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Studio series 20-band stereo equaliser This completely new stereo equaliser is intended for home or professional use. It has a performance equal to or better than the finest commercial models but you can build it for a fraction of their price. By LEO SIMPSON & BOB FLYNN Have no doubt about it. This new equaliser has a performance that gives away nothing to the very best commercial equalisers. It has very low residual noise and harmonic distortion - much lower than any previously published design. The new stereo equaliser is a half-octave design, with 20 bands per channel. As a half-octave unit, it is a compromise between the high resolution of a third-octave design, as published in our March and April 1989 issues, and the normal octave band arrangement of a stereo equaliser. Ideally, we would have liked to produce an upgraded stereo ver26 SILICON CHIP sion of our third octave design mentioned above but it just would have been too big and unwieldy - hence the compromise of a 20-band stereo design which still fits into a standard rack mount case. But while the number of bands is a compromise, the performance is not. It rivals the performance of compact disc players. As you can see from the accompanying specification panel, the harmonic distortion is particularly low while the signal to noise ratio is very good - better than - 104dB unweighted with respect to 1V RMS. That is better than many compact disc players. The half octave band frequencies are as follows: 28Hz, 39Hz, 55Hz, 78Hz, 110Hz, 156Hz, 220Hz, 312Hz, 440Hz, 625Hz, 880Hz, 1.25kHz, 1.75kHz, 2.5kHz, 3.5kHz, 5kHz, 7kHz, lOkHz, 14kHz and 20kHz. Adjacent frequencies have a relationship between them which is close to 1.414 [the square root of 2), giving the half octave factor. By contrast, in an octave band equaliser, the bands increase by a factor of 2; eg, 625Hz, 1.25kHz, 2.5kHz, 5kHz and so on. With a half octave equaliser, you have much finer control over the equalisation which is desirable whether you are doing PA work, customising your own tapes or equalising rooms and loudspeakers. Special slider pots So each channel has 20 slider pots. These are the special slider pots which were first featured in our third octave equaliser mentioned above. They are specially imported by Jaycar Electronics. select the equaliser function or pass the signal through completely unmodified. The back panel is completely bare except for a group of 12 RCA phono sockets. Why so many? Two pairs are for the inputs and outputs to your stereo amplifier while another two pairs are to duplicate the Tape Monitor function on your amplifier, as already mentioned. Another pair is for equalised outputs which are always available, regardless of the settings of the front panel buttons. These equalised outputs can be very useful if you want to custom equalise your tapes when dubbing, say for use in your car. The remaining pair of RCA sockets is not used. 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 audible 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 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 of travel, antilog in 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. Apart from the 40 sliders, there are three switches on the front panel. On the righthand side is the push-on push-off mains switch and above it is the red LED power indicator. On the left hand side are two push-on push off switches. The top one is the Tape Monitor loop. It replaces the Tape Monitor function on your stereo amplifier. This is necessary because normally you would connect the equaliser into the tape monitor loop for convenience of use. That is not to say that you can't connect the equaliser in the signal path between a stereo control unit and power amplifier. However, if you want to use it for equalising tapes when dubbing, it is more convenient if it is in the tape monitor loop. The lower pushbutton switch is a bypass control which allows you to Chassis details Inside, virtually all the wiring is taken care of by three printed circuit boards. There is one long board to accommodate the 40 slider controls and another large board to accommodate the active equaliser circuitry. Finally, a smaller board takes care of the power transformer and power supply circuitry. This latter board is exactly the same as used in the third octave Specifications Frequency Response Equaliser out Equaliser in Boost and cut Flat 1 0Hz-20kHz ±0.5dB; -3dB at 60kHz ±12dB Signal Handling Gain Maximum input and output Unity 8 .5 volts RMS (all controls flat) Harmonic Distortion <.005% for frequency range 1 0Hz to 20kHz; typically better than .001% Separation Between Channels With respect to 1 V RMS - 77dB at 1 0kHz; -95dB at 1 kHz ; -98dB at 100Hz Signal to Noise Ratio With respect to 1 V RMS 1 04dB unweighted (20Hz-20kHz) 1 05dB A-weighted Input Impedance 100kD Output impedance 470D AUGUST 1989 27 10k INPUT~Mf.