The previous (and only) subwoofer controller described in SILICON CHIP was
featured in the December 1995 issue.
Since then we have had quite a lot of input from readers and
this completely new design is our response to our readers’ comprehensive wish
Adding a subwoofer to a home theatre or hifi system can achieve
a dramatic improvement in listening enjoyment, by extending the response of the
system down into the low bass frequencies. But this improvement can only be
fully realised if various basic conditions are met:
1. The crossover between your main system speakers and the
subwoofer is smooth, with no obvious peak or dip in overall frequency response
during the transition; otherwise the system will sound either boomy or weak in
2. The subwoofer level is correctly balanced or matched with
the level from the rest of the system speakers.
3. The response of the subwoofer itself is smooth (ie, without
pronounced peaks or dips) over its operating frequency range.
4. Very low (subsonic) frequencies are prevented from reaching
the subwoofer, as these can cause its cone to ‘flap around’ – which can cause
unwanted noises and possible damage to the subwoofer.
The Subwoofer Controller unit we’re describing here caters for
all these conditions. It provides:
A convenient adjustment of subwoofer upper frequency rolloff,
so you can achieve the smoothest possible crossover transition.
Easy adjustment of subwoofer level, for optimum overall tonal
A parametric equaliser circuit which allows you to compensate
for any response peaks or dips which the subwoofer may have in its operating
range, to achieve a smoother response.
There’s also a built-in subsonic high pass filter, which rolls
off the response steeply below 15Hz to protect the subwoofer from
Three signal sources
Subwoofer Controller Specifications
Input impedance: Line level and LFE signal inputs 47kΩ
Speaker line inputs 10kΩ with an 11:1
to +8dB, variable
Low pass filter: Corner frequency variable between 41Hz and 200Hz
12dB/octave rolloff slope
Parametric equaliser: Centre frequency variable between 30Hz
Cut/boost variable between ±12dB at centre frequency
Q approximately 5
High pass (subsonic) filter: Corner frequency 15Hz, rolloff slope
Signal to noise ratio: -80dB unweighted relative to 1V RMS
2V RMS output
Maximum output signal: 2.4V RMS
Output impedance: 1kΩ (both outputs)
Amplifier hold-on time: Approx 11 minutes after end of
Power requirements:. Operates from 12V DC, from a battery or regulated plugpack supply.
Current consumption 45mA in standby mode, less than 60mA in active mode.
In addition, there is the ability to select between three
possible sources for the subwoofer signal: line level outputs from your main
amplifier; speaker level outputs or the ‘LFE’ (low frequency effects) channel
output from your DVD player or surround sound decoder; finally there are normal
and inverted subwoofer output signals, so you can easily use a stereo amp to
drive the subwoofer in bridge mode.
It also has an auto turn-on circuit to switch on your
subwoofer’s amplifier automatically as soon as it detects the presence of audio
signals. Then it holds the amplifier’s power on while ever audio signals are
being fed to the controller and only turns it off again after waiting about
11-12 minutes from when they are no longer detected.
So you no longer have to worry about remembering to turn on the
power to the subwoofer amplifier or off again afterwards.
All of the controller’s circuitry fits inside a compact
low-profile instrument case and operates from a single 12V DC supply from either
a battery or a regulated mains plugpack. The current drain is modest too – less
than 60mA when active.
How it works
You can get a good overview of the way the controller works
from the block diagram in Fig.1. As you can see, the source select switch is
right at the inputs, allowing you to choose between the left and right channel
line outputs of your main amplifier if it has them, from the speaker outputs if
it doesn’t, or from the LFE output of your surround sound decoder or DVD
The line and speaker level stereo inputs are each mixed
together to produce a mono signal for the Subwoofer Controller but the LFE
signal is already mono so mixing isn’t required.
The signals are selected by switch S1, then passed through an
input buffer stage which allows you to adjust their level (and hence the
subwoofer volume) for tonal balancing.
The input buffer uses a feedback-type level control, which can
either attenuate or amplify by up to 8dB either way, giving a 16dB adjustment
range which will be more than adequate.
Next the signals move to the low pass filter stage which can be
adjusted between 41Hz and 200Hz. This allows you to ‘fine tune’ the crossover
frequency where the subwoofer takes over from the main system speakers, to
achieve the smoothest transition.
(This filter is not needed when you are using the LFE signal to
drive the subwoofer, so in this case you just set the low pass filter frequency
to maximum, where it will have minimal effect.)
Next is the parametric equaliser stage, which allows you to
compensate for any peaks or troughs (dips) in the subwoofer’s own frequency
response. It does this by allowing you to produce a counteracting trough or peak
at any frequency in the range from 30Hz to 200Hz, and with an amplitude of up to
12dB either way.
This should smooth out most likely peaks or dips in the
subwoofer’s performance - provided that it’s in a reasonably damped
Fig.1: block diagram of the Subwoofer Controller.