linkwitz wm-61a mic + preamp picking up country music station

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I built the mic and preamp in the above link and followed the design 100%. It is picking up interference from a country music station. The interference is loud enough that I'm able to follow the lyrics when I have enough gain to amplify quiet music in the room to -3db.

I'm using it to apply digital room correction so having extraneous music signals will throw my calibration off.

I can think of a number of possible causes but I'm not sure which is the right one if any.

1. Mic Cable is acting as an antenna because it's the wrong type or because I left the wires parallel and haven't twisted them.
2. Lack of enclosure to shield from RF.
3. Parasitic capacitance/inductance from the layout I used.
4. Other.

I'm looking for suggestions on how to fix this. Thanks.





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Look at the Linkwitz schematic again.

It specifies less than 30 feet of co-ax with less than 50pF/ft.

Look at the photograph of the assembled circuit.

The circuit is enclosed in a diecast aluminium enclosure. The entry and exit points to the enclosure are an SMA connector and a BNC connector which both have an integral ground connection to the diecast box.

Now look at your circuit...

Make yours look like his and it will probably work.


I'm not joking
I'm sure you can get the amp by itself to perform well without an enclosure. You may have to dress the switch leads and battery wires by twisting and keeping lengths to a minimum.
The microphone if it's not modified for higher SPL's will need extra attention to the circuit and supply differences to related your input circuit. Perhaps mount the mic capsule in a small dia. metal tube using the tube for the outer shield to the output connector, Otherwise rethink your whole packaging concept.
I didn't realize that there is a schematic symbol for coaxial cable. I also referenced this site in my build where a guy recommended to use "kynar wire" from radio shack. The wire I used is probably about the same. It is alpha corp 18 awg stranded. I tried twisting the wires (gave myself awful blisters) for the whole length which is around 11ft and it may have decreased the interference a tiny amount but it's still there. I guess, I'll try some of the other suggestions now.
Your cable should use a twisted pair for the signal (to cancel magnetic induction) with an overall screen (to protect from electric fields). You need to think about a good solid RF ground from input to output, best achieved by putting it in a metal box. You may need RF filters on the input and/or output. RF can also come in via the power supply.

A bare bit of board with a few wires hanging off it is guaranteed to pick up RF, as you now know.
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