I have a CW filter from New England QRP the NESCAF switched capacitor audio filter. Mine works great with the uBITX and others have had good success as well.? It lets you very both center frequency and the bandwidth.? Mine will go from a bandwidth of about 1.5 kHz down to about 80 Hz. In many cases it helps eliminate some of the higher frequency audio noise for SSB operation.? ??
