What are these inductors doing here?

[pucho812]

Here is the output of a microphone.

It’s basically a Jensen di transformer which is a 12:1 what is the purpose of the inductors after the transformer?

 

[Tubetec]

Im only guessing because theres no schematic ,
equalisation ?

 

[pucho812]

Im only guessing because theres no schematic ,
equalisation ?

Opps. Forgot to attach it.
I just updated the op and attached it.
Inductors are in series between the transformer secondaries and xlr connector.

 

[Tubetec]

LPF ?

 

[Murdock]

LPF to filter out RF? Cut off frequency seems to be 80KHz, right?

 

[pucho812]

LPF to filter out RF? Cut off frequency seems to be 80KHz, right?

Interesting.

 

[abbey road d enfer]

EMI/RFI protection. Somewhat weird unless there is particularly tricky circuitry in the mic.
It prevents EMI/RFI produced IN the mic to be transmitted to the world.
This type of protection is usually fitted at the input of the mic pre, where it filters interference transmitted to the cable.

 

[Matt Syson]

As Abbey Road says this sort of filter is often (or should be) included on micpreamp inputs where it deals with HF interference (where the notion of 'balanced input' breaks down) so LPF filtering is necessary to stop demodulation. Whether the inductors shown are really 'inductance only' or have ant resistive damping as well could be interesting as they might give a slight boost to HF before the capacitors quash that effect. demodulation occurring at the OUTPUT of an amplifuier is a 'thing' as the negative feedback loop of an amplifier is not necessarily wideband. I have had a radiio station breakthrough because the speaker cables picked up interference and the power amplifier output stage simply retified (demodulated) it!

 

[JohnRoberts]

Radio station interference caused by too slow active circuits is often called "rectification".

One effective solution is passive LPF on inputs (and outputs). I have seen RF come in through speaker wires in high RF environments. Many power amps use passive output filters.

JR

 

[abbey road d enfer]

Whether the inductors shown are really 'inductance only' or have ant resistive damping as well could be interesting as they might give a slight boost

If you take into account a source impedance of 200 ohms, the response is petty much B'worth.
Now, in practice, the actual source impedance of a mic is anyone's guess.

 

[aurt]

LPF to filter out RF? Cut off frequency seems to be 80KHz, right?

Would the inductance of the secondary come into play?

 

[Philbo King]

Possibly a common mode choke

 

[abbey road d enfer]

Would the inductance of the secondary come into play?

Yes. As well as the reflected impedance of the capsule, which dominates.

 

[abbey road d enfer]

Possibly a common mode choke

It could be. However, the way it's drawn, it appears more like two separate inductors.
A properly drawn schemo would represent a CM choke as two windings on a common core.
How faithful the drawing is is open to conjecture.
Anyway, what would be the justification for a CM choke here? As much as it makes sense at the mic pre input, at the output of a mic it is questionable. Unless, as I hinted previously, there is something in the mic that generates or is sensitive to EMI/RFI, which I doubt.

 

[Tubetec]

Wouldnt two differential mode chokes placed in very close proximity cause an interaction between the two coils and provide some cancellation of common mode noise ?
a bit like a humbucking pickup.
I found theres a possibility of unwanted interaction between any coiled components ,even screened ones , depending on physical orientation and spacing .

 

[abbey road d enfer]

Wouldnt two differential mode chokes placed in very close proximity cause an interaction between the two coils and provide some cancellation of common mode noise ?

Yes, but not as efficiently as a real CM choke. You an't except more that 20dB CMRR with separate coils. Proper CMRR relies on very strong coupling coefficient.

 

[Tubetec]

Does the type of inductor , com or dif mode , have any effect on the equalisation curve ?

 

[abbey road d enfer]

Does the type of inductor , com or dif mode , have any effect on the equalisation curve ?

They shouldn't really, as long a they are properly sized for the actual source and load impedances.
Assuming the mic in post #1 is 150r nominal, the filter introduces less than 0.2dB variation at 20kHz.

 

[Walter66]

Exactly this kind of noise supression filter is used to suppress the AC line noise in front of the power transformer. One can buy it in capsules including the AC connector and a fuse. I use it, too.

 

[abbey road d enfer]

Exactly this kind of noise supression filter is used to suppress the AC line noise in front of the power transformer. One can buy it in capsules including the AC connector and a fuse. I use it, too.

Noise suppressors use a CM choke (with very high coupling between sections). The filter in the mic is not clear as it's drawn.