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A completely new design for stage or studio with equalisers on each input channel 16-CHANNEL IXING DESK In the entertainment business, nothing stays still for long. Where 8-channel mixers used to be regarded as pretty fancy stuff, they are now just not big enough. Sixteen input channels are now necessary, with the ability to mix down to two or four output channels. This new low noise design does just that. By JOHN CLARKE This design is the result of collaboration between SILICON CHIP magazine and Ja year Electronics Pty Ltd. The starting point was Jaycar's very popular and reliable 8002 mixer which, as its name suggests, has 8 input channels and two output channels (for stereo). The aim was to expand the basic design to 16 input channels and mix down to 2 or 4 output channels, making it compatible with the many 4channel recorders presently available. Quite a few extra features were added as well. Each input channel can now be switched to provide for balanced or unbalanced line inputs, as well balanced microphones. The various channels can also be switched right out of circuit when not in use, so that they make no contribution to the residual noise in the system. A LED overload indicator is fitted to each channel so that the mixer operator can keep distortion to an absolute minimum. As well, the 18 SILICON CHIP & LEO SIMPSON operator can separately monitor any of the 16 input channels via the headphone sockets so that levels can be absolutely "spot on". Signal monitoring in the four output channels is pretty fancy too. Instead of small VU meters, this new mixer has vertical LED bargraphs with average and peak indication over a range of 60dB. While the Jaycar 8002 was the starting point as far as the operating facilities are concerned, the circuit design has been completely revised and is new from the chassis up. Where the previous design was largely based on 5534 op amps, our new design uses LM833 dual low noise op amps. This has allowed us to keep the overall component count and cost under control while considerably increasing the facilities. And what about the cost? At the time of writing this first article in what will be a series of three or four, the all up cost for a kit of this 16-channel mixer as published is expected to be close to $1400. This makes it a real bargain when compared to any commercial mixing desk with a comparable range of facilities and performance. By the way, the main performance parameters are summarised in a panel accompanying this article. We think you'll agree that the specs are very good and more than comparable with the commercial mixers. We also understand that there will be a 12-channel version of the mixer which will have the same size control panel and chassis but with the unused controls blanked off. As well as this, kits of the various modules will be available separately for those readers who want to produce their own customised versions of the mixer (see the Jaycar adverts in forthcoming issues for the details). Our thanks to Jaycar for their assistance throughout the design of this complex project and for supplying all the componentry. Now, let's get down to the nitty-gritty of this instrument. Physical layout There is no way around it - a 16 channel mixing desk of this complexity is a large instrument. With timber end pieces, our prototype measures 865mm wide and 440mm deep. It weighs about 14kg. While the dimensions of the final production version are likely to be slightly different, the mixer panel itself is 828mm wide and 435mm deep. It is mounted on a chassis to place it at a comfortable angle for the operator - about 85mm high at the front and 180mm high at the back. Those latter dimensions depend to a small extent on the size of rubber feet used. All told, there are 156 knobs, 20 60mm sliders and 17 toggle switches, making a total of 193 controls on the panel. Each knob is colour coded, so that its function in each channel is clarified, making it less likely that you'll reach for the wrong knob. In addition to all the knobs and switches, there are 18 6.5mm jack The new mixer features switchable balanced or unbalanced line inputs, equalisers on each input channel, and effects and foldback control facilities. The 4 output channels are monitored using LED bargraph VU meters. FEBRUARY1990 19 ii / Four different PCB designs are used in the mixer: one for each of the input channels, one for each of the output channels, one for the effects and foldback control circuitry, and one for the power supply (not shown here) sockets for headphone monitoring and 80 LEDs in the four bargraph signal indicators. Even so, while there are a great number of knobs and switches, this is largely due to duplication of the features in each of the 16 input channels. After a short period of familiarisation, an experienced mixer operator will feel at home with this unit. Mixing features The 16 input channels are arranged in two groups of eight columns, starting from the lefthand side of control panel. At the very top of each column is a 3-position rotary switch to select microphone (mic), balanced line or unbalanced line input. The next knob down is the prefade attenuator (atten). This allows