Spec'ing current reqs on trafo secondaries

[brucew268]

In spec'ing a replacement transformer for the failed one in my preamp, I'm being asked for the current requirements on the secondaries. Every calculator I find online is different to the others and am a bit confused at this point. I don't want to come up to speed on all the theory right now, I just want to the right value to give the transformer manufacturer, or a calculator I can easily use to get it.

The original Trafo is 15VA with a 1amp fuse on the live input, even at 110vac.

Secondaries:
23vac > diode bridge > 10k uF cap > 15v reg (+) > 10k uF cap
__________________> 10k uF cap > 15v reg (-) > 10k uF cap
9vac > diode bridge > 4700uF cap > 5v reg (single)> 4700uF cap

The 11DQ10 diodes each show a max handling of 1.1amp,and the regs are LM2940/LM2990 with slide on heat sinks.
It looks like the 5v rail merely handles the remote motor on the volume pot. The motor uses 150ma plus circuit. So I imagine most the current is for the 15v legs.

[RothwellAudio]

What's the pre-amp? 15VA would be enough information to go on if there was only one secondary winding. With a 15VA rating and two secondaries you just need to know the split between the current requirements of the two. If all the current was for the 23VAC winding it would work out at about 650mA. If all the current was for the 9VAC winding it would work out at about 1.7A.
Half each would be 325mA for the 23VAC winding and 1.35A for the 9VAC winding. You could look at the failed transformer to get a clue. If the two windings have different wire gauges the thicker wire would suggest a higher current rating for that winding.
To be honest it would probably be ok to just go with a 50/50 split.

[brucew268]

Well, the 9v secondary merely handles the circuit feeding the remote motor on the volume pot. The motor itself can handle 150ma max at @5v. So I imagine most the current is for the 15v legs (23v secondaries). Classe Audio CP-30. (I tried to upload the schematic, but AOS doesn't allow PDF's of 20k or more.)

[RothwellAudio]

The motor itself can handle 150ma max at @5v.

Well how about 300mA for the 9VAC winding? That would be 2.7VA, leaving you with 12.3VA for the 23VAC winding. That works out as 535mA and should be plenty for a pre-amp.

[brucew268]

The specs on the motor say power = 0.05w 4-6v. Motor current at end slip: 150mA at 4.5Vdc. So that would mean I would not need more than 2.5-4.5VA from transformer?

[brucew268]

Well how about 300mA for the 9VAC winding? That would be 2.7VA, leaving you with 12.3VA for the 23VAC winding. That works out as 535mA and should be plenty for a pre-amp.

Sounds about right.

[brucew268]

I ended up getting a small cheap 4.5v x2 transformer in order to measure the current draw from the remote volume control circuit, which includes the LED and mute switch. Since the trafo is rated at 230vac, the actual readings came out to just under 6vx2 without load The current readings came out to:

Idle current: 145mA
Usage current: 180mA
Peak current: 193mA

So it seems I should be safe spec'ing this transformer winding for 200mA @ 9v, which comes out to 1.8VA. That leaves 13.2VA for the windings feeding the main circuits = 22-23v x2.