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
Specifications
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High-pass (subsonic) filter:
- 20Hz cutoff
- 18dB/octave rolloff slope
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Low-pass (subwoofer) filter:
- 40Hz to 200Hz cutoff (variable)
-12dB/octave rolloff slope
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Lower frequency limit:
- DC (with HP-Filter switched out)
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Gain:
- +/- 12dB (variable)
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Equalizer:
- +/- 12dB boost/cut (variable)
- 30Hz to 190Hz centre frequency (variable)
- Q of 5 (set by resistors)
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Distortion and noise:
- 0.03% THD, -92dB unweighted (ref 1V)
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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
4. Comments
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.
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