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Balanced input & output stages for the Studio Series equalisers Have you built the Studio Series Third Octave Equaliser or the Studio Series Half Octave Equaliser? Or do you have a piece of audio gear which could benefit from balanced lines? If so, you'll want this little board which gives balanced input and output stages. Design by JOHN CLARKE When we featured the Studio Series 32-Band Graphic Equaliser in the March and April issues of this year, we were well aware that many users would want to build the unit with balanced inputs and outputs. Here then, is the add-on board we promised in the April issue. If you have to run audio signal cables for long distances, say more than 5 or 6 metres, balanced lines can give a big improvement in the amount of hum and noise in the system. They can also obviate most of the hum problems which occur due to earth loops. What we propose to do in this article is to present a printed circuit board which has balanced inputs and outputs and which can be built in mono or stereo form. We'll show how it is installed in both the mono 32-band equaliser presented in March and April and the stereo 20-band equaliser presented in August and September. After reading this article, it will be fairly obvious how to use this board in other applications, such as adding balanced inputs to a stereo amplifier or say, balanced outputs to a stereo mixer. For the present though, let's just consider the equalisers mentioned above. Now refer to the circuit of Fig.1. Fig; 1 shows two separate circuits, one for balanced inputs and one for balanced outputs. Let's look at the balanced input stage first. Balanced input This view shows the mono version of the balanced input/output board installed in the chassis of the 32-Band Third Octave Equaliser. Note the use of shielded cable for the audio input and output connections. 74 SILICON CHIP The balanced input stage involves a dual op amp, ICla and ICl b. Strictly speaking, the circuit should be called a balanced to unbalanced stage since it effectively converts balanced lines to unbalanced output. Alternatively, you could think of ICl as providing a differential input and a single ended output. The method of connecting the op amps of ICl is a little unusual. ICla is connected as a conventional noninverting amplifier with feedback from the output to the inverting input via R2 and Rl. Since Rl and R2 have equal values, the gain of ICl is 2. I Cl b does double duty. For signals from the output of ICla, it functions as an inverting amplifier with a gain of 1 and so gives an overall gain of - 2. However, for R2 1k R4 1k FIX-A-KIT Kit Repairs - $15 per hour. ~,~:2f,- Normal Rate - $35 per hour. 33 INPUT 3 + 1 33 OUTPUT TO EQUALISER INPUT - 10k .,. ~ '":" i -15V R1 1k 10k No charge for kits that can't be repaired. 3 month Warranty on repairs. Construction - fixed or hourly 1 2 Month Warranty on Manufactured Kits. +15V + LM833 10:r Custom Designing, Manufacturing, Large or Small Quantitites. .,. INPUT FROM EQUALISER OUTPUT i 100k HYCAL .,. BALANCED LINE OUTPUT 10k INSTRUMENTS Design, Manufacture, Repair of Electronic Equipment. (02) 633 5897 Unit 4, 62 Great Western Highway, Parramatta, NSW 2150. BRACKETS SHOW SECOND CHANNEL ICS IF REQUIRED Trading hours: Barn to 3pm Monday to Friday. BALANCED LINE INPUT AND OUTPUT FOR EQUALISERS BARGAIN COMPONENTS PRICE Fig.1: the circuit consists of two separate parts, one to provide the balanced inputs (IC1) and the other to provide balanced outputs (IC2). The ± 15V rails are derived from the equaliser circuitry. signals to its non-inverting input (pin 5) it functions as a noninverting amplifier with a gain of + 2. Both input signals therefore-have a gain of 2 via IC1. To enable them to be fully floating, both input signals are AC-coupled via 33µ.F capacitors. Ideally, for best balance in the circuit, resistors Rt, R2, R3 & R4 should be as closely matched as possible. This will give the highest possible rejection of common mode signals which means best rejection of hum and noise. In practice, with t % resistors for Rt to R4, the common mode rejection will be around - 80dB which is pretty good. Now let's look at the balanced output stage which involves IC2. It could be correctly thought of as an unbalanced to balanced line driver. IC2a is relatively straightfor- ward to understand. It is connected as a non-inverting voltage follower with the negative feedback being virtually direct from the output at pin 7 to the non-inverting input at pin 