Topic: DPA 4060's acting as radio antenna?

[tooldvn]

I have a friend that has been using her DPA 4060's to record her piano sessions at home.  She has been picking up a local radio station that broadcasts at 100.7 on some of her recordings...  what can be done about this?  Can I add additional shielding (some short of extra sheath?) to the cables to prevent this?  Anyone even hear of this before?

Thanks
-dvn

[SparkE!]

Bypass the signal path with a ceramic 100 pF chip capacitor of the NPO variety and whose series resonant frequency is above the frequency of interest.  Most small 100 pF chip caps will be of the NPO variety and will have an a sufficiently high series resonant frequency.  Essentially, this produces an RF short to ground from the signal path.  If you look inside a label mic of the type that is used for VHF portable radios, you'll usually see a 100 pF cap right across the electret mic element itself, at least if it's a good mic.  The NPO rating means that the capacitance is constant over temperature, but that's not the reason for specifying NPO dielectric material.  The reason that you want a NPO cap is that NPO dielectric materials tend to have a very low series resistance so the capacitors have a very high Q.  COG dielectric is also a good choice.  You can pick these up from DigiKey.com.  I've found that the 0603 size is large enough to handle with tweezers and yet they are small enough that they aren't inductive at VHF frequencies.