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Five band equaliser uses two low cost ICs Liven up your keyboard, guitar or music system with this 5-band equaliser. It only uses a few low cost parts and will enable you to “customise” the sound of your system just by twiddling a few knobs. By JOHN CLARKE These days, many home music systems have equalisers and so do the more expensive car sound systems. They can be used to tailor the sound quality by removing unwanted frequency peaks and boosting frequency troughs, to flatten or enhance the overall frequency response. They are also used during recording sessions to enhance the sound of particular instruments or even to change the sound of vocalists. An equaliser can be thought of as an expanded tone control where the audio spectrum is divided up into several sections or frequency bands. 22  Silicon Chip Each of these bands can be boosted or cut independently. Some equalisers can control 30 or more bands but this design is more modest with just five frequency bands. These bands are centred on 63Hz, 250Hz, 1kHz, 4kHz and 16kHz. A potentiometer is used to boost or cut the signals in each frequency band and so the PC board has five pots but no other controls. These are standard rotary pots and not the linear sliders which are often used on equalisers. However, you can substitute slider types if you wish. This equaliser does not use fancy hard-to-get ICs but its performance is quite respectable, as detailed in the accompanying spec panel. Its overall boost and cut performance is detailed in the composite graphs of Fig.1. This shows the response of each band separately as it is set to the extremes of boost and cut. As can be seen from Fig.1, the maximum boost and cut is ±12dB. Also shown on the graphs is the frequency “ripple effect” when all controls are set to boost and cut. This is an unrealistic setting but it indicates what happens to the frequency response when two adjacent bands are set for boost or cut – you get a dip or a peak between the bands. The circuit is very quiet at better than -94dB with respect to 1V and has very low distortion, typically less than .001%. Equaliser principles Typical equalisers do not work the same way as tone controls which boost or cut frequencies above or be- low a certain frequency. As already indicated, an equaliser boosts or cuts defined frequency bands which have particular “centre” frequencies. Thus, we have the notion that each band is tuned on a particular centre frequency and that requires a circuit which is tuned or resonates, as defined by an LC (inductor-capacitor) network. This can be seen in Fig.2 which can be thought of as a one-band equaliser. In essence, we have an op amp (IC1a) connected as a non-inverting amplifier and a feedback network with a potentiometer (VR1) with its wiper connected to ground via an LC network. This LC network sets the centre frequency of the band. With VR1 centred, the op amp has unity gain and the tuned series LC circuit has no effect on the frequency response. In other words, an input signal passes through the circuit unchanged and with a flat frequency response. This is the “flat” setting for the equaliser. When VR1 is rotated to its boost setting, the LC network is connected directly to the inverting (-) input of the op amp, shunting the negative feedback to ground. At the resonant frequency, the impedance of the LC network is at a minimum. Thus, the feedback will be reduced and the gain will be at maximum, at the resonant frequency. Conversely, when potentiometer VR1 is rotated to the maximum cut setting, the LC network is connected to the non-inverting (+) input, and tends to shunt the input signal to ground. This results in a reduction (cut) in gain at the resonant frequency. Naturally, at intermediate settings of the potentiometer, the boost or cut is reduced in proportion. The centre frequency of the circuit can be obtained from the formula: f = 1/2π√(LC) We could design an equaliser using inductors and capacitors as shown in Fig.2 and that is exactly how equalisers were made more than 20 years ago. However, inductors for audio circuits tend to be quite heavy and bulky and they have tendency to pick up hum which we don’t want. So instead of using inductors we use gyrators. A “gyrator” is a pseudo inductor using an op amp and a capacitor. This circuit is shown in Fig.3. In an inductor, the current lags or is delayed by 90° with respect to the AUDIO PRECISION FREQRESP AMPL(dBr) vs FREQ(Hz) 20.000 25 OCT 95 10:59:54 15.000 10.000 5.0000 0.0 -5.000 -10.00 -15.00 -20.00 20 100 1k 10k 20k Fig.1: this composite graph