If I understood the situation, the keyer outputs a 3V square wave that goes into the filter. Trying to understand how much do you need to attenuate the signal to accomodate it to the filter, I looked at
the filter schematic. At first sight it's a typical, single supply, three stage low pass Sallen-Key, then one high-pass stage, followed by an LM386 power amplifier. But there is something I don't understand about the schematic, the bias after the capacitor coupled stages (the first and last). Do these stages have the common mode set at the upper rail? There are two components, a resistor and a zener, that seemed to set the common mode at 3.9V, but they were removed. Why not use a couple of resistors?
According to the datasheet, the TLC274 the common mode input voltage range doesn't reach the top rail. Is this schematic right?