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Studio series 32-band 1/3-octave equaliser If you 're running a disco, doing your own recordings, or involved in a rock group or PA work, you will be interested in this new design for a 1/3-octave equaliser. It has 32 separate frequency bands and excellent audio performance. By LEO SIMPSON & JOHN CLARKE Most people are probably familiar with the stereo graphic equalisers used in home hifi systems. Generally these have 10 frequency bands or less but this results in too coarse a control over the audio bands for more serious applications, particularly for PA work. If there are nasty peaks or troughs in a a system's overall response, due to room acoustics or whatever, you really need a 1/3-octave equaliser to cure the problem. It can provide a boost or cut to a very narrow band of frequencies and thereby provide fine 44 SILICON CHIP acoustic tuning which is just not possible with a 10-band equaliser. Since the equaliser to be described here is intended for semiprofessional use, it is a mono instrument only. For use in stereo systems, two equalisers will be required. Note that while the Studio series 1/3-octave equaliser is specifically intended for semi-professional use there is no reason why it cannot be used in domestic stereo systems. If you want 1/3-octave control, it is the only way to go. In most stereo systems the easiest way to connect two of these equalisers [one for each channel) would be via the Tape Monitor loop or between the preamplifer and power amplifier. 32 bands are used to cover the audible frequency range. The centre frequencies of the bands are as follows: 16Hz, 20Hz, 25Hz, 32Hz, 40Hz, 50Hz, 63Hz, 80Hz, l00Hz, 125Hz, 160Hz, 200Hz, 250Hz, 320Hz, 400Hz, 500Hz, 630Hz, 800Hz, lkHz, 1.25kHz, 1.6kHz, 2kHz, 2.5kHz, 3.2kHz, 4kHz, 5kHz, 6.3kHz, 8kHz, lOkHz, 12.5kHz, 16kHz and 20kHz. For a strict relationship of 1/3 of an octave between each band, the centre frequencies should increase in the relationship 1:1.26 [actually 1:1.259921 to be precise). However, the centre frequencies we have chosen are suitably precise and easily recognised. They are also the same as used in commercial equalisers. The equaliser is housed in a standard 2-unit high rack mounting case [ie, the front panel is 435mm wide by 88mm high). In all, there are 33 sliders on the front panel, 32 for the individual 1/3 octave bands and one as a master level control. Apart from the sliders, there are only two switches. One is a bypass control which passes the signal through completely unmodified while the other is the push-on pushoff mains switch. The back panel is completely bare except for two RCA sockets, one for the input signal and one for the output. Inside, virtually all the wiring is taken care of by three printed circuit boards. There is one long board to accommodate the 33 slider controls and another large board to accommodate the active equaliser circuitry. Finally, a smaller board takes care of the power supply circuitry. The slider board and the main board are linked together by five short multiway cables with plugs and sockets at each end for easy removal. To ensure that no problems are likely to occur with earth loops, the entire circuit of the equaliser is completely isolated from chassis although the chassis itself is connected to mains earth. tion performance and with plenty of signal overload margin, even if full boost is applied. Full details of the performance are noted in the specifications panel. In one very important respect though, the performance of the equaliser is not apparent from the spec panel and this involves the slider pots. In the past, graphic equalisers have been designed with linear pots and this has led to a problem whereby the boost and cut for each slider is concentrated at the extremes of travel. In other words, to obtain an audi- ble effect from .a particular slider, you had to push it a fair way from the centre detent setting (which gave a flat response) before an audible effect was heard. This characteristic is inevitable with linear pots. To solve it, the potentiometer manufacturers in Asia have come up with a new design of resistance element for sliders intended for graphic equaliser use. Called the 4BM taper, it is effectively a centre tapped