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 list.

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 bass.

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 balance.

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 damage.

Three signal sources

Subwoofer Controller Specifications Input impedance: Line level and LFE signal inputs 47kΩ Speaker line inputs 10kΩ with an 11:1 mixing divider Gain: -8dB to +8dB, variable Low pass filter: Corner frequency variable between 41Hz and 200Hz 12dB/octave rolloff slope Parametric equaliser: Centre frequency variable between 30Hz and 200Hz Cut/boost variable between ±12dB at centre frequency Q approximately 5 High pass (subsonic) filter: Corner frequency 15Hz, rolloff slope -18dB/octave Signal to noise ratio: -80dB unweighted relative to 1V RMS input, 2V RMS output Maximum output signal: 2.4V RMS Output impedance: 1kΩ (both outputs) Amplifier hold-on time: Approx 11 minutes after end of signals 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 player.

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 enclosure.