6. So IC2a gives an output (at pin 3 of the XLR socket) which is in phase with the input. To get the 180° out-of-phase output, the signal from pin 7 of IC2a is fed to IC2b which functions as an inverting amplifier with a gain of unity. Its output is fed to pin 2 of the XLR socket. Both of the output lines are AC-coupled via 47µ.F capacitors. The low noise LM833 dual op amp from National Semiconductor has been specified for both !Cl and IC2. First used in the Studio 200 Stereo Control Unit published in June and July 1988, this op amp has proved itself as a very quiet unit and it is ideal in this application ITEM COMMENTS AUST. 74LSOO Nand Gate 0.60 74LS32 Or Gate 0.60 7 4LS244 Octal Driver 2.00 7 4LS3 7 3 Octal latch 1.80 4001 Nor Gate 0.50 4011 Nand Gate 0.50 4013 Dual Flipflop 0.75 4017 Decade Counter 1.20 4040 12-Stage Counter 0.95 4066 Analog Switch 0.75 4082 And Gate 0.70 1488 RS-232C Trans. 1.20 1489 RS·232C Rx. 1.20 LM555 Timer 0.65 UA324 Quad Op Amp. 0.95 LF351 =TL071 Op Amp. 1.10 LF353 =TL072 Dual 1.60 LF347 =TL074 Quad 3.35 LM1458 Dual Op Amp. 0.95 LM13600 Dual Transcon. 3.10 LM393 Dual Comparator 1.05 LF833 LIN. Op. Amp. 3.05 LM3914 Led Bar Driver 5.75 LM317T 2·30V ; 1A Adj. 1.75 LM340T5, T12, T15. Reg. 1.05 LM7905, 12, 15. Neg. Reg. 1.15 MPSA06 80V NPN Trans. 0.90 MPSA56 80V PNP Trans. 0.90 RED LEDS 0.15 GREEN LEDS 0.30 50 Mixed Diodes 2.40 4 MEG . Crystal 2.40 10 of one item & receive one free NZ customers add 12.5% GST All orders add $5.00 P&P; airmail extra. Send cheque, money order to: N.Z. 0.75 0.75 2.50 2.25 0 .65 0.65 0.95 1.50 1.20 0.95 0.95 1.50 1.50 0.85 1.20 1.40 2.00 4.20 1.20 3.95 1.35 3.85 7.20 2.20 1.30 1.45 1.15 1.15, 0.20 0.40 3.00 3.00 SAMBOR EL CTRONICS 143 BAMB0R0UGH STREET INVERCARGILL, NEW ZEALAND Phone: 021 77456 DECEMBER1989 75 INPUT -----------.BYPASS ~50k Sl One point should be noted about the balanced input stage, particularly as it applies to the stereo equaliser. Since the balanced input stage has a gain of 2, it is twice the gain of a conventional equaliser with all controls set for flat response. With a gain of 2, the maximum input signal that the equaliser can handle without overload will be reduced by 2 also. In most situations this will not cause a problem but you should be aware of it. OUTPUT ~.,. -:- (a) Fig.2a: basic block diagram for the 32-band equaliser. BALANCED INPUT AMPLIFIER BALANCED OUTPUT AMPLIFIER ..... ,--'-',--03 / BA~:Pi~ED 30---d""".d •-.;,...,.-02 BaM~8¥D 1 (b) Fig.2b: how the balanced input and output stages are added to the 32-band equaliser shown in Fig.2a. since it is capable of driving 6000 lines. Fig.2 shows how the circuits of Fig.1 are connected into the 32-band graphic equaliser referred to above. Fig.2a shows the basic block diagram of the equaliser while Fig.2b shows how the balanced inputs and outputs are added in. Essentially what happens is that the existing input and output cables are disconnected from the RCA sockets and connected to the balanced circuits instead. Fig.3 is more complex and shows how balanced inputs and outputs can be added to the the 20-band stereo equaliser. Fig.3a shows the block diagram of the stereo equaliser while Fig.3b shows how the balanced inputs and outputs are added. Building the board To enable you to add the balanced input and output stages to the mono or stereo equalisers, we have designed a two channel board. That way, you build one half of the board to suit the 32-band mono equaliser or both halves to suit the stereo equaliser. Fig.4 shows the board layout for one channel while Fig.5 shows the layout for two channels. The parts list shows firstly the parts for the mono version and then the extra parts needed to complete the stereo version. The PC board measures 87 x 75mm and is coded SC 01112891. It should be fitted with PC pins at both ends to take the input, output and power supply connections. BYPASS TAPE PLAYBACK OUTPUT LINE INPUT EQUALISED OUTPUT -;- TO TAPE .,. 00 Fig.3a: block diagram of the 20-band stereo equaliser (one channel only shown). BALANCED DUTPUT AMPLIAER TAPE ~ PLAYBACK+ BALANCED INPUT BYPASS 100k '::c" ~EQUALISED OUTPUT TO~ TAPE (b) Fig.3b: how th~ balanced input and balanced output stages are added to the 20-band stereo equaliser. 76 SILICON CHIP ~ ~~ ::-:--=--.._..• v V INPUT FROM ALISER OUTPUT BALANCE INPUT CHANNEL 1 OUTPUT TO EQUALISER INP ALANCEO LINE OUTPUT 47 t l.?8 <; II I~ :)2 I Fig.4: this diagram shows the parts layout on the PC board for the mono version. Use PC stakes to terminate the external connections and note carefully the orientation of the ICs and capacitors. 