Subwoofer amplifier

Subwoofer amplifier

The main properties of this subwoofer amplifier are:
  • Hypex UCD400 class D output stage
  • 2x56V power supply with 6x 6800uF buffer
  • Differential audio input
  • 4th order high-cut filter
  • 2nd order low-cut (subsonic) filter
  • 3-channel parametric equalizer



The main job for the filter is to limit the bandwidth sent to the power amplifier. It consists of a differential input amplifier, a 24db/octave low-pass filter at 160Hz, a 12db/octave high-pass filter at 20Hz and a phase shifter.

The input amplifier converts the differential input signal into a single ended signal. Since the signal levels at the subwoofer output of a surround reveiver are relatively small and the distance between the amplifier and the receiver can be high, differential signal transmission is a way better way to transmit the audio signal than a single-ended transmission.

The low-pass filter removes any frequencies above 160Hz. This frequency is chosen higher than the standard 80Hz crossover frequency, since many surround recivers already incorporate a crossover filter. If the -3db frequency of the filter would have been chosen at 80Hz, the output level at 80Hz may be too low.

The high-pass filter removes any frequencies below 20Hz. Frequencies below this frequencie are not reproduced by the subwoofer anyway and may even damage the subwoofer by excessive excursion. The strange effect of this relatively high frequency is that the subwoofer seems to sound much deeper than with a cut-off frequency at 5Hz.

The phase shifter lets the user invert the phase of the subwoofer.

Schematics and PCB

Input + main filter board (schematic + PCB)
Parametric equalizer board (schematic + PCB)
Main (UCD) supply board (schematic + PCB)
Auxilary (filters) supply board (schematic + PCB)