Sub-bass Processor

  0. Introduction

The Sub-bass Processor was designed by Rob Evans and was published in the September issue of Electroincs Australia magazine. Details and fullsize PCB artwork were provided with the issue, but it would be best to purchase a kit from Dick Smith Electronics or Jaycar Electronics.

1. Schematic Diagram


High-pass (subsonic) filter:

- 20Hz cutoff
- 18dB/octave rolloff slope

Low-pass (subwoofer) filter:

- 40Hz to 200Hz cutoff (variable)
-12dB/octave rolloff slope

Lower frequency limit:

- DC (with HP-Filter switched out)


- +/- 12dB (variable)


- +/- 12dB boost/cut (variable)
- 30Hz to 190Hz centre frequency (variable)
- Q of 5 (set by resistors)

Distortion and noise:

- 0.03% THD, -92dB unweighted (ref 1V)

Here is the schematic diagram; taken from the assembly manual of the kit. The circuit can be used with a speaker-level or line-level signal. Moreover, an extra out-of-phase output is available.

2. Simulations

Circuit diagram used with Circuit Maker 6 (student version)

Frequency response with all controls centered.

Output level is at zero gain when all controls are centered while cutoff frequency is around 64Hz. Output is -6dB at 83Hz.

3. Pictures


The kit from both Dick Smith Electronics and Jaycar Electronics includes LM833 opamps. I found this particular "opamp's" midbass response to be noticeably thin! I have a preamp kit which uses the same opamps and found the same lack of midbass bite.

I have changed one of the LM833 opamp to a TL072 (or LF353) and now every note is back to normal. I left the inverting LM833 opamp in the PCB since I don't need the out-of-phase output anyway. I will try NE5532 opamps soon and will comment on the resultant sound.