element with a log/antilog resistance taper; log in one direction, antilog ~n the other. The new element concentrates more of the boost and cut action in the slider travel immediately either side of the centre detent setting and thereby gives a better control action. As far as we know, this is the first design using these pots to be published in a magazine. They are already being used in the best commercial 1/3-octave equalisers. They really do give a much better apparent response from the equaliser controls. In fact , we regard these pots as being one of the key features of this new design. (We are indebted to Jaycar Electronics for their efforts in sourcing these pots from Asia). Circuit principles The circuit principle used in virtually all of today's graphic equaliser designs is the same. Each Specifications Frequency Response Equaliser out Equalis8r in Boost and cut Flat 5Hz-20kHz ± 1dB ; - 3dB at 45kHz ±12dB Signal Handling Gain Maximum input and output Unity (see text) 1 O volts RMS (all controls flat) Harmonic Distortion <.05% for frequency range 1 OHz to 2GkHz Signal to Noise Ratio With respect to 1 V RMS 95dB unweighted (20Hz-20kHz) 97dB A-weighted Special slider pots Input Impedance 33k0 The entire circuit has been designed for low noise, low distor- Output impedance 1 kO MARCH 1989 45 R2 1k l Vout Fig.1: this circuit demonstrates the basic principle of a graphic equaliser with only one slider control. The tuned LC circuit shunts signal to ground to give either boost or cut. In practical circuits, inductor L is a gyrator. frequency band requires its own resonant circuit, as shown in Fig.1. This resonant circuit is connected into the negative feedback circuit of an operational amplifier connected in the inverting mode. Fig.1 shows the op amp with just one resonant circuit. A real circuit has a resonant circuit for each frequency band but we show just one to keep things simple. Now consider how it works. With the 50k0 slider control in the centre setting, the op amp provides unity gain and the tuned LC circuit has virtually no effect on the frequency response. When the slider pot is set to the le:::::::--..,_ ~ _,,,,.---- loul~"--- Fig.3: this diagram shows the voltage and current relationships around the gyrator circuit of Fig.2. 46 SILICON CHIP Fig.2: the circuit configuration of a gyrator. The op amp effectively transforms capacitor C into an inductor which is proportional to Rt, R2 and C. boost end, the negative feedback tends to be shunted to ground by the tuned circuit. Since it is a series tuned circuit it will have a low impedance at its resonant frequency. Hence, the feedback will be reduced at the resonant frequency (and for the narrow band of frequencies on either side of resonance) and so an increase in the gain will result. Thus, the signal will be boosted over a narrow frequency range. When the slider is set to the cut end, the negative feedback is at a maximum and the tuned LC circuit actually tends to shunt the input signal to ground. This results in a reduction in gain at the resonant frequency. Naturally, the amount of boost and cut is proportional to the slider setting and reduced settings give reduced amounts of boost and cut. Gyrators instead of inductors Tuned LC circuits mean inductors should be used throughout the circuit; 32 in fact, one for each frequency band. But if you look at the complete circuit or at a photo of the inside of the chassis, you will see no evidence of inductors. Indeed there are none and nor will you find any in current commercial equalisers (as far as we know). Instead, we use an op amp circuit which simulates the performance of an inductor. This is known as a gyrator. Inductors are not used these days because they are bulky and expensive components to make (compared with resistors and capacitors) and they are also prone to hum pickup and mutual interaction. In short, they are bad news compared to gyrators. Fig.2 shows the circuit of a gyrator using an op amp. It effectively transforms a capacitor into an inductor. It does this by altering the phase of the current through the capacitor for a given applied signal voltage. In an inductor, the current lags the voltage (ie, the current is delayed in phase by go 0 ) while in a capacitor, the voltage lags the