MFJ-1026 mods

December 28, 2025 - Reading time: 2 minutes

The MFJ-1025/1026 noise cancellers use a bridge style phasing network on one input only (the aux input). Here's a Qucs simulation of the 'low freq' setting, sweeping the phase control logarithmically from 1R to 1k:

Notice the magnitude is flat over the full range, which requires the difference of the two voltages to be taken. In practice, there is a balanced to unbalanced conversion done in T1 (see circuit below), which achieves the same thing as the VCVS in the simulation.

Unfortunately, you will also see that a full 180 degrees of phase shift isn't obtained, so despite the phase inversion facility we can't rotate the full 360 degrees. At 3.5MHz we only get about 2x168 = 336 degrees. More expensive noise cancellers use this bridge circuit on both inputs and they are operated differentially with one control. Occasionally you might find that the MFJ-1026 cannot quite get to the optimum setting for this reason, but there is a simple solution: provide a facility to swap the inputs. I've done this using a DPDT toggle switch, as shown in the modified schematic below:

You might be able to spot the reinstatement of two other tracks that were cut. Initially I had implemented the swap prior to the gain control pots. This is a simpler mod, but it's more intuitive for the gain control to be always associated with the same source. This is the front panel, which I think looks OK:

It works as expected, but maybe someday I'll build my own tandem bride noise canceller!