you to set the channel signal level independently of the main fader control - a very handy feature. And in between the input selector and pre-fade attenuator is the red LED (light emitting diode) overload indicator. This allows you to have signal levels under control at all times. Specifications 20 Below the pre-fade attenuator is a 3-band equaliser (EQ) giving a range of boost and cut of ± 12dB at treble, middle and bass frequencies. This will augment the tone controls that are on most electronic instrument inputs and give adequate tone adjustment for microphone signals. The sixth knob down, below the 3-band equaliser, is the 'pan' control which allows the input signal to be directed to output channels 1 or 2 or any combination between the two. The first 8 input channels can be panned to output channels 1 and 2 while the second 8 input channels can be panned to output channels 3 and 4. Below the pan control is a pair of knobs which provide for 'Effects' (sends) and 'Foldback'. The Effects control determines how much of the signal is fed to an 'effects' output which may provide reverberation, echo, phaser, flanger or possibly a digital processor which can provide any or all of these effects and a lot more besides. The 'Foldback' control determines how much of the channel signal is fed to a foldback amplifier and speakers. Foldback speakers are placed on stage so that musicians can actually hear their own playing or the lead player. Below the eight control knobs is a toggle switch. This allows each channel to be switched right out if it is not being used and so helps to minimise the total noise from the mixer. Right at the bottom of each column is a 60mm slider which is the Fader for that particular channel. And immediately above the Fader is the 6.5mm jack socket which may be used with any pair of stereo headphones. So far then, we have described the physical control features of each of the 16 input channels. Moving across the control panel we come to the columns of knobs for the output channels of which there are six: four main outputs, effects and foldback. Signal To Noise Ratio (with respect to 1 00mV input and output) 90dB from 20Hz-20kHz; 93dB with A weighting Sensitivity For 1 V Output 60mV for balanced and unbalanced inputs; 6mV for microphone Frequency Response -3dB at 15Hz & 38kHz, -1.5dB at 20Hz, -:-0.8dB at 20kHz Distortion 0.015% with respect to 1 V in and 3V out at 1kHz Equaliser ±12dB Output channels Bargraph Display Peak hold and VU meter; 60dB range with 3dB steps As with the input channels, the four output channels are arranged in columnar form, with the 20-LED SILICON CHIP All potentiometers except for the fader (slide) controls are mounted directly on the PC boards but there is still a fair amount of wiring. Much of this wiring is repetitive and involves hooking up the supply rails and making connections to the fader controls and input/output sockets. VU meters at the top. As each LED comes on, it indicates a signal increase of + 3dB and as noted above, the display gives both average (bargraph) and peak (single LED) display. Below the LED bargraph is a 5-band equaliser (EQ) giving maximum boost and cut of ± 12dB for frequency bands centred on 60Hz, 240Hz, lkHz, 3.5kHz and lOkHz. Next, below the 5-band equaliser, are the Faders, one for each of the output channels. On the far righthand side of the control panel are two columns of four knobs. These are the Effects return and sends controls, panning controls for the two effects channels between output channels 1 and 2 or between 3 and 4, and the Foldback channel level controls. In the bottom righthand corner of the mixer panel is a pair of 6.5mm sockets for stereo headphone monitoring of output channels 1 and 2 and 3 and 4. Above the headphone sockets is a toggle switch to provide for stereo or 4-channel outputs. Rear panel On the rear panel are 16 female XLR sockets for the inputs and 4 male XLR sockets for the balanced output channels. Above the latter group are six 6.5mm sockets: two for the Foldback channel outputs, two for the Effects return line and two for the Effects send line. There is also the mains fuse and the mains toggle switch, although we expect that production models of this mixer will have an il- luminated rocker switch in place of the power LED at the top righthand corner of the control panel. Block diagram Having discussed the physical layout of the controls, you can get an idea of the electronic layout of the mixer by having a look at the block diagram of Fig.1. Even though it is a large diagram, it shows only two of the input channels. However, all of the output channels are depicted, so that you don't have to leave too much to the imagination. As you can see from the block diagram, the physical layout of the controls does not really relate to the flow of signals through the circuitry. For example, the Pan and Effects sends pots come after the main Fader for each input channel which you would not necessarily expect by looking at the control panel. This is because good control layout (ergonomics) and good cirFEBRUARY 1990 21 '"Cl ..... z n ::c: 0 n t== ..... er. N N PROGRAM INPUTS 9-16 PROGRAM INPUTS 1-8 1 : MIC (BAL) 20dB .. 