current [by go 0 ). Consider an AC signal source, Vin, connected to the input of Fig.2. This causes a current to flow through the capacitor and through the associated resistor Rl. The voltage impressed across Rl, as a result of the capacitor current le, is fed to the non-inverting input of the op amp which is connected as a voltage follower [with inverting input connected directly to the output). Because it is a voltage follower, the op amp reproduces its input voltage exactly at its output. Vout then causes a current to flow through resistor R2. This current, lout, then adds vectorially with the input current le and the resultant current which flows from the source lags the input voltage. As far as the signal source is concerned then, the gyrator looks like an inductor, not like an op amp with PARTS LIST 1 rack mounting case, 483 x 88 x 200mm (from Jaycar) 1 30V 1 50mA centre-tapped transformer (Altronics Cat. M-2855) 1 DPDT 250VAC toggle switch 33 50k0 45mm slider pots with 4BM taper, Jaycar Cat. RP-3914 1 2-pole push on/push off switch with mounting bracket 1 cord-grip grommet 1 0 18mm PC board spacers 1 0 3mm x 25mm screws 22 3mm nuts 10 3mm x 12mm screws 2 3mm x 6mm screws (to mount transformer) 2 insulated panel mount RCA sockets 4 stick on rubber feet 1 solder lug 9 8-way pin headers (Jaycar Cat. HM-321 0) 9 8-way connector socket (Jaycar Cat. HM-3220) 1 5 1 mm PC pins Cable 1 3-core mains cord and moulded 3-pin plug 1 800mm length of 8-way rainbow cable 1 metre shielded audio cable 1 metre of 250VAC rated insulated hookup wire two resistors and -a capacitor connected to it. The inductance is given by the formula: L = Rl x R2 x C where L is in Henries, R is in ohms and C is in Farads. With the use of quad op amp ICs (four op amps in a package), gyrator circuitry can be made much more compact than equivalent tuned LC filters. Which is a good thing otherwise this 1/3-octave band equaliser would use much larger circuit boards. To make the tuned LC circuit shown in Fig.1, all we need do is to connect a capacitor in series with the input to Fig.2. Now refer to the main circuit diagram. We quite understand if you have just opened the two pages of the full circuit diagram, shuddered and Printed Circuit Boards 1 main equaliser PCB, code SC01103891 1 power supply PCB, code SC01103892 1 equalizer control PCB, code SC01103893 Semiconductors 8 LF347N quad op amps 1 LM833 low noise op amp 1 7 81 5 3-terminal regulator 1 7915 3-terminal regulator 4 1 N4004 rectifier diodes 1 5mm red LED Capacitors 2 2200µF 25VW PC electrolytic 4 220µF 25VW PC electrolytic 2 1 00µF 25VW PC electrolytic 5 1 0µF 16VW PC electrolytic 2 1µF metallised polyester (greencap) 1 0 .68µF metallised polyester 1 0.56µF metallised polyester 3 0. 4 7 µF metallised polyester 1 0.39µF metallised polyester 2 0.33µF metallised polyester 2 0. 2 7 µF metallised polyester 2 0.22µF metallised polyester 1 0.18µF metallised polyester 3 0.15µF metallised polyester 1 0.12µF metallised polyester 14 0.1 µF metallised polyester 1 . 082µF metallised polyester then closed -it again. However, it really isn't all that complicated. It basically is just one gyrator circuit repeated 32 times, albeit with different values for Rl, R2 and C. The key op anip in the circuit is IClb and it performs the same function as the one in Fig.1. 32 50k0 slider pots are connected in parallel in the feedback network of IClb and each has an associated gyrator and additional series capacitor. For example, the gyrator for the 20Hz 1/3-octave band is IC2d and this is connected to the wiper of the slider via a lµF capacitor. Similarly, for the 2kHz band, the gyrator is IC7a and it is connected to the wiper of its slider via a .OlµF capacitor. Apart from the 32 gyrators and their common unity gain feedback 2 1 2 1 2 2 2 1 3 1 2 1 2 1 2 1 3 2 2 1 3 1 2 1 1 1 1 1 2 .068µF metallised polyester .056µF metallised polyester .04 7 µF metallised polyester .039µF metallised polyester .033µF metallised polyester .027 µF metallised polyester .022µF metallised polyester .018µF metallised polyester .015µF metallised polyester .012µF metallised polyester .01 µF metallised polyester .0082µF metallised polyester .0068µF metallised polyester .0056µF metallised polyester .004 7 µF metallised polyester .0039µF metallised polyester .0033µF metallised polyester .0027 µF metallised polyester .0022µF metallised polyester .0018µF metallised polyester .0015µF metallised polyester .0012µF metallised polyester .001 µF metallised polyester 680pF polystyrene 560pF polystyrene 4 70pF polystyrene 330pF polystyrene 270pF polystyrene 33pF disc ceramic Resistors (0.25W, 1 %) 1 x 1 Mn, 32 x 22okn, 1 X 1 00k0, 2 x 1 0k0, 1 x 3.3k0 0.5W (5%). 