Mic pre buzz (Magnetic field?)

[ForthMonkey]

Hi,

I designed mic pre based API312.Added some features...For now i built with e.balanced input and impedance balanced output.Today tried DI input.I can get sound but it's very noisy.Tried get rid of noise.Then i found it's about magnetic field.When i put board vertical,noise going away but not OK yet.When put my hand on the board,noise going away but not enough.It has no case right now.I believe it causes noise.And noise not hum.Just buzzing.Like unshielded guitar.When i put this into case,will noise go away fully?

Thanks.

 

[ruffrecords]

Hi impedance inputs like DI usually are sensitive to electrostatic interference. Proper grounding ans screening will usually fix it.

Cheers

Ian

 

[ForthMonkey]

Hi impedance inputs like DI usually are sensitive to electrostatic interference. Proper grounding ans screening will usually fix it.

Cheers

Ian

Thanks for reply.Can you explain "proper grounding"?

And about shielding,for now i don't use XLR pin 1 cause preamp is nude right now.Has no chassis.But when i put it into case,i will use shielded cable for input and output XLR.Hot to pin 2,cold to pin3 and chassis to pin 1.Right?Is it some kind of shielding?

 

[My3gger]

Ian already wrote nice paper about grounding, it is attached. Unfortunately i don't have his writing about phantom grounding in easy to read format.

 

[My3gger]

Another one, goes into more details and complements Ian's.

 

[ForthMonkey]

Thanks for files My3gger.And thanks to ruffrecords for making awesome things!

It's my first mic pre project.And first time i'm getting noise like this.Other eqs and compressors working good and no noise.

Now first of all i need case to get rid of noise because of magnetic field.Am i right?

And my power supply has one ground for +/-18V and +48V.And PSU's ground(0V) going to signal ground.I will left them as it.

Then i will use screened cable for in and out XLRs.Screen will go to pin 1.And screen will go to chassis.

Earth bond of power inlet will go to chassis to.(Somewhere near to inlet.)

Let me know if i'm wrong...

 

[ForthMonkey]

And another thing about my premap,i'm gettin too much high freqs.Sounds like added lots of air freq.Is it good thing?What about changing C2 with bigger values to cut some highs?

http://i.hizliresim.com/a075od.png

You can hear sound samples.

It's direct input of soundcard.

https://dl.dropboxusercontent.com/u/91809016/CHEAPRE/TRI-CAPTURE.wav

And it's mic pre.

https://dl.dropboxusercontent.com/u/91809016/CHEAPRE/312-HALF%20GAIN.wav