phatom power from +24V, -24V

[gusfinley]

Hi,

I'm building a mic preamp with +12V and -12V on the rails of the opamp. How can I convert this voltage difference to 24V phantom power?

I know it has to do with how you hook the resistors (usually 6.81K) up... Do i just send +12V through on resistor and -12V through the other? without connecting them you would with a single supply?

I saw a schematic somewhere on the net but i just can;t seem to find it now!!

 

[dgatwood]

First, it's 48V phantom, not 24. 24V might run some mics, but it won't be reliable for a large percentage of modern mics.

Second, to step 12VDC up to 48VDC requires a DC-DC inverter board (which consists of an oscillator (ideally high frequency), either a transformer or voltage doubler/tripler/* rectifier, a transformer to step the voltage up, and filter caps to clean up the resulting voltage). Either way, you can't do it with a passive circuit, and it's not a very good idea to use a DC-DC inverter anywhere in the general vicinity of audio gear because unless you use a very high frequency inverter (harder to design), you're likely to end up with audio interference as a result.

My advice is to build a 48VDC supply circuit into your power supply design instead so that you can start with AC. For a fairly simple 48V power supply schematic from +/- 15VAC, look here:

http://sound.westhost.com/project96.htm

That's probably your best bet. Presumably your power supply is providing about 15VAC at some stage of the game.

If you really insist on trying to go the DC-DC route (which I would strongly discourage in something as noise-sensitive as a mic preamp), for 12V to 48V, you'd need something more like this:

http://silenceisdefeat.org/~lgtngst...p_Inverter/12_to_24v_DC_Step_Up_Inverter.html

...except with different turn counts on the transformer to get 48V instead.