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16-Channel Mixing Desk, Pt.2 In this second article on our new 16-Channel Mixing Desk, we present the details of the circuitry. The design is based largely on the LM833 low noise dual op amp instead of the 5534 used in previous mixer designs. By JOHN CLARKE & LEO SIMPSON To be able to easily follow the circuit description, you will need to refer to the description of the block diagram, Fig.1, in the first article in this series. Now let's look at Fig.2 which is the circuitry used on each of the 16 input channel boards. This is depicted schematically along the top of the block diagram, Fig.1. The input signals come from pins 2 and 3 of a 3-pin XLR socket and are fed via 33µF capacitors to the inputs of ICla which is half of an 60 SILICON CHIP LM833 dual low noise op amp. Switch S1, a 3-pole 3-position switch, switches the feedback network (Slc), the input attenuator (Slb) and pin 2 of the input socket for the balanced or unbalanced input condition (Sla). Let's see how these conditions come together. When switch S1 is in setting 1, the input channel is connected to suit balanced microphone outputs. In effect, ICla operates as an inverting amplifier for signals from pin 2 of the XLR socket and as a non-inverting amplifier for signals from pin 3 of the XLR socket. In both cases, the gain is determined by the two resistors connected to the noninverting input, pin 2; ie, 10k0 and lkO. Gain for the inverting signal path is 10k0/1k0 = 10 while gain for the non-inverting signal is 100/lkO + 1 = 11. To compensate for this difference in path gain, a resistive attenuator is connected in series with the signal to pin 3 of ICla. This attenuates the signal by a factor of 0.909. When multiplied by the nonFig.2 (opposite): this is the circuitry for each of the 16 input channels. IC1a & ICtb are input preamplifier and buffer stages, IC2a a 3-band tone control stage, IC2b the post fade buffer and IC3a, D1, D2, Qt & Q2 the headphone driver stage. IC4, IC3b & IC5 provide overload indication. ~ ~ 0) 0 CD ....CD :r: n !Jl 10kVR6 LOG ~ - · FADER~• ■ - 16VW 100 - ' . " WI+ ' . 0.1! o•.w.I I l 4.7!! IC1 t8 0.1 IC2 I- \, .,. 6.8k ':'" 0. 1 I v- POST FADE BUFFER 1 OF 16 .,. 16VWI - I 1 V+ 011 1 2 ~ BUS EFFECTS CH2/CH4 BUS HEADPHONE MONITOR 1 OF 16 j 100pF EFFECTS~10k VR7 10k LOG S2b . DlI .,. I ~ 1L ., r . r r- 0·1 10k Sl: 1: MIC 2 : LINE BAL 3: LINE UNBAL 1.2k 1¾ 2k 1°1. CH1/CH3 BUS ATTE~~~TOR 10k LOG INPUT .,. "~h L 330 ~ .,. PEAK DETECTOR 1 OF 16 INPUT BUFFER AND AT~E~~WR -- -i. 1 OF 16 v- 3-BANO EQUALISER .0027 33k VIEWED FROM BELOW B ' EOc v+ .. ,. S2a ' .~- S2 : 1 : CHANNEL OUT 2 : CHANNEL IN 10k LOG FOL~i~CK~ FOLDBACK BUS All the circuitry ,shown in Fig.2 is accommodated on a single PCB. There are 16 of these boards altogether - one for each input channel. Note how the pots are mounted directly on the board to cut down on the internal wiring. inverting gain of 11, it gives an overall gain of 10, the same as for the inverting gain. Hence, the circuit provides a perfectly balanced input. The input impedance is 1.BkO which comprises the lkO input resistor network in the inverting signal path and the lkO input divider in the non-inverting signal path, with both shunted by lOkO resistors to earth. Line balanced & unbalanced In position 2 of switch S1, the input channel is connected to suit balanced line inputs. The gain is reduced by a factor of 10 by switching the 1.2k0 resistor across the 1OkO resistor between pins 1 and 2 of ICa (Slc). This provides a gain of 1.07 for the inverting