[R-390] Filter test response

[quartz55]

Well, I got a response, I only checked it every 0.2 KHz, it appears to be skewed to the high side, but the BW is about right. The IL I measure around 23db, but it may be more than that given my setup.  Here's a chart. I only get about 40 db of range on my setup before things start feeding through.

http://i251.photobucket.com/albums/gg287/DogTi/R390A/filterresponse_zps6dc20bb8.jpg