Help picking out power transformer

[nosamiam]

I'm working on J. Haible's clone of the Compact Phasing A. Fun project so far but sourcing some parts has been tricky.

Here's the project: http://jhaible.com/legacy/compact_clone/

And here's the schematic: http://jhaible.com/legacy/compact_clone/jh_krautrock_phaser_sch.pdf

The circuit runs on +/-15v. It has the rectification and smoothing built into the PCB but needs a transformer mounted off-board that puts out 2x 18VAC. I'm not really sure what to buy. How do I know how much current the circuit draws if I can't get it running to measure it?

I did a Mouser search to get me in the ballpark: http://www.mouser.com/Power/Transformers/Power-Transformers/_/N-8u9n5?P=1ysxitiZ1ysxiynZ1yzuxs6&Ns=Pricing|0 but I'm not sure which one I should pick.

Any help?

ps: Don't worry about my well-being what with me asking an AC question. I'm ok there. I've built tons of stuff over the past 10+ yrs, have good soldering hygeine, and have worked with AC mains stuff. I'm familiar with the risks and safety practices of dealing with high voltage.

[antonis]

A "lazy" way is to slighty overcome the lower of max current ratings for your rectification/regulation components.. 

(which in your case is 1A for 1N4002 Diodes & 1.5A for LM317/337)

Of course, no designer sizes components at their maximum current (or voltage/power) ratings so a transformer of 1A should be more than adequate..

But as I've said, it's a very rough estimation..

[nosamiam]

That makes sense! I'll go with the lazy way! Thanks.

[R.G.]

The short answer is that you need 36Vct at more than 250ma. The 12VA ones rated at 36Vct/350ma should do nicely.

If you're mounting the transformer inside the same box, think about using a toroidal transformer instead of an EI or flatpack. They radiate less magnetic field to cause hum.

If it were me, I'd change out pairs of LM741s for duals - TL072 is a good choice. The LM741 is marginal for audio.

[PRR]

It's got light-bulbs, it's got relays, and a lotta opamps.

It appears to be 100mA lamps, two in series. While they "blink", I would assume they may draw most of 100mA most of the time, hence 100mA.

I do not know what the external connection "lamp" goes to. In vintage gear this would be more lamp(s) at whatever current. (However this seems to turn-on in opposition to the internal lamps, so maybe this does not add to toal load.)

I don't see a part-number for the relay. Surely you do, and will look it up. Assume relay could be on for long time, full mA.

Over a dozen opamps. Agree that today you use TL072. IIRC a '072 will idle near 3mA for two opamps, and I do not see any heavy dynamic load (no headphones or such; lamps have transistor buffers). Round-up to 5mA per pair, 35mA for opamps, call it 50mA.

So at-least 150mA, more if the relay is a pig.

You double the DC mA to get AC mA. (The theoretical factor is 1.6 to 1.8 depending on rectifier conditions; at the one-off few-Watt level it is not worth even a 2-digit precision-- double it.) 300+mA AC.

18VAC is ample headroom for 15V DC. If you go a little light on the AC mA it will likely work.

Agree that 250mA is a practical low-limit, 350mA is a better bet.
____________________

> worked with AC mains stuff

Even so... faced with a similar demand, I went-safe. Got a 24VAC 1A "Alarm" wall-wart. Made for 24/7 duty. Left the hum at the wall where it belongs. Feed two "AC" pins, this will voltage-double +/-35V DC. (Check those first caps.) I'm not afraid to stick my hand in a live fuse-box, but sometimes there's no good reason to go there.

[nosamiam]

Thanks for the help, folks. Got some good info here! I think I'm on my way.

[PRR]

> no center-tap. How do I get +/-35vDCs from one 24VAC?

http://www.diystompboxes.com/smfforum/index.php?topic=102313.msg906284#msg906284

[nosamiam]

That's lovely! Thanks!!!