signal path and a gain of 2.07 for the non inverting signal path. This disparity of gain is compensated for by the switched attenuator (Slb) in the non-inverting signal path which reduces the signal by a factor of 0.4825. When multiplied by the non-inverting gain of 2.07, the overall gain is 0.999 which is pretty close to the inverting gain of 1.07. Thus, by virtue of This board accommodates all the effects send, effects return & foldback circuitry shown in Fig.3 .. There are two such boards in the mixer. As before, the pots are soldered directly to the PCB to simplify the wiring. 62 SILICON CHIP the selection of feedback resistors and the attenuator associated with S1b, the circuit gives balanced input conditions for both line and microphone sources. In position 3 of switch S1, the input channel is connected for unbalanced inputs. The feedback and input attenuator is the same as for the balanced condition while the difference is that pin 2 of the XLR socket is grounded. Pre-fade attenuator Following the preamplifier stage is a non-inverting op amp stage comprising ICl b. This is labelled on the circuit as an input buffer and attenuator but most mixer operators would think of it as a "prefade attenuator". It enables the signal levels to be set up on each input channel. The gain of this stage is variable from 100 to unity by means of VRl. This stage is followed by IC2a which is a 3-band Baxandall tone control stage. VR2 provides the bass control function and has no effect on frequencies above about 300Hz by virtue of the .015µF capacitor shunting it. VR4 provides the treble control function and has little effect on frequencies below about 2kHz, by virtue of the .0015µF capacitor in series with the pot wiper. VR3 provides a midchannel control by virtue of the .002 7µF capacitor shunting the control and the .012µF capacitor in series with the pot wiper. 2.2 16VW o-4 ..---.~w.-....---"1 EFFECTS RETURN + CH1/CH3 BUS EFFECTS RETURN AND PAN (2) CH2/CH4 BUS . 22k PAN VR10 10k LIN 15k 6.Bk 2.2k 39k EFFECTS jjlJs +15V EFFECTS SUM AND LINE AMPLIFIER (2 ) 2.2k 100k 47 16VW ,. + -15V EFFECTS SEND LINE OUTPUT 39pF + 6.8k 39k FOLOBACK BUS F-0 1k .,. 2.2k FOLOBACK SUM AND LINE AMPLIFIER (2) 100k • +15V0 l 0.11 t 1001 16VW+ IC6 - 15V0 I o.,I 47 16VW + 7 F-o +15V FOLDBACK LINE OUTPUT ,. iB 6.8k IC8 r ,oo -I r .a + 1k -15V e- 16VW+ .,. Fig.3: the effects and foldback circuitry. IC6a & IC6b process the effects return signal while VR10 pans the signal between the CH1/CH3 and CH2/CH4 buses. IC7a & IC8a are the effects sum and foldback sum amplifiers, while IC7b and IC8b function as line amplifiers. Headphone driver The output of IC2a then feeds the foldback send control VR5 and the main fader control VR6 (bottom, left of the diagram). Following VR6 is the post fade buffer, IC2b, which has a gain of 2. It feeds the effects send control VR7, the pan control VRB and the headphone monitor stage, IC3a. This uses two transistors, Ql and QZ, to boost the output current to give sufficient drive for any stereo headphones, high or low impedance, that may be used. Diodes Dl and D2, in conjunction with the 330 emitter resistors, set and stabilise the quiescent current through Ql and QZ, to eliminate crossover distortion. Peak detector As well as driving the headphone monitor for each channel, the signal from the emitters of Ql and Q2 also feeds the peak detector which monitors for signal overload. IC4 and D3 function as a precision full wave rectifier. Its output is fed to the inverting ( - ) input of IC3b which functions as a corn- parator with