-......- - - - 1 R2 lk 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. equaliser although some of the filter capacitor values used in the circuit are different. 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. Circuit principles The circuit principle used in virtually all of today's graphic equaliser designs is the same. We have already talked about this principle in our previous equaliser articles but for the sake of completeness, we will repeat the description here. Each 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 one just to keep things simple. Now consider how it works. With the 5Dk0 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 boost end, the negative feedback 28 SILICON CHIP Fig.2: the circuit configuration of a gyrator. The op amp transforms capacitor C into an inductor which is proportional to Rt, R2 and C. 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 le .............. ~ .c--: Fig.3: this diagram shows the relationship between the voltage and current in the gyrator circuit of Fig.2. 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; 40 in fact, one for each frequency band, in each channel. But instead of inductors, our circuit follows normal design practice and uses gyrators instead. 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 R1, 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. V0ut then causes a current to flow The new equaliser is easy to build with virtually all the circuitry accommodated on three printed circuit boards. Note the use of the miniature encapsulated 5% tolerance capacitors which not only enable a much smaller printed circuit board but also give improved performance. Plug in wiring connectors take care of most of the wiring between the two main boards. 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 two resistors and a capacitor connected to it. The inductance is given by the formula: L = Rt x R2 x C where L is in Henries, R is in ohms and C is in Farads. To make the tuned LC circuit shown in Fig.t, all we need do is to connect a capacitor in series with the input to Fig.2. Now refer to the main circuit diagram. This shows just one channel of the stereo equaliser which is basically just one gyrator circuit repeated 20 times, with different values for Rt, R2 and C. The key op amp in the circuit is IC2a and it performs the same function as the one in Fig.1. 20 50kQ slider pots are connected in parallel in the feedback network of lC2a and each has an associated The power supply PCB is adjacent to the power switch and delivers regulated ± 15V rails to power the equaliser circuitry. gyrator and additional series capacitor. For example, the gyrator for the 55Hz ½-octave band is IC3c and this is connected to the wiper of the slider via a tµ.F capacitor. Similarly, for the 1.75kHz band (immediately below IC3a on the main AUGUST 1989 29 TAPE PLAYBACK +15V r- LINE ~r-.r. I~ LINE INPUT 11 II 0.47 DUTPUT · 100k EQUALISED TAPE OUTPUT .,. II I -=J!l TO TAPE FIGURES IN BRACKETS INDICATE RIGHT CHANNEL DEVICES. 220pf 50k 50k 0.47 820() 750() 680Q 680() -15V 28Hz 55Hz 39Hz 680() .068 +15V 91k -15V 110Hz 78Hz .,. 50k 50k 620() 50k· 50k 620() 6200 50k 620P. +15V 880Hz 1.25kHz 1.75kHz 62011 .0022 2.5kHz 3.5kHz IC3-1C7: LF347 ONLY LEFT HAND CHANNEL SHOWN. ALTER ICS HAVE SAME NUMBERS IN RIGHT CHANNEL STUDIO SERIES HALF OCTAVE EQUALISER Fig.4: the circuit shows one channel of the new stereo equaliser. Each channel has 20 gyrator circuits connected in parallel into the negative feedback loop of IC2a. ICla functions as an input buffer stage. circuit}, the gyrator is IC5a and it is connected to the wiper of its slider via a .033/.lF capacitor. Apart from the 20 gyrators and their common unity gain feedback amplifier, IC2a, there is only one other op amp, ICla, which func30 SILICON CHIP tions as an input buffer stage with a gain of unity. ICl and IC2 are LM833 low noise dual op amps made by National Semiconductor. IClb and IC2b are not shown on the circuit but they provide the identical circuit func- tions in the other channel. The excellent characteristics of the LM833 (previously featured in the Studio 200 Stereo Control Unit published in the June and July 1988 issues of SILICON CHIP) are a major factor in obtaining the high performance of the circuit. It not only has very low noise and distortion, but can also drive 6000 lines which is S3 01 -04 4x1N4002 OUT 240VAC +15V A + LED1 .,. nh7 1000 25VW CASE + 4, 100 16VW + - - - 220 16VW -15V OUT p. p. 0.33 0.15 0.22 6800 6800 .068 ii'"' + 220 16VW + 4x 100 16VW 10 16VW 4x0.1 "·"' 680!l ii'"" 0.1//.022 .022 0.1 620!l +15V .015 P™ 620!1 .01 110k 156Hz 312Hz 220Hz p. p. .00.. .01 .015 6200 P·· 620!1 .0015 .0022 +15V 440Hz ·""" 620!l P·· .ooaa 620!l 680pf .001 625Hz ff'"" 620!l 680pf 47k 5kHz 7kHz 10kHz 5 ffi IN OUT GND an advantage in this circuit. The other major factor in obtaining the high performance is the use of 5 % metallised plastic capacitors for all the critical audio filter stages. More particularly, except for the very largest values, all the capacitors specified are metallised polycarbonate. These have a better power factor than the more corn- 14kHz 20kHz 5 ffi GND OUT IN mon metallised polyester capacitors, particularly at the higher frequencies, and this is an important factor in the very low distortion figures obtained. All the 5 % metallised plastic capacitors in our prototype were kindly supply by Adilam Electronics Pty Ltd who are the Australian agents for Wima capacitors. Another benefit obtained from specifying 5% capacitors is that the tuned frequency and Q of each gyrator stage is much more precisely defined. In fact, to be really sure of obtaining the correct Q and the specified boost and cut figures at the higher audio frequencies, 5 % polycarbonate capacitors must be used. Some varieties of "greencap" AUGUST 1989 31 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 2 2-pole push on/push off switches with mounting brackets 40 50k0 45mm silder pots with 4BM taper, Jaycar Cat. RP-3914 1 cord-grip grommet 8 1 2mm PC board spacers 8 10mm PC board spacers 8 6mm PC board spacers 8 3mm x 25mm countersunk screws 4 3mm x 1 5mm countersunk screws 4 3mm x 1 5mm roundhead screws 6 3mm x 6mm screws (to mount transformer and RCA socket panel) 24 3mm nuts 1 insulated panel with 1 2 RCA sockets 4 stick-on rubber feet 1 solder lug 8 10-way pin headers 8 1 0-way connector sockets 4 4-way pin headers 4 4-way connector sockets 20 1mm PC pins rated insulated hookup wire (for power switch) Printed Circuit Boards 1 •main equaliser PCB, code SC01103891 , 262 x 150mm 1 power supply PCB, code SC01103892 , 113 x 7 4mm 1 equaliser control PCB, code SC01107892 , 370 x 78mm 1 mains switch shield, made of PCB copper laminate (see text) Semiconductors 10 LF34 7N quad op amps 2 LM833 low noise op amps 1 7 81 5 3-terminal regulator 1 7915 3-terminal regulator 4 1N4004 rectifier diodes 1 5mm red LED Capacitors 1 2200µF 25VW PC electrolytic 1 1 000µF 25VW PC electrolytic 4 220µF 16VW PC electrolytics 8 100µF 16VW PC electrolytics 2 10µF 16VW PC electrolytics 2 2.2µF 50VW bipolar electrolytics 8 0 . 1µF monolithics 4 220pF ceramics Cable 1 3-core mains cord and moulded 3-pin plug 1 800mm length of 8-way rainbow cable 1 1 -metre length of figure-8 shielded audio cable 1 400mm length of 250VAC are quite poor in their high frequency power factor and thus can significantly degrade gyrator performance. 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. Nor should the pin-forpin replacement LM837 be used. This is superficially a quad version of the LM833 but it does not per32 SILICON CHIP Audio Filter Capacitors (5% - see text) 6 1µF MKS2/5/63 polyester 4 0.68µF MKS2/5/63 polyester 4 0.47µF MKC2/5/63 polycarbonate form as well in this circuit as the specified LF347s. Power supply Power for the circuit is provided by a 30V centre-tapped mains transformer feeding a bridge rectifier. The positive supply is filtered by a 2200µF 25VW capacitor while the negative supply has a lOOOµF 25VW capacitor. This produces unregulated supplies of about ± 21 volts which are then fed to 3-terminal regulators to produce 6 0 .33µF MKC2/5/63 polycarbonate 6 0.22µF MKC2/5/63 polycarbonate 4 0 .15µF MKC2/5/63 polycarbonate 6 0.1 µF MKC2/5/63 polycarbonate 6 .068µF MKC2/5/63 polycarbonate 4 .033µF MKC2/5/1 00 polycarbonate 6 .022µF MKC2/5/1 00 polycarbonate 6 .015µF MKC2/5/100 polycarbonate 4 .01 µF MKC2/5/ 100 polycarbonate 6 .0068µF FKC2/5/100 polycarbonate 6 .004 7 µF FKC2/5/ 100 polycarbonate 4 .0033µF FKC2/5/100 polycarbonate 4 .0022µF FKC2/5/100 polycarbonate 2 .0015µF FKC2/5/ 100 polycarbonate 2 .001 µF FKC2 /5/ 100 polycarbonate 4 680pF FKC2/5/ 100 polycarbonate Resistors (¼W , 1 %) 2 1MO 2 51k0 6 1 10k0 2 47k0 8 1OOkO 4 5.6k0 6 91k0 2 8200 6 82k0 2 7500 4 75k0 12 6800 4 68k0 24 6200 2 62k0 2 4700 2 56k0 1 3.3k0 , ½W 5% 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 , lOOµF and O.lµF capacitors. A light emitting diode in series with a 3.3k0 ½ W 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. ~