Calculating current-consumption?

[Coriolis]

Right: I'm looking to build a powersupply (for a micpre), and I was wondering how to chose the right transformer.
I know it has to do with the VA rating, but if I don't know how much current the micpre eats, how can I chose the right tranny?

So I need to know how to calculate that, I guess, but how? I mean, figuring out the current used in a circuit consisting of a couple of resistors, I can probably manage, but caps? Transistors? Opamps? Do I just look in datasheets and add up the numbers?

I'm certainly am puzzled...and it's not that I haven't been searching ! 

Hope someone can help 

Regards, C

[Mike Burgundy]

If you build a disctrete amp, it's the added current of every stage (which can be calculated from the transistors datasheet and the biasing conditions you set) and any passive (resistive)paths between ground and anything carryng supply. The currents through actives can vary with signal load, so this might be tough. If you want to be safe, take the maximum current ratings and use those. If you've calculated and designed correctly, these will not be exceeded. Then build a PS that can do twice that.
Calculating all this can be a pain if you're not building/designing from scratch (otherwise it's necessary) but it is good excersize. Do a search on transistor bias theory or something like that and read to your hearts content.
Easy method however is to use an external (low-voltage) supply, and *measure*, both with and without signal applied. Make sure to check low-frequency signals.. Also build a PS that handles at least twice that.

[Coriolis]

Thanks a lot Mike! That gives me a couple of methods to try...
I guess when you say "low-voltage" supply, you mean something in the area of what I would be using to power the circuit with?
Like  +/- 15V? I suppose I should get (or build) myself some sort of variable tone generator as well?

Which leads to another question: To get a picture of current consumption under realistic working conditions of the circuit, I would need to give it an input source that looks like what it would get in real life? IOW: what sort of voltage range would I be looking at for a micpre? Oh wait, I can check out specsheets for mics and preamps for that info...

Now look what you did...made me think for my self and all! 

Thanks again!

C