If you want to eliminate the "loading effect" that occurs - that causes the signal at the tachometer terminals - to be smaller that what you want it to be, you can "buffer" the tachometer output.  That means that you attach a circuit - a buffer amplifier - to the tachometer that will reproduce the signal whilst drawing no noticeable current.  That's shown below, with two buffer amplifier.  One that gets rid of the loading effect on the tachometer, and another that will eliminate any loading effect on the diode filter.

• You can use this circuit to eliminate the loading effects, BUT, nothing here will eliminate the effects of the diode threshold!!!

      Here is a link to a note on using the 741 Operational Amplifier as an Isolation Amplifier.  Note the drawing of the circuit board and the connections.  And, you may want the pinout for a 741.  That's shown below.