shows the boost and cut performance of the equaliser at the five centre frequencies. Maximum boost and cut is ±12dB. Also shown is the frequency “ripple effect” when all controls are set to boost and cut. Fig.2: this is the essence of a graphic equaliser. A series resonant LC network and potentiometer is connected into the op amp feedback network for each frequency band. Fig.3: the circuit of a gyrator. The op amp simulates an inductor by a vector transformation of the current through the capacitor C. The resulting inductor is equal to the product of R1, R2 and C. Fig.4: these waveforms show the phase differences between current and voltage for the various points on the circuit of Fig.3. Notice that the output current IOUT lags the input voltage VIN by 90 degrees. Thus, as far as the signal source is concerned, the circuit behaves as an inductor. December 1995  23 +15V 0.47 INPUT 12 100k 13 4 IC1b TL074 10k 14 10 33pF 11 1k 8 IC1a 9 10 OUTPUT 22k -15V 10k 47  33pF 47 250Hz VR2 50k LIN 63Hz VR1 50k LIN 0.22 0.82 3 2 1 220k 6 7 220k IC2a 2 TL074 4 2k 270pF 68pF 11 3 IC1d 1.8k -15V .001 5 IC1c .0033 1.8k .0047 16kHz VR5 50k LIN .015 2k .018 4kHz VR4 50k LIN .056 2k 220k 1kHz VR3 50k LIN 10 5 1 220k 6 IC2b 7 220k 9 IC2c 8 +15V REG1 BR1 1B04 REG2 15V 240VAC 0V 15V I GO IN 470 25VW GND 5-BAND EQUALISER Fig.5: the final circuit uses five gyrators (IC1c,d & IC2a,b,c) to give centre frequencies of 63Hz, 250Hz, 1kHz, 4kHz and 16kHz. Note that the fourth op amp in IC2 is not used. voltage waveform. With a capacitor, however, the voltage lags the current by 90°. To simulate the inductor, the voltage lag of the capacitor must be converted to a leading voltage compared to the current. Consider an AC signal applied to the input of the circuit (Vin) of Fig.3. Current will flow through capacitor C and resistor R1. Because it is connected as a voltage follower, the op amp will reproduce the voltage across R1 at its output. This voltage will now cause a current to flow in R2 and it adds vectorially with the input current and the resulting total current lags the input voltage. The waveforms in Fig.4 show the phase differences between current and voltage for the various points on the circuit. Notice that the output current IOUT lags the input voltage VIN by OUT +15V 10 16VW 0.1 GIO 470 25VW 24  Silicon Chip REG1 7815 90°. Thus, as far as the signal source is concerned, the circuit behaves as an inductor. The value of simulated inductance is given by the equation: L = R1 x R2 x C where L is in Henries, R is in Ohms and C is in Farads. By substituting the gyrator for the inductor in the circuit of Fig.2, we have the basis for a complete equaliser. In our circuit, we need five gyrators and their accompanying potent­ iometers and capacitors. The complete circuit is shown in Fig.5. It comprises two quad op amps and associated potentiometers and gyrator components. The gyrator op amps are IC1c, IC1d, IC2a, IC2b and IC2c. Note that the fourth op amp in IC2 is not used. IC1b is a unity-gain buffer for the input signals. These are AC-coupled GND IN REG2 7915 10 16VW OUT -15V via a 0.47µF capacitor to the non-inverting input at pin 12. The output of IC1b is applied to the equaliser circuit via a 10kΩ resistor. The 33pF capacitor provides high frequency rolloff and prevents instability in the circuit. Similarly, the 33pF capacitor in the negative feedback path for IC1a provides some high frequency rolloff. The five potentiometers are connected between the inputs of op amp IC1a and the overall boost and cut range for each frequency band is restricted to about ±12dB with the 47Ω resistors at pins 9 & 10. As you can see, the capacitor values used in the resonant networks are large for the low frequency bands and small for the high frequency bands. The output of IC1a is AC-coupled via a 10µF capacitor and a 1kΩ resistor. The resistor is there to prevent instability in the op amp if it is connected to long lengths of cable. The op amps are run from ±15V This 5-band mono equaliser operates at line levels (ie, CD, tape and tuner levels) and gives a maximum boost and cut of 12dB at the centre frequencies of 63Hz, 250Hz, 1kHz, 4kHz and 16kHz. Fig.6: the parts layout for the PC board. Note that the pot cases must be earthed via a length of tinned copper wire. TABLE 1: RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 5 1 1 2 3 2 1 2 Value 220kΩ 100kΩ 22kΩ 10kΩ 2kΩ 1.8kΩ 1kΩ 47Ω 4-Band Code (1%) red red yellow brown brown black yellow brown red red orange brown brown black orange brown red black red brown brown grey red brown brown black red brown yellow violet black brown 5-Band