Band-stop filter in U87 P48 (or MXL style) condenser mic? Any ideas?

[ln76d]

Hi guys,

do someone have any concept to apply band stop filter in U87 style circuit.
More options (i think) would be with MXL circuit (emitter folower + 2:1 transformer on the output).
I need to cut a little bit 500Hz area.
Any ideas?

 

[RuudNL]

How much do you need to cut (dB), and how 'wide' should the dip be?

 

[ln76d]

3 -10dB (depends on wideness and complications in building). Best would be 125Hz - 500Hz (f0) - 875Hz
Do you think, that simple RLC would be possible?

 

[ln76d]

RuudNL where are you?

Most important for me right now is the place in the circuit, where i could put the filter.
In deemphasis circuit part? I need  LPF too
Maybe after the fet in emitter follower section? Audix make here switchable HPF in CX112 for example.

 

[gyraf]

Wouldn't it make more sense to do this after the microphone - e.g. a passive filter in a M/F XLR adapter?

Jakob E.

 

[RuudNL]

I agree that it would be much easier to do the filtering later in the circuit, at a higher level and with much more control.
If you really would like to do the filtering in the microphone, my first thought would be a parallel L/C/R circuit in the drain source of the FET. This would need some experimentation I suppose. The L/C combination would define the 'dip' frequency, the R would control the Q of the dip.

 

[ln76d]

Wouldn't it make more sense to do this after the microphone - e.g. a passive filter in a M/F XLR adapter?

Jakob E.

Since this is not existing microphone, but project which i want to build from begining with my own PCB, why not go that way?
Using filtering in different place of the circuit can give the different results in sound. I could put filter in that way inside mike, but maybe before transformer could sound better? LPF in deemphasis circuit sound better than applying LPF after microphone.

 

[ln76d]

I agree that it would be much easier to do the filtering later in the circuit, at a higher level and with much more control.
If you really would like to do the filtering in the microphone, my first thought would be a parallel L/C/R circuit in the drain of the FET. This would need some experimentation I suppose. The L/C combination would define the 'dip' frequency, the R would control the Q of the dip.

My skills to recalculating circuit and knowledge of what additional changes it can do in the circuit  are little bit limited.
Usual am making simulations, to check the circuit, but right now my computer with licensed simulation software  is broken, i need a while to fix it. Parallel to drain? Hm...
There are two options in attachment, sorry if these are stupid, but without simulation it's only guessing for me.

 

[RuudNL]

My mistake! Drain should be source.
With a parallel resonant circuit the impedance in the source would be higher at the frequency the L/C circuit is tuned to.
That would mean less gain at this frequency.
The problem is that the frequency you need is rather low, meaning that a high L value is required.