I have an audio amplifier with a regulated power supply putting out around 34V DC. I would like to add a preamp which requires on 15V. How would I draw 15V from the main power supply?
How to get 15V from a 34V power supply
amplifieraudiopower supply
Related Solutions
Your circuit using 30k to -15 will work just fine, as long as you keep in mind the output limits. Depending on the op amp and the output load, an amp using +/- 15 volts will only be able to drive its output to somewhere in the range of +/- 10 volts to +/- 15 volts. Look up "rail to rail op amp".
So, if you're using an op amp with a +/- 10 volt output range, it will only respond to inputs in the range of -1 to +0.5 volts. For an op amp with +/- 15 volt capability, your input must be limited to the range -1.5 to +1.0 volts.
If you ever get to building circuits for real, this is not a good idea. Power supply levels are not usually held to high precision, and the voltage can vary with the loads being driven. For theoretical exercises like this it's OK, but in the real world can get you into all sorts of trouble.
If you can find a 10VAC mains powered transformer, then you can make a simple power supply like this:
simulate this circuit – Schematic created using CircuitLab
Failing that, two 12VDC-output switching adapters will do a fine job, at the cost of some convenience (two plugs have to be put in the wall). Put a diode (eg. 1N4004) across the output of each adapter (reverse biased) so that if just one is plugged in the other one does not see much reverse voltage at the output.
The power supply voltage for your amplifier is not critical, a bit lower does no harm other than lowering the maximum power output (for a given speaker) a bit.
Laptop supplies may not work because some of them have the minus lead common with earth, so connecting them as required will short the outputs.
Best Answer
If as I'm guessing your preamp needs a reasonable amount of current then a step down switching regulator will be the best solution (if I'm wrong and it's <100mA or so then a standard e.g. 7815 as Olin mentions would do okay)
The reason a switcher is better in this scenario is the large input/output voltage difference. The linear regulator simply "burns off" the excess voltage whereas the switcher does not. To give an example, let's say your preamp uses 500mA:
With a 7815 you have: (34V - 15V) * 0.5 = 9.5W dissipation.
This is a LOT of heat to get rid of, more than the load itself is drawing.
With a 90% efficient switcher, you get:
(7.5W / 0.9) - 7.5W = 0.83W. (the 7.5W comes from 15V * 0.5A, the 0.9 is the 90% efficiency)
This is clearly far more efficient.
Switching regulators are a little more complex and need a few more external components than a linear regulator, so depending on your experience/confidence level you can either build a circuit from scratch or purchase a ready built module.
If this is a one off and you are not looking to learn anything by building your own, then something like this from eBay is hard to beat:
If you want to look at designing your own, then the datasheets and app notes of ICs like the above LM2596 are a good place to start reading.