Code (1%) red red black orange brown brown black black orange brown red red black red brown brown black black red brown red black black brown brown brown grey black brown brown brown black black brown brown yellow violet black gold brown December 1995  25 Fig.7: this is the actual size artwork for the PC board. Check the board carefully for etching defects before mounting any of the parts. supply rails and these are provided by the 3-terminal regulators REG1 and REG2. The input voltage can be a centre tapped 30V AC supply or a DC centre tapped source which is greater than ±18V but less than ±35V. The AC input is applied to the bridge rectifier BR1 and two 470µF capacitors to provide plus and minus DC rails for the 3-terminal regulators. PC board assembly The PC board is coded 01309951 and measures 167 x 65mm. The component overlay diagram is shown in Fig.6. Begin assembly by checking the PC board against the published pattern in Fig.7. Look for possible broken TABLE 2: CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ Value 0.82µF 0.47µF 0.22µF 0.1µF .056µF .018µF .015µF .0047µF .0033µF .001µF 270pF 68pF 33pF IEC Code EIA code 820n 824 470n 474 220n 224 100n 104 56n 563 18n 183 15n 153 4n7 472 3n3 332 1n0 102 270p 271 68p   68 33p   33 Specifications Frequency Response All controls centred ....................... 20Hz to 20kHz within ±0.5dB Boost and cut ............................... ±12dB (see graph of Fig.1) Centre frequencies ....................... 63Hz, 250Hz, 1kHz, 4kHz & 16kHz Signal Handling Gain .............................................. Unity Maximum input & output .............. 8V RMS (all controls centred) Input impedance ........................... 100kΩ Output impedance ........................ 1kΩ Harmonic Distortion <.005% for frequency range 20Hz to 20kHz; .0017% <at> 1kHz and 3V, typically less than .001% Signal to Noise Ratio With respect to 1V RMS ............... -94dB unweighted (20Hz-20kHz); -97dB A-weighted Power Supply ................................. ±15V at 30mA 26  Silicon Chip tracks or shorts. Fix any defects before inserting any of the components. First, insert the PC stakes located at all the external wiring points. There are seven in all. Next, do the wire links and resistors. Table 1 shows the resistor colour codes but it is always a good idea to check each value with a digital multimeter as some of the colours can be difficult to distinguish. Take care when installing the 3-terminal regulators and whatever you do, do not get them swapped around otherwise they’ll self-destruct as soon as power is applied. Each regulator is secured to the PC board using a screw and nut but no heatsinks are required. The bridge rectifier (BR1) looks like an 8-pin IC with 4 pins missing. Make sure you insert it the right way. The same remark applies to the two ICs. When installing the MKT capacitors, use Table 2 if you have any doubt about the coded values. Make sure that the electrolytic capacitors are installed the right way around, as shown on Fig.6. The five potentiometers are PC mounting types which are simply inserted and soldered into the board. When they are all soldered in, solder a length of tinned copper along the top of each pot and to the earth terminal as shown in the photo. This will prevent hum pick-up. As mentioned earlier, the circuit can be powered from a centre-tapped 30V AC supply transformer or from balanced DC rails of more than ±18V but less than ±35V. Once the board is finished, it should be checked over carefully. This done, apply power and check that +15V is present between pin 4 of IC1 and IC2 and ground. Also check for -15V BOOKSHELF – CONTINUED FROM PAGE 15 PARTS LIST 1 PC board coded, 01309951, 167 x 65mm 5 50kΩ linear PC mounting pots 5 knobs 7 PC stakes 1 320mm length of tinned copper wire 2 3mm screws, star washers and nuts Semiconductors 2 TL074 quad FET-input op amps (IC1,IC2) 1 7815 3-terminal regulator (REG1) 1 7915 3-terminal regulator (REG2) 1 1B04 1A 400V bridge rectifier (BR1) Capacitors 2 470µF 25VW PC electrolytic 3 10µF 16VW PC electrolytic 1 0.82µF MKT polyester 1 0.47µF MKT polyester 1 0.22µF MKT polyester 1 0.1µF MKT polyester 1 .056µF MKT polyester 1 .018µF MKT polyester 1 .015µF MKT polyester 1 .0047µF MKT polyester 1 .0033µF MKT polyester 1 .001µF MKT polyester 1 270pF ceramic or MKT polyester 1 68pF ceramic 2 33pF ceramic Resistors (0.25W 1%) 5 220kΩ 3 2kΩ 1 100kΩ 2 1.8kΩ 1 22kΩ 1 1kΩ 2 10kΩ 2 47Ω between pin 11 and ground of IC1 and IC2. Installation When installed into audio equipment, the input and output lines should be run in shielded cable. To avoid hum loops, the