Using a 24v SMPS for a 12v board?

[clawmd]

I have an amplifier and a preamp in the same enclosure that I want to power with a 27v SMPS. The preamp requires 12v and draws very little current, 1 amp max. The amplifier needs 27v. I suspect I can use a simple circuit with a voltage regulator, but designing it exceeds my capabilities

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1 amp max seems a little high to me, are you sure it's 1A? Many are under 100mA. I ask because if it's much lower you can use some smaller regulators and easier heatsinking.

Anyway a common, easy way to do it is take an LM7812 (fixed 12V) or LM317 adjustable regulator (lower noise than LM7812), you simply follow the respective datasheet schematic with the input being the output from your 27V SMPS, common ground, and output going to the preamp. It would look something like the "typical applications" circuit on the first page of the datasheet, except most people would use a larger capacitor in parallel with the 0.1uF C1, input capacitor and the 1uF, C2 output capacitor. For example a 10uF tantalum or 100uF electrolytic capacitor in parallel in the circuit at both locations.

LM317 datasheet: http://www.national.com/ds/LM/LM117.pdf

Voltage drop (output - input voltage) x current the preamp uses = heat
If heat is around 1W or higher, put a heatsink on the regulator. How much heat determines how big a heatsink to use. You can buy ready-made regulator boards at some electronics hobbyist type sites or on ebay (but your profile doesn't show your global location). Here are some examples:

LM317 | eBay

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Oh, since the input is from a SMPS, you may find putting ferrite beads on the input and output pins of the regulator reduces HF noise.