13 0 O PREAMPLIFIER +20d8 OdB 3u------------ 1~ 2 1 1 : MIC (BAL) 20dB 2 : LINE (BAL) OdB 3 : LINE (UNBAL) 30--------- 1~ INPUT" SELECT 20----0 3 : LINE (UNBAL) 2 : LINE (BAL) OdB PREAMPLIFIER +20dB OdB INPUT BUFFER AND ATTENUATOR 6-40d8 FOLOBACK SEND .,. !• INPUT BUFFER AND AITTNUATOR 6-40dB 3-BANII EQUALISER :!:12tl8 POST FADE BUFFER +6dB .,.. FOLDBACKf., SEND 3-BANO EQUALISER :!:12dB 1 OF 8 . 1 OF 8 CH1 BUS CH4 BUS CH3 BUS POST FADE BUFFER +6dB FOLDBACK 9-16 BUS EFFECTS 9-16 BUS .,. FOLDBACK 1-8 BUS EFFECTS 1-8 BUS CH2 BUS .,. .,. J• hh uu Win uu PAN PEAK DETECTOR EFFECTS SEND HEADPHONE MONITOR EFFECT~f • SEND HEADPHONE BUFFER OdB .... Hti fh ,. ',\ PEAK DETECTOR .,. C1 O' CH2 ' fY CHJ IN/OUT 0 IN/OUT CH1 ~ ~ N 0 cc ....cc >-<: :Jj §: ti:, :Jj EFFECTS 9-16 RETURN 0-----1 EFFECTS o----t 1-8 RETURN ':' / I [,/ I I . ( I V I I,,, I r I,,, I ~f~~~&:-7 4/2 OUTPUT BUFFER +6dB FOLDBACK 9-16 FOLDBACK 1-8 EFFECTS 9-16 EFFECTS 1-8 J;.IM CH2 CH3 CH1 CH4 SUM AMPLIFIER .,. 5-BAND LED VU METER EFFECTS 1-8 SEND LINE OUTPUT >--------------------------OF~t3:~~~pt{6 EFFECTS 9-16 SEND ILINE OUTPUT CH3/CH4 HEADPHONE MONITOR 9-16 l PROJGRAMS 4 BALANCED CH '-------0 LINE OUTPUT BALANCED LINE AMPLIFIER +6dB >------------------------0 5-BAND EQUALISER METER LED VU BALANCED CH3 '-------0 LINE OUTPUT BALANCED LINE AMPLIFIER +6dB elu~~IER ______,_________ METER LED VU 1-8 >------------------------v FOLDBACK LINE OUTPUT >-------------------------0 HEADPHONE BUFFERS 1-8 PROJGRAMS 2 BALANCED LINE OUTPUT CH l CH1 BALANCED LINE OUTPUT CH 1/CH2 HEADPHONE MONITOR ~ BALANCED LINE AMPLIFIER +6dB L----0 .BALANCED LINE AMPLIFIER +6dB E~~~ER _ _""T_ _..,._ _ _ _ _ _ _-t EQUALISER LED VU METER Fig.1: here's bow the various circuits are linlced together. Note that only 2 of the 16 input channels are shown. EFFECTS 9-16 ATTENUATOR CH3 SUM AMPLIFIER CH1 SUM AMPLIFIER The power supply is mounted in the bottom righthand corner of the chassis. It uses a toroidal transformer to keep mains hum to an absolute minimum. cuit layout aren't necessarily related. Now let's follow the signal though the block diagram for one of the input channels, as shown along the top of the 2-page diagram. The input signal goes to a 3-pin female XLR socket and then to the input switch which provides for balanced or unbalanced input operation of the preamplifier. Not shown is the other pole of the input switch which changes the gain of the preamplifier, depending on whether microphone or line input is desired. For microphone input, the preamp has a gain of + ZOdB [lOx) while for line inputs, the gain is unity. Following the preamplifier is the input buffer and pre-fade attenuator stage which, depending on the setting of the control knob, can have a gain between + 6dB (Zx) and + 40dB [lOOx). Note that the prefade attenuator is a feedback control which means that it minimises the residual noise for any gain set- ting and also can't be set down to zero, which could cause confusion for the operator. After the pre-fade attenuator stage is the 3-band equaliser, followed immediately by the main channel fader and the foldback level controls. Following the main fader is the "post fade buffer" stage with a gain of + 6dB. This feeds the headphone buffer, the effects send control and the associated pan pot. The headphone buffer also drives a peak detector stage which drives the overload LED indicator for each channel. In the lower half of the block diagram you can see all the output channels depicted. In each of the four main output channels, the signal is picked up from the associated signal bus and fed to a summing amplifier [gain OdB or unity) which then drives one of four master faders. Each master fader is followed by a buffer with a gain + 12dB (4 times) which then drives the associated 5-band equaliser. The equaliser then drives LED VU meter stages and the balanced output amplifiers which are terminated in 3-pin male XLR sockets. Immediately below the Chl and ChZ summing amplifiers are the effects 1-8 and foldback 1-8 summing amplifiers. These both drive their associated master faders and line amplifiers to provide the send and foldback outputs. The rest of the diagram is pretty much self-explanatory although it needs to be looked at in conjunction with the individual circuit diagrams which will be published in next month's issue. Mechanical design The mechanical design of the new mixer is fairly simple. Essentially it is just a large panel with a whole lot of boards attached to it via their potentiometers. The control panel is supported on a large shallow chassis which has the power supply in the righthand rear corner. A toroidal transformer is used to keep hum at a minimum even though the total power drawn is fairly high. Four different PC designs are used, including that for the power supply. All potentiometers except for the Fader controls are wired directly into the boards which helps keep the wiring at a minimum. However, there is still quite a lot of wiring in the unit, as you can see from the photographs. Next month Next month we shall present the circuit details of all the different printed circuit boards and the complete parts list. ~ The rear panel holds 16 female XLR input sockets, 4 male XLR output sockets and 6 6.5mm sockets for effects & foldback. 24 SILICON CHIP