hysteresis set by the 220k0 feedback resistor from pin 7 to pin 5. The non-inverting ( +) input is fed from a lOkO trimpot. This is adjusted to set the input signal level above which the comparator output, pin 7, goes low. When this happens, the 7555 timer, IC5, is triggered and turns on LED 21 for around 24 milliseconds which is quite long enough to be seen. Thus even the briefest of overloads will be registered by the circuit. And because IC4 functions as a full wave rectifier, peak MARCH 1990 63 There's quite a lot of wiring inside the mixer, despite the fact that all rotary pots are directly soldered to the boards. Most of this wiring is repetitive and includes power supply connections, wiring to the 20 slider contr.ol~, and input/output socket connections. Note that several boards are m1ssmg from this photo. overloads on both positive and negative half cycles of the audio waveform will be registered. Power supply connections Note that the positive and negative power supply connections for ICl and ICZ are not shown on their respective op amp symbols. They were omitted for clarity. However, these connections are shown on the supply rails, on the lefthand centre of the diagram, together with the various bypass capacitors on the rails. Note the 4. 70 resistors in series with both the positive and negative supply rail inputs. These provide a degree of supply decoupling, in conjunction with the 100µF bypass capacitors. As well, the 4.70 resistors will act as fuses in the unlikely event of a short on the board. If this happened, only one input channel would be disabled rather than possibly the whole mixer circuit. Effects and foldback The effects and foldback buffer stages are shown in Fig.3. Note that all of Fig.3 is accommodated on one printed board, of which there are 64 SILICON CHIP two. These stages are all based on the LM833 dual low noise amplifier. They are all quite straightforward but you should look at the block diagram, Fig.1, to understand how they fit into the overall scheme. IC6a, the effects return circuit, is a non-inverting unity gain buffer. IC6b, the effects pan driver, is a non-inverting amplifier with a gain of 2. IC7a and IC8a, the effects sum and foldback sum amplifiers, are inverting amplifiers, each with a gain of 17. IC7b and IC8b, the effects and foldback line amplifiers, are non-inverting amplifiers, each with a gain of 7.8. Note that the supply connections to IC6 and IC8 have again been omitted for clarity but they are shown on the supply bus, at the lower lefthand corner of the circuit. Now let's have a look at the 2-page circuit diagram, Fig.4. This is the circuitry for the 5-band equalisers and LED VU monitors. It also includes the master fader, the sum and post fade amplifiers and the headphone monitor stages. All of Fig.4 is accommodated on one printed circuit board, of which there are four. IC9a is shown as the "sum amplifier" connected to the CHl bus but remember that there are four of these boards so it could equally apply to the other three output channels. That is why it is shown as 1 of 4. You will find this note on several parts of the circuit as a reminder that the same circuit is on four boards. IC9a is an inverting amplifier with a gain of 34 (ie, 75k0/2.2k0). It drives the master fader VR13 and then the post fade amplifier IC9b which is a non-inverting amplifier with a gain of 4 (ie, 6.8k0/2.2k0 + 1). Five band equaliser IC10b, IC12 (a TL074 quad Fetinput op amp) and IC13 (a single Fet-input op amp) make up the five band equaliser. The five op amps in IC12 and IC13 are