I have a preamp from a friend with a rail-to-rail power supply (-9V and +9v) that runs on batteries. I'm trying to mod it for him so that it can run on a power supply (I've built the charge pump for this purpose) but would like to keep the use of batteries.
It's already got a switched stereo input jack. I'm just trying to figure out how I can use the switched DC jack to interrupt both batteries at once.
To be honest, it may be easier to run them in series for +18 0 instead of +9 -9. I was testing using FET switching to get one of those working. It was more trouble than it was worth, it offset the rails.
RG has an article on using BJTs to switch supply rails. It may be of some use.
I decided to just run the single battery into the charge pump. Works fine now. :)
There is one way to do what was originally requested: Use a shorting jack at the input to connect or disconnect the positive battery to ground as is commonly done and use another shorting jack at the output to connect or disconnect the negative battery to ground. If you take off the input and output cables, the batteries float. When you plug them in, they connect. You just have to make sure there is no circuit issue that can cause trouble with only one supply connected (and some op amps may latch up with inputs taken to ground with only one supply connected). The choice of which jack is used for the positive or negative battery is arbitrary - either choice will do.
Geofex. The pertinent part is in the dashed lines.
(http://geofex.com/FX_images/splitter.gif)
Note that you may need series diodes to keep the additional power supplies from making the transistors conduct in the reverse direction.
Keeping two batteries as well as a DC power supply and a charge pump inverter complicates things.