1• BALANCE INPUT CHANNEL 2 OUTPUT TO EQUALISER INP --= 1,,-L~ ~~ I INPUT FROM UALISER OUTPUT BALANCED LINE OUTPUT 5V 5V BALANCE INPUT INPUT FROM UALISER OUTPUT CHANNEL 1 OUTPUT TO EQUALISER INP BALANCED LINE OUTPUT I Fig.5: the stereo version of the project uses the same PCB pattern as the mono version. All you have to do is add two more ICs plus their associated resistors and capacitors. When assembling the board take particular care with the polarised capacitors and the ICs. All four ICs are oriented in the same direction. Once the board has been fully assembled, it should be put aside so that work can be done on the equaliser chassis. In the case of the mono 32-band equaliser, it should be fitted with one female 3-pin XLR socket (for input) and one male 3-pin XLR socket (for output). Similarly, the stereo equaliser should be fitted with two female 3-pin XLR sockets and two male 3-pin XLR sockets. These are wired to the board using shielded cable. When the XLR sockets are mount- PARTS LIST 1 PC board, code SC 01112891, 87 x 75mm 12 PC pins 4 plastic PC mounts 1 3-pin chassis female XLR socket (Jaycar Cat. PS 1030) 1 3-pin chassis male XLR socket (Jaycar Cat. PP 1020) 2 LM833 low noise dual op amps 2 4 7µF 35VW PC electrolytic capacitor 2 33µF 35VW PC electrolytic capacitor 2 1 OµF 35VW PC electrolytic capacitor 1 100k0 ¼W 4 10k0 1 % ¼W 4 1k0 1% ¼W 2 330 ¼W Extra parts for stereo version 1 3-pin chassis female XLR socket (Jaycar Cat PS 1030) 1 3-pin chassis male XLR socket (Jaycar Cat PP 1020) 2 LM833 low noise dual op amps 2 47µF 35VW PC electrolytic capacitor 2 33µF 35VW PC electrolytic capacitor 1 100k0 ¼W 4 10k0 1 % ¼W 4 1k0 1% ¼W 2 330 ¼W Miscellaneous Shielded cable, screws, nuts, washers, solder. DECEMBER1989 77 "''~ --D -~(~ MAIN BOARD POTENTIOMETER BOARD ....~ C ....~ '"""'~ \0 SK3aO '"""'~ ""~ "'B SK3bo ""~ .,,> m•~ + "'~ ~, -~:~ >c > "'"'.,, ~"' + o'\_ A ~ I +15V POWER SUPPLY BOARD ).0 TRP~~i:~~MER ~ r-r--~~\ \ \ H G F N (BLU) E A (~RN) ( / CORO CLAMP GROMMET rN ~~(GRN/YEL) ~ ~ EARTH LUG Fig.7: here's how to wire the balanced input/output stage into the 32-band graphic equaliser. The best place to mount the PCB is at the front of the chassis, immediately behind the potentiometer board. The ± 15V rails are derived from the existing power supply board. 78 SILICON CHIP C~ G ,------_,U-1-~ .. J~ ~ H - D~ ,... ~I S2 ..... K +15V ov 1c:::::::J SK6a 1c:::::::J SK5a <at>,~, LEFT ~213 ~ OUTPUT SK6b 1c::::J .{lj (i::) 2 E - 1(=:::J SK5b INPUT RIGHT MAIN BOARD OUTPUT F -~ +15V OV -15V BALANCED INPUT/OUTPUT BOARD POTENTIOMETER BOARD A ~LEO K Fig.a: for the 20-band stereo equaliser, the balanced input/output board can be mounted on the rear panel, along with the four XLR sockets (female for the inputs, male for the outputs). The PCB can be supported on pillars and secured using screws and nuts, or you can use plastic PC supports. DECEMBER 1989 79 This view shows how the two XLR sockets were mounted on the rear panel of the 32-band equaliser. Use shielded cable for the connections to these sockets. ed, the balanced board should be mounted inside the equaliser chassis using four metal pillars, screws and nuts or alternatively, using plastic clip-in PC mounts such as those made by Richco. The female XLR socket at left is used for the input connections while the male XLR socket at right provides a balanced signal output. Follow the wiring diagram of Fig.7 for the mono version and Fig.8 for the stereo version. When all your wiring is complete, check it carefully and then apply power. Check that + 15V is present at pin 8 of each IC and that - 15V is present at pin 4. Check also that pin 1 and pin 7 of each IC is close to OV (ie, within ± 10mV). That being the case, everything is apples and you are in business. · ~ Protect vour valuable issues! SILICON CHIP BINDERS These beautifully made binders will protect your copies of SILICON CHIP. In distinctive two-tone green high-quality vinyl specially selected for SILICON CHIP, and with heavy board covers, each binder holds a year's issues (the 14 issues of Vol.1 or the 12 issues of Vol.2). It will look great on your bookshelf. * High quality * Hold up to 14 issues * 80mm internal width * Gold printed with SILICON CHIP logo on spine •;:·:} ~~~ ;~~~..... :,~ ~,,,... ..,,~ 80 SILICON CHIP . '~,;~,;!~.!·..., . "' ... Price: $All.95 plus $A3 p&p each (NZ p&p $A6). Send your order to SILICON CHIP Publications Pty Ltd, PO Box 139, Collaroy Beach, NSW 2097; or use the handy order form on page 115.