help with mic preamp

[MrNudge]

I have a condensing microphone that requires the normal 48v phantom power, and a i want to make a preamp for it that unbalances the signal so i can use it with my pc.

I've pieced together what i know and come up with the following schematic.
Just want to know if there are any glaring issues with it, and also a basic way to drop 48v to 12v.

any help would be great!
thanks guys.

[stratus46]

Originally Posted by MrNudge

I have a condensing microphone that requires the normal 48v phantom power, and a i want to make a preamp for it that unbalances the signal so i can use it with my pc.

I've pieced together what i know and come up with the following schematic.
Just want to know if there are any glaring issues with it, and also a basic way to drop 48v to 12v.

any help would be great!
thanks guys.

Not a good idea on several fronts. You really don't want to have a volume control at the milliVolt level particularly followed by a variable gain amplifier. Turn down the volume and turn up the gain and the level may be OK but the noise could be considerably higher than it should be. I suspect you're doing that because of the very low power supply voltage you're running. You'd have better results with a higher Voltage on the opamp power rails. When you're working with 'raw' audio you have very little idea what kind of levels you're going to get in contrast with dealing with commercial recordings where somebody else got rid of the large signal changes. You need a lot more headroom with unprocessed audio so higher power rails are an advantage.

Having done something similar (but a long time ago) I found that with AKG and Sony condenser mics, a preamp gain of 40-50 dB (I use 46) will cover nearly all the bases with very good results. The only time it was too much was in a small room with a brass band. Dang that was LOUD.

You're not showing any protection diodes for when the phantom power is turned on. That 48 Volt surge into your opamps may very likely kill the inputs. Also you want to add some RFI filtering on the inputs. I was recording in a church near O'Hare in Chicago and while my stuff was fine, another guy was there and he was picking up the control tower. You don't want things like that.

As for your power supply you'd have better luck with 3 supplies - dual transformers or one with dual secondaries. They all need to be well regulated. On my preamp I'm running +/- 15 to the opamps along with the 48.

Good luck

G²

[samsagaz]

someone know where can i found an review of opamps to see which to choice? i ordered a few that 1570, but want to test different ones to see wich will be works better, i wwant to build an pre amp with 2 fixed values 20db 40db 60db.