1 x 1.2k!1 (see text), 4 x 1 .1 kn, 1 7 x 1 kn, 13 x 91 on . amplifier, IClb, there is only one other op amp, ICla, which functions as an input buffer stage. It can be configured for a gain of unity or 2.2, as we shall see later. ICl is an LM833 low noise dual op amp made by National Semiconductor. Its excellent characteristics (previously featured in the Studio 200 Stereo Control Unit published in the June and July 1988 issues of SILICON CHIP) are largely responsible for the high performance of the circuit. It not only has very low noise and distortion, but can also drive 6000 lines which is an advantage in this circuit. Fig.4 (next page): the circuit has 32 ► gyrator circuits connected in parallel into the negative feedback loop of IClb. ICla functions as an input buffer stage. MARCH 1989 47 +15V INPUT~ BYPASS 10 16VW S1 ,--0 1.,.~1~-r;r- ~OUTPUT >,:+-~+Ul1..-M .... 1 .0033! 10k 33pF 50k 50k 50k 50k 50k 1.1k +15V 1k 910!l 1k 0.22 0.27 0.391- 0.47 0.68 910!l +15V 0.18 0.12 220k .,. 20Hz 16Hz 32Hz 25Hz 40Hz JC2·1C9 : LF347 ONLY 50k 50k 50k .082 0.1 910!l 1k +15V .027 ... .047 .068 1.1k 320Hz 50k 1.1k 50k .0047 910!l .0022 500Hz 50k .0068 1k +15V 400Hz 50k .0082 .01 +15V .015 250Hz 50k 1k 1k .018 200Hz 50k 50k .0039 .0015 .0018 1k 1k .0012 .00331-.001 220k .,.. 2kHz 2.SkHz 3.2kHz 4kHz STUDIO SERIES THIRD OCTAVE EQUALISER 5kHz POWER 01-04 4x1N4002 .,oa:U.;.T...._ _ _ _.,__ _ _-+---t----+---t----+-----+15V + E tm7 2200 25VW CASE 10 16VW _ "o"'u=r....____________.....,....._ _...._ _.....,....._ _..........__ 15v -~"' . ill"' GND IN 0.27 910!:l 1k f+· 0.22 fi""' 0.15 910\l 1k .056 .068 f+· 0.15 i3""' 910(] 1k .033 .047 220k 160Hz 63Hz 50Hz .033 r+· .027 fi""' .022 1k 910!:l 125Hz 100Hz 80Hz ft·· .015 910!:l fi""' .015 f+· 910!:l 1k .0068 1.6kHz 800Hz p. . 630Hz 50k ,.,; p. . '"" 1k 910!:l 680pF 1kHz +15V 910\l 560pF ·'"'" FI·. 1k 470pF 1.25kHz ·'"'" Ff""' .001 910\l 330pF ff""' 1.1k 270pF +15V 20kHz 6.3kHz 8kHz 10kHz 12.5kHz 16kHz The new equaliser is easy to build with virtually all the circuitry accommodated on three printed circuit boards. Plug in wiring connectors take care of most of the wiring between the two main boards. The gyrators are all based on LF347 quad FET-input op amps, made by National Semiconductor. It is important that these are used and not the ostensibly equivalent TL074s made by Texas Instruments. The reason for this is that op amp gyrato-r circuits have a tendency to misbehave when the power is turned off. As the supply rails to the gyrators drop to very low values, they can burst into high frequency oscillation which then dies away as the supply rails drop further. The effect of this misbehaviour is that the equaliser emits a loud chirp, about a second or so after the power is turned off. Since this sort of behaviour is undesirable, it is essential that LF347s be used instead of TL074s. While these op amps are functionally equivalent they are quite different in their internal circuitry and so behave differently as their supply rails are reduced to very low values. Power supply Power for the circuit is provided 50 SILICON CHIP The power supply PCB is mounted on the rear panel and delivers regulated ± 15V rails to power the equaliser circuitry. by a 30V centre-tapped mains transformer feeding a bridge rectifier and two 2200µF capacitors. This produces unregulated supplies of about ± 21 volts which are then fed to 3-terminal regulators to produce balanced supply rails of ± 15 volts. The outputs of the regulators are bypassed on the power supply board with lOµF capacitors and on the main circuit board with 220µF, 100µF and O. lµF capacitors. A light emitting diode in series with a 3.3k0 0.5W resistor across the ± 15V supply rails functions as the power indicator on the front panel. That's all we have space for this month. Next month we'll present the full details of construction. ~