all connected as gyrators which are the equivalent of inductors. The gyrators act together with a series capacitor to form broad tuned circuits centred on 60Hz, 240Hz, lkHz, 3.5kHz and lOkHz, and give boost and cut of ± 12dB. Fig.4 (overleaf): the circuitry for the 5-band equalisers, output channel monitors and LED VU meters. The VU meter uses cascaded LM3915s (IC14 & IC15) and these are multiplexed between dot and bar modes to display peak and average signal levels. . drasticallY ust be . ,, Everything d -- stocks rn . of profit! .. reduce d irrespective cleare SH1 SH2 SH3 SH4 SH5 SH6 SH7 SH8 SH9 SH10 SH11 SH12 SH13 SH14 SH15 SH16 SH17 SH18 SH19 SH20 SH21 SH22 SH23 SH24 SH25 SH26 SH27 SH28 SH29 SH30 SH31 SH32 SH33 SH34 SH35 SH36 SH37 SH38 SH39 SH40 SH41 SH42 SH43 7 400 Quad 2-lnput NANO Gate 10 for 7 403 Quad 2-lnput Open Drain NANO Gate 10 for 7 404 Hex Inverter 10 for 7 408 Quad 2-lnput AND gate 10 for 7 41 0 Triple 3-lnput NANO gate 10 for 7 442 BCD to Decimal Decoder 10 for 7476 Dual J-K Flipflops with Preset and Clear 10 for 7 41 56 Dual 2-Line to 4-Line Decoder/Demultiplexer 10 for 7 4188 256-Bit PROM, Open Collector Output 10 for 75136 10 for 75451 Dual Peripheral Driver Positive AND Gate 10 for 7 4LS02 Quad 2-lnput NOR Gate 10 for 7 4LS04 Hex Inverter 10 for 74LS27 Triple 3-lnput NOR Gate 10 for 74LS51 Dual 2 Wide 2-lnput AND/OR Invert Gate 10 for 7 4LS86 Quad 2-lnput Exclusive OR Gate 10 for 7 4LS92 Low Power Divide By 12 Counter 10 for 7 4LS113 Dual J-K Flipflop with Preset 10 for 7 4LS1 23 Dual Monostable Multivibrator 10 for 74LS157 Quad 2-lnput Multiplexer 10 for 74LS161 Synch 4-Bit Binary 10 for Counter 7 4LS 1 63 Synchronous 4-Bit Binary 10 for Counter 7 4LS169 Synchronous 4-Bit Binary 10 for Up/Down Counter 74LS175 Quad D Flipflop 10 for 74LS367 Hex Bus Driver With 3-State 10 for Outputs 7 4S04 Ultra High Speed Hex 10 for Inverter 7 4S112 Fast J-K Flipflop With Preset and Clear 10 for 7 4S113 Fast J-K Flipflop With 10 for Preset 7 4S188 Schottky, 256-Bit PROM, 10 for Open Collector Output 7 4S241 Octal Buffers/Line Dr/Rx, 3-State Outputs 10 for 4020 CMOS 1 4-Bit Binary Counter 10 for 4049 CMOS Hex Inverter/Buffer 10 for $ $ $ $ $ 2.00 2.00 2.00 2.00 2.00 $ 2.00 BLACKTOWN i ,~2, STORE ~~ENING SOON SH44 SH45 SH46 SH47 SH48 SH49 SH50 SH51 SH52 $ 2.00 $ 2.00 $ 2.50 SH53 SH54 SH55 SH56 $ $ $ $ 2.50 2.00 2.00 2.00 $ 2.00 SH57 SH58 SH59 SH60 $ 2.00 SH61 $ 2.00 SH62 $ 2.00 $ 2.00 $ 3.00 SH63 SH64 SH65 SH66 $ 2.50 $ 2.50 $ 3.00 $ 2.00 SH67 SH68 SH69 $2.00 SH70 $ 2.00 SH71 $ 2.00 SH72 $ 2.00 $ 3.00 $ 2.50 $ 2.00 $ 2.00 6800P IC-CMOS 8-Bit Microprocessor, $ 1.50ea 1MHz 6821 IC-CMOS PIA Peripheral Interface $ 1.50ea Adaptor 2114P20 IC-MOS Static 4K RAM, 200ns $ 0.80ea $ 0.60ea 2114P30 IC-MOS Static 4K RAM, 300ns ML8205 Dual Tone Ringer 10 for $ 1.00 TR1865 Disc Controller $ 1.00ea WD2795A Disc Controller $ 1.00ea 10 for $ 1.00 8T26AN HT1-201-5 10 for $ 1.00 $ 0.60ea 7805/LM309, T03 Voltage Regulator $ 0.60ea 7815, T03 Voltage Regulator AMPLE PARKING IN ADJACENT STREETS ~ $ 2.00 SH73 SH74 SH75 SH76 SH77 SH78 SH79 SH80 SH81 SH82 SH83 SH84 SH85 SH86 Transistor, B0679 NPN, T0220 10 for Transistor, B0680 PNP, T0220 10 for IC, LM348 Op. Amplifier 10 for Benelec Analog Multimeter 20kohm Per Volt, With Case and Leads IEC Panel Mounting Socket IEC Switched & Fused Panel Mounting Plug IEC Panel Plug with Filter & Fused PSU Smoothing & Filter Unit 'Amiga' Computer Lead, Centronics to 25 'D' Female Hand Held Cassette Microphone-Stop/Start 5-pin 'DIN' In line Male Plug 10 for 5-pin 'DIN' In line Female Plug 10 for 5-pin 