Crane Song Insigna tube equalizer Insigna is a tube equalizer in a 500 Series package, using a dual triode circuit with a 12AX7 tube It has three bands and filters. The EQ has 24 db/oct high pass and low pass filters each with seven frequencies. The top and bottom band are shelving each with eight frequencies. The mid band is peaking with eight frequencies. The three bands work by using buffered RC circuits in the negative feedback path around the tube amplifiers. The result is a fat sounding EQ rich in second harmonic triode tube color.