How to guess the value of this filtering inductance?

[LastStarDust]

Hello! I am in the process of reverse engineering a Klipsch ProMedia Bluetooth 2.1 computer speaker system.

To expose some tracks on the control pod PCB, I had to remove a blob of epoxy glue. While removing the glue I also removed what I think was an inductor under the glue.

Eventually, I was able to reverse engineer the circuit and now I would like to replace the inductor. However, I have no idea what kind of inductor to purchase.

As you can see from the photos the inductor is very small and is damaged, so little hope to measure the inductance directly.

My questions are: what is the purpose of the L1 inductor? By looking at the schematics can you guess what is the original value of the inductance L1?

The inductance dimensions are about 3mm x 1.5mm x 1.5mm.
The LEFT_IN and RIGHT_IN signals come directly from the jack input. The WIRED_LEFT and WIRED_RIGHT signals go on to the input of an OPAMP amplifier.

Here are part of the schematics and some photos of the inductor before and after the glue removal. If you need further info please let me know.

 

[Mark Tillotson]

Can't see the images.


Small inductors (or ferrite beads) are often used for stopping RF and digital hash from getting into the circuit.

 

[LastStarDust]

I have fixed the image links. I think that the choke is used for stopping RF too. In such a case should I use a differential choke or common mode choke? Which impedance and current rating do you suggest?

 

[abraxalito]

Its a common-mode choke. The precise impedance won't matter. Try one like this : DLW31SN222SQ2(DLW31SN222SQ2B,DLW31SN222SQ2L)|Common Mode Choke Coils/Common Mode Noise Filters|Noise Suppression Products/EMI Suppression Filters|Murata Manufacturing Co., Ltd.

 

[LastStarDust]

@abraxalito thank you very much for even providing a link to a choke with the correct 1206 package! Very appreciated.

 

[abraxalito]

You're welcome. It was easy for me to find that 1206 CMchoke as I've used them myself and have a reel somewhere in my collection....

 

[LastStarDust]

The last minor question would be: why do you think they put the glue on top of that choke? To avoid mechanical stress?

 

[abraxalito]

I can only guess - the socket next to it looks rather too close. Perhaps there was a chance that jiggling that socket could touch the choke. Rather than re-spin the PCB they decided to use goo to stabilize it.

 

[LastStarDust]

Probably it is like you say. Thank you again.

 

[Mark Tillotson]

The last minor question would be: why do you think they put the glue on top of that choke? To avoid mechanical stress?

Possibly to stop it singing.

 

[Fast Eddie D]

Common mode impedance @ 100 mHz is specified (2200 ohms 25% tolerance). You can calculate the inductance from that.

 

[LastStarDust]

Just to confirm. Assuming an ideal inductor the impedance would be Z = 2 pi f L, where f is the frequency and L is the inductance. So by plugging in your values (assuming 100mHz is a typo for 100MHz), the inductance would be 3.5uH, right?

 

[abraxalito]

I got the same value. Seems about right.

 

[Fast Eddie D]

(assuming 100mHz is a typo for 100 MHz)

That would be quite a large inductor.

 

[LastStarDust]

I have both a good iron and a good hot air station. What method do you recommend for soldering the new choke?

 

[Vovk Z]

Its a common-mode choke. The precise impedance won't matter.

+1. Any of that size should do the job.

 

[LastStarDust]

I soldered them with the iron. Boy it was difficult with all the other components around.