I am interested about common mode chokes at the input of a microphone preamplifier. ( Not interested here about CM chokes at a Power Supply input, nor general theory about CM chokes ).
What caracteristics should I look for ?
Sources, models ?
Application notes about common mode chokes at microphone preamplifier input ?
What caracteristics should I look for ?
Sources, models ?
Application notes about common mode chokes at microphone preamplifier input ?
Last edited:
So the questions is: Why one might want a common mode choke in that position? Is the connection balanced? If so, I imagine the only thing you might want the choke to reject would be frequencies that are outside the CMRR range - i.e. RF.
That being the case, you need to start thinking in RF terms in choosing the ferrite and the windings.
There is a vast amount of info on the web written by RF amateurs and professionals on this subject.
If we're talking "unbalanced" it's a whole different can of worms..
That being the case, you need to start thinking in RF terms in choosing the ferrite and the windings.
There is a vast amount of info on the web written by RF amateurs and professionals on this subject.
If we're talking "unbalanced" it's a whole different can of worms..
I am talking balanced.
I am implementing a double balanced preamp as from Graeme Cohen:
http://leonaudio.biz/double.balanced.mic.amp.notes.pdf
Another implementation from Samuel Groner: Analog Circuit Design · Samuel Groner · Resources · Microphone Preamplifiers. See:
Monte Generoso
http://www.nanovolt.ch/resources/microphone_preamplifiers/pdf/monte_generoso_r1.pdf
I am implementing a double balanced preamp as from Graeme Cohen:
http://leonaudio.biz/double.balanced.mic.amp.notes.pdf
Another implementation from Samuel Groner: Analog Circuit Design · Samuel Groner · Resources · Microphone Preamplifiers. See:
Monte Generoso
http://www.nanovolt.ch/resources/microphone_preamplifiers/pdf/monte_generoso_r1.pdf
In order to engineer a solution, you first have to identify the problem. What problem do you have?
The information on the Monte Generoso seems sufficient. The value is small, and very un-critical. So small that you can't find a standard part with enough DCR to be an issue. Or so big that stray capacitance makes trouble.
So I should only care about the 2 x 3.3mH.
What is DCR ?
Common mode rejection is not an easy engeering topic here, because of too many unknowns: RF environment ? microphone impedance value and balance ? câble lenght, caracteristics. A microphone preamplifier must cope with all those unknowns.
Millions of mike preamps have worked fine without that network.
Floating mikes on decently twisted cable have huge CMRR. Telephone systems working next to 10,000 Volt 50/60Hz power lines mostly worked well despite thousands of times as much potential interference.
I think it comes in partly because of increased exposure to WiFi and cellphones, and also because the coil has now become a 3-buck part.
The 3.3mH 1nFd network does about nothing below 87KHz. It is strictly an RF filter. In RF-benign spaces (like your studio should be) it would not be needed.
Floating mikes on decently twisted cable have huge CMRR. Telephone systems working next to 10,000 Volt 50/60Hz power lines mostly worked well despite thousands of times as much potential interference.
I think it comes in partly because of increased exposure to WiFi and cellphones, and also because the coil has now become a 3-buck part.
The 3.3mH 1nFd network does about nothing below 87KHz. It is strictly an RF filter. In RF-benign spaces (like your studio should be) it would not be needed.
- Status
- Not open for further replies.