ElectroMix 842 (Part 2)
The second and final part of our superb professional quality studio mixer project
This month we continue our superb mixer project with construction and setting up details.
Although construction should be fairly straightforward, there are a few points worth noting. Before assembling any of the boards, slots for the buss wires have to cut in the edge using a hacksaw and/or small file. The buss wire should be 16swg, stretched to form straight lengths. IC sockets should be used although none were used in our prototype. A socket for IC1 of the Channel Module can be made by carefully cutting a 16 pin socket in half.
Insert and solder the required links on each board as shown in Figures 1, 2, and 3, with reference to the photographs of the completed boards. Next insert the veropins; on the component overlays these are shown with crosses when they protrude from the track side and with dots when they protrude from the component side. The resistors and capacitors can now be inserted and soldered, taking care with electrolytic capacitor polarities. If you are using IC sockets these can now be positioned and soldered. The switches and pots come next, not forgetting the pre-set on the monitor board. After this stage the IC's can be pushed into their sockets or soldered into place.
As PCB mounting jack sockets seem almost unobtainable except to equipment manufacturers, normal types with solder tags are used which are fixed to the boards with short lengths of tinned copper wire, (soldered first and then inserted into the PCB's). The sockets and the switches must be seated squarely on the board to avoid problems when mating the PCB's to the front panel. There is a link to be added on the routing switches of each channel board and one on the monitor board which are soldered directly to the switch tags (see overlay diagrams). There is also an earthing link from each PCB to the back of one of its potentiometers. The LED's are bent and soldered to Veropins after the PCB's have been mounted to the face plate. The bulk of the wiring is to the Aux, Group and Monitor sockets which is made with screened wire. The returns of the Group faders should be screened although this is not necessary for the Channel faders (see wiring details). As the power supply is remote, miniature 3-core mains cable was used to feed the Channel supply busses. These should be linked to the Group and Monitor busses (see Figure 4).
After checking wiring, IC, diode and LED orientation, power up and test supply voltages. If all is well, connect a suitable amplifier and speaker set-up to the Monitor outputs. Turn the Monitor level to maximum, little or no background noise should be heard. Plug a signal source (tape deck, radio etc.) into Channel 1 line input and select line input. Bring the Channel and Group faders to 0VU and the Panpot to central position. Press Group 1 and 2 switches on the Monitor module and adjust Monitor level. Run through all the functions on the Channel i.e. Input select (check Mic input is more sensitive), EQ, Routing, Pan, Aux sends, etc. This is repeated for the other Channels selecting the appropriate outputs on the Monitor select switches each time. If all inputs and outputs appear to operate correctly, disconnect the signal source and turn Monitor level to maximum. Run through all controls and switches which should operate quietly. Any thumps or clicks produced by switches indicates a DC level, which should be checked out with an oscilloscope or voltmeter. If any controls produce a small amount of noise this should be compared to normal listening levels to see if any further investigation is necessary.
The only preset which has to be set on the mixer is the Reference for the clip LED's. Set all faders to 0VU and insert a signal. If using as a level indicator for recording adjust the LED's to come on at a 0VU reading on your Tape recorder. In a P.A. situation the LED's could indicate a signal level for full output power of the amplifiers used. Alternatively as a clipping indicator, reduce the level of signal with the fader and increase the gain until definite distortion is heard. Reduce the gain so the distortion is no longer audible and adjust the LED's to just light. If you have access to an oscilloscope Table 1 gives peak to peak values for various signal levels.
The mixer requires a ±15V regulated supply at approx. 200mA. A supply designed specifically for the ElectroMix is shown in Figure 5. This circuit uses standard voltage regulators IC1 and 2 fitted with on board heatsinks. This supply will also power the meter bridge.
Construction is straightforward with components inserted in the usual order; veropins, diodes, capacitors and IC's. (— correct polarities are very important!). See Figure 6.
Double check all connections before connecting to the mains supply.
Please note; LF351 and LF353 IC's were specified on the circuit diagrams last month. Certain samples of LF353 seemed to burst into high frequency oscillation, so alternative types TL071 and TL072 were tested and proved to be more suitable, with the added bonus of being a lot cheaper.
The Retro-fit input channel with an electronically balanced Mic input and the Meter Bridge will be described in a forthcoming issue.
All parts except panel and PCB's are available at most electronic component retailers. Details of complete kit suppliers will be published next month.
The fully finished panel and a set of PCB's are available from Electronics and Music Maker, (Contact Details). The prices are £24.95 for the drilled, sprayed and screen printed panel and £24.95 for a full set of PCB's inc. p&p and VAT. Order as ElectroMix 842 Panel and ElectroMix 842 PCB set. Please allow 28 days for delivery.
Feature by Paul Bird
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