shields of these cables should normally only be connected at one end. For stereo use, two equaliser boards will be needed. Also, the ±15V power output from one equaliser can be connected to the power rails of the other SC and the regulators deleted. The Motorola Impedance Matching Program (MIMP) is discussed in chapter eight. Available free of charge from ter eight. Available free of charge from Motorola, this program provides a simple method for entering and analysing impedance matching circuitry. A standard library of passive circuit elements is provided by MIMP, including various combina­tions of capacitors, inductors and transmission lines, in both series and shunt configurations. Chapter nine, titled “After the Pow­er Amplifier Output”, discusses the protection needed for solid state amplifiers. Most failures occur due to load mismatch, which causes a high current in the output transistors. Since the temperature time constant for a typical RF transistor is 0.5-1.0 millisecond, any protec­tion must be faster than this. The most common method for load sensing is the reflectometer VSWR. This sensor is usually located in series between the output stage and the load. A voltage, proportional to the amount of mismatch, is supplied by the re­ flectometer and this is used to reduce the drive, or shut down the power amplifier, depending on the design brief. Most RF power amplifiers require a low pass filter to ensure that any harmonics generated by the amplifier will not be radiated. The various types of filter, the design procedure and the types of components constitute the balance of this chapter. The 10th chapter covers wideband impedance matching which is usually done with transformers. The transformer types covered are conventional, twisted wire and transmission line. A conven­ tional transformer is defined as one with two windings, often on a ferrite core. The twisted wire type is exemplified by the humble balun used in most TV set antenna circuits. The transmis­sion line transformer is the one most likely to be unfamiliar to many readers. In practice, it can be realised with twisted enamel wires, coaxial cables, parallel flat ribbons or a micro-strip. The main identifying feature is that the power transferred from input to output is not coupled through a magnetic core but rather through the dielectric medium separating the line conductors. Various examples of each type are detailed. “Power Splitting and Combining” is the title of chapter 11. If the power output requirements exceed the capabilities of one output device, multiple stages can be combined to produce the required power. These com­ biners are similar to wideband transformers in design and construction, the main difference being the way the windings are connected. A splitter is simply a low power combin­er used in reverse. Combiners covered include the 0° and 180° devices, the 90° hybrid and the Wilkinson combiner. Chapter 12 is titled “Frequency Compensation and Negative Feedback”. As the input impedance of a BJT or FET varies much more with frequency than the output impedance, it is usual to only compensate the input. Methods used include series chokes, series resistors shunted with small capacitors in the base drive circuit or series chokes between base and ground. Negative feedback, similar to that used in audio amplifi­ers, can be used to broaden the frequency response of HF amplifi­ers but as the impedances are so much lower, considerable power can be dissipated in the feedback network. With a 300 watt 175MHz broad-band amplifier, the power loss at 10MHz could be in the order of 10%. Various methods of feedback using R, L, C (resis­tors, inductors, capacitors) and input and output transformers are discussed. The final chapter, titled “Small Signal Amp­lifier Design”, de­scribes a straight­forward approach to this topic. The three basic ingredients are the selection of a bias point, then the use of scattering parameters and noise parameters to complete a specific circuit. The authors cover each of these points in some detail and recommend the use of one or two computer programs, should the design require controlled noise and gain performance over a band of frequencies. Fourteen pages of worked examples complete the book. In summary, in view of the dearth of good current textbooks on RF design, this book can be highly recommended. Our copy came from Butter­ worth-Heinemann Australia, PO Box 5577, West Chats­wood, NSW 2067. Copies can be obtained from SILICON CHIP. The ordering details are shown in the SILICON CHIP Bookshop adver­ tisement in this issue. (R.J.W.) SC December 1995  27