Expandable Graphic Equaliser circuit

Posted on Feb 2, 2013

The project described in this article is a constant Q, fully expandable graphic equaliser. Where most `conventional` graphic EQ circuits have a Q that is dependent on the setting of the pot, this one maintains the same Q at all settings. This is achieved by using MFB (Multiple Feedback Bandpass) filters, instead of the more common `gyrator` tuned circuit. As always, there are pros and cons for the approach described here. Phase shifts tend to be a little more radical, and the passband has more ripple than a conventional circuit, but only where a number of sliders are set to boost or cut. On the positive side, specific frequencies are dealt with specifically regardless of the level, and not with a variable Q. The constant Q circuit makes room equalisation and feedback reduction far better behaved.

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The filters used are the same as in the Instrument Graphic Equaliser and subwoofer equaliser (see Project 64 and Project 84), and are multiple feedback bandpass types. An example of this filter is shown in Figure 1, and more details are available from the project page for the MFB filter (Project 63). Depending on the configuration you ultimately decide upon, you will need between 10 and 30 of these filters - per channel for stereo! This circuit is reproduced from the original article for convenience - the actual filter circuits used are slightly different, and are shown in Figure 3. Building 60 of these may sound like an awful chore, which is perfectly reasonable, since it will be just that. With this knowledge at hand, this may go some way to help you make some. There is one thing of special note in this circuit. R6 (39k as shown) determines the maximum amount of boost and cut, and if you wanted to, you can make it variable. With the filter circuits shown below, 39k allows a boost and cut of 12dB - which is about right in most installations. A value of 10k will allow a maximum of a little over 5dB. Any value between these limits will provide the optimum for a given environment, and this can be preset. This is a very useful feature, and one that I believe is unique to this circuit. The slide pots are wired with all the end connections in parallel, and the "Sig" output above must drive all the filter inputs, which are also...

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