+/- 9V DC Power Supply
- LM741 is a terrible op-amp for audio. The NE5532 is something like 15 dB quieter, and still cheap. Notes on Audio Op-Amps
- You should be using rails as high as possible for best dynamic range. The mixers I work on are always ±15 V, because that's the highest voltage most op-amps are recommended for.
- Power supplies with split center-tapped outputs exist, though I can't find any right now. That's essentially the same thing as stacking two isolated supplies. Also, you could always build your own with a center-tapped transformer and learn how to build linear supplies. It's not too complex: High Quality Audio Mixer - Stage 3 - Power supplies
Assuming at least one of the power supplies is isolated (which will be true if they are good power supplies, but maybe not if they are cheap), you can make a stack of two supplies, so you have outputs at 0 V, 9 V and 18 V. Then, connect your op-amp circuit like this:
- Power supply 18 V -> 9 V op-amp pin
- Power supply 9 V -> 0 V op-amp pin
- Power supply 0 V -> -9 V op-amp pin
Another way to think about this is to understand that ground (0 V) is a relative choice; you can change your choice, as long as you're consistent within one circuit.
One other note-- I should explain what I mean by "isolated." A cheap power supply will have its ground pin connected to the neutral wire of its power cord. Most of the time, that's fine. But if you want to stack supplies, you need an isolated supply, i.e. one where no output pin is connected internally to the power cord.
If you do try to stack non-isolated supplies, you'll be connecting the positive output to the neutral wire, which is the same as shorting out the supply, so its internal fuse will blow. (If it's a really cheap supply, this will destroy it.)
One other solution
If you can't lay your hands on an isolated supply, you could use a switched capacitor voltage converter. This is a chip that charges up a capacitor, and then quickly shifts the pins so what was previously the positive lead is connected to ground, and the old ground becomes a negative output. It does this charge/switch routine at 10-100 kHz, so to humans, it looks like a negative supply.
One thing you can do when you aren't going to source or sink a lot of current into your 0V, and only have a single supply, is to use a 'virtual ground'.
For example, suppose you only have an 24V supply - two leads, supply and return. You can get -12V/0V/+12V by dedicating an op-amp to stabilize the half-way point and then use that as a 0V reference. In other words, the 24V supply becomes your +12V line, the 24V supply return becomes your -12V line, and the dedicated op-amp produces the 0V line. All you do is put a 50% voltage divider across the 24V to give you the half-way point, and then buffer it with a voltage follower configuration.
Basically your op-amps are powered from the +V/-V rails anyway, the problem is not having a ground. The half-way point voltage follower effectively 'regulates' the midpoint for you. The limitation is that you can't source or sink more current into the virtual ground than the op-amp delivering it can handle, or you lose the reference.