Switching between 9v and 12v with a voltage doubler

[mremic01]

I would like to put together a ruby that's switchable between 9v and 12v. I was thinking of running 9v into it and then just switching between having it powered directly by the adapter/battery and having the power source going through this (http://www.geofex.com/circuits/+9_to_33.htm) to bump it up to 18/17v and then knocking it down to 12v. There's two things I'm unsure of:

First, can I get down to 12v with just resistors? My gut is telling me diodes need to be involved.

Secondly, does switching like this create any potential issues?

[Seljer]

Be aware that those capacitor based voltage doubler's aren't really good once you start drawing more than a couple of ten milliamperes of current out of it.

[R.G.]

Seljer is correct, more than tens of milliamperes will be an issue with many voltage doublers.

And to address your other questions:
- You can get down to 12V with just resistors, but it's impractical to drive any significant load out of a resistive divider. The divider needs to be conducting about ten times the load current to keep the output voltage reasonably close to the desired one. Worse yet, the output voltage changes as the current drawn changes. Use of diodes won't help much.
- To do what you're thinking about, the thing that makes more sense is to drive the whole thing with 14-18Vdc all the time and then use an adjustable regulator to make the voltage change up to 12 or down to 9. The LM317 can do this nicely.
- Switching the power supply voltage probably causes big steps in the DC level of the output as it moves from half of one level to half of the next one. This comes out of the output as a pop or thump. You'll have to worry about how to mute it while you switch and until it settles.
- There may be other issues depending on the rest of the circuit.

Just out of curiousity, do you need to switch this on the fly, or could you try both ways, then only use the one you like better?

[mremic01]

I don't need to do it on the fly, I just want to have a 9v/12v switch on the amp if it's possible. I've only got 9v adapters, so I want to use one of those to power it, with an on-board option to increase the voltage for extra headroom without changing adapters or running off batteries.