Unipolar PSU Question

i got a non shruthi related question:

is there an easy/cheap way of getting an +9v / -9v power supply?
maybey a way of getting -9v, 0v and 9 from a 18v psu. maybe change the schematic from shruthi psu from/-5 to +/-9v ??

at the moment i am using 2 9v batteries for this, but they don`t fit into the enclosure i got and i want to get rid of them anyways.

all boutique unipolar psu i found via google are quite expensive.
maybe someone has some links to schematics or (even better )cheep ready build ones or can give a hint, where to start reading.
i don`t need much current. (haven`t messured, but can`t be much)

You can even have +/- 9V from a 9V AC source with a half wave rectifier provided you don’t load it much.
I’ll draw a schematic… wait a sec.

Oh oh, no monospace font available…
Simplified ASCII-schematic attached ->

Use a LT1054 to get a 9V rail from your 9V positive rail. Limited to 100mA. You can do it the Shruthi way (input> LT1054 > 7909 ; input> 7809) but if you already have a stable 9V source from a battery, you can just do battery -> LT1054 to get the negative rail. Keep in mind that -9V is not a very common voltage, you’ll find more easily 7908 Vregs.

I am recommending this because portability seems to be a concern in your case… And an AC supply with half-wave rectifiers (ie, if your input transformer is an AC wall-wart, not a proper transformer with a center tap) needs quite large reservoir caps to hold the input voltage during the half-cycle. You might also have to put proper heatsinks on your Vreg. So two 2200uF caps + fancy heatsinks and we have already exceeded the size of a 9V battery! All this is true assuming you are drawing currents in the hundreds of mA.

Yupp, large caps (like at least 4x2200u on the inputs and 4x1u around the regulators) you’ll need but I don’t know bout the fancy heatsinks as you will not have a big voltage drop from the rectified and filtered voltages. Still both solutions will be bigger than 2x9V batteries…

I’ve worked on something like that recently and there’s definitely a trade-off to find between using smaller caps + a larger input voltage and thus get more heat at the Vregs and possibly more residual ripple ; or use large caps, a smaller input voltage and little heat dissipation at the Vregs.

One could almost argue that the size of the solution stays the same.
Anyway, I think Ray has half rectifier PCBs, made for the Sound Lab, over at MFOS if one wants to try that.

thanks a lot for your fast help!
i thinks i`ll try it the one-battery-with-LT1054 way. (i think i got a LT1054 laying around somewhere)
portability is not that important. it is all about fitting this thing into the enclosure, which is already done. (i didn`t leave enough space for the 2 batteries…)