Mic pre-amp

Hi all,

I'm currently designing a balanced mic preamp for a project. My two major constraints are
1) I work with 0-5VDC single supply.
2) I want phantom power (48VDC)

From the datasheets I've read, and from what I found on the Web, here is what I have so far.



Any comments, suggestions, about my design ?

I especially want to validate the following:

1) I plan on using the AD623 Inst. op-amp. Good ? Bad ? Ok ?

2) As shown in the diagram, the total gain of my design can vary from +20dB to +60dB. Is that enough ? Should I get it all from the inst. op-amp and forget about the extra gain stages of my codec ?

3) Is my DC coupling from 48V to 0V to 2.5V using 47uF caps ok ? Should I look into transformer coupling instead ?

4) Are the diode bridge and limiting 10Ω resistors ok to protect the op-amp from the 48V phantom power ?


Thanks a lot !
Any comment will be appreciated !

Th AD623 is far to noisy for a mic pre. Go for the INA217 or THAT1512. Unfortunately they require a higher rail voltage. I did a design using the Murata NDTD series DC-DC converter (5v to +/- 15V). You will need 45 to 65dB of gain. Have a look at the datasheets and app notes of the parts I mentioned for phantom power and input protection.
Using the DC-DC converter will also give you the phantom power as most condensor mics will work down to 12 V. E

Thanks for all the part suggestions. I've been looking at all that but that proves to be a challenge. I'm in total lack of space !! And the DC-DC converter is quite large (24 DIP). But I'll keep reworking the circuit.

But I'm still wondering if there are any decent instrumentation op-amps out there that would work in single supply and still have good noise characteristics ?

Thanks !

Sorry, I gave you the wrong part# for the DC to DC. It is VASD1-S5-D12(or 15)-SIP, .25"x.8"x.4". About $8 from Digikey. E

Thanks ! I was actually looking at those smaller part, indeed !

Did you see the other question I posted ? I was wondering about the quality of the output of such a converter for audio. Do they need special filtering/regulation of are the outputs "clean" and "quiet" enough to power the audio circuit directly ? Any idea on that ?

THanks !
TG

I use a .1 cer. and a 1u tantalum close to the +/- output pins and have no problems. A lot depends on the pc layout though. Good luck TG! E

Well, I'll give it a try !! Thanks again !! TG