Bipolar PSU question

[MartyMart]

I need a +/- 15v supply for a mic preamp ( IC based with audio transformer balanced I/O )
I have a (UK) 230v Primary "Block" transformer with 2 X 15v secondary sets of pins
rated at 2,3VA
(Also have a larger 3,2VA version with only 1 set of secondary pins)

Is it fairly straight forward / possible to use these two secondaries as +/- 15v points ?
a little clarification needed !
It's a small 2x2x2 inch block, not a torodial transformer

Thanks,
Marty.

[R.G.]

See "Power Supply Basics" at GEO.

Yes, straightforward. The biggest issue is voltage. Your transformer produces nominally 15Vac, which gives 15*1.414 = 21.21Vdc, plenty of room for a regulator. The problem you'll have is that small transformers sag in voltage a lot. You don't say how many VA the transformer is rated at, nor how much current your preamp needs. It is possible that the transformer produces only 1 or 2 VA. With 42Vdc coming out, 2VA is 47ma, so the current is quite restricted.

[MartyMart]

I did say that it's rated a 2.3VA , the circuit requires only 100Ma so it looks like
there will be enough ... ?

Thanks RG

MM.

[R.G.]

Sorry - yes you did and I missed it.

If it will do 2.3VA at 30Vct, that's 2.3/30 = 76ma of AC that it will put out under full load.

That's the RMS rating of the wires inside. If you full wave rectify it, the current pulses are much higher in current and shorter in duration than full-cycle AC. The "effective" RMS current into a full wave bridge is 1.6 to 1.8 times the DC average out of it. If your circuit uses 100ma of DC, then the RMS drawn from the transformer for that is 160 to 180ma, and your transformer is undersized by about 1/2. What that probably means is that the voltage will sage quite a bit and it will get hotter than it was expected. It will be marginal if it works at all on a load that is really, truly 100ma.

There's another way to look at this too. The output of the transformer at DC is 42Vdc, and that is what voltage goes to the regulators. They eat the difference between that and your output 30Vdc as heat, so the transformer sees a 42Vload at 100ma. That is 4.2watts of DC delivered, and that had to come from your transformer as well. So it's undersized by that measure as well.

I think that you need a bigger transformer or a smaller load current. Is your preamp rated at 100ma and perhaps actually uses less? That happens sometimes.

[MartyMart]

Thanks RG, I had no idea that AC/DC conversion meant so much "loss" like that
.... I'm a PSU/Transformer newbie  !!
I'm guessing at the 100Ma required, just being safe, its made from 4 X NE5532AN's and
of course a bunch of other "parts" and there's a 48v phantom power option, which I'll have
to supply from another transformer ...... so it may well be less than 100Ma required, more like
50/60 perhaps ....for 4 opamps ?

Regards,
Marty.

[R.G.]

There's no good way to tell without measuring or trying it. I thought you had a spec on how much current it needed. I agree, four dual opamps should be under 100ma, but without knowing what else in in there, it's hard to tell. You can look up the idle current for the NE5532 in its datasheet. The NE5532 is often used because it can drive a lot of output current, though.

Is there any way to test the thing out with exterior supplies temporarily and measure the currents?

[MartyMart]

It's not built yet !
It's an SSL 9000 series mic-pre, which is mostly opamps and descrete components, nothing
too wild apart from the 48v DC phantom power board, but that will have its own transformer etc.
It's balanced/line in and a balanced output ( option of Jensen I/O transformers too )
I was just trying to use up a few PCB transformers that I have, it may well be enough and
it's quite small .... which helps !
It will be in a 19inch rack, with stepped gain, 80hz roll off switch and either a meter or `LED
ladder.
I'll read up on PSU's .... thanks a bunch

MM