Destiny Modular Mixer (Part 2)
Output & Auxiliary Modules
In last month's look at Tim Orr's Destiny mixer design, we described the facilities and construction of the input module. In this second part, we take a look at the output and auxiliary modules. These three modules then form the basis of this versatile and flexible modular mixer. The separate power supply unit will be described in a later part.
First to correct a statement made in the first part; each output module will only process signals put on to one of the group busses, and as such can only be considered as having one output, not two as mentioned last month. So, for a stereo output, two output modules will be needed.
Referring to the block schematic Figure 1, and the circuit diagram Figure 2, IC1b of the output module operates as a virtual earth amplifier to collect signals put onto the appropriate group buss. The advantage of using a virtual earth amplifier is that, since its input impedance is theoretically zero, no voltage is developed, hence no current is bled off by any non-contributing buss inputs. In theory then, any number of input modules could be added to the buss without lowering the throughput gain of the other modules. The gain of IC1b is set by VR1 so that a sensible mixed signal level can be achieved regardless of the number of input channels contributing. VR4 allows the output signal to be put onto the PFL buss, while VR3 provides, via IC1a, a pair of studio output sockets with a monitoring signal. IC2 picks off the output signal from the fader, VR2, and supplies the Record output sockets, JK3 and 4 with signals up to +18dBm at 20KHz!
IC3a and b form a precision full wave rectifier and peak detector for driving the peak programme meter (PPM). The LED driver, IC4 does all the necessary logarithmic decoding and driving of the ten LEDs, D8-17. The device runs in the dot mode for low current consumption, driving the LEDs in series between the +ve and -ve supply rails to keep any 'dirty' current off the 0V rail.
Although not an essential part of the mixer, the auxiliary module is certainly a worthwhile addition since it provides headphone monitoring, talkback microphone, auxiliary outputs for effects or foldback, a PFL output, and a PPM display for the PFL/headphone signal, as shown in Figure 3. Referring to Figure 4, the signals on the PFL, Aux1 and Aux2 busses are collected by the virtual earth amplifiers IC3a, IC1a and IC1b respectively.
The amplified PFL signal is passed to the PFL output JK4, and to the PPM circuit, IC5 and 6. Since IC3a operates with unity gain, the signal level in any input channel PFL switch. The headphone amplifier, IC4 makes use of a synthetic ground rail produced by TR1 so that the 'dirty' headphone current is kept off the true ground rail. VR1 and VR2 allow the auxiliary output levels to be adjusted. S1 and S2 additionally allow the auxiliary signals to be put onto the PFL buss.
The talkback plus Aux 2 output socket, JK1 normally carries the Aux 2 signal. When the talk switch, S3 is operated however, the Aux 2 signal becomes attenuated by some 20dB and allows the amplified talkback microphone signal to be mixed in.
The output and auxiliary modules are both assembled in much the same way as described for the input module last month, ie. wire links first, then resistors, IC sockets, semiconductors and capacitors. The rotary pots and sockets are then added and soldered. Before the panels are fitted, the auxiliary module has two slider switches which must be located, but not soldered.
The evenness of assembly of the PPM LEDs on both the output and auxiliary modules will have a significant effect on the overall appearance of the mixer, so it's worth taking plenty of time in doing it. Firstly, the LED leads have to be pre-formed so that the leads are bent down by 90 degrees at a point about 20mm from the face, after making sure of polarity. This is best done using a specially prepared jig of wood or plastic or whatever, which should ideally include an end-stop for accuracy. Now pop the LEDs into the PCB but before soldering them and the switches in place, the panel should be fixed onto the pot bushes so that the LED and switch positions can be optimised. The output module fader can then be connected using a piece of screened cable.
The talkback microphone should be found to fit snugly into the auxiliary panel hole, although some epoxy adhesive applied to the rear will fix it permanently. The bezel should be fitted to the talkback switch, S3, before it is clipped into the front of the panel. Make sure that the orientation is as shown in the manual component overlay.
The buss address of the output module is determined by the position of a link near the buss connector. The auxiliary module microphone, and the headphone socket JK5 are connected using twisted pairs of insulated hook-up wire. The talkback switch S3 though, can be connected to the PCB using straight tinned copper wire.
The final stage is to fit the ICs into their appropriate sockets, making sure of polarity. Note that the 8 pin IC near the buss connector of the output module is in fact IC1, not IC4 as shown in the manual.
The output and auxiliary modules can be tested independently by connecting them to a twin rail power supply, as detailed for the input module last month. On the output module, a signal injected via a 47k resistor to the address link should appear on the studio outputs at a level determined by VR3, the studio level control. The signal also appears on the record outputs when the fader is lifted, as well as being displayed on the PPM.
A pair of headphones plugged into JK5 of the auxiliary module will enable you to monitor a signal injected via a 47k resistor to pin 2 of the buss connector. This signal will also be displayed on the PPM. A signal similarly injected into pins 3 or 4 of the buss connector will become available from the Aux 1 or Aux 2 sockets respectively. The Aux 2 signal should also appear on JK1, the foldback output. When the talk switch is operated, the Aux 2 signal on JK1 should be attenuated, and the talk-back microphone become active.
Many readers will have noticed that the Destiny does not make direct provision for individually controlled groups. There are ways around the problem though. A group is useful when, for instance, you have a drum kit miked up into four inputs. To control the overall level of the drums in the mix, it is obviously inconvenient to have to ride four faders. If the drum channels were routed to a group fader, then this single control would affect the overall drum level setting.
If you are using your Destiny to mix to a multitrack recorder, then obviously the drums could all be routed to one output channel (or across a pair for stereo), which is then used for one track on the recorder. When mixing a live show down to stereo, the solution is to route the drums in this instance to a spare output channel, or the auxiliary module. This combined signal is then routed around to a spare input channel, whose fader then controls the overall drum level... easy!
Feature by Paul Williams
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