The 9 KHz AM filter is for high fidelity audio when listening to AM broadcast with a nice wide range. You have to hear it to believe it!NOTE: Normally I have this filter in stock, but there are times when I may have a stock order being built for me and if this is the case, it can take up to two weeks before I can ship.These Curry "Longwave" filters are designed as direct replacements for the original mechanical filters which are now 50 years old and failing. The Longwave filters look almost identical to the originals but in use they are vastly superior, having steeper skirts and much lower insertion loss. this 9kc filter is made with cascaded Murata ceramic fiters installed in a stainless metal tube package, with a 9 pin header made from Brass and VERY expensive Glass bead insulators. They will fit just like the originals!Everyone who has purchased these filters is delighted with the way they perk up their 75A-4 receivers. Experts such as Howard Mills, W3HM and Ray Osterwald, N0DMS have endorsed the Longwave filters and have stated in theirreviews that they are superior to the originals. NOTE: I have these filters in stock and ready to ship!