Microphone Electronics Question?

[barefoot]

Is it ok to float the common pin negative relative to the shield in a phantom power scheme?

I want to build a portable battery powered mic preamp. I have 4 very powerful compact rechargeable 12V batteries. The simplified circuit layout is shown here:

The amplifier will run from +/- 24V supplies. Then, instead of having to add 2 more 12V batteries to reach 48V for the phantom power, I'd like to just pull pins 2 and 3 up to +24V and pull pin 1 down to -24V.

Will this work, or could floating the common harm the mic?

Thanks!

barefoot

http://barefootsound.com

 

[longsoughtfor]

Barefoot - I'm sure it is microphone dependent, but I thought the phantom power spec was something like 11-48V... Have you tried grounding pin 1 as normal but supplying +24 to both 2&3 ? You can pick the current limiting resistor to be something other then the usual 6.8K and still power the mic.

Kevin.

 

[barefoot]

Kevin,

Well, I checked out my Oktava MC012 and there is only one center pin in the capsule that electrically connects it to the preamp section. So the other connection must be the mic body. And since the body is ground to the capsule, it's more than likely ground to the preamp as well (didn't have my Ohmmeter handy). My circuit would then either bias the mic body -24V relative to my portable preamp case, or even worse, it might short the battery.

I still want to use the full 48V because that's what my Oktava calls for in it's spec. Plus, I think most good mics are made to work most optimally with 48V.

The solution then is to connect the phantom power in the normal way, and float the amplifier by capacitively coupling it's output. Another cap in the circuit is not ideal, but it does make the preamp more robust by protecting it's output - probably a good idea for a portable anyway.

Thanks,

barefoot