How do I get 15V from a 34V power supply?
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 &lt;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:



Specification:
Input voltage:4.5-40V
Output voltage:1.5-35V(Adjustable)
Output current:Rated current is 2A,maximum 3A(Additional heatsink is required)
Dimension(L x W x H): 43 x 20 x 12mm
Weight: 11.6g

Package Included:
1 x LM2596 adjustable power module


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.