A variable 2 band Equaliser with an exceptional ±30dB range that will transform cymbals, remove hums, improve vocals and clean up your instruments!
When recording vocals or instruments particularly acoustic instruments using a microphone, the results can often be very disappointing, lacking life and character. Invariably the problem is due to the microphone not picking up all the natural warm resonances of the instrument or singers voice. The microphone itself can exhibit peaks and dips in its frequency response which aggravate the problem. Another familiar problem is when the recorded instrument exhibits an unnatural resonance, or boominess at a particular frequency.
What's obviously needed is a means of controlling the contour of the frequency response; to lift the dips and drop the peaks. A graphic equaliser may provide the answer, if you can justify the cost of one, and if the +10dB or so of control range is sufficient. A parametric equaliser can also be very useful, although considering its complexity would be an overkill in most situations. Since every parameter of the parametric filter's response can be varied, the operator is confronted with quite an array of controls to adjust, just for one centre frequency!
The E&MM Sweep Equaliser provides a simple, low cost, but very effective solution. It provides two stages of equalisation whose centre frequency can be varied. Each stage has its own boost/cut control with an astounding range of +30dB at the centre frequency. The busy musician/operator/engineer is presented with only the essential controls necessary to overcome equalisation problems quickly.
The use of the Sweep Equaliser is certainly not limited to frequency response correction. It should also be considered an invaluable creative tool in the studio, or on stage for use with any sound source, acoustic or electronic. Professional studio engineers use Equalisers (EQ) constantly in the production of the recordings we hear today, for both corrective and creative purposes. When you have a Sweep Equaliser at your fingertips, a whole new spectrum of sounds become available at the turn of a knob.
Active filter theory is too complex a subject to investigate in depth in these pages, but the following discussion gives some insight into the development of the bandpass filters which are the heart of this project. Some useful active bandpass filter design equations are also given.
The filters are based on the well known multiple feedback bandpass filter, sometimes known as an inductorless resonator for obvious reasons! Figure 1a shows the general circuit. Since we need to vary the centre frequency without changing the gain or Q factor, it can be seen from the equations that R1 and R2 must be varied simultaneously. In any practical circuit, R1 and R2 will have to be equal so that a standard dual gang potentiometer can be used.
Re-writing the equations for the equal value resistor circuit shown in Figure 1b, we find that the maximum value of Q is 0.5 which occurs when C1=C2. A filter with such a small Q factor would be of very limited use since it would not be nearly selective enough.
The final solution shown in Figure 1c uses positive feedback to peak up the response at the centre frequency to give a design target Q factor of 1.8. Note that an additional stage is required to amplify and invert the signal fed back to the inverting input of the filter amplifier. This configuration is not suitable for high Q filters since it would be unstable in such applications.
Figure 2 shows the complete circuit diagram for the Sweep Equaliser. IC2a & b form a bandpass filter for the high frequency range, while IC2c & d form the filter for the low range. RV3 and RV4 control the filter centre frequencies. With the controls flat, the input signal, buffered by IC1a is passed directly to the output amplifier, IC1b via R15. The signal will not be affected, and maintains its original polarity since it is inverted twice; once by IC1a and again by IC1b.
When RV1 is turned clockwise, the input signal is fed to the input of the high range filter, the output of which is amplified by IC1b to produce an output with a peak of 30dB at the centre frequency. When RV1 is turned anticlockwise, the high range filter is effectively placed in the feedback path of the output amplifier IC1b, causing its gain to decrease dramatically at the centre frequency, producing a dip, or notch 30dB deep. The low range filter is controlled by RV2 in exactly the same manner. RV1 and RV2 have been given a relatively high value compared with R3 and R4 so that the resultant loading on the potentiometers imparts a pseudo logarithmic response to the controls, providing finer resolution near their central positions.
Capacitor C6 ensures that IC1b maintains high frequency stability at all control settings.
The ease of construction of this project relies on the use of a single PCB to mount all the components, including the potentiometers. Assembly of the PCB should commence by inserting and soldering the vero-pins, used to connect the flying leads to the PCB. Next insert the two wire links and all the resistors. Bend the leads at 45° to secure the components, then solder and crop. Fit the capacitors in a similar manner, taking care with the polarity of the electrolytic types. Solder in the IC sockets next, but don't fit the IC's at this stage.
After preparing the case panels as shown in Figure 5, fit the DIN sockets to the rear panel and the jack sockets to the front panel. Now locate the potentiometers in the PCB, but before soldering, secure them to the front panel so that strain is not put on the joints. At this stage check the assembly and all the soldering carefully, preferably with an eyeglass, looking for dry joints and bridged tracks. The IC's can now be fitted in their sockets, taking care with their orientation. Slide the completed panel assemblies into the case base and complete the project by wiring the PCB to the sockets as shown in Figure 4.
The Sweep Equaliser requires no setting up, and should be ready for use. It was designed to be used with the Twinpak DC power supply described in E&MM Sept. '82 P.59, although almost any twin regulated DC supply will suffice provided that it delivers regulated ±9 to ±15v at a few tens of milliamps. Connect the DC supply to one of the DIN sockets, the other socket being intended to extend the supply to another unit.
Equalisation should ideally be performed during the recording process rather than on playback, as long as you can monitor the signal being recorded. Not only will this usually yield a higher signal-to-noise ratio, but also allows each track to be individually equalised using only one equaliser. To find the required frequency, set the relevant section to full boost and adjust the frequency control until the sounds requiring alteration become prominent. The boost/cut control can then be set as required to adjust the relative level of the selected sound.
Instruments such as guitars, synthesisers or organs can be connected directly to the unit, the output of which will then be routed to your amplifier, mixer, or tape recorder. To provide more equalisation stages, two or more Sweep Equalisers can be connected in series. Three or four units could be used together to form a powerful graphic equaliser!
A complete set of parts for the Sweep EQ including components, PCB, case, screen-printed panel and hardware is available from E&MM, (Contact Details), at a cost of £28.95, including postage, packing and VAT. Please order as: Sweep EQ kit.
Feature by Paul Williams
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