Share a parametric equalizer circuit

In this article, we discussed a parametric equalizer circuit that can be used to enhance the music output to excellent levels, much better than any standard graphic equalizer output.

Advantages

The main advantage of a parametric equalizer is that it allows you to modify the frequency and "Q factor" over a wide range while controlling the gain at the same time, which is not possible with most graphic equalizer circuits.

Furthermore, a few parametric equalizers per channel can outperform a complex graphic equalizer in most cases. Therefore, the circuit design discussed in this article can produce very beneficial results by improving the quality of the music input to an excellent level.

Specification

The parametric equalizer mentioned is a high-performance gadget that can be used with even the most unconventional hi-fi systems. The device has a boost and cut range specification of 0 to just over 20dB, a Q factor variable of 1 to 25, and a frequency range of 50Hz to 11kHz.

Gain and Q testing was done at 1kHz, but response uniformity was found to be excellent across the frequency range.

Since frequencies below 50Hz are mostly never preferred, the lower range specs for this circuit are great for most audio applications. The upper spec of 11kHz is also great because it reduces the chances of harming the tweeter if the EQ is set for a strong boost in the high frequencies (unless corrections are being made for the dips in the response). However, as described later in this article, the frequency range of this circuit can be easily changed if desired.

High-gain, high-Q parametric EQs are very effective for "enhancing" recordings. The most typical change might be to boost frequencies that were not available in the original music and remove unwanted frequencies, such as noise.

Parametric equalizers like this one are also very suitable for electronic music. For starters, such parametric equalizers can be used as so-called passive filters for illustrative music synthesis, to enhance the music spectrum or the typical frequency spectrum of the instrument being simulated.

Because of the rather astonishing frequency changes they can produce, especially at higher gain levels, these EQs are now often used as effects units.

Circuit Description

The following figure depicts the complete circuit schematic for one channel of the parametric equalizer. The main feature of the equalizer is the bandpass filter built around ICs 3a, 3b, and 3c, which are configured in a "state variable" topology.

The passband is obtained from the output of IC3c and the center frequency of the filter is controlled by capacitors C7 and C8 which work like integrators and the resistance defined by the ganged potentiometer RV3.

The frequency range using a 1500pF capacitor is 50Hz to 11kHz, this range can be changed by adjusting each of the C7 and C8 values. For example, if a 1000pF capacitor is used, the frequency range will be between 75Hz and 16.5kHz, while a 2000pF capacitor will result in a frequency range of 35Hz to 7.5kHz.

Returning to the input stage, the music input is connected to IC1, which is basically a buffer stage that provides a low impedance to the gain control RV1. Resistor R1 determines the input impedance of the circuit.

The processed signal then passes through unity gain amplifier IC2 to the output.

When switch SW1 is flipped to short the noninverting input of IC2 to ground, the filter section is disabled.

The filter receives its input through RV1, which has one end connected to the original inverted signal at the output of IC1 and the other end connected to the inverted signal at the output of IC2.

RV1's wiper provides the signal input to the bandpass filter, while RV3 determines which frequency components are processed by the filter and fed back to the noninverting input of IC2.

Therefore, when the cursor of RV1 is closer to the output of IC1 (original input music signal), the specified frequency band of the original signal at IC2 is added (enhanced) to the original signal.

On the other hand, when the RV1 wiper is brought close to the IC2 output, the result is subtraction (cut-off) of the original signal in IC2.

While the effect is similar to a traditional graphic equalizer, the flexibility to modify frequencies across the entire range is a major advantage of this parametric EQ.

The variable Q is another element that must be included in a parametric equalizer; the larger the Q value, the higher the peaking of the filter response.

Having a large amount of feedback increases the Q, and obtaining a high Q without allowing the filter to oscillate is one of the key advantages of implementing a state variable filter.

The ratio of R8 and R17 controls the Q of the circuit (if RV2/R is not included). However, changing these resistors will also change the gain of the circuit unless the configuration is changed and a dual potentiometer is introduced to change the two resistor values. This problem can be solved by increasing the gain before R8 and then changing the Q by passivating the feedback signal through RV2.

How to use

When the gain control potentiometer's wiper is in the middle position or the bypass point is grounded, the parametric equalizer circuit has unity gain, which means the signal output value is the same as the signal value. Therefore, the input signal needs to be pre-amplified, and for many audio amplifiers, the monitor output is the best starting point.

The maximum signal level that can be used is determined partly by the power supply and partly by the application.

The Clip and Boost controls are configured simply to correct for frequency response shortcomings in high fidelity and many recording scenarios, ensuring a basically flat response.

Independently of the power supply used, the parametric equalizer circuit will handle the signal levels commonly found in these types of Hi-Fi systems.

If this parametric EQ is used for special effects and the gain is adjusted to either extreme, the maximum input music signal levels shown in the table below are recommended to prevent clipping.

If you don't have an audio analyzer device with you, you may want to start by adjusting the knobs and judging the response by ear, just as you would with the tone controls.

Start with the Q control at or near its lowest setting, then gently increase or decrease the gain from the center position and slowly move the Frequency control clockwise.

The results will amaze you, and you'll quickly find the ideal setting for your setup. Because the frequency spectrum that is boosted or cut is much smaller as Q is raised, the effect of a gain control is usually not very noticeable to the ear unless it is concentrated on certain consistent frequencies in the music.

Bypass switch

The SPST switch can be connected between the "Bypass Switch, SW1" and "0V" terminals as shown in the circuit diagram (in parallel with C6). Whenever the switch is on, the non-inverting input of IC2 is grounded and the original input signal flows through the equalizer, unaffected by the potentiometer adjustment.

This switch is very useful for examining the effects of a circuit, simply toggle SW1 on/off. This switch will allow you to quickly analyze the difference in the output results of this parametric equalizer circuit, just by dialing the switch in and out.