'DIN' 2-metre Lead, Male to Male, Flat Cable 5-pin 'DIN' 5-metre Lead, Male to Male, Flat Cable 5-pin DIN 9-metre Lead, Male to Male, Flat Cable IEC Mains Lead, Female In-Line to 2-Pin Plug 9-Way 'D' Female Plug Lead to 7-Pin Fem.Ski. 2 Metres Telephone 'Curly-Cord' with US Telephone Plug Transformer, 240V to 15V 200mA Power Pack, 240VAC to 15VDC 500mA ('OZ' Adapt. Req) 50-Way 'D', Male & Female Connectors 1 2 Assorted Multi-turn Trimpots SONY 3.5-inch Single Sided Disc Drive 400K MITSUMI 3.5-inch Single Sided Disc Drive 360K CROUZET Syncronous Motor, 240VAC 60 RPM Motorized Paper Tape Assy. With 30VDC Motor (OK for 1 2V) Telecom 'Commander' Interface Boards, Lots of 1 2V Relays 1 2-Digit Numeric Array Displays , MFTD by NSD Burroughs 12-Key Keypad in Case With Lead 'SHURE' Precision Stylus Force Gauge SUPER PACKS 1 OO's 1/4, 1/2 and 1 watt resistors SUPER PACKS 1 OO's Assorted Electro's SUPER PACKS Approx. 30 Assorted Potentiometers SUPER PACKS 1 OO's Assorted Disc Ceramic Caps SUPER PACKS 100 Power Diodes, 100V 1.5A Mini Relays, 5V DPDT Changeover Mini Relays, 12V Single Pole Changeover 'SAFT' Sub 'C' Nicad Batteries, 1.5Ah 'SAFT' 'AA' Lithium Batteries, 3.6V, Wire Ends 30 Assorted Resistor Networks, SIL & DIL Philips Drive Motors, Dual Spindle, 15V DC 40 x 40mm BNC Male to Female Leads, 3 Mir Length 41 64 DRAM, 1 50 n/sec Mail Charges: $15-$19.95 . . .. . . ..•.... $5 $20·$39.95 ... .. .•...... $7 $40·$69 .............. . $8 $70-$99 ..... . ........ $10 $100 and over ... ...... $15 Large or overweight articles sent Skyroad Freight Forward, Trading Hours: Mon-Frl . . . . . . . 9am•5.30pm Sat ........... 9.30am-1 pm $ 0.95 $ 0.70 $ 3.75 $33.00 $ 1.00ea $ 3.95 $ 6.00 $ 6.95 $ $ $ $ 6.50 2.50 1.50 1.50 $ 2.75 $ 3.25 $ 4.75 $ 2.00 SH47 Benelec 20kohmlvolt " '=t~· IEC switched & fused panel mounting plug V. ' H50 IEC panel plug with filter & fuse $ 1.25 $ 1.00 $ 1.95 · SH66 Sony 3.5-lnch single sided disc drive (400K) $ 9.50 $ 0.75pr $ 4.00 $12.50 $15.00 . 7 Mitsumi 3.5-lnch single sided drive (360K) $ 3.00 $13.50 $ 6.00 $ 0.50 ~ ';;' >'' $ 4.00 $ 2.50 ... _ _h ..... - - - - - - ' _ ,...., "' ,0 12-diglt numeric dlsp~~~ 1 $ 3.00 $ 5.00 . ' $ 3.00 2 $ 3.00 $ $ $ $ 3.00 1.00 1.00 2 .00 Burroughs 12-key keypad ·-a-·· · · .. SH84 Philips dual spindle drive motors $ 3.50 $ 2.00 $13.50 $ 3.00 $ 2.95 S~51 PSU smoothing & filter unit SHERIDAN ELECTRONICS 286 Cleveland Street, Surry Hills, NSW 2010 Ph: (02) 699 5922 or (02) 699 6912. Fax: (02) 698 3694 Mail orders to: PO Box 226, Strawberry Hills, NSW 2012 NOTE: Wo accept Bankcard, Mastercard and Visa. Howerer, we cannot glre quantity discounts on credit card purchases or account orders, Minimum tor account orders Is $15 excluslre of postage and packing. All prices Include Salos Tax. All prices quoted are tor this month only or unt/1 stocks last :r: ::a z n n 0 t""' gi C) C) .,. f CH1 BUS I I r ' -15V 1n1, -15V 910\l 60Hz VR14 100k LIN 16VWi ~ I .,. 1nr.1 240Hz VR15 100k LIN 2.2k .,. I FIVE BAND EQUALISER 1 OF 4 r 9100 CUT I .,. .018 T 4.7k:S I l - 100kl .,. .047 ~ 1kHz VR16 100k LIN BOOST 4.7k 100k i F :-i .,. 33pF.I I .0012: µ 1 011 -=:= I .022 0.22 47k SUM AMPLIFIER ANO POST FADE AMPLIFIER 1 OF 4 16VW:r: 100 r--------~-+15V 100 -... 75k 22pF _,,- -- ■■ T I I I I T _J_ I 1.,. 100k 560pF1 .0047 HEADPHONE MONITOR 1 OF 4 rV I I I l + T _.91011 ........ ¥hi 3.5kHz VR17 100k LIN ■ 33pF ..\D_.k_. I\, I UU 10kf ,. 10k I +15V BALANCED LINE AMPLIFIER 1 OF 4 I ., .,. II 3 -- BALANCED LINE OUTPUT ~ J I CHANNEL MONITOR 16VW+ 10 "ii L----~---+-+------15V ~ 16VW 10 ,------tl------------- +15V .......... 