You'll only need a divider that big for setting gains on amplifiers, when you have to drive the amp to full output. You can leave this connected to the amp's output all day with no problems.
The current through this circuit at 30V would be about. The values I've suggested are standard values and should be pretty easy to find. Why a 10k resistor? Because we want the impedance of this circuit to be high so that it doesn't affect the rest of the circuit by drawing a bunch of current and so that we can use 1/4-watt resistors. If you want to test a device with a higher voltage, make R2 smaller, but try to leave the 10k resistor alone. If your sound card can accept up to 1V (this is the case with the creative card I use), then this divider will allow you to safely test devices that output up to 30V-that's 225 watts at 4 ohms. So, in the diagram above, about 1/30 of the applied voltage is dropped across the 330 ohm resistor and about 29/30ths of the voltage is dropped across the 10k ohm resistor. Kirchoff's Voltage Law states (I'm paraphrasing) that the sum of the voltage drops across resistors in a series circuit are equal to the source voltage. I don't know the input voltage of the mobilepre so that's something that should be addressed before folks go plugging their cable to the line in on the preamp, and especially laptop soundcards. This preamp has what I call a gain on it, so I just watched that to make sure the light didn't turn read (clipping). Someone in another thread called to attention the input voltage of the preamp and that doing this could blow the preamp. If anyone has ANY comments on something I may not be doing right here, PLEASE let me know and speak up! This passed the sanity test but it still doesn't mean I'm exactly right. Just make sure to get the kind with the ring (top connector in diagram at the beginning of this post). My radio shack had a ton of these when I was there earlier.
The ONLY difference you should have is that you'd use a 1/8" TRS type patch cord instead of the 1/4" like I used. **For those who do not have the m-audio mobile pre, and are instead using laptop soundcards, I think you can still just as easily do this same test. Against my better judgment I left them at the halfway point, but will definitely drop them down, now. I noticed that lower than that things sounded better.
When I had the gains to about the halfway point (straight up and down on the dial) things sounded compressed.
Yesterday I did a lot of tinkering with the gains.