. Use a relay to switch on the primary of the big transfo. There is much less current on that side.

[PM650]

Practically all comercial amplifiers use what pinkmouse suggests, a small transformer to run a soft-start or even a simple relay switch for the main amp transformer(s) and other standby curcuits. Unless you're using a complex smps supply to get the voltages you need for the microprocessor and the amp supplies, it would be best to run only what is needed to switch the amp on and off from another smaller transformer, as pinkmouse said.

The relay contact rating may become irrelevant should you connect the relays between the caps and supply. Larger power supply capacitors effectively represent a short circuit to the power supply before they begin to charge; connecting a discharged capacitor to a supply could instantly destroy relay contacts, but (I would guess) this all depends on how much peak current the supply could source and the value (size) of the capacitors (and of course the relay contacts).

Someone else will probably be able to give you a definitive answer with more details on your project, though.