910[) h'h 10kHz VR18 100k LIN -15V .... ..,. ?"'-- +15V s::: > (") :r: "-I Cf) C) co ....co !Jj + + ":" EOC VIEWED FROM BELOW 8 I w >- v2 18 117 10 .J.i" 16VW+ f""" +15V 3.9k +A¥, ± 1% 30k 0.11 SIG K -57dB -15V , PEAK/AVERAGE LED VU METER 1 0F4 ~ 10 10 16VWI 16VWI +5V hs -:- +15V -:- RL0 4 115 - 680!! 1% 113 REF ADJ 8 .011 + 10 .,JJ' .,. 16VW.I 10 111 IC20 7555 ""=' 1% 1k 13 16VW:r } 112 ), t--L l 100k 0.1-I RHI 6 REF OUT 7 IC14 LM3915 114 ), -:- l l 1% l I 1k v+ 3 MODE g 110 }- ~ 11 + 1 16VWI v+ 3 g MOOE sGtl,c21c 100k !- ~ V2 l1a 1% 910!! ~ 8 REF ADJ 680!! 1'/, l1s 10 16VWi -: RL0 4 In ~ 5 r:i\11c21b 10kt +15V REF OUT 7 RHI 6 1,4 IC15 LM3915 l1s 113 IC21d 12 l,2 (ll (10 SIG ~,//wI 0dB '1~rn 2ol,o , tlf l f l f l f ... LED1-20 The power supply board provides ± 15V rails to power the op amps, together with a + 5V supply to run all the LEDs in the VU meters. The three 3-terminal regulators are bolted to small heatsinks. For a detailed description of how gyrator and equaliser circuits work, see the articles on the Studio Series 1/2-Octave 20-Band Stereo Equaliser described in the August and September 1989 issues. Balanced output stage The output of the 5-band equaliser, at pin 7 of op amp IC10b, feeds the balanced output stage which comprises op amps ICl la and ICl 1b. This acts in the following way: ICl 1b acts as a non-inverting unity gain buffer which feeds pin 3 of the XLR output socket. It also feeds IClla which is connected as an inverting op amp with a gain of 68 SILICON CHIP - 1. This gives a signal 180° out of phase with the input. It drives pin 2 of the XLR socket. Both output signals are coupled via 4 7µF capacitors. As well as driving the balanced output stage, the output of IC10b also drives the headphone monitor, comprising IC10a, Q3, Q4, D4 and D5. This stage is identical to those used for each input channel, as described above. VU metering The signal metering circuitry is based on the National Semiconductor LM3915 dot/bar display driver. Used by itself, the LM3915 is capable of driving 10 light emitting diodes to give a moving dot or bargraph display with a signal range of 30dB. To give a moving dot display (ie, only one LED on at a time), pin 9 is pulled below the V + line at pin 3. To give a bargraph display, pin 9 is connected directly to pin 3. In the circuit under discussion, we have two LM3915s cascaded to cover a signal range of 60dB. To enable them to display a bargraph for the average signal level and a dot (single LED) for the peak signal level, the LM3915s are switched rapidly between the two modes and . the signal fed to their inputs (pin 5) is switched at the same rate. This switching, or multiplexing, is accomplished by a 4066 4-way analog switch, IC21. Its four switch sections are opened and closed by the square wave signal generated by IC20, a 7555 timer. This is connected to give a 50% duty cycle square wave by charging and discharging the .0lµF capacitor at pin 6 via the 100k0 resistor connected from pin 3. The frequency of this waveform is about 450Hz. Signal detection The signal from the output of the equaliser (pin 7, IC10b) is fed to IC16, a non-inverting amplifier with gain adjustable by trimpot TRIM2. IC16's output is fed to ICl 7 which is Below: four of these 5-band equaliser plus LED VU meter boards are used · in the mixer (one for each output channel). This board carries all of the circuitry shown in Fig.4. OUT 2500 25VW 2500 25VW 240VAC + 2500 25VW + - - 1 25VW - 1 25VW + + - .,. - 2500 25VW OUT E • +15V -1 5V FO"-'-UT_ _ _ _ _ ___,,_ +5V nh, CASE 