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| Elliott Sound Products | Project 30b |
In this article, the Mixing Modules, Line Output stages and the start of the power supplies are described. There are two main types of mixing module - a stereo unit to accept the outputs of the Mic/Line modules, and a mono version for the Auxiliary sends that includes an Aux return as well (these are typically used for effects, or as a simple foldback mix).
The number of modules depends entirely on the final configuration you are aiming for. A typical unit might have four stereo sub-groups, one master mixer and two auxiliary send mixers.
The master modules do not have tone controls, as it is anticipated that the final mix will be sent to an outboard graphic equaliser or parametric equaliser for overall acoustic tuning. Simple tone controls in the master sections are worse than useless.
A block diagram of a 'typical' configuration is shown below. This is based on my original premise of four sub-groups, two auxiliary sends, and a master. All sub-groups outputs are re-mixed to the master mix bus, and optionally can be connected to balanced line output stages as well. The aux sends are not shown, but work in much the same fashion (except they are mono, not stereo).
Figure 1 - 'Typical' Configuration
As shown, channels 1 & 2 are switched to Mix Bus 1, channel 3 to bus 2, channels 4 & 5 to bus 4, and channel 'n' to the master bus. The outputs of each sub-master (or group) all connect to the master bus. All buses are stereo, but this is not shown for clarity. This is to demonstrate the flexibility of using the multiple sub master groups, a 6 channel mixer with 4 groups and a master would not be worth the effort, but when you have a 36 channel unit this will change !
The mixer modules are 'virtual earth' types, meaning that the actual mixing bus carries signal current, but that little or no voltage is measurable. This is the most common type of mixer, as it means that there is no interaction between the various input level controls (the faders).
The input capacitors should be high quality aluminium electrolytic types, as these are capable of operating safely with no bias (or a slight reverse bias). The Gain control is used to trim the Master Module gain in the same way as the input modules. This allows the operator to get all faders in a position where gain can be increased or decreased by the desired amount, and returned to a known starting position. A setting of -6dB on all faders is a good starting point, and this allows the operator to be able to change the level of any channel or group, and set the level back to the 'standard' starting position again.
An output is provided for peak detectors (the circuit for these is in Part 1) - their use is recommended, but not mandatory. Even better is to use good quality meters (to be described in a later article) for all Sub-Groups and Masters.
Figure 2 - Stereo Mixing Module (Groups & Master)
Figure 3 - Mono (Auxiliary) Mixing Modules
The line output stages (like the mic / line inputs) can be either transformer of electronically balanced. Both types are described so you can choose the one you prefer (or can afford - good transformers are expensive). For recording work, the line output stages can also be connected to as many of the mic/line input modules as needed to suit your recorder. Typically, line out modules would be connected to the output of the tone control section for each channel. The fader is used only for mixdown, and to set the correct mixing level during recording. Some engineers prefer to record flat (no equalisation) to ensure that nothing is lost in the original. Adding a switch to connect the line out pre- or post-equalisation allows for all possibilities.
Figure 4 - Active Line Output Module
I suggest that the signal is reduced by 6dB before it is applied to the active line out module, otherwise the level would be double that expected due to the balanced circuit. The transformer version (below) is unity gain, since transformers do not have this problem - they have others instead.
Figure 5 - Transformer Line Output Module
There are a great many different circuits for balanced line drivers, but the simple approach is often the best. The simplest is to use a transformer, but unfortunately, if you want good quality you will pay for it. Before deciding to use a transformer output, I strongly recommend that you read Transformers For Small Signal Audio, as this article describes the various options as well as traps for the unwary.
Click on the PCB image to look at Project 87 - Balanced Line Driver & Receiver - PCBs are available.
There are two sections to the power supply, the main +/-15 Volt supplies and the 48V phantom feed supply. The main supplies require considerable current capacity, due to the large amount of electronics involved, so conventional 3-terminal regulators will not work unassisted.
The 48V phantom supply needs to be as quiet as possible, as any noise will appear at microphone inputs. In theory, the noise is simply cancelled because it is common mode, but I believe that a quiet power supply is worth the effort.
Figure 6 - Power Supply Transformers and Filters
The transformer and bridge rectifiers are shown in Figure 6 for the two supplies. The size of the transformer for the phantom power supply depends on the number of phantom powered devices you want to operate. Since each will take a current of (typically) 7.0mA or 14mA worst case, we need to design for the worst. If 10 phantom powered devices were to be used at once, we will need a supply capable of 140mA at 48V. Before regulation, we will need about 60V minimum, and I suggest a 20VA transformer which will be more than enough.
For the +/- 15V supplies, I have used an estimate of 100mA per module (this is a bit of overkill, but it is far better to have it and not need it than to need it and not have it). Allowing for a 36 into 4 into 1 mixer with 4 Aux sends and 20 line outputs, this means a total current of about 4A. To be on the safe side, a supply capable of 5A is the design goal, so using a 20-0-20 transformer (40V centre tapped) at 1.8 times the DC current means that we will need a 360VA transformer as a minimum (use 500VA). (In case you missed it, the AC current into a capacitor input filter is 1.8 times the DC current for a full wave bridge rectifier.) With any transformer over 300VA I recommend that you include a Project 39 soft start circuit.
The minimum power supply components are listed below, feel free to increase the ratings on transformers and capacitors, and adjust the fuse rating accordingly.
I strongly recommend that the power supply be built in a separate box, so that no transformer hum is injected into the circuitry. The virtual earth mixing buses are particularly susceptible to magnetic flux leakage from transformers, and a separate supply will eliminate this problem, and helps to keep the weight down as well. The power supplies will require generous heatsinks, so a small enclosure is not an option.
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