012-015 4x1N4002 2200 16VW + 2200 _ 16VW + - 1 16VW .,. Fig.5: the power supply uses a large toroidal transformer with two secondary windings. A 30V centre-tapped secondary feeds bridge rectifier D8-D11 and this drives positive and negative 3-terminal regulators to give ± 15V supply rails. The other secondary feeds bridge rectifer D12-D15 and this drives a 7805 3-terminal regulator to give a + 5V supply. another fullwave precision rectifier (ie, it responds to both the positive and negative half cycles of the waveform and it has very good linearity). Its output is fed via a 68k0 resistor to a lµF filter capacitor to give a DC voltage which is proportional to the average signal level over any time period of approximately 68 milliseconds. This DC voltage is the one used to produce the bargraph display. To obtain the peak signal level at any time, the rectified signal from the cathode of D6 is fed to IC18 which functions as a sample and hold circuit. The l0µF capacitor, combined with the 36k0 and 62k0 resistors, has a time constant of one second so that even brief signal peaks are "caught" and displayed. Switching between display modes To understand how the switching between the two modes occurs, essentially all you need to know is that each 4066 analog switch is switched on when its control pin is taken high. Hence, when the output of IC20 is high, switches IC21a, IC21b and IC21c are on and IC2ld, is off. This places the LM3915s in bargraph mode and the averaged signal from the lµF capacitor on pin 8 of IC21c is fed to pin 5 of IC15, the LM3915 displaying the top 30dB of signal range. The same DC signal is fed to ICl 9, an op amp with a gain of 31 ( + 30dB) to drive pin 5 of IC14, the LM3915 displaying the lower 30dB of signal range. When the output of IC20 is low, switches IC2la, IC21b and IC21c are all off (ie, non-conducting) while IC21d is on. This feeds the peak DC signal through to the LM3915s, which are now in dot mode. Power supply Fig.5 shows the power supply which has more in common with a power amplifier than a small signal device. It uses a hefty toroidal power transformer and a substantial amount of filter capacitance. A centre-tapped 30V (15V + 15V) secondary feeds a bridge rectifier consisting of diodes DB to Dl 1. The positive and negative outputs of the bridge rectifier each feed two 2500µF filter capacitors. The resulting positive and negative DC rails feed 3-terminal regulators to obtain balanced supply rails of ± 15V. A separate regulated 5V supply rail is required to run all the LEDs of the four dot/bargraph displays. This is obtained from an BV winding on the transformer which feeds a bridge rectifier consisting of diodes D12 to D15. The DC output is filtered by two 2200µF capacitors which then feed a 7805 3-terminal regulator. RF breakthrough To minimise RF breakthrough in all parts of the circuitry, you will notice that many of the feedback resistors of the op amps are shunted with capacitors ranging between 39pF and 220pF. This is to roll off the response a hove the audio range. You will also note a number of rolloff networks involving lkO series stopper resistors and 150pF shunt capacitors. Again, these are there to roll off any supersonic and RF signals so that they do not break through into audibility. This is most important for a professional mixer design which is often used in proximity to vicious electrical noise sources. Next month we will describe the construction of the printed circuit board modules and, if space permits, describe the wiring of the mixer control panel. ~ MARCH 1990 69